ic2

NEBOSH International Diploma in Occupational Health and SafetyPlease be advised that the course material is regularly reviewed and updated on the eLearning platform. SHEilds would like to inform students downloading these printable notes and using these from which to study that we cannot ensure the accuracy subsequent to the date of printing. It is therefore important to access the eLearning environment regularly to ensure we can track your progress and to ensure you have the most up to date materials. Version 1.3a (18/02/2013) Element - IC2: Principles of Fire & Explosion. Learning outcomes. On completion of this element, candidates should be able to: 1. Outline the properties of flammable and explosive materials and the mechanisms by which they ignite. 2. Outline the behaviour of structural materials, buildings and building contents in a fire. 3. Outline the main principles and practices of fire and explosion prevention and protection. 4. Outline the contribution to typical mechanical and systems failures to major accidents. Relevant Standards  International Labour Office, Safety in the Use of Chemicals at Work, an ILO Code of Practice, ILO, 1993. ISBN: 9221080064 Section 6: Operational control measures Section 7: Design and installation Minimum hours of tuition 6 hours. 1.0 - Properties of Flammable & Explosive Materials & Mechanisms by which they Ignite. The States of Matter - Solid, Liquid and Gas. Each of the classical states of matter, unlike plasma for example, can transition directly into any of the other classical states. Solid. The particles (ions, atoms or molecules) are packed closely together. The forces between particles are strong enough so that the particles cannot move freely but can only vibrate. As a result, a solid has a stable, definite shape, and a definite volume. Solids can only change their shape by force, as when broken or cut. In crystalline solids, the particles (atoms, molecules, or ions) are packed in a regularly ordered, repeating pattern. There are many different crystal structures, and the same substance can have more than one structure (or solid phase). For example, iron has a body-centred cubic structure at temperatures below 912 °C, and a face-centred cubic structure between 912 and 1394 °C. Ice has fifteen known crystal structures, or fifteen solid phases which exist at various temperatures and pressures. Glasses and other non-crystalline, amorphous solids without long-range order are not thermal equilibrium ground states; therefore they are described below as nonclassical states of matter. Solids can be transformed into liquids by melting, and liquids can be transformed into solids by freezing. Solids can also change directly into gases through the process of sublimation. Solids are used where we need something to keep its shape or to support something. This is because they keep their shape, stay where they are put and cannot be compressed. Imagine a chair. Try to imagine what it would be like if the chair was made from a liquid. Would it keep its shape? Would it support your weight? Liquid. A liquid is a nearly incompressible fluid which is able to conform to the shape of its container but retains a (nearly) constant volume independent of pressure. The volume is definite if the temperature and pressure are constant. When a solid is heated above its melting point, it becomes liquid, given that the pressure is higher than the triple point of the substance. Intermolecular (or interatomic or interionic) forces are still important, but the molecules have enough energy to move relative to each other and the structure is mobile. This means that the shape of a liquid is not definite but is determined by its container. The volume is usually greater than that of the corresponding solid, the most well known exception being water, H2O. The highest temperature at which a given liquid can exist is its critical temperature. Liquids are used where we need something to flow e.g. for making a drink, or when we need something to take up the shape of a container such as a mould. A good example of this is making a jelly. The jelly (solid) has to be turned into a liquid (in this case by dissolving) so that it takes up the shape of the mould. It is then left to set (i.e. go solid again) so that it keeps its shape when removed from the mould. Gas. A gas is a compressible fluid. Not only will a gas conform to the shape of its container but it will also expand to fill the container. In a gas, the molecules have enough kinetic energy so that the effect of intermolecular forces is small (or zero for an ideal gas), and the typical distance between neighbouring molecules is much greater than the molecular size. A gas has no definite shape or volume, but occupies the entire container in which it is confined. A liquid may be converted to a gas by heating at constant pressure to the boiling point, or else by reducing the pressure at constant temperature. At temperatures below its critical temperature, a gas is also called a vapour, and can be liquefied by compression alone without cooling. A vapour can exist in equilibrium with a liquid (or solid), in which case the gas pressure equals the vapour pressure of the liquid (or solid). A supercritical fluid (SCF) is a gas whose temperature and pressure are above the critical temperature and critical pressure respectively. In this state, the distinction between liquid and gas disappears. A supercritical fluid has the physical properties of a gas, but its high density confers solvent properties in some cases which lead to useful applications. For example, supercritical carbon dioxide is used to extract caffeine in the manufacture of decaffeinated coffee. Gases are used when we need something to spring back after it has been squashed e.g. in a tyre, or when we need something to spread out e.g. fly killer spray. This is because gases can be greatly compressed and, when you release the force that is compressing the gas, it will go back to its original state. If a gas is released from a container, it will spread out wherever it can so if you spray some fly killer at one end of a room, given time, it will spread throughout the room. Solids    Keep their shape unless they are broken. Do not flow. Cannot be compressed (keep the same volume). Liquids    Do not keep their shape, they take the shape of the container they are in. Flow. Cannot be compressed (keep the same volume). This molecular energy is transferred to other fuel and oxygen molecules.1 . and the emission of visible and invisible radiation. a reducing agent (the fuel) and an oxidising agent (usually oxygen) must be present. smouldering or glowing embers. not the liquid. Flames are often visible. in the strict chemical sense. accompanied by the evolution of heated gaseous products of combustion. The flash point of a liquid fuel is the lowest temperature at which it can form an ignitable mix with air. Oxidation occurs all around us in the form of rust on metal surfaces. the ignition source. For an oxidation reaction to occur. Oxidation. Solid fuels. spread out quickly from where they are to start with (this is called diffusion). It is also the minimum temperature at which there is enough evaporated fuel in the air to start combustion. The combustion process is usually associated with the oxidation of a fuel in the presence of oxygen. However. we refer to this process as a controlled fire and if it is a building on fire. when the unburned fuel is heated up to its flash point and then fire point. with the emission of heat and light. The combustion process occurs in two modes:   The flaming. they completely fill the container that they are in. It is the vapour that burns.Reactions in Fire. when the output of flammable gases from the material is too low for persistent presence of flame and the charred fuel does not burn rapidly anymore but just glows and later only smoulders 1. Therefore. The act of combustion consists of three relatively distinct but overlapping phases: Preheating phase. the key word that sets combustion apart from other forms of oxidation is the word "rapid". 1. means the loss of electrons. which creates a chain reaction. when the mix of evolved flammable gases with oxygen is ignited. As heat is added. we refer to this process as an uncontrolled fire. If the fire is in a fire grate or furnace. and in our bodies by metabolising the food we eat. Charcoal phase or solid phase. Heat transfer from the combustion to the solid maintains the evolution of flammable vapours. Flow. One generally-accepted definition of combustion or fire is a process involving rapid oxidation at elevated temperatures. A reaction takes place where the fuel loses electrons and the oxygen gains electrons. The non-flaming. Combustion of a liquid fuel in an oxidising atmosphere actually happens in the gas phase. Liquid fuels. . a liquid will normally catch fire only above a certain temperature: its flash point. the fuel molecules and oxygen molecules gain energy and become active. Energy is produced in the form of heat and light.Gases    Do not keep their shape. Can be compressed (squashed into a much smaller volume). This exothermic electron transfer emits heat and/or light. Distillation phase or gaseous phase.Combustion.2 . Flammable gases start being evolved in a process similar to dry distillation. which is characterised by a continuous deceleration in the heat release rate.3 . the fire is coming under control. during which fuel and oxygen are virtually unlimited. The induction stage is the precursor to ignition where preheating. This phase is characterised by an exponentially-increasing heat release rate. the fire will extinguish. or when suppression activity begins to impact on the fire. the Growth stage and the Decay stage. This phase is characterised by a heat release rate which is relatively unchanging. Continued burning in the flaming mode requires a high burning rate. If the heat loss is greater than the energy output of the fire. The early stage of a fire. convection. In this (final) stage. The flaming stage is a region of rapid reaction that covers the period of initial occurrence of flame to a fully-developed fire.4 . Invisible aerosol and visible smoke particles are generated and transported away from the source by moderate convection patterns and background air movement. Gas minute particles are generated and transported away from the source by diffusion. emitted from the fuel is referred to as pyrolysis. "the fire is burning itself out". It is also useful to be aware of the Steady State Phase. The decay stage does eventually occur naturally. air movement and weak convection movement. 1. the fighting of a fire is about getting to this stage as quickly as possible. 1. by various means or methods. however. The middle stage of a fire is the Steady State Phase. the fire is reducing in heat in a constant manner. .For the flaming mode. it is necessary for solid and liquid fuels to be vaporised. The final stage of a fire is the Decay Phase. is the Growth Phase. Heat transfer from the fire occurs predominantly from radiation and convection from the flame.Growth (Smoulder & Flame). The solid fuel vapours are thermally driven off or distilled. This gas or vapour production. distillation and slow pyrolysis are in progress. heat transfer from the flame to the fuel surface continues to drive off more volatile gases and perpetuates the combustion process. leading to fire extinguishment. and the heat loss associated with transfer of heat from the flame area by conduction. 1. There are three generally-recognised stages to a fire:    the Induction stage. and the liquid fuel vapours evaporated. Consider why this stage can occur naturally. It is this volatile vapour from the solid or liquid fuels that we see actually burning in the flaming mode. the combustion is coming to and end. The smouldering stage is a region of fully-developed pyrolysis that begins with ignition and includes the initial stage of combustion.Decay. and radiation must be less than the energy output of the fire.5 . this is the phase when. Transition from the Growth Phase to the Steady State Phase can occur when fuel or oxygen begins to be limited.Induction. Once a flame has been established. Spontaneous combustion can begin without any flame. the greater the potential fire hazard.8 C. The temperature is usually higher than the flash point. (Specialised extraction. when saturated cleaning cloths are not correctly disposed of and are placed along with combustible materials in 'regular' waste bins. spark. the vapours from the saturated cloths spontaneously ignite. A quite common everyday occurrence of this type of combustion happens with paint or cleaning solvents.7 . A flammable liquid has a flash point below 37.Relative Density.9 . .8 C while a combustible liquid has a flash point greater than 37. as further explanation. and when the vapours are ignited the heat of the flash raises the temperature of the liquid surface to a point where sufficient vapour is given off to sustain combustion.Fire Point. Auto-ignition temperature is the minimum temperature at which the vapour/air mixture over a liquid spontaneously catches fire. The cabinet is a semi-contained area with specialised extraction which ventilates the fumes directly away.Auto-Ignition Temperature. Vapour Density is defined as the ratio of the density of the gas or vapour to the density of air (vapour density of air = 1). it is the temperature when the vapour pressure of a substance becomes high enough to allow the air/vapour layer over the substance to be ignited.8 . This is what happened at Buncefield in 2005 (see section 7. Fire point can be defined as the lowest temperature at which the application of an ignition source will lead to continuing burning.6 of this module). vapours from flammable liquids are denser than air and thus tend to sink to ground level.6 . The lower the flash point. This means that no additional heat needs to be applied to the fuel source for the flash point to be reached. Generally.10 .Vapour Density. the process of spontaneous combustion.1. such vapours can travel some distance and encounter ignition sources remote from the point at which the vapour first started. such as Ether and Acetone have flash points below room temperature. 1.Definition of Flash Point. The flash point of a fuel is the temperature at which vapour given off can be ignited. 1. Flash point: a liquid is classified as flammable or combustible depending on its flash point. heat or other ignition source. also local exhaust ventilation or LEV) 1. This means that the vapour mix ignites without the need for a flame or a spark. Subsequently. which makes them very dangerous. Certain substances. An example of a control measure when working with heavier-than-air vapours is a fume cabinet found in laboratories. 1. Maximum Explosion Pressure. 1. possibly. 1. 1. Combustion occurs when the three elements are combined in the right mixture: . The fire is prevented or extinguished by removing any one of them. Please note that flash point and fire point apply only to liquids.Density (at 20°C/68°F) of a solid or liquid relative to (divided by) the maximum density of water (at 4°C/39. a fire requires three elements heat. and is a measure of a liquid's volatility. which provides a more complete model.12 .2°F). In general. The boiling point is the temperature at which the vapour pressure equals atmospheric pressure.Rate of Pressure Rise.14 . a low boiling point indicates a high vapour pressure and.13 . Vapour Pressure is defined as the partial pressure of a gas in equilibrium with a condensed form (solid or liquid) of the same substance.Critical Temperature. and whilst the fire triangle model still remains valid. an increased fire hazard. fuel and oxygen. or one that readily vaporises. the tetrahedron is also described below. The critical temperature is the temperature above which it is no longer possible to liquefy the substance in question by increasing the pressure.11 . The fire triangle is a simplistic graphic representation for understanding the components necessary for most fires. The relative density of a gas is its density divided by the density of hydrogen (or sometimes dry air) at the same temperature and pressure. 1. such that the pressure of the atmosphere can no longer hold the liquid in a liquid state and bubbles begin to form.Mechanism of Fire & Explosion. in the last few years it has started to be replaced in by the Fire Tetrahedron. The fire triangle illustrates the rule that in order to ignite and burn. while ignition temperature and maximum explosion limit apply to both liquids and gases. In addition to the triangle model. A high vapour pressure usually is an indication of a volatile liquid. The maximum explosive limit is the greatest concentration of flammable gas or vapour in the air which is capable of ignition and subsequent flame propagation under prescribed test conditions. a fire cannot begin. but when the fire involves burning metals. it will stop. Such techniques form the basis for most major fire-fighting tactics. In this type of fire. In the majority of circumstances the fire will fail to ignite. The following diagram is a two-dimensional representation of the tetrahedron. the old fire triangle model works well enough. The fire triangle remains an effective teaching tool. or smothering a fire with a fire blanket. etc. A simple method of oxygen removal is the technique of blanketing. Without sufficient heat. 'Figure 2' Combustion is the chemical chain reaction that feeds a fire more heat and allows it to continue. there are certain chemical fires where knowledge of the fire tetrahedron is essential.Outline of the Effects of Atomisation & Oxygen Content. liquid stream or by mechanical means. such as magnesium. and it cannot continue.15 . This can happen as the process continues and the fuel is consumed "the fire burns itself out". carbon dioxide. the water turns to steam. therefore extinguishing the fire. examples of blanketing include covering a pan fire with a dampened cloth. The fuel can be physically removed. however. 1.'Figure 1' When a fire runs out of fuel. Putting water on such a fire could increase the heat of the fire and could also be the catalyst for an explosion. or is extinguished whichever element is removed. Another example of removing heat from the fire triangle is separating burning fuels from each other. A good example of fuel separation is cutting fire breaks when fighting a forest fire. With most types of fires. but does not demonstrate the fourth element of fire. It is useful to visualise the fire tetrahedron as a pyramid having four sides including the bottom. the sustaining chemical reaction. lithium. Turning off the electricity in an electrical fire removes the heat source. Atomisation Definition: The dispersion of a liquid into particles by a rapidly moving gas. (although other fuels may have caught fire and continue burning. it becomes useful to consider the chemistry of combustion. Heat can be removed by dousing (some types of fire) with water. dry chemicals. taking the heat with it. This has led to development of the fire tetrahedron. Combustion or burning is a chemical process. or by chemically removing the fuel from the fire. specialised chemicals must be used to break the chain reaction of metallic combustion. and this can be an effective way to reduce the heat. an exothermic reaction between a substance (the . or enclosing it so that the fire will quickly use up all of the available oxygen.) Oxygen may be removed from a fire by smothering it with aqueous foam. When a hydrocarbon burns in oxygen. there is an inadequate supply of oxygen for the combustion to occur completely. the chemical equation for burning a hydrocarbon (such as octane ) in oxygen is as follows: CxHy + (x + y/4)O2 → xCO2 + (y/2)H2O For example.Complete Combustion. When a hydrocarbon burns in oxygen. these by-products can be quite lethal and damaging to the environment. Incomplete combustion is much more common and will produce large amounts of by-products and. Carbon will yield carbon dioxide. the reaction will only yield carbon dioxide and water. 1.18 .19 . In incomplete combustion. In a complete combustion reaction. producing a limited number of products. and the products are compounds of each element in the fuel with the oxidising element.fuel) and a gas (the oxidiser). Generally. a compound reacts with an oxidising element. sulphur and iron are burned. Rapid combustion is a form of combustion in which large amounts of heat and light energy are released. usually O2. Nitrogen will yield nitrogen dioxide. 1.17 . they will yield the most common oxides. In complete combustion. Respiration is an example of slow combustion. Complete combustion is generally impossible to achieve unless the reaction occurs where conditions are carefully controlled (e. and in thermobaric weapons.20 . nitrogen. such as internal combustion engines. 1.g. in the case of burning fuel in automobiles. For example: CH4 + 2 O2 → CO2 + 2 H2O + heat CH2S + 6 F2 → CF4 + 2 HF + SF6 + heat 1.Rapid Combustion. This is used in forms of machinery. Slow combustion is a form of combustion which takes place at low temperatures. to release heat. the reaction will yield carbon dioxide. the burning of propane is: .Slower Combustion. 1. water. carbon monoxide. and various other compounds such as nitrogen oxides. in a lab environment). the reactant will burn in oxygen. The reactant will burn in oxygen. When elements such as carbon. but will produce numerous products.16 . This often occurs as a fire.Chemical Equation. Iron will yield iron (III) oxide. Sulphur will yield sulphur dioxide.Incomplete Combustion. Combustion of a liquid fuel in an oxidising atmosphere actually happens in the gas phase. the stoichiometric air ratio. the liquid will not evaporate fast enough to sustain the fire.Combustion Temperatures. Therefore. oxygen or air). when the output of flammable gases from the material is too low for persistent presence of flame and the charred fuel does not burn rapidly anymore. the heat capacity of fuel and air.22 . Typically. .23 . Charcoal phase or solid phase. Energy is produced in the form of heat and light.g. The formula that yields this temperature is based on the first law of thermodynamics and takes note of the fact that the heat of combustion (calculated from the fuel's heating value ) is used entirely for warming up fuel and gas (e. Assuming perfect combustion conditions. around 2000 °C for oil and 2200 °C for natural gas. 1. when the unburned fuel is heated up to its flash point and then fire point.21 . The adiabatic combustion temperature increases for higher heating values and inlet temperatures and stoichiometric ratios towards one. The act of combustion consists of three relatively distinct but overlapping phases:    Preheating phase.C3H8 + 5O2 → 3CO2 + 4H2O The simple word equation for the combustion of a hydrocarbon is: Fuel + Oxygen → Heat + Water + Carbon dioxide. the combustion temperature depends on     the heating value.e.Oxidising Materials. 1. not the liquid. flame is often visible. i. it is the vapour that burns. 1. when the mix of evolved flammable gases with oxygen is ignited. air and fuel inlet temperatures. a liquid will normally catch fire only above a certain temperature . but just glows and later only smoulders. 1.0).24 . Below that temperature. In the case of fossil fuels burnt in air. the adiabatic combustion temperatures for coals are around 1500 °C (for inlet temperatures of room temperatures and λ = 1. such as an adiabatic (no heat loss) and complete combustion.Combustion of Solid Fuels. Flammable gases start being evolved in a process similar to dry distillation. Distillation phase or gaseous phase. the adiabatic combustion temperature can be determined.Combustion of Liquid Fuels.its flash point. Can explode if exposed to slight heat. They also include materials that react chemically to oxidise combustible (burnable) materials. This reaction may be spontaneous at either room temperature or may occur under slight heating. this means that oxygen combines chemically with the other material in a way that increases the chance of a fire or explosion. Oxidising liquids and solids can be severe fire and explosion hazards. Will increase the burning rate of combustibles. Can cause combustibles to ignite spontaneously. Will cause sustained and vigorous decomposition if contaminated with a combustible material or if exposed to sufficient heat. However.Oxidising materials are liquids or solids that readily give off oxygen or other oxidising substances (such as bromine. Some oxidising materials are also incompatible with non-combustible materials. and gives off heat. many plastics and textiles Finely divided metals Other oxidisable substances such as hydrazine. flammable and combustible liquids. These oxidizers can undergo dangerous reactions with water. When a combustible substance burns. shock. shock or friction. The MSDS for a particular oxidizing material should explain what other substances the oxidizer is incompatible with (reacts in a dangerous fashion) and any other conditions. such as heat. greases. or friction. Class 4 Oxidizers:        Can explode when in contact with certain contaminants. they can produce very flammable or explosive mixtures when combined with combustible materials like: Organic (carbon-containing) materials such as paper. Class 1 Oxidizers:   Slightly increase the burning rate of combustible materials. Class 3 Oxidizers:   Severely increase the burning rate of combustible materials with which they come in contact. waxes. Burning involves the oxidation of a combustible (burnable) substance. Examples of NFPA Class 1 oxidizers include: Aluminium nitrate. Although the following list is taken from the North American National Fire Protection Association (NFPA). chlorine. Do not cause spontaneous ignition when they come in contact with them. inorganic acids or even other oxidising materials. Class 2 Oxidizers:   Increase the burning rate of combustible materials moderately with which they come in contact. hydrides. Ammonium persulfate. that could result in dangerous chemical reactions. it gives a good idea of the relative danger of various chemicals in fire situations. May cause spontaneous ignition when in contact with a combustible material. or fluorine). hydrogen. silicon and ammonia or ammonia compounds. oxidising materials can supply combustible substances with oxygen and support a fire even when air is not present. . The usual source of oxygen for burning is air. wood. a chemical reaction occurs in which the substance (fuel) combines with oxygen. and often light (flames). sulphur or sulphur compounds. Although most oxidising materials do not burn themselves. gases. phosphorous. Potassium nitrate.5% by weight). Sodium chlorite (greater than 40% by weight). Perchloric acid solutions (greater than 72. Potassium permanganate. . Nitric acid (40% concentration or less). Sodium peroxide Examples of NFPA Class 3 oxidizers include: Ammonium dichromate. Persulphuric acids (H2SO5 and H2SO8). Sodium dichloroisocyanurate dihydrate.5% by weight). Tetranitromethane. Hydrogen peroxide (H2O2) and other inorganic peroxides. Calcium hypochlorite (50% or less by weight). Strontium nitrate. Ozone (O3). Sodium permanganate. Sodium dichloroisocyanurate. Perchloric acid solutions (60 to 72% by weight). and other halogens. Sulphuric acid (H2SO4). Magnesium perchlorate. Fluorine (F2). Sodium dichromate. Hydrogen peroxide (27.                                                        Barium peroxide. Strontium peroxide. Ammonium permanganate. Hydrogen peroxide (52 to 91% by weight). Sodium chlorite (40% or less by weight). 1. Examples of NFPA Class 2 oxidizers include: Calcium chlorate. Sodium chlorate. Common oxidising agents: Oxygen (O2). Sodium nitrate. Trichloroisocyanuric acid. Sodium nitrite. Sodium perborate (and its monohydrate). Potassium chlorate. Potassium dichloroisocyanurate. Zinc peroxide. Potassium dichromate. Hydrogen peroxide (greater than 91% by weight). Chromic acid (chromium trioxide). Examples of NFPA Class 4 oxidizers include: Ammonium perchlorate (particle size greater than 15 microns). Magnesium nitrate. chlorine (Cl2).5 to 52% by weight).5-dimethylhydantoin. Sodium persulfate. Hydrogen peroxide solutions (8% to 27. Nitric acid (concentration greater than 40% but less than 86%). Perchloric acid solutions (less than 50% by weight). fuming (concentration greater than 86%). Nitric acid. Sodium perchlorate (and its monohydrate). Potassium bromate. Nitric acid (HNO3) and nitrate compounds.3-dichloro-5. Silver nitrate. g.g. e. think of a burning wood fire where the wood itself may be embers but 'gas flames' burn with it. halogen lamps. and arson. electrical. hot surfaces and obstruction of equipment ventilation.          Chlorite.g. For ignition to occur with liquids. machinery. including household bleach (NaClO). cigarettes and matches. Regardless of whether a fuel was originally a liquid or solid. Tollens' reagent. and other analogous halogen compounds. 2. Most solid materials take up energy from any outer ignition source either by conduction. The process of pyrolysis is temperature reliant so therefore if the source of the heat (energy) is .2'-Dipyridyldisulphide (DPS). lighting equipment. Hexavalent chromium compounds such as chromic and dichromic acids and chromium trioxide. pyridinium chlorochromate (PCC). hot processes (such as welding or grinding work). 1. perchlorate. or are heated up as a result of the heatproducing processes taking place internally that start decomposition on their surfaces.Identification of Ignition Sources. e. Hypochlorite and other hypohalite compounds. You can identify the potential ignition sources in your workplace by looking for possible heat sources that could get hot enough to ignite materials. gas or oil-fired heaters (fixed or portable). convection or radiation (mostly by their combination). It is the change of state of materials to become gaseous. Sodium perborate.this is defined in the context of flame and fire as the stage of ignition during which energy causes gas molecules given off by a heated fuel to vibrate and break into pieces. cooking. engines or boilers. A process of ignition is Pyrolysis . e. naked flames. These sources of heat could include:               smokers' materials. faulty or misused electrical equipment.g. in that it does not necessarily require an open flame or spark if the correct conditions are met. e.25 . The vapours released and the gaseous decomposition products mix with the air above the surface of liquid or solid material. friction. Silver oxide (Ag2O). Osmium tetroxide (OsO4). the overall burning process will gasify the fuel. Permanganate compounds. Nitrous oxide (N2O). from loose bearings or drive belts. typically by evaporation. office equipment. metal impact (such as metal tools striking each other). chlorate. static electricity. Ignition can be spontaneous. these must have the formation of a vapour space above their surface that is capable of burning. and chromate/dichromate compounds. then a flame can propagate through the combustible mixture at high speeds.Explosions.e.removed. pressure can build up until the containing walls rupture. If the amount of heat released is sufficient to cause further combustion. These can be classified as detonations if the speed is greater than the speed of sound in the explosion medium. a UCVE may result. 1984) Unconfined Vapour Cloud Explosions (UCVCE) If a flammable gas/air cloud burns in free space with sufficient rapidity to generate pressure waves. storage container. and the concentration of the fuel. If this is rapid enough. 1.28 . then the reaction will continue. forming a flammable cloud and a flame propagates through it. A dust explosion occurs when a combustible material is dispersed in the air. in mentioning heat. . then the fire will not just continue. Dust explosions are unlikely to cause detonations due to the relatively slow process of combusting solid particles. deflagrations. or. (Abbeystead.Specific Surface Area. when the flame front speed is less than the speed of sound. CVCEs can cause considerable damage and could affect nearby plant where serious secondary explosions could follow. gas phase mixing and gas phase combustion. when a volume of a flammable mixture is ignited. Please consider other aspects of the fire equation in the control or avoidance of ignition. The maximum pressure in a dust explosion is typically around 5-12bar(g). i.26 . it will cause the fire to grow very rapidly. then the explosion will not occur. If the area available for combustion is high enough. resulting in a rapid pressure increase and fire moving through the cloud. The explosion occurs in three stages. Explosions are sudden releases of energy.27 . one aspect of the fire triangle has been nominated. 1. that lead to sudden and significant pressure rise. then the fire will become an explosion. If either of these are too high or low. in this context resulting from a chemical reaction.BLEVE see 6. A dust explosion is very similar to a gas or vapour cloud explosion.Dust Explosions.g. 1. Confined Vapour Cloud Explosions (CVCE) If a flammable vapour cloud is ignited inside a container (e. Detonations are much more destructive than deflagrations.Vapour Phase Explosions. or even a building). (Nypro UK plant at Flixborough 1974) Boiling Liquid Expanding Vapour Explosions . or the particle is vaporised). If the heat release produces more combustion than it took to generate. a process vessel. Any solid material that can burn in air will do so at a rate that increases with increased surface area. devolatilisation (where volatiles are evaporated from the particles. This of course also depends on the supply of oxygen to the fire.29 .7 1. which propagate through the vapour cloud and into the surrounding atmosphere. the ignition can not take place. A useful measurement is the specific surface area. Turbulence in the dust cloud. Such explosions can be devastating. 1. resulting in loss of life and plant.32 . Concentration distribution in the dust cloud. there are other factors that will influence it:          Chemical Composition of the dust (and its moisture content). will have a specific surface area S that is equal to: S = 6 x2 / x3 = 6 / x This also holds true for spherical particles (where x is the diameter): S = x2 / ( x3 / 6) = 6 / x For flake-shaped particles the specific surface area is: S=2/x For fibres where the length is much greater than the diameter: S=4/x Therefore. In the UK. we have a convenient way of estimating the specific surface area quickly from the diameter and basic particle characteristics.Factors Affecting Ignition Sensitivity and Explosion Violence. They can arise during normal operation. Degree of dispersion of the dust cloud. the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) came into force for new plant in July 2003 and existing plant by 2006. Particle shape and size distribution. 1. Pressure and temperature of the gas. Essentially.Gas and Vapour Explosions. although it is possible for very small particles to agglomerate into lumps. thus reducing the likelihood of a dust explosion. Flame front disturbance by mechanisms other than turbulence. which allows us to see exactly how much surface area there is per unit mass. For example a cube of edge x. the higher the specific surface area the more danger there is of dust being involved in a dust cloud explosion. Operators of plants handling flammable gases and vapours will be required to undertake specific risk assessments and make improvements where required to ensure explosion risks are being adequately managed. .31 . Gas and vapour explosion risks are present in a wide range of process industries.Liquefied Propane Gas (LPG) Tank Hazards. 1. Amount of turbulence caused by the explosion in unburnt parts of the cloud. Although particle size/specific surface area is the main factor in the estimation of the likelihood of a dust explosion. but more often occur as a result of process abnormality or during startup and shut-down.We can see that the degree of subdivision of a solid dictates the likelihood of an explosion.30 . Radiative heat transfer from the flame (dependent on chemistry). there was a massive vapour cloud explosion which caused extensive damage and started numerous fires on the site. Mexico City (1984). Flixborough (1974). San Carlos (1978). Hickson and Welch (1992)). It is recognised that the number of casualties would have been far higher if the incident had occurred on a weekday.5 was leaking cyclohexane. causing a nearly instantaneous transition from liquid to vapour with a corresponding energy release. . The plant was subsequently shut down for an investigation.LPG tank fires are structural fires. and equipped with a volume of water adequate to safely attack the fire. During the late afternoon on 1st June 1974. BLEVEs are a major hazard to emergency responders. Eighteen fatalities occurred in the control room as a result of the windows shattering and the collapse of the roof. it is not necessary for the liquid to be flammable to have a BLEVE occur. and a further 36 suffered injuries. However.35 . The fires burned for several days and after ten days. those that still raged were hampering the rescue work. Tackling them should only be attempted by trained personnel using full structural personal protective equipment. as the main office block was not occupied. it was discovered that a vertical crack in reactor No. This identified a serious problem with the reactor.34 . http://www. involving hazardous materials.uk/file.6 so that the plant could continue production. since an external fire impinging on the vapour space of a pressure vessel is a common BLEVE scenario. There were fifty-three reported injuries off-site. and the decision was taken to remove it and install a bypass assembly to connect reactors No. At about 16:53 hours on Saturday 1st June 1974.Discussion of Examples of Actual Incidents.4 and No.Video: BLEVE. Causes & Effects (e. Flixborough (Nypro UK) Explosion 1st June 1974 Accident summary. 1. A BLEVE is often accompanied by a large fireball if a flammable liquid is involved. Property in the surrounding area was damaged to a varying degree. a 20 inch bypass system ruptured. At about 16:53 hours.flv 1. This resulted in the escape of a large quantity of cyclohexane. The cyclohexane formed a flammable mixture and subsequently found a source of ignition. No-one escaped from the control room. 1. Prior to the explosion. A type of rapid phase transition in which a liquid contained above its atmospheric boiling point is rapidly depressurised. Twenty-eight workers were killed. which may have been caused by a fire on a nearby 8 inch pipe.co. the Nypro (UK) site at Flixborough was severely damaged by a large explosion. on 27th March 1974.g.php/4/videos/BLEVEs. A BLEVE or sudden complete failure of the tank is the most recognised hazard with LPG tanks.Boiling Liquid Expanding Vapour Explosions (BLEVE).33 .sheilds-elearning. Control Room Design: structural design to withstand major hazards events.1. Accident summary. Mexico. A drop in pressure was noticed in the control room and also at a pipeline pumping station. 19th November 1984. Those concerned with the design. the operators could not identify the cause of the pressure drop. causing violent ground shock. At approximately 05:35 hours on 19th November 1984. The plant was being filled from a refinery 400 km away. Mexico City. Isolation: emergency isolation means. taking various actions. Workers on the plant now tried to deal with the escape. 1. An 8inch pipe between a sphere and a series of cylinders had ruptured.               A plant modification occurred without a full assessment of the potential consequences.PEMEX LPG Terminal. Two large spheres and 48 cylindrical vessels were filled to 90%.Failings in Technical Measures. construction and layout of the plant did not consider the potential for a major disaster happening instantaneously.38 . The explosions were recorded on a seismograph at the University of Mexico. some 500 individuals were killed and the terminal destroyed. estimated at 200m x 150m x 2m high. The release of LPG had been going on for about 5-10 minutes when the gas cloud. 1. as on the previous day it had become almost empty. As a consequence of these events. Plant Layout: positioning of occupied buildings. Mexico City.Pipework: use of flexible pipes.   Plant Layout: positioning of the vessels. the first BLEVE occurred. Maintenance Procedures: recommissioning. a major fire and a series of catastrophic explosions occurred at the government-owned and operated PEMEX LPG Terminal at San Juan Ixhuatepec. which included the layout of the plant and emergency isolation features. In particular. No pressure testing was carried out on the installed pipework modification. Three refineries supplied the facility with LPG on a daily basis.37 . Plant Modification / Change Procedures: HAZOP. About fifteen minutes after the initial release. For the next hour and a half. Operating Procedures: number of critical decisions to be made Inerting : reliability/back-up/proof testing. Only limited calculations were undertaken on the integrity of the bypass line.36 . there followed a series of BLEVEs as the LPG vessels violently exploded. Design Codes . somebody pressed the emergency shut-down button. drifted to a flare stack. No drawing of the proposed modification was produced. .Failings in Technical Measures. Unfortunately. No calculations were undertaken for the dog-legged shaped line or for the bellows. The total destruction of the terminal occurred because there was a failure of the overall basis of safety. and 4 smaller spheres to 50% full. The incident happened during start-up when critical decisions were made under operational stress. LPG was said to rain down and surfaces covered in the liquid were set alight. It ignited. A number of ground fires occurred. At a late stage. the shortage of nitrogen for inerting would tend to inhibit the venting of off-gas as a method of pressure control/reduction. A vertical jet of burning vapours shot out of the top rear vent to the height of the distillation column nearby. was organised in order to remove residues. a number of employees involved in the raking left the still base to get on with other tasks. nor was the atmosphere inside the vessel checked for a flammable vapour. As he leapt from the scaffold. 1. water deluge. The plant had no gas detection system and therefore. In order to soften the sludge. The installation of a more effective gas detection and emergency isolation system could have averted the incident. known as "60 still base". steam was applied to the bottom battery. The jet severely damaged this building and then impacted on the north face of the main office block. it was probably too late. when the emergency isolation was initiated. which turned instantly to an orange flame. This projected horizontally towards the Meissner control building.40 . access of emergency vehicles. Leak / Gas Detection: gas detection. The force of the jet destroyed the scaffold. insulation thickness. A total of 22 fire appliances and over 100 fire fighters attended the incident. The material was tar-like and had liquid entrained in it. which built up as local residents sought to escape the area. with the consistency of soft butter. Castleford. Accident summary. One person left on the scaffold had stopped raking and noticed a blue light. Also.Failures in Technical Measures.The Fire at Hickson & Welch Limited. Hindering the arrival of the emergency services was the traffic chaos. No sample was sent for analysis. 21st September 1992. An operator dipped the sludge to examine it and reported to management that the sludge was gritty. It was mistakenly thought that the material was a thermally stable tar.      The terminal's fire water system was disabled in the initial blast. The vessel's temperature gauge in the control room was reported to be reading 48°C. Employees started the clean-out operation using a metal rake.39 . The jet fire lasted for approximately one minute before subsiding to localised fires around the manhole and buildings nearby. a longer rake was used to reach further into the still. 1. Advice was given not to exceed 90°C. which was estimated to have a depth of 34 cm (14 in). causing a number of fires to start inside the building. This vessel had never been cleaned since it was installed in the nitrotoluenes area in 1961. an incandescent conical jet erupted from the manhole. Instructions were given to isolate the steam. Emergency Response / Spill Control: site emergency plan. Approximately one hour into the cleaning process. the water spray systems were inadequate. . in the process propelling the manhole cover into the centre of the Meissner control building. At approximately 13:20 hrs. A clean-out operation of a batch still. Active/Passive Fire Protection: survivability of critical systems. an oil storage facility located near the M1 motorway by Hemel Hempstead in Hertfordshire. Control Systems: sensors. but these may have been persons missing. Witnesses many miles from the terminal observed flames hundreds of feet high.4 on the Richter scale. The British Geological Survey monitored the event. Subsequent explosions occurred at 06:27 and 06:28. because of the position of the temperature probe. Further explosions followed which eventually overwhelmed 20 large storage tanks. Damage from the blasts included broken windows at various buildings.000 imperial gallons (272. Use of a metal rake to clean the sludge from a still containing flammable vapours. Buildings in neighbouring towns also suffered.765. It was reported that people were woken in south London. Inadequate measurement of sludge temperature. and as far west as Wokingham (about 28 miles (45 km)). The Buncefield fire was a major conflagration caused by a series of explosions on 11th December 2005 at the Hertfordshire Oil Storage Terminal. France. Finchampstead. and from as far north as Lincolnshire (about 70 miles (113 km)) away. A government inquiry held jointly by the Health and Safety Executive (HSE) and the Environment Agency was started. Casualties all from within control room and administration block. with breathing difficulties. the smoke cloud was visible from space. Failure to analyse the sludge and atmosphere within the vessel prior to cleaning. Permit to Work Systems: safety management systems. There were 43 reported injuries. there were reports that they were audible in Belgium.                The decision to clean out the vessel for the first time after 30 years of operation. Failure to isolate the still base inlet prior to the work commencing.41 . Two permits were issued for the removal of the manlid and one permit for the blanking of the still base inlet. The Board included Lord . numerous people felt the shockwave after the initial explosion. The first and largest explosion occurred at 06:01 UTC near container 912. Maintenance Procedures: isolation. two people were deemed to be seriously injured enough to be kept in hospital. faulty pressure-reducing valve on steam supply supplied steam at a higher temperature than anticipated.400 l) of fuel. Some early media reports spoke of eight fatalities. and another in Hemel Hempstead Hospital. and the Netherlands.Buncefield. Because of an inversion layer. Also. Maintenance Procedures: flameproof tools. and an entire wall being removed from a warehouse more than half a mile (800 m) from the site. blown-in or warped front doors. although they were not in a life-threatening condition. Hazardous Area Classification / Flameproofing: ignition sources identification and elimination. A window was blown out of St Albans Abbey (about 5 miles (8 km)). effect of impurities. 1. with a capacity of about 60. which measured 2. Reaction / Product Testing: thermal stability.000. the explosions were heard up to 125 miles (201 km) away. but calls for a full public enquiry were declined. Raw Materials Control / Sampling: identification of potentially hazardous impurities. where in its southern suburb. one in Watford General Hospital. it seems to have been an unconfined vapour cloud explosion of unusually high strength-also known as a fuel-air explosion. Maintenance Procedures: maintenance systems. Plant Layout: positioning of occupied buildings. England. No permit to work was ever issued for the task of raking out the still base. All members of staff from the terminal were accounted for. From all accounts. The terminal was the fifth largest oilproducts storage depot in the United Kingdom. Around 2. which found everything normal. A further announcement was made on 9th May 2006 about the sequence of events which caused the explosion. Initially the fuel was pumped in at 550 cubic metres (19. published on 11th April 2006. Forty-one minutes later.423 cu ft) per hour. An initial progress report by the Major Incident Investigation Board on 21st February 2006 did not go into the causes of the explosion. milk powder. 1. affecting all types of companies. From about 03:00 the level gauge for Tank 912 began to indicate an unchanging level reading. By 06:01. flour-based goods. Any environment in which dust or powder is allowed to gather on hot surfaces or could be ignited . but it increased to about 890 cubic metres (31. Its aim was to identify the immediate causes of the explosion. CCTV footage showed such a vapour flowing out the bund wall from around 05:38. and a check was made of the contents of tanks. failed to operate. fodder and tobacco. which should have detected that the tank was full and shut off the supply. The investigators did not believe that it was caused either by the driver of a fuel tanker. grain. a medical expert. An overflow such as this results in the rapid formation of a rich fuel and air vapour. Environment Agency and HSE staff were also on the board.430 cu ft) per hour. By 05:50 the vapour already started flowing off the site. so as not to prejudice further legal proceedings. when the first explosion occurred. At midnight the terminal closed. It was felt unlikely that the explosion had a widespread effect on air quality at ground level. or by anyone using a mobile phone. the cloud which was initially about 1 metre (3 ft) deep. was filled with unleaded petrol from the Lindsey oil refinery in Lincolnshire. As well as these manufacturers. A second progress report.423 cu ft) per hour. towards the north west of the main depot. looked at the environmental impact. Normally the gauges monitor the level of the fuel in the tank as it fills from the particular pipeline. rather than consider who was to blame for any deficiencies. but it too appears to have failed. Around 05:50 the rate at which fuel was being pumped into the tank increased dramatically. There is evidence suggesting that a high-level switch. coal. an estimated 300 tonnes of petrol would have spilled down the side of the tank through the roof vents onto the ground inside a bund wall-a semienclosed compound surrounding several tanks.Newton of Braintree. despite it being filled at 550 cubic metres (19. The extent of the damage meant it was not possible to determine the exact source of ignition.Dust Explosions. more than five per day. thickened to 2 metres (7 ft) and had spread beyond the boundaries of the site. as had been speculated. Professor Dougal Drysdale-an authority on fire safety and Dr Peter Baxter.42 .000 dust explosions occur in Europe every year. but possibilities include an emergency generator and the depot's fire pump system. Starting at 19:00 on the evening of 10th December Tank 912. cocoa. wood and metal processing companies are also vulnerable. but summed up the event and the immediate reaction from the emergency services. tea. Calculations show that the tank would have begun to overflow at about 05:20. including manufacturers of          sugar. The switch failure should have triggered an alarm. From 2006. For electrical equipment in areas at risk from dust explosions. one of the main protection types for electrical equipment at risk from dust explosions in accordance with ATEX Directive 94/9/EC. owners are governed by the compulsory European directive 1999/92/EC. The dust explosion killed 13 people and injured many more. as a result of a dust explosion can be enormous. put controls in place to reduce the effects of any incidents involving dangerous substances. control them. in terms of lives lost and damage to plant. use suitable production equipment. create an explosion protection document. define zones in areas at risk from dust explosions. flammable gases. is the protection by enclosures type "tD". for example) in those areas. this directive also includes 'old' as well as 'new' equipment. a new series of stricter standards. cause harm to people as a result of a fire or explosion. In order to ensure that electrical equipment is compliant with the latest standards. This includes members of the public who may be put at risk by work activity. What does DSEAR require? Employers must:       establish what dangerous substances are in the workplace and what the fire and explosion risks are. USA. create a test plan. if not properly controlled. have been introduced. such as liquid petroleum gas (LPG). dusts from machining and sanding operations and dusts from foodstuffs. They can be found in nearly all workplaces and include such things as solvents.by a spark from electrical equipment is a potential risk. incidents and emergencies involving dangerous substances. varnishes. In February 2008. which came into effect from the 1st October 2008. The cost. Within EN61241. where this is not possible. identify and classify areas of the workplace where explosive atmospheres may occur and avoid ignition sources (from unprotected equipment. make sure employees are properly informed about and trained to control or deal with the risks from the dangerous substances. Owners of electrical equipment are required to       identify the risks of explosion. prepare plans and procedures to deal with accidents. paints. implement explosion protection measures such as avoiding sources of ignition (secondary explosion protection). What is DSEAR? DSEAR stands for the Dangerous Substances and Explosive Atmospheres Regulations 2002. a catastrophic dust explosion occurred at the Imperial Sugar Refinery in Port Wentworth. DSEAR puts duties on employers and the self-employed to protect people from risks to their safety from fires. put control measures in place to remove those risks or. implemented in the UK as the 'Dangerous Substances and Explosive Atmospheres Regulations 2002' (DSEAR). What are dangerous substances? Dangerous substances are any substances used or present at work that could. This protection type is where the . explosions and similar events in the workplace. The standards supersede the existing standard EN 50 281. EN61241. The science of explosions A dust explosion is the fast combustion of dust particles suspended in the air in an enclosed location. The released custard powder ignited as a dust cloud within the building. Zone 20 Explosive atmosphere in air with a cloud of combustible dust present continuously or for long periods of time . Zone 22 Explosive atmosphere in air with a cloud of combustible dust which is not likely to occur in normal operation. Zone 2 Explosive atmosphere in air with gas. electrostatics and ageing resistance. vapour or mist not likely to occur in normal operation.electrical equipment is protected by an enclosure to prevent dust penetration and where measures have been taken to limit the surface temperature. The primary changes to this standard are concerned with impact energy. Why does dust explode? A mass of solid combustible material as a heap or pile will burn relatively slowly owing to the limited surface area exposed to the oxygen of the air.       Zone 0 Explosive atmosphere in air with gas. the surface area exposed to the air is much larger. On the 6th April 2008. Zone 21 Explosive atmosphere in air with a cloud of combustible dust likely to occur in normal operation occasionally. vapour or mist likely to occur in normal operation occasionally. will cause the pressure to rise to levels which most industrial plant is not designed to withstand. This means that companies' internal control of health and safety is a business risk that senior managers can no longer regard as a minor matter. Coal dust explosions are a frequent hazard in underground mines. In areas where explosions can occur these areas should be divided into zones. the Corporate Manslaughter and Corporate Homicide Act was introduced in the UK. vapour or mist present continuously or for long periods of time. There is an ignition source. If the same solid is in the form of a fine powder is suspended in air as a dust cloud. A fault in a pneumatic conveying system caused a holding bin to overfill and the air pressure caused the bin to fail. the absence any of these five conditions means there can be no dust explosion or deflagration. But if it does it will be for a short period only. The dust is confined. This means that companies found guilty of negligence or non-compliance could face severe fines and/or imprisonment. The concentration . There is an oxidant (typically atmospheric oxygen). In this case. in the case of a contained dust cloud. and if ignition occurs. There are five necessary conditions for a dust explosion:      A combustible dust. Although a cloud of flammable dust in air may explode violently. not all mixtures will do so. but dust explosions can occur where any powdered combustible material is present in an enclosed atmosphere. In 1981 an explosion at a plant in Banbury which manufactured custard powder injured nine men and caused substantial damage to an external wall of the building. A recent report from the HSE stated that on average there are between 70 and 100 incidents each year in the UK that involve explosive atmospheres. This results in a rapid release of heat and gaseous products and. The dust is suspended in the air at a high concentration. the result will be quite different. the whole of the cloud may burn very rapidly. but if it does it will be for a short period only. In one year. the HSE investigated five fatal accidents involving explosive atmospheres and 16 major accidents. Zone 1 Explosive atmosphere in air with gas. With the exception of 4. preventing the explosive atmosphere by inerting. present periodically during normal operations or not present during normal operations . A dust cloud of this concentration resembles a very dense fog. friction. The LOC must be experimentally determined for the dust concerned. This minimum oxygen concentration for the dust is called the Limiting Oxygen Concentration (LOC) and is typically in the range of 8% to 11% oxygen (where atmospheric air has an oxygen concentration around 21%). it is often difficult to determine the exact source of ignition after the explosion. and plant controls.of dust and air must be within the upper and lower explosive limits for the dust involved. They can be grouped under the headings of:     controls over dust cloud formation. Upper explosive limits are difficult to measure accurately. the source of ignition must have sufficient ignition energy and/or a surface at a hot enough temperature to ignite the dust cloud. Part 3: Classification of areas where combustible dusts are or may be present. For an explosion to occur. hot surfaces. it will often be cited as static electricity. but there are occasions where granular or pasty products can be used with advantage. glowing material often remains after the explosion.50g/m3. Only weak explosions are likely where the mean particle size of the dust exceeds 200 microns. Many products have to be handled as fine powders. avoiding ignition sources. However. For an explosion to occur. arcing from machinery or other equipment. which may have various purposes. and other factors. When a source cannot be found. If the oxygen concentration in the air can be reduced below LOC by the addition of additional inert components to the 'air'. and for many organic materials the limit is in the range 10 . Hazardous areas due to clouds of combustible dust are classified as zones 20. Measurements of the lower explosive limits of many materials are available. fire. This means that the oxygen available in the air cannot burn all the dust. Such a dense dust cloud will generally appear to be opaque. There are many sources of ignition and a naked flame need not be the only one: over one half of the dust explosions in Germany in 2005 were from non-flame sources. strongly affects the force of the explosion and the explosive limits. Common sources of ignition include      electrostatic discharge. and most of the control measures concern conditions inside the dust handling system. Appendix A contains information about methods of testing dusts. The supply of oxygen has to have a minimum oxygen concentration above the minimum required to sustain an explosion what the particular dust. 21 or 22 depending generally on whether the release of dust is always present. the cloud of dust has to have a density of at least 30 g/m3 and probably at least 60 g/m3. then an explosion can be prevented. Sources of ignition in the vicinity of dust clouds are normally controlled by the classification of hazardous areas according to the recommendations in BS EN 50281-3: 2002 Electrical apparatus for use in the presence of combustible dust. The most violent explosions usually result from dust/air mixtures that are fuel rich. and have little practical importance. The great majority of dust explosions start inside the process plant. This can reignite if more air becomes available. The risk of an explosion may also be effectively eliminated if the quantity of dust present is sufficiently small. and partly burnt. or the moisture content exceeds 16%. The shape and size of the dust particles. and the risk cannot be eliminated. For example. generally either thermal or electrical. many otherwise mundane materials can also lead to a dangerous dust cloud such as grain. The dust can arise from activities such as transporting grain and indeed grain silos do regularly have explosions. if the fuel/air ratio increases above the upper explosive limit there is insufficient oxidant to permit combustion to continue at the necessary rate. combustion can then proceed very rapidly and the flame front can also travel quickly. if they are generated. there is simply insufficient dust to support the combustion at the rate required for an explosion. Sources of dust Many materials which are commonly known to oxidise can generate a dust explosion. Examples of methods of preventing dust explosions are as follows:     Inerting where inert gases are added to the 'air' in the system to ensure that the oxygen level is always maintained below the LOC. Many powdered metals like aluminium and titanium. Similarly. but finer dust will present a much greater hazard than coarse particles by virtue of the larger surface area. the pressure increases. A figure 20% lower than the LEL is considered safe. the lower explosive limit (LEL). 120 micrometres in diameter will have a surface area of 50 square metres or 540 square feet. However. These sources of ignition may be electrical or non-electrical. Due to the thermal expansion of the gas. Suppression where the presence of either a spark (sensed by infra-red detectors) or a developing explosion (sensed by a pressure sensor) is sensed and extinguished by the very rapid injection of a quenching material (which might decompose to generate an inert gas) to quench the explosion before it has the opportunity to cause damage. Typically this control is exercised by the definition and classification of appropriate hazardous areas. and so will support combustion. In an enclosed space this leads to the . Mechanism of dust explosions Different dusts will have different combustion temperatures and dust of various types will either suppress or elevate this temperature in relation to the stoichiometric concentration of the dusts. Concentration Below a certain value. can form explosive suspensions in air. Containment where the plant is made strong enough to withstand the explosion without suffering serious damage. sawdust and magnesium. powdered milk and pollen. It is necessary that sufficient energy. flour. sugar. Limiting generation or release of dust clouds: If the process can be designed so that dust clouds are not generated or. Dust is defined as powders with particles less than about 500 micrometres in diameter. such as coal. 1 kg of powder. Due to the small volume in relation to the large surface area. The dust must also consist of very small particles. Control of Sources of Ignition where all potential sources of ignition in areas where dust clouds might form are controlled such that ignition energies and surface temperatures are always inadequate to ignite the dust cloud. be applied to trigger combustion. Mining of coal leads to coal dust and flour mills likewise have large amounts of flour dust as a result of milling. where the surface area is very large. Methods of mitigating the effect of dust explosions include:   Relief Panels where one area of the equipment is made deliberately weaker than the remainder and which preferentially bursts to relieve the pressure of the explosion before it damages the remainder of the plant. are not released into areas where the oxygen level is above the LOC and/or sources of ignition are present. then explosions should be avoided.but present at other times. Spontaneous ignition temperature Otherwise known as ignition temperature.Ignition Energy. carbon dioxide or argon. In the coal mining industry. Protection and Mitigation from dust explosions Much research has been carried out in Europe and elsewhere to understand how to control these dangers. or suspended from trays in the roof. molecules are actually colliding with one another. Some industries exclude air from dust-raising processes. but explosions still occur. 1. There are two explosive limits for any gas or vapour. In all cases. Generally. at concentrations above the UEL. some form of ignition source has to be present which is sufficient to create the necessary energy to raise a volume of combustible material to its ignition temperature. an inert gas such as nitrogen or argon which dilutes the explosive gas before coming in contact with air. the Abel and the Pensky-Martin. When the chemical reaction of combustion takes place. Ignition energy of liquids and gases can be measured in three ways: A . fire is spread by a range or combination of factors. Methods used to control the concentration of a potentially explosive gas or vapour include use of sweep gas. which are incombustible gases and so inhibit combustion. This is the lowest temperature at which the substance will ignite spontaneously. C . known as "inerting". the lower explosive limit (LEL) and the upper explosive limit (UEL). Controlling gas and vapour concentrations outside the explosive limits is a major consideration in occupational safety and health. The same method is also used in large storage tanks where inflammable vapours can accumulate. There are two sets of prescribed conditions.Explosive Limit. Concentrations of explosive gases are often given in terms of percent of lower explosive limit (%LEL). B . The alternatives for making processes and plants safer depend on the industry. Typically this uses nitrogen . a methane explosion can initiate a coal dust explosion. 1. so as to dilute the coal dust raised ahead of the combustion zone by the shock wave. This collision energy is the physical representation of the initiation energy required for a fire to start. also called the explosion limit. of a gas or a vapour is the limiting concentration (in air) that is needed for the gas to ignite and explode.44 . which can then engulf an entire pit working. At concentrations in air below the LEL. to the point where it cannot burn. Use of scrubbers or adsorption resins to remove explosive gases before release is also common. The explosive limit.Fire Point This is the lowest temperature at which the heat from combustion of a burning vapour is capable of producing sufficient vapour to sustain combustion.43 .Liquids and Gases.Flash Point The flash point is the minimum liquid temperature at which sufficient vapour is given off to form a mixture with air capable of ignition under prescribed test conditions. Stone dust is spread along mine roadways.condition called overpressure. Mines may also be sprayed with water to inhibit ignition. the fuel has displaced so much air that there is not enough oxygen to begin a reaction. Ignition energy . there is not enough fuel to continue an explosion. Gases can also be maintained safely at . concentrations above the UEL, although a breach in the storage container can lead to explosive conditions. The explosive limits of some gases and vapours are given below. Concentrations are given in percent by volume in air. Substance Acetone Acetylene Benzene Butane Ethanol Ethylbenzene Ethylene Diethyl ether Diesel fuel Gasoline Hexane Heptane Hydrogen Hydrogen sulphide Kerosene Methane Octane Pentane Propane Propylene Styrene Toluene Xylene LEL 3% 2.5% 1.2% 1.8% 3% 1.0% 2.7% 1.9% 0.6% 1.4% 1.1% 1.05% 4.1% 4.3% 0.6% 5.0% 1% 1.5% 2.1% 2.0% 1.1% 1.2% 1.0% UEL 13% 100% 7.8% 8.4% 19% 1.7% 36% 36% 7.5% 7.6% 7.5% 6.7% 74.8% 46% 4.9% 15% 7% 7.8% 9.5% 11.1% 6.1% 7.1% 7.0% 1.45 - Primary and Secondary Explosions. The concentrations needed for a dust explosion are rarely seen outside of process vessels, hence most severe dust explosions start within a piece of equipment (such as mills, mixers, screens, dryers, cyclones, hoppers, filters, bucket elevators, silos, aspiration ducts and pneumatic transit systems). These are known as Primary Explosions. It is important to note that one of the main differences between the dust explosion and flammable gas hazard is that gas/vapour explosions rarely happen inside vessels, due to a lack of air to support explosions. However, dust is generally suspended in air in process equipment, which can allow dust explosions to occur (unless the vessels are operated in pure nitrogen atmospheres, which can still pose a problem with metal powders). This can then cause the vessel to rupture if it has insufficient pressure release devices/venting, or if its design pressure is too low. An important aspect of dust explosion avoidance is the limiting of the possibility of primary explosions. However, more important is to reduce the possibility of a secondary explosion occurring. Secondary explosions are caused (in this context) when lying dust is disturbed by the primary explosion and forms a second dust cloud, which then is ignited by the heat released from the primary explosion. The problem is that small amounts of lying dust occupy very little space, but once disturbed - can easily form dangerous clouds. A 1mm layer of dust of 500kg/m3 can give rise to a 5m deep cloud of 100g/m3 dust. 1.46 - Humidity and Dust Explosions. Humidity and the frequency of dust explosions in industry are closely related. Statistics on dust explosion accidents in the United States from 1979 to 1986 show that an overwhelming majority of such explosions occurred in the winter months (November to February, October to March for the high latitude states), and in inland areas (100 miles or farther from coast lines). Low humidity in these months and areas certainly contributes to the causes of dust explosions in the grain industry, since humidity of the ambient air directly affects the moisture content of dusts. During the experimental study of explosions, a similar situation was experienced. i.e., most of the runs that resulted in deflagration/detonation transition (DDT) were observed during the winter months rather than in the summer (especially in July and August), since the dust layers were in direct contact with the ambient air. 1.47 - The Imperial Sugar Incident. Shortly before 7:15 p.m. on February 7th, 2008, the new Imperial Sugar Company CEO was touring the facility with three employees. Walking through the refinery toward the south packing building, they were startled by what sounded and felt like a heavy roll of packing material dropped from a forklift somewhere in the packing building. Three to five seconds later, a loud explosion knocked them backward, and debris was thrown through the large packing building doorway. The security guard at the gatehouse, and other workers in nearby buildings, also heard a loud explosive report. Outside, massive flames and debris erupted above the packing buildings and silos. A security camera at the Georgia Ports Authority located south of the facility recorded the first of many fireballs erupting from the Imperial Sugar facility. Three-inch thick concrete floors in the south packing building heaved and buckled from the explosive force of the sugar dust-fueled explosions as they progressed through the packing buildings and into adjacent buildings. The wooden roof on the palletizer room was shattered and blown into the bulk sugar rail loading area. Workers in the packing buildings had little or no warning as walls, equipment, and furniture were thrown about. Superheated air burned exposed flesh. Workers attempting to escape struggled to find their way out of the smoke-filled, darkened work areas. Debris littered the passageways. Some exits were blocked by collapsed brick walls and other debris. The fire sprinkler system failed because the explosions ruptured the water pipes. Intense fireballs advanced through the entire north and south packing and palletizer buildings as sugar dust, shaken loose from overhead surfaces, ignited. Heaving and buckling floors opened large crevasses. The piles of granulated and powdered sugar that had accumulated around equipment rained down and intensified the fires burning below. Fireballs advanced through enclosed screw conveyors and ignited fires in the refinery and bulk sugar building hundreds of feet from the packing buildings where the incident had begun. Violent fireballs erupted from the facility for more than 15 minutes as spilled sugar and accumulated sugar dust continued fueling the fires The Chemical Safety Bureau (CSB) concluded that the Port Wentworth manufacturing facility incident was a combination of a primary and multiple secondary dust explosions. The CSB further concluded that the secondary dust explosions would have been highly unlikely had Imperial Sugar performed routine maintenance on sugar conveying and packaging equipment to minimise dust releases and sugar spillage, and promptly removed accumulated dust and spilled sugar. When sugar dust and sugar escaped the equipment in the packing buildings, timely housekeeping activities should have been performed to remove accumulations from elevated horizontal surfaces and spilled granulated and powdered sugar on the floors before the sugar accumulated to hazardous levels. Pre-explosion Sugar Dust Incident History The Port Wentworth facility operated for more than 80 years without experiencing a devastating dust explosion. Workers told the CSB investigators that small fires sometimes occurred in the buildings but were quickly extinguished without escalating to a major incident. Less than two weeks before the February incident, a small explosion in a dry dust collector on the roof of the packing building damaged the dust collector, but was safely vented through the deflagration vent panels. The dust collector had not yet been returned to service at the time of the incident. even though photographs dating from the 1970s to 2007 and internal correspondence dating to the early 1960s confirmed that significant sugar dust and spilled sugar in the packing buildings and silo penthouse were long-standing problems. In the year preceding the February 2008 explosion, quality audits and worker injury reports documented spilled sugar at "knee deep" levels in some areas. Operators reported that the buckets inside bucket elevators throughout the facility sometimes broke loose and fell to the bottom of the elevator. However, they could recall only one incident, some 10 years earlier, where a falling bucket was thought to be the ignition source of a fire involving a bucket elevator in the raw sugar warehouse. That event did not escalate to a major facility fire or explosion. The CSB investigators learned through worker interviews that conveying granulated sugar on the steel belt conveyor in the tunnel under silos 1 and 2 generated some sugar dust. When sugar lumps lodged in a silo outlet pipe and blocked the movement of the sugar on the belt, as operators reported sometimes occurred, sugar spilling off the belt would release airborne dust. Before the company enclosed the steel belt conveyor, the dust was released into the large volume of the tunnel. Airflow through the tunnel would also keep the airborne dust concentration low; thus, airborne sugar dust likely never accumulated to an ignitable concentration. In 2007, Imperial Sugar enclosed the granulated sugar belt conveyors in the penthouse and the tunnel under the silos to address sugar contamination concerns. However, the company did not evaluate the hazards associated with generating and accumulating combustible dust inside the new enclosure. They did not install a dust removal system to ensure that sugar dust did not reach the MEC inside the enclosure. Furthermore, the enclosures were not equipped with deflagration vents to direct overpressure safely out of the building. Primary Event Location if airborne sugar dust ignited. For more than 80 years, the facility operated the open steel belt conveyor in the silo tunnel without incident. Less than a year after the enclosure was completed, however, the silos and packing buildings were destroyed by devastating explosions and fires. The CSB examined the blast patterns and damage inside the tunnel, at the east and west tunnel openings, and rooms inside the base of the silos directly above the silo tunnel and concluded that the primary dust explosion most likely occurred in the silo tunnel, approximately midway along the length of the steel belt. Examination of the equipment in the silo tunnel found that every panel used to enclose the 80foot long steel belt conveyor had been violently blown off the support frame. Panels east of the approximate midpoint of the conveyor (directly under the silo 1-silo 2 abutment) were deflected eastward, and panels west of the midpoint were deflected westward. Pressure damage inside the pantleg rooms above the tunnel was greatest inside the silo 2 pantleg room and least inside silo 3 pantleg room. Major equipment damage was observed at the east entrance of the silo tunnel with all equipment deflections eastward, away from the tunnel. The equipment under silo 3 and outside the west end of the tunnel had damage patterns clearly indicating that a pressure wave travelled west out of the tunnel. Pressure waves blew the wood walls and doors off the west and east tunnel entrances. Mangled steel belt cover panels were blown out of the east end of the tunnel. The pressure wave also travelled up between silos 1 and 2 and blew the brick walls that enclosed the south stairwell into the work areas on each floor of the south packing building Primary Event Combustible Dust Source The CSB learned that, during the 3 to 4 days preceding the incident, workers were clearing sugar lumps lodged in the silo 1 sugar discharge holes above the steel belt. Through access ports in the pantleg room, they used steel rods to break and dislodge sugar lumps found above and inside the sugar discharge holes. Workers also used access ports located on the discharge chutes to break lumps that lodged on the steel belt, blocking sugar flow from the upstream discharge it is highly likely that sugar lumps lodged between the moving steel belt and one or both silo 1 discharge chutes. and into the bulk sugar station. below. Contaminated land. Explosion pressure waves travelled between silos 1 and 2 and exited into the south stairwell. Building Control is the framework within which the Building Regulations and associated legislation are administered. and then ignited. The steel belt return loop was fewer than 10 inches above the tunnel floor.chutes. but too large to pass between the end of the chute and the steel belt would create a "dam" that would cause sugar travelling from silo 2 to spill off the steel belt upstream of the silo 1 discharge chute. and sugar dust generated as the steel belt travelled through the sugar pile.Building Regulations and Fire. palletizer room. In this element. Sugar lumps small enough to enter the discharge chute. Sound insulation. we are principally involved with the parts that appertain to fire protection. sugar dust accumulated above the MEC inside the enclosed conveyor. Sugar dust released into the air from the sugar spilling off the conveyor. Building Regulations deal with health and safety issues for people who are in or about the building. blowing the brick walls out into the packing building. The regulations are also intended to deal with a variety of environmental issues. Spilled sugar would then accumulate inside the covered conveyor. Prevention of condensation in roofs. the list below summarises the issues with which the Building Regulations are concerned. whilst others have a slightly more indirect connection :        Structural stability. Room ventilation. including means of escape in case of fire and access for the fire brigade. This is done by controlling how buildings are constructed and how certain physical services are installed. sugar continued to flow from silo 2 onto the steel belt upstream of the silo 1 discharge chutes. 2. During the silo 1 "rodding" activities. likely covering parts of the steel belt. Some have an immediate and direct relation to Fire protection.0 . The fireballs continued to be fuelled by sugar dust dislodged from overhead equipment. Secondary Dust Explosions The primary dust explosion in the silo tunnel sent overpressure waves into the three silo pantleg rooms. The explosion triggered a series of secondary explosions that rapidly progressed through the packing buildings. The CSB concluded that as granulated sugar spilled off the moving steel conveyor at the blocked outlet. The pressure waves also violently heaved concrete floors upward throughout the south packing building ( and then out into the first floor of the Bosch building. Spilled sugar that had accumulated on the floors around conveyors and packaging equipment rained into the rooms below adding more fuel to the advancing fireballs. However. . Sugar dust accumulations on elevated surfaces and spilled sugar on the floors around the packaging equipment contributed to the explosion energy. Fire safety. Powdered and granulated sugar added fuel as sugar was thrown into the air by the advancing pressure waves. Workers reported that spilled granulated sugar sometimes covered the belt return loop and support bearings. Prevention of damp. could accumulate in the unventilated enclosure. Based on the statements from the workers who were clearing the sugar lumps from the silo outlets. . hence they are classed as 'passive protection'. Heat-producing appliances. ramps. following which Charles II decreed that walls between buildings (party walls) must be constructed of brick or stone. From 1858 on. The Buildings Regulations provide detailed information specific to a variety of premises such as dwellings and commercial properties. retard and isolate flames and the associated smoke and fumes: they do not in themselves extinguish flames. including measures for cleaning and opening windows safely.does the ventilation design contribute to fire growth? The Building Regulations are made by the Secretary of State for Communities and Local Government under powers delegated by Parliament under the Building Act 1984. Waste disposal. including hot and cold water. Compliance with the detailed guidance of the Approved Documents is evidence that the Regulations themselves have been complied with. and that buildings with timber claddings must be far enough apart to prevent the spread of fire from one to another. The design and construction stages of a building present the opportunity to provide built-in. The level of safety and standards acceptable are set out as guidance in the Approved Documents.do any chemicals and materials increase the potential for combustion and fire growth. in this respect qualified as the guarantee that a specific resistance/retardation time increases the time available for the evacuation of a building. local councils made bye-laws under various Public Health Acts to control private building in the interest of public health and safety. It is easy to identify 'Fire safety including means of escape in case of fire and access for the fire brigade' along with 'heat-producing appliances' as specific aspects of fire protection. Foul and surface water drainage. Times range from 30 minutes to 4 hours retardation.are windows of the correct design and type present as routes for escape if other exits are unusable? Insulation and prevention of damp . and measures to prevent falling. the implementation of Building Regulations is a historic precedent and can be traced back to the Great Fire of London in 1666. Alternate ways of achieving the same level of safety are also acceptable. The fundamental requirement of passive fire protection systems concerns public safety. Stairs. or passive fire protection.        Provision of toilets. Passive fire protection construction components are used to resist. As a brief aside. other matters can be identified as (almost) principal factors:    Glazing . Conservation of fuel and power. those properties having specific requirements subject to use. These Regulations are framed as basic performance standards. collisions with doors and windows. Passive fire protection is achieved by the use of suitable construction materials and the design of construction itself. Other headings do not initially seem to be aspects of fire protection in buildings. Access and facilities for disabled people to and within all buildings. including dwellings. Safety glazing. or can materials be utilised that minimise growth or contain fire spread? Room ventilation . but when a more holistic assessment of the situation is undertaken. 1 . the fire performance is therefore even more enhanced. Wood. and the intumescent coating expands to many times its original thickness. the increase in temperature causes a chemical reaction. This provides an insulating foamlike coating or 'char' which protects the substrate. 2. In a fire. contents and surroundings by flames or smoke may be significantly reduced by a passive fire protection system. damage to the building. These coatings are designed to expand to form an insulating and fire-resistant covering when subject to heat. thus providing an inherent or passive fire protection capacity. Intumescent coatings provide an appearance similar to that of a paint finish. the potential for these elements if 'untreated' to transfer heat is quite high. Intumescents are designed to reduce heat propagation. the basic elements of a structure are typically steelwork. Excluding wood at this point. Intumescent coatings are made of a combination of products. however. This in turn protects the steel and other materials from the high temperatures of fires. thus preventing or retarding structural damage during fires. Intumescent technology is not only restricted to material coatings. for example a steel girder. The flame-retardant and smoke-stopping capabilities of these construction components also enable rescue and emergency services to undertake their jobs in a more controlled environment. . the intumescent coating is designed to insulate the steel and prevent the temperature of the steel from rising above a certain point. applied to the surface like paint.Behaviour of Materials cont. other products use intumescent technology to provide fire retardation and to inhibit the spread of fire. now mentioned as an element of construction in many instances. Intumescent coatings are also used on wood but work in a different manner. in a fire situation. offering good insulation protection to the material coated. and remain stable at ambient temperatures. the coating expands to a thick non-flammable layer of bubbles. intumescent systems are now being incorporated into certain plastics. However. concrete and brickwork and (in normal circumstances) do not readily support combustion.dependent on the particular product and application. and reduce the spread of flame. some of which are shown in the following sections. timber structures are more susceptible to the surface spread of flame and heat propagation. Finally. As well as being used to protect flammable materials and structural elements. In applying treatment to these elements. can be treated with the similar fireproofing/fire retarding materials to achieve improved fire performance itself. In the protection steelwork. There are numerous examples of treatment materials. . Another method used frequently in construction is the encasement or 'boxing' in of steelwork and wooden beams. therefore greatly reducing the potential for the steel to become heated. This in turn reduces the potential for the metal to conduct heat elsewhere. the material used to encase the steelwork is of a high fire rating and is used to insulate the metal from the heat source. the potential for the structure to warp and fail is also greatly reduced. 'Figure 2' This illustration depicts the application of an intumescent coating to a building slab through which electrical cabling and metal conduits are fitted.'Figure 1' A widely-used product is the intumescent 'pillow' This illustration shows several of these pillows being layered in combination to fill an electrical cabling route through a block-built wall. Also by reducing the amount of available heat to the steel construction. typically. In this application. this happens more so on steelwork. such expansion can lead to the collapse of the structure. but the hardened cement paste also shrinks as a result of loss of moisture by drying out and internal stresses can be set up within the concrete. it expands due to thermal expansion of the materials. The materials used to make them are of low heat conductivity. and is aggravated if the hot concrete is suddenly chilled. it may have been fabricated to meet a specific fire resistance standard. thus protecting the steel beams and columns from the heat source.'Figure 3' In this illustration. A 10-metre steel joist. loses two-thirds of its strength at 593°C and. indeed. will expand 60mm for a 500°C rise of temperature. It should now be identified that the insulation of steel work is paramount to the level of resistance to collapse of the building or structure. thermal insulation and fire protection. unprotected metal surfaces can constitute a serious risk in a fire because all metals heat up and expand when exposed to fire and are a potential cause of fire spread by conduction. . which is used to carry a load presents the even more serious danger of rapid collapse when excessively heated. for example with a jet of water. 'spalling' (or breaking off) of the surface material occurs. Concrete can be produced to have a wide range of properties. resulting in collapse. This is by no means an abnormal temperature in even a moderate fire . we can see how the encasement of both a cross-member and an upright steel girder has been achieved. durability. Concrete made with limestone or lightweight aggregates is very much less susceptible to spalling than those made with more dense aggregates. begins to sag and twist. When concrete is heated. Unprotected metal.B. The encasing 'boards' are depicted in pink. such as high compressive strength. and although metals commonly used in the construction of a building are not combustible and present no risk of fire spread from direct burning. N. In a severe fire. for example. hence the fire resistance of structural concrete is of a different classification. and where it is built into a load bearing wall.the danger of the failure of unprotected load-bearing metal work cannot be over-emphasised. in proportion to the amount and direction of the load to which it is subjected. for example. Concrete is also a commonly-found material used in building structures and is inherently noncombustible. Structural steel. lofts. the build-up of charcoal on the surface of burning timber limits the availability of oxygen. The pitched roof design is the most common method of construction. Even given the fact that timber is combustible. In principle. In some type of houses. Generally. this process 'drives' the liquid into the timber and is a type of permanent treatment. over the whole of a block of properties. attics voids etc. for example terraced properties and rows of cottages. between the ceiling of the rooms below and the weather covering. and presents problems due to the large unused spaces that exist. Roof coverings are finished with materials that are not readily combustible. twisting and softening is less likely.using a combination of pressure and vacuum. Structural members of timber are less likely to enable the spread of fire from one compartment to another due to thermal conductivity. thereby insulating the remainder of the section. 2. there are various flame-retardant treatments that can assist in making wood more difficult to ignite. Failure of structural members of timber due to sagging.2 . in some cases. so a roof is not normally vulnerable to fire from an external source. Timber remains a fundamental material in building construction. It has been established that the burning or charring rate is predictable and varies only slightly with species of timber and not on the severity of the fire. it can be that the performance of timber in real fires is frequently far superior to unprotected. it is the manner in which roofs are constructed rather than the material used that is the major concern when dealing with the potential spread of fire.e. Sacrificial timber built into the construction may be consumed by a fire before the structural core is attacked..Behaviour of Materials cont'd. . these voids can extend unbroken. Surface coatings . it may shrink slightly) Wood has the inherent ability to protect itself. and although wood is combustible. non-combustible materials such as steel and aluminium for the following reasons:      Timber does not expand significantly under the influence of heat (in fact.2. there are two types of flame-retardant treatments for timber: 1.painted onto the timber surface with little or no penetration into the wood. over several dwellings or. Impregnation . i. There are direct similarities between the internal compartmental method and reducing the potential for the spread of fire through external walls The cavities between walls can be filled with noncombustible. This takes on a greater level of significance when properties are joined via a party wall. Most modern residential buildings with this type of roof use a method known as "fire blocking" where required in construction. This method is when fire-resisting foam blocks are inserted to prevent fire spreading through cavities up into the roof. Method is applied to detached properties and also rows of houses. . The roofs are also compartmented and precautions are taken to stop the spread of fire from the roof void of one property to another. fire-retardant material.'Diagram 1' The diagram shows a typical roof construction. there was heavy damage to the roof structure of the Royal Kitchen. closer examination revealed that the surviving deeply charred timbers were not. Point of references: the Fire Service Manual Volume 3 Fire Safety.the original oak structure having been . the point of reference for specific and definitive information is section 14 of the Building Regulations Approved Document B (Fire Safety) 2006 edition.g. Although not strictly the purpose of this element. What is easily apparent from the information is that there is no one set distance of separation between buildings. there may be problems with large panels (over 4 metres) of brickwork due to differential expansion and movement. However. type and construction. Basic Principles of Building Construction.'Image 1' This picture shows the wall cavity with 'rockwool' installed. Background During the disastrous fire at Windsor Castle on 20th November 1992. To initiate the subsequent repair and restoration operations. In fact. brick panels in concrete frame buildings. Such is the detail provided by the document that it should be read by the candidate. This improves the fire rating of the external structure and further reduces the potential for transmission of heat via the wall(s). In these instances. perhaps the candidate should also consider the distance between properties in the event of structural failure and collapse. the volume of detail precludes the use of the material in a verbatim manner in this element. Brickwork is generally a very good fire-resisting material.Windsor Castle Fire 1992. of a single 19th century date. they were of two periods . the restraints being applied to the edges of the panels become critical e. orientation to other properties and the intended use of the building. A key element in the external spread of fire is the distance between buildings. Repair Programme Although it had initially been thought that an entire replacement might be necessary. 2. this roof was inspected in detail. the stability of the material being due to the high temperatures to which it has already been subjected during manufacture. as had been thought.3 . some of which are the designed fire rating of the building. It is quite possible to achieve periods of resistance of up to four hours. and that the calculation of such imports data and details. whilst the missing coving and the outer timber roof were also reinstated. with emergency services from East and West Sussex." 2. destroying six carriages and one locomotive. including smoke inhalation. although fire crews continued to smother minor fires nearby for a further two hours. in matching oak. In certain positions. Eyewitness accounts state that two loud bangs.perhaps to make the ceiling seem "more Gothic". a toxic product used in the pharmaceutical industry. however this is discovered to be an error and only 100g was carried. Now rebuilt with every available original timber. evidently introduced by Wyatville .ornamented and clad in decorative softwood.4 . the fire-damaged timbers were thoroughly de-charred. this was augmented by concealed steelwork. The shuttle was carrying 27 vehicles. The train came to a grinding halt. to permit closer structural examination. More than 300 firefighters from both sides of the English Channel helped tackle the blaze. The crew and passengers also hear several explosions. Madrid 2005. it was evident that the roof presented a very important historical record. fourteen people suffered minor injuries. were heard and then thick smoke swept through the carriage. 2. Hence the decision was taken to repair the extensive damage by traditional splicing methods. The report into the fire includes an observation into the cause of the explosions. . The open vents had long been lost. The restored structure was incorporated within Wyatville's extensions of the original lantern. The earliest carpentry was dendrochronologically dated to soon after 1337. at approximately 14:57 BST.Windsor Tower Fire. The source of these explosions cannot be determined with certainty. although it is probably connected with the explosion of tyres and or fuel tanks. It was evident that beneath the charring. After careful protective measures. The fire was reported on 11th September 2008. In view of the above dating discovery. the lantern displays its initial length. and the lights went out. described as explosions. The fire continued to burn overnight and was reported to have been put out by 06:00 GMT the following day. The latter was particularly required to carry the modern air-conditioning equipment that is essential for a busy modern working kitchen.Channel Tunnel Fire 2008. The temperature in the tunnel was described as "very hot". was initially thought to be close to the seat of the fire. It was conducted alongside the creation of a detailed historical record. This was recognised as having formed part of the structure that was rebuilt in 1489. The blaze spread to other trucks on the train during the evening. The professional team worked on behalf of English Heritage to undertake this initial survey work. giving the impression of a major fire burning behind that vehicle and/or on the vehicles immediately behind.8 miles from the French entrance to the tunnel in the North Tunnel. and analysis. braces and cusping. making retention of such historic fabric feasible. and one passenger smashed a window with a hammer in order to climb out. 6. Thirty-two people on board the train were led to safety down a separate service tunnel. and were taken to hospital. London and Essex providing support. A lorry carrying 100 kg of phenol (carbolic acid). some of the timberwork possessed usable residual sectional properties and strength. internally. Further eyewitness accounts suggest that the emergency exit was jammed. but new green oak timberwork completed the roof.5 . "The chef de train and some of the passengers see that the first road vehicle on the shuttle (a HGV) is outlined against the flames. including ties. the perimeter columns and internal steel beams were left unprotected in accordance with the Spanish building code at the time of construction The building featured two heavily reinforced concrete transfer structures (technical floors) between the 2nd and 3rd Floors. The building was totally destroyed by the fire. Commercial. As a result. the fire protection for all steelwork below the 17th floor had been completed except a proportion of the 9th and 15th floors. Spain Fire Event: 12 February 2005 Fire started at the 21st Floor. not all the gaps between the cladding and the floor slabs had been sealed with fireproof material (Dave 2005). the Spanish codes did not require fire protection to steelwork and sprinkler fire protection for the building. spreading to all floors above the 2nd Floor. The Fire . Also fire stopping to voids and fire doors to vertical shafts were not fully installed. the fire compartmentation could only be floor-by-floor (about 40 x 25m). No sprinklers. However. The building was subjected to a three year refurbishment programme of works when the fire broke out. The gap between the original cladding and floor slabs was not firestopped as well.Overview Location: Madrid. the vertical compartmentation might not be fully achieved due to the lack of firestop system in floor openings and between the original cladding and the floor slabs. Since the building adopted the "open plan" floor concept. The original cladding system was fixed to the steel perimeter columns and the floor slabs. A typical floor was two-way spanning 280mm deep waffle slab supported by the concrete core. Originally. these weak links in the fire protection of the building was being rectified in the refurbishment project at the time of the fire. Fire Resistance: Passive fire protection. In fact. The major works included the installations of:      Fire protection to the perimeter steel columns using a boarding system Fire protection to the internal steel beams using a spray protection A sprinkler system A new aluminium cladding system The refurbishment was carried out floor-by-floor from the lower floors upwards. The perimeter columns were supported by the transfer structures at the 17th and 3rd Floor levels. and between the 16th and 17th Floors respectively. Fire Protection System The Windsor Tower's original structural design complied with the Spanish building codes in the 1970s. However. effectively. The Building The Windsor Tower or Torre Windsor (officially known as Edificio Windsor) was a 32-storey concrete building with a reinforced concrete central core. internal RC columns with additional 360mm deep steel I-beams and steel perimeter columns. Building Type: 106 m (32 storeys). the original existing steelwork was left unprotected and no sprinkler system was installed in the building. Fire duration: 18 ~ 20 hours Fire Damage: Extensive slab collapse above the 17th Floor. with perimeter steel columns which were unprotected above the 17th Floor level at the time of the fire. Construction Type: Reinforced concrete core with waffle slabs supported by internal RC columns and steel beams. At the time of the construction. By the time the fire broke out. so they left the train and ran the remaining mile to the south portal (where they knew there was a direct telephone connection to the signaller) to raise the alarm.m. A large portion of the floor slabs above the 17th Floor progressively collapsed during the fire when the unprotected steel perimeter columns on the upper levels buckled and collapsed (see Figure 1). The estimated property loss was €72m before the renovation. those protected perimeter columns survived.50 a. Nevertheless. The actual cause will be difficult to be found due to the collapse of the break-out floor. However. all floors above the 21st Floor were on fire. One-third of the way through the tunnel. . It is clear that the structural integrity and redundancy of the remaining parts of the building provided the overall stability of the building. Within one hour. such as automotive sprinklers the "open plan" floors with a floor area of 1000m2 the failure of vertical compartmentation measures. a defective axle bearing derailed the fourth tanker. in the façade system and the floor openings It was believed that the multiple floor fire. Analysis The main factors leading to the rapid fire growth and the fire spread to almost all floors included:    the lack of effective fire fighting measures.000 gallons of four-star petrol in thirteen tankers entered the tunnel on the Yorkshire (north) side. the reinforced concrete central core. The total fire duration was estimated to be 18 ~ 20 hours. structural fire analysis should be carried out before such a conclusion can be drawn. they did not cause any structural collapse. the applied loads supported by these buckled columns had been redistributed to the remaining reinforced concrete shear walls. It was reported that the fire started at 23:00 at the 21st Floor. In the following hours. The whole building was beyond repair and had to be demolished. However. The three train crewmembers could see fire spreading through the ballast beneath the other track in the tunnel. triggered the collapse of the floor slabs above the 17th floor. which promptly knocked those behind it off the track. When the fire spread below the 17th floor. The Damage The Windsor Tower was completely gutted by the fire on 12 February 2005. One of the derailed tankers fell on its side and began to leak petrol into the tunnel. except for the unprotected columns at the 9th and 15th floors which all buckled in the multiple floor fire. waffle slabs and transfer structures performed very well in such a severe fire. along with the simultaneous buckling of the unprotected steel perimeter columns at several floors. some facts under investigation point that it could be induced by arsonists. 2. the fire gradually spread downwards to the lower technical floor at the 3rd Floor. The fire occurred at 5. Obviously.The fire was believed to have been caused by a short-circuit on the 21st floor. The reduced damage below the 17th floor might provide a clue.6 .Summit Tunnel Fire 1984. on 20th December 1984 when a goods train carrying more than 220. On the other hand. It was believed that the massive transfer structure at the 17th Floor level resisted further collapse of the building. Only the locomotive and the first three tankers remained on the rails. The fire protection on the existing steelworks below the 17th floor had been completed at the time of fire except for the 9th and 15th floors. columns. Vapour from the leaking petrol was probably ignited by a hot axle box. Approximately half a mile of track had to be replaced. The brigades continued to fight the fire for a further two days. The train crew were persuaded to return to the train. At the same time. where they uncoupled the three tankers still on the rails and used the locomotive to drive them out. the bricks in the tunnel wall began to spall and melt in the flames and the breathing apparatus crews from both brigades decided to evacuate. Petrol continued to leak from the derailed wagons through the tunnel drainage and ballast and the vapour sporadically re-ignited when it came into contact with the hot tunnel lining. The gases are estimated to have flowed up these shafts at 50 metres per second (110 mph). pillars of flame approximately 45 metres (148 ft) high rose from the shaft outlets on the hillside above. Left to itself. perhaps because they had both participated in an emergency exercise in the tunnel a month before. crews from West Yorkshire fire brigade entered the tunnel and began fighting fires in the ballast at the north end of the train. The wall deflected the flames both ways along the tunnel. The fuel supply to the fire was so rich that some of the combustibles were unable to find oxygen inside the tunnel to burn with: they were instead ejected from vent shafts 8 and 9 as superheated. small globules of metal were found on the ground surrounding shaft 9 . swept up with the exhaust gases. The vented vapour caught fire and blew flames onto the tunnel wall. until West Yorkshire fire brigade issued the stop message just after 6:30 p. They also lowered hoselines down one of the ventilation shafts to provide a water supply. In the clearup operation afterwards. the fire brigades forced foam into ventilation shafts far from the fire. as did all the electrical services and signalling. fuel-rich gases that burst into flame the moment they encountered oxygen in the air outside the tunnel. However. It also became apparent that petrol vapour had leaked into the nearby river Roch. By midafternoon the next day the inferno was no longer burning. Greater Manchester fire brigade then loaded firefighting equipment onto track trolleys and sent a crew with breathing apparatus in to begin their firefighting operation at the south end of the train. all ten tankers discharged petrol vapour from their pressure relief valves.these had been melted from the tanker walls. These set much of the vegetation on fire and caused the closure of the A6033 road. . the pressure in one of the heated tankers rose high enough to open its pressure relief valves.m. the fire burned as hot as it could. Although some bricks in the tunnel and in the blast relief shafts had become so hot that they vitrified and ran like molten glass. The damage done by the fire was minimal.530 °C and discharged their remaining loads as floods. The biggest surprise was how well the brick lining had stood up to the fire.m. Air at this speed is capable of blowing around fairly heavy items: hot projectiles made from tunnel lining (rather like lava bombs from a volcano) were cast out over the hillside. Unable to get close enough to safely fight the fire directly. possibly creating explosive atmospheres in the nearby towns of Summit and Todmorden. at 9. At the height of the fire.40 a. Fire crews remained at the site until 7th January 1985. They managed to leave just before the first explosion rocked the tunnel. As the walls warmed up and the air temperature in the tunnel rose. though the fire was by no means dead. which were partially evacuated in response. This created blockages that starved the fire of oxygen.Crews from Greater Manchester Fire Brigade and West Yorkshire Fire Brigade quickly attended the scene. on Christmas Eve. Two tankers melted (at approximately 1. most of the brickwork lining of the tunnel was scorched but still serviceable. and dropped out onto the grass around the top of the shaft. Co-ordination between the brigades appears to have worked well. 2. The amazing luck of those who fought it. the compartmentation of a roof void in a terrace property: . The factory. which owned the site. The Kader Toy Factory Fire case study by Casey Cananaugh Grant is available on the following link: http://www. the building was reinforced with un-insulated steel girders which quickly weakened and collapsed when heated by the flames. Furthermore.org/safework_bookshelf/english?content&nd=857170498 3. Most of the fatalities were young female workers from rural families. For example.The structures that were destroyed in the blaze were all owned and operated directly by Kader.ilo. The Kader Toy Factory fire was a fire on 10 May 1993 at a factory in Thailand which killed 188 people and over 500 were seriously injured.The factory was poorly designed and built. Compartmentation is the process or design of isolating zones of a building to ensure a fire in one area does not spread uncontrolled to another area. The firefighters in breating apparatus sets who were in the tunnel when it did fill with flames were saved by the fact that blast relief shafts 8 and 9 acted as flame vents (a function their designer never envisaged).   The size of the fire: it is probably the biggest underground fire in transportation history. which manufactured stuffed toys and licensed plastic dolls. and the existing external doors were locked. certainly bigger than the 1996 Channel Tunnel fire (a relatively meagre 350 megawatts) and probably bigger than the ill-defined Salang tunnel fire in Afghanistan. Kader has two sister companies that also operated at the location on a lease arrangement. a Thai transnational corporation and one of Asia's largest agribusiness firms.Kader Toy Factory Fire 1993. Fire exits drawn in the building plans were not in fact constructed. although some of the bricks melted. The BR train crew who returned to the site to rescue a locomotive and three tankers left the fire site shortly before one of the other petrol tankers filled the tunnel with flames. The amount of damage to the primary structure of the tunnel was minimal.Fire & Explosion Prevention & Protection.The Summit Tunnel fire is worthy of note for several reasons.0 .7 . was owned by the Charoen Pokphand (CP) Group. gaps between ceilings and floors and cavity walls etc. timbers passing through the barrier are protected for some distance on either side. and the roof space is kept clear of debris. the reduction of fire spread. walls or an internal lining or a roof. etc. vermiculite. In order to reduce heat transmission in hollow spaces or cavities such as those between double partitions. Roof voids. the barrier is set back from the roof truss.'Image 1' Picture shows the general cavity/void area 'Image 2' This enlarged section of the photograph shows more clearly the cavity barrier in place. they are frequently filled with materials which are of a loose fibrous nature and have a low conductivity. This typical cavity barrier installation in a roof space has been retro-fitted to an existing property and is designed to give a specific integrity performance. These are typically non-combustible products such as rock or glass wool. To prevent heat radiation igniting adjacent material. . are often both referred to and described as concealed spaces. foamed slag. but can be applied to most areas creating specific fire protection zones within a building. As mentioned previously.Fire & Explosion Prevention cont'd. again. in simple terms an example is the difference between a room with either on open or closed door. The division of a building into these fire zones offers perhaps the most effective passive means of limiting fire damage. ceilings and floors) which by design or by modification.'Image 3' This picture provided by Rockwool Ltd. It also delays the spread of fire prior to the arrival of the fire brigade. the size of the damaged area is mainly dependent upon the layout of the fire-resisting barriers within the building.1 . and often an hour or more. this approach provides at least some protection for the rest of the building and its occupants. 3. almost every building has its own natural compartment lines (walls. or internal partition wall.create the boundaries for compartmentation. advertises the fire insulation of their rockwool product 'Image 4' This illustration shows the installation of fire-retardant materials in a 'stud'. The design technique of compartmentation is not only applied to roof voids and other concealed spaces. constructed or modified . A simplified analogy is that the property has been divided up into 'boxes'.e. .if suitably designed. each of which intend to inhibit fire growth. are capable of providing upward of 30 minutes' protection against a fully-developed fire. The potential for such fire protection zones is in fact inherent to the building design i. Designed to contain the fire within the zone of origin. In the event of a fire within a building protected by compartmentation. the walls or rooms of the structure . glass would have constituted a major weakness in a wall. compartmentation does need to be planned and implemented properly.Fire compartmentation is therefore an important part of any damage limitation strategy. Thus. This is particularly true in fire doors and has led to the extensive use of wired glass in these locations. Glass is non-combustible and will not. To be effective. but also protection against radiant heat in a fire. halls and landings would be separated from staircases to prevent a fire from travelling vertically up the stairwell to all floors. Combined with a fire-rated door. therefore contribute fuel to a fire or directly assist a fire to spread. When a fire is developed. At one time. or even through to the adjacent room. a window in a compartment wall could be a weak link. 'Figure 1' This illustration shows an opening in a area using compartmentation (the fire door is absent to allow easier definition) The candidate need not be too concerned with the finite detail of the construction. clear vision through a wall or door can also provide safety benefits in the event of a fire. There is no point in upgrading the fire resistance of a door and then not protecting a ventilation duct by the side of it which runs through to the floor above. high levels of transmitted radiation through glass may present a hazard to people escaping or possibly cause the ignition of combustible materials resulting in fire spread. However. this design would be used a service shaft or perhaps a corridor as part of a evacuation route . door or screen because it would break and fall out. Fire-resisting glazing can provide not only an effective barrier to the spread of smoke and flames. enabling the location and safe evacuation of occupants from burning buildings. However. there are practical limitations on the number of compartment lines because an over-compartmented building can become restrictive in its daily use. Ideally. but should identify that increased spread-inhibiting materials (shown as pink boarding) have been used around the opening. 'Figure 2' Here, fire growth-retardant material has been sandwiched between the joists to fill the cavity between a ceiling and upper floor, improving the compartmental aspect of each room. When applying containment methodology to original or older buildings, the most important elements to be upgraded are the doors, floors and walls, penetrations (metal conduits etc.) through floors and walls and cavity barriers in the roof spaces. Click on this link to view more in-depth information of the preceding sections. Once fire takes hold within a building, the surfaces of wall and ceilings will contribute to the fire if they are ignited. Ensuring that these surfaces are non-combustible or, as a minimum, difficult to ignite means that, if a fire does start, its rate of growth or spread will be reduced. It is also desirable to keep to a minimum the amount of smoke or toxic fumes given off from this type of surface if involved in a fire, particularly those lining escape routes. 3.2 - Fire & Explosion Prevention cont'd. The following paragraphs outline some of the materials and methods that can used to line room surfaces. Fibre building boards are manufactured in a wide range of sheet materials. They are made from actual wood fibres and derive their basic strength and cohesion by the felting together of the fibres themselves, and from their inherent adhesive properties. Bonding, impregnating or other agents, including fire retardants, may be added during or after manufacture to modify particular properties. Building boards of this group are not easily ignitable but typically are combustible. Plaster boards for interior use are composed of a core of set gypsum firmly bonded to two sheets of heavy paper to increase their tensile strength. In a fire, the exposed paper face may burn away, making it relatively easy to break up the noncombustible gypsum core, but until this happens, the plaster board will retard the spread of fire. 'Image 1' Plaster boards can be used to 'line out' a wall by fixing directly to the wall Gypsum plaster boards are also used to construct partition walls by fixing to stud work. This process in also know as Dry-lining. As this illustration shows this method results in a cavity or concealed space; the illustration also shows the use of insulation material fitted between the wall studs 'Image 2' The candidate should review early paragraphs and consider other materials that inhibit combustion and restrict fire spread, and their application as linings. 3.3 - Plant Layout. General principles.    Plant layout is often a compromise between a number of factors such as: The need to keep distances for transfer of materials between plant/storage units to a minimum to reduce costs and risks. The geographical limitations of the site.         Interaction with existing or planned facilities on site such as roadways, drainage and utilities routings. Interaction with other plants on site. The need for plant operability and maintainability. The need to locate hazardous materials facilities as far as possible from site boundaries and people living in the local neighbourhood. The need to prevent confinement where release of flammable substances may occur. The need to provide access for emergency services. The need to provide emergency escape routes for on-site personnel. The need to provide acceptable working conditions for operators. The most important factors of plant layout as far as safety aspects are concerned are those to:     prevent, limit and/or mitigate escalation of adjacent events (domino); ensure safety within on-site occupied buildings; control access of unauthorised personnel; facilitate access for emergency services. In determining plant layout, designers should consider the factors outlined in the following sections. 3.4 - Inherent Safety. The major principle in Inherent Safety is to remove the hazard altogether. The best method to achieve this is to reduce the inventory of hazardous substances such that a major hazard is no longer presented. However, this is not often readily achievable, and by definition no COMAH facility will have done so. Other possible methods to achieve an Inherently Safer design are:      intensification to reduce inventories; substitution of hazardous substances by less hazardous alternatives; attenuation to reduce hazardous process conditions i.e. temperature, pressure; simpler systems/processes to reduce potential loss of containment or possibility of errors causing a hazardous event; fail-safe design e.g. valve position on failure. Plant layout considerations to achieve Inherent Safety are mainly those concerned with domino effects. 3.5 - The Dow / Mond Indices. These hazard indices are useful for evaluating processes or projects, ranking them against existing facilities, and assigning incident classifications. They provide a comparative measure of the overall risk of fire and explosion of a process, and are useful tools in the plant layout development stage since they enable objective spacing distances to be taken into account at all stages. The methodology for undertaking a rapid ranking method that is based on the Dow / Mond index is detailed in ILO/PIACT Major Hazard Control: A Practical Manual, 1988. Although these are useful rule-of thumb methodologies for first consideration of plant layout, they do not replace risk assessment. The distances derived between plant units using these systems are based upon engineering judgement and some degree of experience rather than any detailed analysis. Consideration should also be given to the spread of flammable material via drains. Further information may be found in BS 5908 : 1990 . particularly against missiles. provision of barriers e.Domino Effects. ensuring that the distances between plant items are sufficient to prevent overheating of adjacent plants.g. Protection against domino effects by convection. Toxic gas releases may cause domino effects by rendering adjacent plants inoperable and injuring operators. Conduction. ducts and ventilation systems. Domino may be by fire. Prevention/mitigation of such effects may be affected by provision of automatic control systems using inherently safer principles and a suitable control room (see section on Occupied Buildings in following pages). Explosion propagation may be directly by pressure waves or indirectly by missiles. The spread of fire from its origin to other parts of the premises can be prevented by vertical and horizontal compartmentation. Delayed ignition following a release may result in spread of flames through such systems via dispersed flammable gases and vapours.Explosion. the latter may not provide practical solutions.8 .Fire. A fire can spread in four ways:     Direct burning (including running liquid fires). using fire-resisting walls and floors. directing explosion relief vents away from vulnerable areas e. As for fires. roadways near site boundaries. . provision of thicker walls on vessels. Where this is not possible due to other restrictions.g.3. and risk analysis may be required to prove adequate safety. conduction and radiation can be achieved by inherent safety principles i. Radiation. 3.g.6 .e.Toxic Gas Releases. location in strong buildings. protecting plant against damage e. Hazard assessment of site layout is critical to ensure consequences of loss of containment and chances of escalation are minimised. blast walls. However.7 . compromising the safety of those plants also. other methods such as fire walls. explosion (pressure wave and missiles) or toxic gas cloud causing loss of control of operations in another location. inherently safe methods that should be considered are:     arranging separation distances such that damage to adjacent plants will not occur even in the worst case.9 . other plants or buildings. Convection. 3. 3. active or passive fire protection may be considered. fitting remote-activated isolation valves where high inventories of hazardous materials may be released into vulnerable areas. hazardous area classification for flammable gases. and personnel with more general site responsibilities should usually be housed in buildings sited in a non-hazard area near the main entrance. siting of plants in the open air to ensure rapid dispersion of minor releases of flammable gases and vapours and thus prevent concentrations building up. This provides guidance on offsite consequence analysis for toxic gases. Risk management techniques should be used to identify control measures that can be adopted to reduce the consequences of on. dykes. locating hazardous plant away from main roadways through the site. occupied buildings should not be sited downwind of hazardous plant areas. In particular. Status of guidance.10 . siting of plants within buildings as secondary containment. fire and toxicity. Plant Layout design techniques applicable to the reduction of the risks from release of flammable or toxic materials include:        locating all high-volume storage of flammable / toxic material well outside process areas.11 . Consideration should be given to siting of occupied buildings outside the main fence.or off-site events.Positioning of Occupied Buildings.Aggregation / Trapping of Flammable Vapours.13 . evacuation routes should not be blocked by poor plant layout. Maintenance procedures should include the displacement of vapours from hazardous areas before work begins (see Technical Measures Document on Permit to Work Systems ).12 . . 3. toxic liquids and flammable substances. buildings should be designed so that all parts of the building are well-ventilated by natural or forced ventilation. See references cited in further reading material. vapours and dusts to designate areas where ignition sources should be eliminated. To avoid aggregation and trapping of flammable/toxic vapours which could lead to a hazardous event. 3. which may lead to flash fires and explosions. 3. In all cases.Reduction of Consequences of Events On & Off Site. In addition to the measures described in the previous sections. provision of ditches.3. Additional material providing much insight into analysis of offsite consequences through a risk management program is now available from the United States Environmental Protection Agency. The distance between occupied buildings and plant buildings will be governed by the need to reduce the dangers of explosion. Further guidance is available in standard references. Flammable storages should be sited in the open air so that minor leaks or thermal out-breathing can be dissipated by natural ventilation.Additional Material on Risk Management of Offsite Consequences. embankments and sloping terrain to contain and control releases and limit the safety and environmental effects. it is important that the personnel involved have the correct combination of technical competencies and experience in order to ensure that all aspects of the design process are being adequately addressed. At each stage. These are then used as the basis for the further. through to the issuing of process flow sheets. specification and chemical engineering design of the equipment. Detailed mechanical. The process design should identify the various operational deviations that may occur and any impurities that may be present. detailed process design and onto detailed engineering design and equipment selection. which covers the steps from the initial selection of the process to be used. In the mechanical design. the magnitude of design factors should allow for uncertainties in material properties. where factors are often added to allow some flexibility in process operation. structural. Evidence of the qualifications. The design may also go through many stages. also need to be considered so that the detailed mechanical design can ensure that sufficient strength is available and suitable materials of construction are selected for fabrication. the materials of construction chosen need to be compatible with the process materials at the standard operating conditions and under excursion conditions.15 . from the original research and development phases. codes and applicable standards for the mechanical design of equipment. On many occasions. Modern engineering codes and standards cover a wide range of areas including: . Plant design should take account of the relevant codes and standards. The process design will often be an iterative process. with many different options being investigated and tested before a process is selected. a number of different options may be available and final selection may depend upon a range of factors. Conformity between projects can be achieved if standard designs are used whenever practicable. Design factors are an essential component in order to give a margin of safety in the design.14 . civil and electrical design of equipment comes after the initial process design. The materials of construction also need to be compatible with each other in terms of corrosion properties. General principles. the environment. fabrication and operating loads. Many varied and complex factors including safety. health. For mechanical and structural design. Such flow sheets will include the selection. experience and training of people involved in design activities should be presented in the Safety Report to demonstrate that the complex issues associated with design have been considered and a rigorous approach has been adopted. Impurities which may cause corrosion. economic and technical issues may have to be considered before the design is finalised. detailed design.Codes and Standards. through conceptual design. 3. and the possibility of erosion. Design factors may be appropriate in either the mechanical engineering design or in the process design. The design of a process plant is a complex activity that will usually involve many different disciplines over a considerable period of time.Introduction to Plant Design. This Technical Measures Document primarily considers the latter stages of the detailed design processes and identifies the detailed design issues. design methods.3. refrigerants and heat transfer media can often be replaced by non-flammable or less flammable (high boiling) materials.     materials.16 . expensive and requires regular testing and maintenance. Substitution . quantities of chlorine.this technique involves replacing a hazardous material (or feature) with a safer one. flammable solvents. properties and compositions. Many companies have their own in-house standards which are primarily based on the published codes. compositions and quality. and hence the risk of a major accident can be significantly reduced. For example. preferred sizes. such as BS5500. Some of the techniques that can be considered are:     Intensification . In the safety report. ammonia and LPG can be stored as refrigerated liquids under atmospheric pressure rather than under pressure at ambient temperature. the operating procedures. regulators. Materials likely to form explosive dusts can be used and stored as slurries to minimise hazards. design methods and inspection and fabrication. Attenuation .Inherently Safer Design. Some companies now have design procedures that require a review of designs and seek to ensure that inherently safer concepts have been addressed.affected by equipment design or changes to reaction conditions rather than by . The principles of inherently safer design are particularly important for major hazard plants and should be considered during the design stage. testing procedures. A Safety Report should demonstrate that consideration has been given to the appropriate standards and codes of practice developed by legislators. Limitation . Protective equipment installed onto standard equipment to control accidents and protect people from their consequences is often complex. the base document for the in-house codes should be clearly stated and the key safety-related deviations or enhancements demonstrated so that the assessor can determine their adequacy. Attempts should be made to reduce the requirement for such protective equipment by designing simpler and safer processes in the first instance. distillation and heat exchange but it may involve different mechanisms and approaches having to be employed to the reaction chemistry and control systems. For example. codes of practice for plant operation and safety. It can be applied to a wide range of unit operations including reactors. The Safety Report should adequately demonstrate that consideration has been given to the concepts. plates and standard sections. Often hazardous processes can also be replaced by inherently safer processes that do not involve the use of hazardous substances. A number of approaches can be considered but basically an inherently safer plant can be achieved by minimising the inventories of hazardous substances in storage and in process.using a hazardous material under less hazardous conditions. professional institutions and trade associations. It should also demonstrate that for any equipment installed. 3. This often means carrying out the reaction or unit operation in a smaller volume. for example for tubes. Inherently safe design should be considered during the design stage in an effort to reduce the hazard potential of the plant. with added extras which cover either technical or contractual matters.this technique involves reducing the inventory of hazardous materials to a level whereby it poses a reduced hazard. or which operate at lower temperatures and pressures. testing regimes and maintenance strategies that are in place meet or exceed these requirements in terms of safety performance. for example for performance. 3. Reliability and availability assessment.for critical equipment. Evidence that Hazard identification and/or HAZOP studies have been carried out should be provided to show that a design has been evaluated and carefully considered before being installed on the plant. Consideration should be given to installing different types of connections on inlet/outlet pipework to avoid the possibility of wrong connections being made. or equipment of a lesser specification being chosen should also demonstrate that the major accident hazard implications of such decisions have been considered. any decisions taken as a result of a value engineering assessment that result in standby equipment not being installed. Hazard identification and assessment. 3. Many runaway reactions can be prevented.17 .Design Assessments. the selection of some types of gaskets can reduce leak rates from equipment in the event of a leak. There are several general topics that are common to the detailed mechanical design of many types of equipment.18 . either by changing the order of addition. Environmental assessment. A number of different features can be examined and assessed.simpler plants are friendlier and safer than complex plants and therefore less likely to have a major accident caused by operator error. Evidence that some system of assessment has taken place should be provided in the Safety Report. Simplification . Occupational health assessment. A design should be subject to a number of detailed assessments throughout its development. plants can be designed so that incorrect assembly is difficult or impossible. Knock-on effects .plants should be designed to reduce the likelihood of incidents producing knock-on effects or domino effects in other areas. These assessments all have a specific individual focus. Avoid incorrect assembly .   adding on protective equipment. hence limiting the hazard.General Considerations. reducing the temperature or changing other parameters. Energy efficiency assessment. Examples are given below:       Value engineering assessment. For example. and these are discussed in greater detail below:   Temperature and Pressure. Materials of Construction. A number of companies have developed detailed procedures for design studies that incorporate many of these assessments into a formalised structure. . For example. but in the context of COMAH it needs to be demonstrated that major accident hazards are not introduced as a result of the assessments that are undertaken. ambient temperatures.Temperature & Pressure. Account should be taken of foreseeable reactions that may occur which are likely to increase or reduce the heat input to the system. It should not adversely affect the mechanical strength and hence integrity. should also be considered. or result in additional process hazards as a result of overheating. Loss of containment may occur due to leaks. The combination of temperature and pressure should be considered since this affects the mechanical integrity of any equipment that is installed. solar radiation. Evidence should be provided in the safety report that the process conditions and environment in . The extremes of ambient temperature should be taken into account for plant situated outside buildings. Consideration needs to be given to the temperature of the fluids that are to be handled. Care should be taken to ensure that the maximum temperature that can be achieved by heating oil systems or the minimum temperature that can be achieved by cryogenic cooling systems do not compromise the design of the equipment. 3. External facilities should be designed to accommodate the cycling of temperatures between extreme weather conditions. If secondary heating and cooling systems are employed.Temperature. the low temperatures that can be achieved under conditions of snow. decomposition or runaway reactions.19 . ice and wind. and any excursions in temperature that could occur as a result of the failure of temperature control systems. A number of potential hazards can be introduced if these are not given adequate consideration. Solar radiation on the exposed surface area of large storage tanks can significantly increase surface temperatures for storage vessels. Any equipment that is to be installed should be designed to withstand the foreseeable temperature and pressure over the whole life of the plant. In determining design temperatures. a number of factors should be considered including:      the temperature of the fluids to be handled.20 . Temperature and pressure are two basic design parameters. all real gases except hydrogen and helium cool upon expansion and this phenomenon is often utilised in liquefying gases). then the maximum and minimum temperatures that can be achieved by these secondary systems should be assessed assuming failure of any control systems associated with these systems. leading to significant thermal expansion of vessel contents. Corrosion/Erosion. Likewise. The strength of materials decreases with increasing temperature and therefore the maximum design temperature should take into account the strength of material used for fabrication. equipment failure. Joule-Thomson effect (The Joule-Thomson effect is the change in temperature that accompanies expansion of a gas without production of work or transfer of heat. fire or explosion and result in a major accident. which can cause solidification of contents in vessels and pipelines. and heating and cooling medium temperatures. 3. At ordinary temperatures and pressures. this is usually taken to be no more than 10% for design purposes) can be expected to increase above the set point for the relief device. and that an appropriate design pressure has been selected. Documentation for relief streams should be available for inspection. This should be considered as part of the mechanical design of the equipment if such systems are to be employed. and that an appropriate design temperature has been selected. During the operation of the relief valve.21 .which the equipment is to be utilised have been assessed. then setting the relief pressure at a high level above the normal operating pressure may allow the reaction to reach a higher temperature and to proceed more rapidly before venting starts. it may not be possible to adequately design the equipment to withstand the maximum predicted temperature and pressure. This is normally 5-10% above the normal working pressure to avoid inadvertent operation during minor process upsets. For vessels under internal pressure. or gas evolution and hence result in increased or decreased temperatures and pressures. The maximum allowable accumulated pressure (MAAP) is specified within the various codes.Pressure. since if the potential cause of pressure rise is runaway reaction. A vessel should be designed to withstand the maximum pressure to which it is likely to be subjected in operation. Where strongly exothermic reactions or runaway reactions are possible. but not when normal minor operating pressure deviations occur. Vessels likely to be subjected to vacuum should be designed for full negative pressure of 1 bar unless fitted with an effective and reliable vacuum breaker device. Discharge of hazardous substances from relief systems under emergency conditions should be routed to secondary containment vessels or to safe locations. ensures that the overall pressure is below the MAAP. Account should also be taken of foreseeable reactions which may occur that are likely to increase the heat input to a system. The set pressure of a relief valve should be such that the valve opens when the pressure rise threatens the integrity of the vessel. and this should be taken into account when the relief valve set point is selected. Under such circumstances. Pressure vessels should be fitted with some form of pressure relief device set at the design pressure of the equipment to relieve over-pressure in a controlled manner. the design pressure is usually taken at that which the relief valve is set. It is necessary to balance a number of factors in the selection of relief valve set pressures. Vessels subjected to external pressure should be designed to resist the maximum differential pressure that is likely to occur. Other codes permit higher MAAPs in certain circumstances. Specific guidance on the recommendations for pressure relief protective devices is given in Appendix J of BS 5500 : 1997 seen in the HSE link. Normally. allowing for the overpressure during a relief event. 3. some form of pressure relief system may be appropriate in order to protect the equipment and prevent a catastrophic failure from occurring. so that additional hazards to personnel or equipment and the possible escalation of an incident do not occur. Evidence should be provided in the Safety Report that the relief systems have been suitably designed and that consideration has been given to the discharge locations. the pressure at the inlet to the relief valve (the overpressure . Evidence should be provided in the safety report that the process conditions and environment in which the equipment is to be utilised have been assessed. Secondary containment facilities may be appropriate for discharge of relief streams. The accumulation in the vessel is the permitted increase in the system pressure above the design pressure in an emergency overpressure situation. Consideration should be given to the possibility of pressure cycling in equipment and subsequent . the relief valve set point is set below or up to the maximum design pressure which. The layout of plant and equipment for corrosive materials is discussed in 'Safety and Management . The life of equipment subjected to corrosive environments can be increased by proper consideration of design details. Availability in standard sizes. Stiffness.electrical resistance. The standard recommends that all possible forms of corrosion such as chemical attack. erosion and high temperature oxidation are reviewed.the Association of British Chemical Manufacturers.Corrosion/Erosion. If materials to be used in the process are corrosive. that particular attention be paid to impurities and to fluid velocities and that where doubt exists. The Safety report should contain evidence that the materials of construction that have been selected are compatible with the process fluids to be handled and the design conditions that have been chosen. magnetic properties. Special properties . protected where possible and regularly inspected if the presence of corrosive materials or a corrosive environment is anticipated.A Guide for the Chemical Industry' . Equipment should be allowed to drain freely and completely. the available materials of construction may constrain the design temperatures and pressures that can be achieved and limit the design of the equipment. The most important characteristics that should be considered when selecting a material of construction are summarised below:             Mechanical Properties. Fatigue resistance. The advice of specialist materials engineers should be sought in the event of difficult applications being identified. The selection of a suitable material of construction is often carried out by disciplines such as process engineers.23 . 3. . Heffer & Sons. Printed by W. rusting. The effect of low and high temperatures on the mechanical properties.failure of the equipment due to metal fatigue. Materials of construction should be carefully selected. and the internal surfaces should be smooth and free from locations where corrosion products can accumulate. In some cases.Materials of Construction. Cost. 1964.22 . Corrosion resistance. Creep resistance. Another important consideration in mechanical design is the selection of the material of construction. then this should be taken into account in the plant design and layout. corrosion tests should be carried out. 3. Ease of fabrication. thermal conductivity. Hardness. Toughness. Tensile strength. Pressure vessels can be divided into 'simple vessels' and those that have more complex features. tees and baffles. It is often the prime cause of deterioration and may occur on any part of a vessel. There are numerous texts available on the details of pressure vessel design.Fluid velocities should be high enough to prevent deposition. but not so high as to cause erosion. Conditions that can cause severe erosion include pneumatic conveying. Other Vessels (including Storage Tanks). Most design codes and standards specify a minimum allowance of 1 mm. testing and scantlings of metal arc welded steel boilers and other pressure vessels". Introduction. 3. a minimum allowance of 2 mm is often used. plastic inserts can be used to protect erosion-corrosion at the inlet to heat exchanger tubes. However. If erosion is likely to occur. Design and manufacture is normally carried out to meet the requirements of national and international standards. the basis of the design of pressure vessels is the use of appropriate formulae for vessel dimensions. 3. The relevant standards and codes provide comprehensive information about the design and manufacture of vessels. Erosion is often localised especially at areas of high velocity or impact. Rotating Equipment. operated and maintained pressure vessel is rare. in conjunction with suitable values of design strength. For example. The other . outright failure of a properly designed. For carbon and low-alloy steels where severe corrosion is not expected. Design issues. Reactor Design.Mechanical Design. In general terms. Erosion is promoted by the presence of solid particles. The corrosion allowance is the additional thickness of metal added to allow for material lost by corrosion and erosion or scaling. Where more severe corrosion is anticipated. Erosion is a particular problem for solids handling in pipework. Occasionally corrosion and erosion combine to increase rates of deterioration. temperature and nature of the corrosive agents in the fluids and the corrosion resistance of the construction materials.1 .Pressure Vessels. then more resistant materials should be specified or the material surface protected in some way. A large proportion of failures in process plant and vessels are due to corrosion. an allowance of 4 mm is often used. wet steam flow. ducts and dryers. and vessel design and fabrication is an area well covered by standards and codes. Corrosion may be of a general nature with fairly uniform deterioration. flashing flow and pump cavitation. bubbles in liquids or two-phase flow.Specific Equipment . or may be very localised with severe local attack. The severity of the deterioration is strongly influenced by the concentration. with one of the earliest being the "AOTC 1939/48/58 Rules for the construction. Heat Exchange Equipment. Furnaces and Boilers.24. It occurs primarily at sites where there is a flow restriction or change in direction. by drops in vapours.24 . including valves. codes and standards applicable to several general categories of equipment have been identified and are discussed below in further detail:       Pressure Vessels. constructed. elbows. principal standards in the UK were BS 1500 and BS 1515 . vaporisers. heat exchangers. heaters. reboilers. Importantly. the two principal codes and standards BS 5500 and ASME VIII are employed in the design and manufacture of pressure vessels within the United Kingdom. in accordance with the standard or code. it is unusual . Corrosion/erosion. Creep. Weight of vessel and contents. condensers. 'Simple unfired pressure vessels designed to contain air or nitrogen'. bullets. This authority is responsible for adherence during both the design and construction phases.for companies and operators to employ their own design codes. Stress concentrations.though not unknown . 3. Ambient and operational temperatures. Joints (bolted and welded). The main code for simple vessels is BS EN 286-1:1991.    Discontinuities such as vessel ends. Buckling. .Complex Vessels. A simple pressure vessel does not have any complicated supports or sections and the ends are dished. changes of cross-section and changes of thickness.25 . Basically. Reaction forces and movements from attachments. Bimetallic joints. any equipment with a "shell" that may experience some internal pressure is covered.27 . piping etc. This section does not cover piping systems (see separate Technical Measures Document on Design Codes Pipework). both of which are now withdrawn and superseded by BS 5500 . both of these demand adherence to satisfaction of an independent inspection authority in the design and manufacturing process. pressure vessel design codes cover equipment such as reactors. 3. Traditionally. distillation columns. 3.Design Considerations. Factors that should be taken into account in the design process for pressure vessels include:            Internal and external static and dynamic pressures. Pressure vessels are subject to a variety of loads and other conditions that cause stress and can result in failure. However. Wind loading. Fatigue. atmospheric storage tanks and rotary machines.26 . These are considered in further detail later. storage drums. localised stress. Generally.Simple Vessels. All aspects of designing and manufacturing the vessel are covered in this code. and there are a number of design features associated with pressure vessels that need to be carefully considered. The other commonly-used design code is ASME VIII. Residual stress. spheres etc. thermal stress etc. trays. Metallic liners are installed in various ways. lead or any other metal resistant to the corrosive agent. Large vessels may have internal bracing and ties. pipe coils.e.e. Liners. mesh or strip type packing. British or European code is used for vessel design and specific materials are quoted within the code.Materials of Construction. a means of determining the internal condition of the vessel by the provision of access openings. insulating material. vessel skirt or support legs. monel alloy. Bolt seating and tightening. A lined vessel is usually more economical than one built of solid corrosion-resistant material. quench lines. Nozzles and connections. Internals. The materials should be selected in order to avoid corrosion effects when the various materials are put together. Supports and lugs. Others have internals such as baffles. it is important that the material selected not only has properties which are suited to that particular application. Flanges. 3. They may be an integral part of the plate material rolled or bonded before fabrication of the vessel. Steel is the most common material of construction. low alloy steel. and stainless steel. and most vacuum vessels have either internal or external stiffening rings. most. low temperature application etc. Consideration should also be given to other parts of the vessel not directly within the pressure envelope. carbon brick. bed supports. glass and plastic. Heat exchangers have internal tube bundles with baffle and .g. Clearly. cyclones. but also has its suitability with regard to fabrication taken into account.28 . however. grids. They should be sufficiently chemically resistant to the fluid contained and not be significantly affected by ageing. but other equally important factors such as corrosion/erosion allowance. rubber.     Holes and openings. Metallic liners may be made of ferritic alloy. and for all test conditions. including mild steel. Vessels. can determine selection. are constructed using welded joints. or they may be separate sheets of metal fastened by welding. Several different methods are used to construct pressure vessels. or to insulate and reduce the temperature on the walls of a pressure vessel. Where carbon steel will not resist expected corrosion or erosion. spray nozzles. vessels may be lined with other metals or non-metals. any failure which could lead to breach of the pressure boundary e. Other factors which require careful consideration include    a means of in-service periodic examination i. or could cause contamination of the product. a means of draining and venting the vessel. but critical to vessel integrity i. The most common materials are reinforced concrete. Where an American. agitators etc. Non-metallic liners may be used to resist corrosion and erosion. Materials used for the manufacture of pressure vessels should have appropriate properties for all operating conditions that are reasonably foreseeable. it is important that the correct materials are used in order that the design is not invalidated. It is often operating process temperature that determines the material used. in the choice of material selection. nickel. Many pressure vessels have no internals. and a means by which the vessel can be safely filled and discharged. brittle failure. These internals may be made from a wide range of materials but care should be taken that the materials selected for the internals are compatible with the materials chosen for fabrication of the main components. .Failure Modes. crevice corrosion. chemical.the American ASME VIII system and BS 5500 in the UK. Two principal codes and standards are employed in the design and manufacture of pressure vessels . The form of deterioration may be electrochemical. The codes and standards cover design. 3. inspection and testing and form the basis of agreement between the manufacturer. creep. These codes relate to vessels fabricated in carbon and alloy steels and aluminium. Any design should take into account the most likely failure modes and causes of deterioration. The most common causes of mechanical failure in process plant are:            faulty materials. Computer programmes to aid the design of vessels to BS 5500 and the ASME VIII codes are commercially available. 3. Mechanical Failure. corrosion failure. overheating. including corrosion beneath lagging. faulty fabrication and assembly.support plates. overpressure. with contaminants. Deterioration is possible on all vessel surfaces in contact with any range of organic or inorganic compounds. customer and the appointed independent inspection authority. steam or the atmosphere. The most common corrosion mechanisms are:       general corrosion. fresh water. This authority is responsible for adherence during both the design and construction phases in accordance with the standard code. Importantly. corrosion pitting. corrosion fatigue.30 . materials of construction. and in certain cases may cause serious failure.Design Codes and Standards. stress corrosion cracking. mechanical or a combination of all. mechanical and thermal fatigue. mechanical shock. Pressure vessels are subject to a variety of loads and other conditions that cause stress. excessive stress.29 . including reaction forces. both of these demand adherence to satisfaction of an independent inspection authority in the design and manufacturing process. fabrication (manufacture and workmanship). external loading. external corrosion. For relief systems. earthquake and others as appropriate. low-pressure storage tanks.Non-metallic Materials of Construction.33 .Reactor Design. The main load to be considered in the design of such tanks is the hydrostatic pressure of the liquid contained within the tank. should it develop.Specification for Design and Construction of Vessels and Tanks in Reinforced Plastics. Tanks should be suitable for their operational duty and all reasonably expected forces such as tank contents. welded. consideration should also be given to other parameters and the wind loading. Vertical storage tanks with flat bases and conical roofs are often used for the storage of liquids at atmospheric pressure. and their design is of utmost importance when considering the safety hazards of a plant. are dependent on the physical properties of the stored material e. . or fibrereinforced plastic (FRP). downstream separation processes. Excessive loss of vapours from vent systems may result from outbreathing and may present a hazard. In addition. and API Std 650 Welded steel tanks for oil storage. vapour pressure. and may vary in size considerably. electrical conductivity etc. and complexity of. pressure relief and blowdown need to be adequately addressed in the design. 3. Although the majority of pressure vessels are constructed from metallic compounds. Some vessels that are used are not designated as pressure vessels. The safety considerations are usually related to fire hazards which. either open to the atmosphere or enclosed. in turn. The selection of the type of tank to be used for a particular duty will be influenced by considerations of safety. and any likely snow loading should also be considered. consideration should be given to the implications of the release of reactor contents and containment and control systems may be necessary to prevent a hazardous situation from developing as a result of the discharge of a relief system. API Standard 2000 gives guidance on the design of vents to prevent pressure changes that would otherwise occur as a result of temperature changes or the transfer in and out of liquids. The effectiveness of the reaction step will often determine the requirement for. Arrangements for venting. 3.Other Vessels (Including Storage Tanks). However. technical suitability and economy. flash point. Reactors are most often considered as pressure vessels and the mechanical design should be in accordance with the codes and standards described earlier. wind and snow loadings. The main relevant standard is BS 4994:1987 .3.31 . The description 'atmospheric storage' is applied to any tank that is designed to be used within a limited range of atmospheric pressure.32 . Reactors are often the centre of most processes. Reactor design should minimise the possibility of a hazardous situation developing and provide the means for dealing with a hazardous situation. ground settlement. pressure vessels can also be constructed from materials such as glass-reinforced plastic (GRP). low conversions may result in large recycles being required. The design of the reactor may affect the efficiency of the reaction process and hence the generation of by-products and impurities. The design of atmospheric storage tanks in general is governed by API Std 620 Design and construction of large.g. frost. Methods for detecting the failure of a mixing/agitation system and/or stopping the flow of reactants into the reactor may be appropriate.     Mixing . the mechanical design of the unit can then be carried out. However.the reaction may take place in the gas. 3. A number of direct or indirect techniques can be employed. which can be designed in many different shapes.D.solid phase.the agitation system selected for the reactor (if appropriate) may directly influence the efficiency of the reaction and hence the generation of by-products. then additional separation steps to remove the catalyst may subsequently be required.Many different types of reactor system are available and some of the important criteria to consider are given below: Addition of reactants . the control of the reaction system and the heat removal systems should be carefully considered. Once the process design has been completed. Consideration should be given to the modes of failure of the control and cooling systems to ensure that the hazards of a runaway exothermic reaction are minimised. the catalyst may present additional hazards and consideration should be given to the selection of the catalyst system in order to minimise the risks associated. chemical addition systems and relief systems have been selected in order to minimise the potential for a major accident. The design of heat exchangers is covered in many texts. evaporation or condensation may all need to be considered and the equipment designed accordingly to account for the differing requirements.Reaction / Product Testing and Control Systems . sizes and configurations necessary to obtain the required heat transfer between one stream and another. International Student Edition. The basic design is commenced by an approximate sizing of the unit.Q.34 . cooling.sometimes . More detailed calculations are then required to confirm and refine the original design and to identify an optimum layout. McGraw Hill. Consideration should also be given to the consequences of agitation failure in the design of the reaction system. Heat removal . The generation of unstable by-products or excessive reaction rates may increase the potential for a hazardous situation to develop. The transfer of heat between two process streams is a common activity and requirement on a chemical plant. Kern.the order and rate of addition of the reactants may affect the rate of reaction and the generation of by-products. Phase . liquid or . A common reference for design engineers is 'Process Heat Transfer . Heating. Catalysts .a reaction may require a catalyst in order to promote the required reaction. . A number of different heat transfer operations are possible. The safety report should describe how the reactor system has been designed with the principles of safe design in mind. based on assumptions made concerning the heat transfer characteristics of the substances involved and the anticipated materials of construction. with some involving a change of phase of one or more component. ISBN 007Y853533. The position of addition of reactants may also be important .for exothermic reactions. especially if there is the possibility of two phases forming on agitation failure which may react exothermically/vigorously when agitation is recommenced. The way in which the reactants are brought into contact may influence the efficiency of the reaction and introduce additional hazards into the reaction system.Heat Exchangers/Reboilers. and how the selection of the mixing. The most common form of equipment used to transfer heat is a heat exchanger. See Technical Measures Documents .sub-surface and directly into an intimate mixing zone within the reactor may result in the minimisation of the generation of reaction by-products. If a catalyst is required. Many companies also have their own standards to supplement these various requirements. tube vibration and tube rupture. materials of construction and testing of shell and tube heat exchangers are covered by 'BS 3274: 1960. Design temperatures and pressures for exchangers are usually specified with a margin of safety beyond the conditions normally anticipated. fuel characteristics (liquid. The TEMA standards give the preferred shell and tube dimensions. destruction of offgases etc.BS 5500 or ASME VIII (Rules for construction of pressure vessels. and specialist designs are often required. and the temperature is commonly 14°C greater than the maximum anticipated service temperature. The safety report should demonstrate that heat exchange equipment has been designed and maintained in accordance with the relevant codes and standards. The design of the furnace or boiler enclosure should be able to withstand the thermal conditions associated with the system. Double block and bleed valves on fuel lines can be considered. and heat input and heat transfer systems. It should be demonstrated that wherever possible. the design pressure may be 170 kPa greater than the maximum anticipated during operation or at pump shut-off. Leaks of fuel can cause explosive atmospheres when ignition is attempted. More specific guidance is given in API RP 520:1990. Careful consideration of the configuration of the pipe work should also be considered to ensure that the flow of fuel into the system after the flame has failed or valves have been closed is minimised.Tubular Heat Exchangers for General Purposes'. and that consideration has been given to the various failure modes that could occur and the implications of such events. solidification. leakage. overheating.35 . Typically. gaseous or solid fuels). Reliance should never be placed upon single valves for isolation. ignition control systems. Isolation systems should be adequately designed to ensure leakage of fuel does not occur. consideration should be given to inerting /ventilation systems prior to ignition sequences to ensure explosive atmospheres are not present. Furnaces and boilers are items of equipment that are often found as part of process plant and are used for a variety of purposes such as waste heat recovery. The design may involve the interaction of many different variables including water/steam circulation systems. instrumentation and control or others. corrosion allowances and the recommended design stresses for materials of construction. The elimination of hazards in burner design is a fundamental design requirement. for example materials which may undergo exothermic decomposition. the design and manufacturing tolerances. Special consideration needs to be given to preventing overheating within heat exchanger equipment. especially if sensitive materials are involved.Furnaces/Boilers. measures have been taken to prevent. control or mitigate the consequences of such events by the appropriate selection of materials of construction. . The standards of the American Tubular Heat Exchanger Manufacturers Association (TEMA standards) are also widely used. Many codes and standards exist for boiler design.The mechanical design features. Major problems associated with heat exchanger design that may affect safety include fouling. Explosions can occur during start-up if ignition design is not carefully considered. fabrication. The shell of an exchanger is normally a pressure vessel and should be designed in accordance with the relevant pressure vessel design code . For these reasons. fabrication methods. steam generation. polymerization. Division 1). 3. compressors. the total dynamic head. centrifuges. the impeller of the pump and the rotor of the motor are mounted on an integral shaft which is encased so that the process fluid can circulate in the space which is normally the air gap of the motor. although positive displacement types (such as reciprocating and screw types) are also used. This type of equipment is a potential source of loss of containment. Process machines are particularly important items of equipment in process plants and in relation to pressure systems. the suc- . Pumps are available in a vast range of sizes and capacities and also in a wide range of materials. the flow rate required and the pressure loss in transmission. Cavitation (the collapse of vapour bubbles in a flowing liquid leading to vibration. In a canned pump.Rotating Equipment. construction and testing details such as material selection. The choice of material of construction is dictated by consideration of corrosion. and numerous standards (API standards. pressure and flow fluctuations may be caused and these can affect the operation of other systems. Many pumps are of the centrifugal type.Pumps. agitators etc. shop inspection and tests. Pumps are particularly vulnerable to bad operation and poor installation practices. In addition. A machine system is any reciprocating or rotating device that is used to transfer or to produce a change in properties within a process plant. clearances. 3. noise and erosion) and dead head running (attempting to run a pump without an outlet for the fluid. drawings. including various metals and plastics. It should be demonstrated that the risks of an explosion occurring have been minimised by the design of the burner control management system and the layout and design of the fuel supply systems.Purging facilities are essential to ensure that the firing space is free from a flammable atmosphere prior to start-up ignition. solids and gases) from one area of operation to another. 3. Proper installation and high quality maintenance is essential for safe operation. Key parameters for pump selection are the liquid to be handled. turbines. Seal-less or 'canned pumps' are often used where any leakage is considered unacceptable. Special requirements for certain industrial sectors may also impose restrictions on the materials of construction to be used or the type of device that can be considered.36 . Problems associated with centrifugal pumps can include bearing and seal failure. Examples may include items such as pumps. A safety report should demonstrate that any furnace/boiler system is designed and maintained to the relevant codes and standards and that consideration has been given to the major hazards associated with the start-up. due to the rotating/vibrating nature of such equipment.37 . ASME standards. The primary advantage of a centrifugal pump is its simplicity. Sealing of pumps is a very important consideration and is discussed later. construction procedures etc. Misalignment between pump and motor is also a common cause of catastrophic failure. The basic requirements to define the application for pumps. ANSI standards) have become available. personnel safety and containment and contamination. erosion. shutdown and operation of the equipment in terms of the fire and explosion potential of such systems. fans. for example against a closed valve) can also result in damage to the pumping equipment. Many designs have become standardised based on experience. These standards often specify design. since they are required to provide the motive force necessary to transfer process fluids (liquids. fans and compressors are usually the suction and delivery pressures. Air compressors for dry air require special consideration. Centrifugal compressors are by far the most common. Numerous factors can result in vibration occurring including cavitation. Failure of a sealing arrangement can lead to loss of containment and a potential for a major accident. There are many factors that govern the selection of seals for a particular application including the product being handled. and can result in a major accident.38 . 3. low pressure applications such as supplying air for drying. the arrangement of the seal. One of the main causes of failure of rotating equipment is vibration. bearing failure. then preventative maintenance is required and should be performed. impeller imbalance. viscosity. specific gravity. They are used in both process gas and refrigeration duties. Both positive displacement and centrifugal compressors are used in the process industry. The main applications for fans are for high flow. it is possible to identify which components of the system are responsible for particular frequencies of the vibration signal. Numerous different types of sealing arrangement exist for rotating equipment.Fans. some of the principal malfunctions include rotor or shaft failure. 3. secondary packing requirements. This should be initially confirmed on installation and then periodically checked. and specific codes and standards exist. It is very important that they are maintained to high operational standards.Seals. They are simple machines but proper installation and maintenance is required to ensure high reliability and safe operation. vibration and surge. These units can be either centrifugal or axial flow type.41 . 3. On centrifugal compressors.Vibration. The materials used for seals should always be compatible with the process fluids being handled. vapour pressure. or in condensing towers. seal face combinations.39 .components in large rotating machinery. This often causes seal damage or fatigue failure and subsequent leakage.40 . temperature.tion and discharge heads. conveying material suspended in a gas stream. If measured levels exceed prescribed values. There are three principal methods of sealing the point at which a rotating shaft enters a pump. loose bearings and pulses in the pipe. seal gland plate arrangements and main seal body etc. 3. the equipment in which the seal is to be installed. ASME standards recommend that pumps should be periodically monitored to detect vibration that should normally fall within prescribed limits as determined by the manufacturer. Reciprocating compressors are utilised for higher compression requirements. It is then possible to identify the component that is deteriorating and responsible for the vibration that is occurring. They are complex machines and their reliability is crucial. removing fumes. the environment in which the seal is installed. By collection and analysis of vibration signatures of rotating equipment.Compressors. although compression is generally lower than that given by reciprocating machines. Seals are very important . They may be either single or multi-stage units.and often critical . liquid corrosion characteristics. . and in systems which are flanged/jointed such as heat exchangers or pipework systems. the presence of solids which may cause erosion etc. Potential consequences of the failure of the process. The aim of such techniques is to identify deterioration and pre-empt imminent failures. Hydrodynamic seal. of which there are a number of techniques. pressure.Critical Machines. They are often fitted as complete cartridge type units. pressure vessel or similar equipment:    Conventional stuffing box with soft packing. Acoustic emission monitoring. and given specific attention during operation including additional maintenance and monitoring.42 . and so secure reliable/available plant. Plant equipment may be monitored during commissioning and throughout its operational life. temperature. Oil analysis. Some product leakage is normal. both lubricating and cooling the packing material. . Mechanical seals are the next most commonly employed arrangement.compressor. particularly for production and safety critical items. 3. Machine systems that have been assessed to present unacceptable consequences if the machine or protective system should fail may be classified as a 'Critical Machine System'. All machine systems should be assessed according to the hazard presented if the machine . The disadvantages are the necessity of frequent attention and the inherent lack of integrity of such a system. Mechanical seals. where rotating vanes keep the shaft free. Some mechanical seals are assemblies of great complexity and consist of components manufactured to very high tolerances. They are used in applications where a leak-tight seal of almost any fluid is required. This monitoring may be carried out on the basis of performance or condition or both. power etc. Potential damage caused by mechanical failure. Shock pulse monitoring. The chief advantages of this type of sealing arrangement are the simplicity and the ease of adjustment or replacement.or any associated protective system . Assessments should be based on:    Potential consequences of any loss of containment. The alternative to performance monitoring is condition monitoring. They can range from the simplest single seal arrangement to complicated sophisticated double seals with monitoring of the interspace. Stuffing boxes and glands with packing are commonly used.should fail. Some sealing arrangements require constant lubrication. Inspection & Monitoring. often from the process fluid itself whilst others require external lubrication arrangements. Some of these techniques are identified below:     Vibration monitoring.Maintenance. 3.43 . Mechanical seals find their best application where fluids should be contained under substantial pressure. The predominant techniques and parameters employed are flow. but may also be stored fully refrigerated (-34°C) at atmospheric pressure. especially for fires involving storage tanks. Structures are required to provide support for plant and should be able to withstand all foreseeable loadings and operational extremes throughout the life of the plant. HSE. 1975. Structural design should take into account natural events such as wind loadings. and guidance is available in BS 6651 : 1992 Code of Practice for Protection of Structures against Lightning. 1986.44 . 1984. Chlorine Institute. The design of systems for chlorine requires special consideration. The HS(G)28 document has replaced earlier guidance from the CIA (Chemical Industries Association) and the Chlorine Institute which included:      Chlorine Manual.Lightning. The above pamphlets from the Chlorine Institute can be accessed by the following link Chlorine Institute Free Leaflets Also see:  HS(G)40 Safe handling of chlorine from drums and cylinders. Pamphlet 1. CIA. if wet. Pamphlet 78. CIA. Pamphlet 5. and specific guidance is given in: HS(G)28 Safety advice for bulk chlorine installations. (the Chlorine Code).46 . Refrigerated Liquid Chlorine Storage. 1980/9. Chlorine Institute. 3. Protection against lightning strikes on process plant located outside buildings is required.Structural Design Considerations.3. . Chlorine Institute. general published codes exist giving full design details for storage and handling.Special Cases. HSE.45 . 3. Non-refrigerated Liquid Chlorine Storage. 1982. This guidance was originally published in 1986 and has been substantially revised. Guidelines for Bulk Handling of Chlorine at Customer Installations (the CIA Chlorine Storage Guide). since lightning is a potential ignition source. snow loadings and seismic activity and also plant excursions. Chlorine storage. since chlorine is highly toxic and. Failure of any structural component could lead to initiation of a major accident. Chlorine is usually stored under pressure at atmospheric temperature. A number of publications are dedicated to the handling of chlorine. Lightning protection should be provided. also very corrosive. Code of Practice for Chemicals with Major Hazards: Chlorine. For the following substances. 1999. Euro Chlor. Ammonia storage. and specific guidance is given in:       HS(G)30 Storage of anhydrous ammonia under pressure in the UK : spherical and cylindrical vessels.org /> ST 79/82. LPG can also be stored under pressure in horizontal cylindrical or spherical pressure vessels. . (The CIA has withdrawn this document). Euro Chlor produces a number of publications. boiling point -33°C. Propane and Butane are referred to as liquefied petroleum gas (LPG) in accordance with BS 4250: Specification for commercial butane and propane. CIA Refrigerated Ammonia Storage Code. Gives advice for the appropriate materials of construction for ammonia storage vessels. Liquefied petroleum gas. EEMUA 147. Part 1 Safety valves. CS16 Chlorine vaporisers. 2000. Part 3: Periodic inspection and testing. LPGA CoP 1 Bulk LPG storage at fixed installations. Further details can be obtained via the website http://www. or at atmospheric pressure in refrigerated facilities. LPG storage. 2000. HSE. LPGA CoP 15 Valves and fittings for LPG service. 2000. CS5 Storage of LPG at fixed installations. HSE. CIA Code of Practice for the storage of anhydrous ammonia under pressure in the UK: Spherical and cylindrical vessels. Fully-refrigerated storage is required at atmospheric pressure and at the boiling points of the substances concerned. LPGA CoP 17 Purging LPG vessels and systems. 1987. Recommendations for the design and construction of refrigerated liquefied gas storage tanks. Part 1: Design. 1986 (Not in current HSE list). LPGA CoP 1 Bulk LPG storage at fixed installations. IP Model code of safe practice: Part 9. 2000. CIA Guidance on transfer connections for the safe handling of anhydrous ammonia in the UK. HSE. A number of publications are dedicated to the handling of ammonia. HSE. is normally stored as a liquid either under pressure. This is a typical industry sector standard. Part 4: Buried/mounded LPG storage vessels. Hydrocarbons storage. containing specific guidance on the use of materials of construction for chlorine systems. LPGA CoP 1 Bulk LPG storage at fixed installations. 'Choice of materials of construction for use in contact with chlorine'.eurochlor. 2000. HSE. CIA Guidance for the large scale storage of fully refrigerated anhydrous ammonia in the UK. Anhydrous ammonia. The Euro Chlor organisation is an affiliate of the European Chemical Industry Council (CEFIC) and represents European chlorine producers at 85 plants in 19 countries. Part 2: Small bulk propane installations for domestic and similar purposes. LPGA CoP 1 Bulk LPG storage at fixed installations. installation and operation of vessels located above ground.            HS(G)34 Storage of LPG at fixed installations. HS(G)15 Storage of liquefied petroleum gas at factories. 2000. This BS supersedes BS 4741:1971 and BS 5387: 1976 both of which are withdrawn. EEMUA 147. American Petroleum Institute. HS(G)176 The storage of flammable liquids in tanks. HSE. CS2 The storage of highly flammable liquids. General principles. HS(G)51 Storage of flammable liquids in containers. vertical. IGE SR7 Bulk storage and handling of highly flammable liquids used within the gas industry. HSE. BS 7777: 1993 Flat-bottomed. 1996. RC 20 Recommendations for the storage and use of flammable liquids. API Std 650 Welded steel tanks for oil storage. low-pressure storage tanks. HS(G)52 The storage of flammable liquids in fixed tanks (exceeding 10000 cu. 1998. 1981 Refining Safety Code. HSE. BS 5387: 1976 Specification for vertical cylindrical welded steel storage tanks for low temperature service: double-wall tanks for temperatures down to -196°C. Refrigerated storage tanks. 1986. HSE. BS 2654: 1989 Specification for manufacture of vertical steel welded non-refrigerated storage tanks with butt-welded shells for the petroleum industry. BS 4741: 1971 Specification for vertical cylindrical welded steel storage tanks for low temperature service: single-wall tanks for temperatures down to -50°C. 1989. cylindrical storage tanks for low temperature service. 1997.A number of standards and codes exist for the storage of petroleum products and flammable liquids generally. 3. 1998. m in total capacity). BS 799: 1972 Oil Burning Equipment. HS(G)50 The storage of flammable liquids in fixed tanks (up to 10000 cu.Maintenance Procedures. Recommendations for the design and construction of refrigerated liquefied gas storage tanks. IP MSCP Part 3. . 1988. 1997. Pressure or refrigerated pressure storage tanks. welded. The relevant standards and codes are:                       API Std 620 Design and construction of large.Above ground storage vessels CS15 The cleaning and gas freeing of tanks containing flammable residues[62]. HS(G)140 Safe use and handling of flammable liquids. 1977.47 . HSE. Part 5 Specification for oil storage tanks. NFPA 30: 1990 Flammable and Combustible Liquids Code. 1990. 1990. HSE. The following aspects should be considered with respect to Maintenance Procedures:  Human factors. m in total capacity). IGE SR14 High pressure gas storage: Part 1 . HSE. A range of different main types of storage tanks and vessels for liquids and liquefied gases can be considered:     Atmospheric storage tanks. LPC. American Petroleum Institute. BS 2594: 1975 Specification for carbon steel welded horizontal cylindrical storage tanks. 1991. Low pressure storage tanks. explosive. Whether the maintenance staff are aware of the type of environment in which they are working (flammable. Whether the procedures to ensure quoted proof check periods for safety critical items are adhered to. attitude) are addressed. Human error during maintenance. risk-based. Static or spark discharge during maintenance in an intrinsically safe zone. fatigue. . Whether sufficient reporting systems are in place so that corrective maintenance can be applied to mitigate a major accident or hazard. running of plant. shift work. corrosive. Whether maintenance schedules are managed and regularly inspected and reviewed. zones 0. Whether sufficient maintenance systems are in place during productive assistance . Whether sufficient precautions are taken prior to maintenance of hazardous plant and equipment (isolation. Whether the maintenance staff use the correct equipment in the workplace during reconditioning. 3. Whether procedures are in place to provide detailed operating instructions for recommission plant after maintenance. Good maintainability principles. which have been subjected to risk assessments. instructed. Poor communication between maintenance and production staff.Contributory Factors for an Assessor to Consider Concerning Maintenance Procedures. plant shutdown and plant breakdown. replacement and re-commissioning (static free.     Poorly-skilled work force.48 . risk assessments.Major Hazards. Knowledge of failure rate and maintainability. intrinsically safe. instrumentation and electrical systems. draining. Unconscious and conscious incompetence. Whether the company Safety Management System includes adequate consideration of maintenance of plant. permits to work. flameproof. condition based or breakdown maintenance) are adequate for each plant item which has a safety function. communication. Whether maintenance staff have been sufficiently informed.49 . servicing. trained and supervised to minimise a potential human failing during maintenance. time allotted for the work). Whether maintenance staff have been sufficiently trained to recognise plant or equipment failing during maintenance inspections. Clear criteria for recognition of faults and marginal performance. PPE/RPE). environmental monitoring. Incompetence of maintenance staff. The Safety Report should address the following points:               Whether the company maintenance regimes (planned. 1 & 2). flushing. Whether proof check periods quoted for safety critical items are adequate to ensure risks are within acceptable limits. reliability-centred. Whether Human factors (stress. 3. The following issues may contribute towards a major accident or hazard:      Failure of safety-critical equipment due to lack of maintenance. Communication to employees is paramount to the success of fire prevention in the workplace. non-return valves orientation incorrect. relief valve springs over-tightened. Combustible & Incompatible Materials.Major hazards could arise from the following:        The lack of control of spares such that incorrect materials or items outside specification (e. bolts torqued incorrectly or bolts missing. fire or explosion.g. . if possible. emergency procedures). 1 Observe the following safety measures with regard to electrical equipment: (a) Avoid temporary connections and joints in cables. (b) Avoid the use of adaptors and extension leads as far as practicable. (e) Do not bring your electrical appliance to use at work. Failure to re-commission plant correctly after maintenance to ensure that operations are not adversely affected in terms of safety considerations (e. 3. Lack of knowledge by maintenance staff of the working environment where maintenance is being carried out (i. warning signs. pipework/flexibles incorrectly connected/installed. causing release of flammable or toxic substances. only competent. one method of improving communication that has been used in the past is to include something similar to the following bullet points in the employees' handbook or induction training. completely isolate equipment at the end of shift. motors running in wrong direction. qualified persons should undertake this work. heat transfer rate changes. lack of risk assessments. respirators) being worn. (d) Ensure that there is a clear space around appliances that are sources of heat. leading to ignition of flammable substances (e. Scheduled maintenance not being undertaken as required or breakdown maintenance inadequate. static and electrical discharge. pipeline spades/orifice plates left in/removed. heat sources such as cigarettes or welding.50 . (f) Switch off all electrical equipment at the end of the working day unless its operation is required. (c) Do not carry out work on electrical installations.Outline of Segregation & Storage of Flammable.g. alignment of couplings on pumps and agitators causing overheating.e. 2 Reduce the risk of arson by adopting the following measures: (a) Ensure that external doors are not left open. leading to unrevealed failures of safety critical items. Maintenance being performed incompetently (particularly alarm/action set points on instruments incorrectly set. Unauthorised staff performing maintenance functions. method statements. flow rate changes. Failure to drain and/or isolate plant prior to dismantling. safety features left disconnected/dismantled. use of non spark-resistant tools) or injury/fatality from incorrect personal protective equipment (e. gaskets left out.g.g. non-flameproof equipment) are used in replacement of plant items leading to increased risk of loss of containment. bursting discs orientation incorrect/left out). contamination. mass transfer rate changes). will it be contained and prevented from spreading to other parts of the working area. finished products containing flammable solids etc? Even something as simple and relatively inexpensive as good housekeeping has an extremely import role to play in the prevention of fire. (d) Make sure that wood stored outside is a sufficient distance from the building. A daily inspection or internal audit of the work area prior to the commencement of and during the working day is a recognised method of engaging employees to be fire safety aware. or the fuel for fire growth. Is it combustible. The following checklist should provide the basis of a daily inspection sheet :        Are all flammable substances kept in suitable containers? In the case of a spill. all smoking materials to be left at the gatehouse. decant over a tray and use work benches with edging lips. . wall or partition? Are flammable materials being stored or used close to heaters or electrical equipment which could run hot and act as a local ignition source? Are gangways and exits from storage and working areas kept clear of packaging materials. and is it flammable? What is the size of the workplace and the operation? Have environmental considerations been taken into account? The list of potential methods of disposal is diverse and therefore almost endless. How the waste is disposed of is reliant upon the material itself. and the actual practicable method or reasonable and practicable method can only be arrived at by risk assessment of the operation and workplace. Such an audit can be carried out by team leaders or supervisors.(b) Report any unknown persons on site. to minimise spillages. (c) Be vigilant. 3 Smoking is not allowed anywhere on site. an operated policy of 'clean as you go' can reduce the potential for the build-up of material that could be a source of ignition. and are spillage catchments trays in place and correctly used? Are flammable substances stored and used well away from other processes and general storage areas? Is there provision by which they can be separated by a physical barrier. The following example scenarios of safe waste disposal are listed: In a garage spray shop. the vehicle for spreading the fire. is spillage equipment readily available? Are lids being replaced on containers. Dispose of such flammable material in a metal lidded container. Empowering the workforce to undertake such inspections is an excellent method of improving fire prevention awareness. 'Clean as you go' has other obvious benefits in health and safety and also the efficiency and organisation of the workplace. A logical extension of 'clean as you go' is the need for methods of safe waste disposal. 4 Keep your work area clean and dispose of waste regularly and safely. Soak up spillages with absorbent material immediately. are the Materials Safety Data Sheets supplied with the substances and materials. avoid the use of solvents. External containers should be designed around the same principle. Avoid breathing vapours. rivers or lakes. a system of internal collection points for these combustible materials can be introduced. A packing and distribution warehouse may be using large amounts of cardboard and polystyrene. Warn others of the dangers present and exclude non-essential personnel. or release from any proc- . the local water company should be contacted immediately. The solution to the safe storage of flammable and combustible materials cannot be arrived at in an arbitrary manner and must be analysed by risk assessment. exclude sources of ignition and ventilate the area. accepting there is inherent diversity. either stand-alone or as part of the risk assessment for the whole process of using such materials. earth. If the product enters drains or sewers.51 . The collection points can be monitored and emptied regularly. cotton wool. However. By applying the following five principles. e. in the case of contamination of streams. V Ventilation: is there plenty of fresh air where flammable liquids or gases are stored and used? Good ventilation will mean that any vapours given off from a spill. diatomaceous earth and place in a clearly-labelled suitable container for disposal in accordance with the company waste regulations. Contain and collect spillages with non-combustible absorbent materials. sand. there remains commonality in the various solutions. 3.g.Safety Principles. the Environment Agency. Clean preferably with a detergent. there can be no one answer or solution to the design and installation of storage facility. The range of materials that are either combustible or flammable is diverse and as such. Do not allow to enter drains or water courses. wipes etc. paper. This commonality is used to good effect in the HSE 'VICES' method. Local waste bins should be constructed of metal and have a self-sealing lid of some type. Such items require a route of disposal. albeit often in an abridged format. leak. Floors may become slippery. please consider why the potential for ignition is reduced in materials that are compressed and become denser by compaction. which details five principles for safe storage and handling. vermiculite. you will be well on the way to making sure that you are working safely with flammable substances. both for fire prevention and environmental considerations. and the waste articles disposed of either by incineration or compaction of the materials. An ideal source of disposal information.For a spillage in a woodworking shop using varnish or adhesives which are flammable. Certain items used in hospitals generate large amounts of combustible materials. This information also appears on the original product containers. Referring to the fire triangle. the ventilation at that location. 2002). could have an automatic fire control system but may also have a detection system installed that can monitor the 'build up' of levels of the flammable parts. The most commonly used standard in the UK for determining area extent and classification is BS EN 60079 part 10 1 . will be rapidly dispersed. as well as this. In other instances. A hazard identification process such as a Preliminary Hazard Analysis (PHA) or a Hazard and Operability Study (HAZOP) should consider these abnormal events. and the Institution of Gas Engineers Safety Recommendations SR25. for example. I Ignition: have all the obvious ignition sources been removed from the storage and handling areas? Ignition sources can be very varied. E Exchange: can you exchange a flammable substance for a less flammable one? Can you eliminate flammable substances from the process altogether? You may be able to think of other ways of carrying out the job more safely. from time to time in normal operation or as the result of some unplanned event. about the hazards and how they should control them. C Containment: are your flammable substances kept in suitable containers? If you have a spill. In addition. Area classification may be carried out by direct analogy with typical installations described in established codes. 3. It contains a simplistic calculation relating the size of zone to a rate of release of gas or vapour. hot surfaces.52 . are the Institute of Petroleum Model Code of Practice (Area Classification Code for Petroleum Installations. Other sources of advice. The starting point is to identify sources of release of flammable gas or vapour. The current version makes clear the direct link between the amounts of flammable vapour that may be released. Safe storage should also take into consideration the transfer or decanting of the materials. which has broad applicability. which describe more sophisticated approaches. (2001). in particular one used for the bulk storage of gas or vaporising liquids. Think about the flammable substances you have in the workplace and apply these five principles wherever possible. Catastrophic failures. a storage facility. can help to prevent spillages spreading. but it is not helpful for liquid releases where the rate of vaporisation controls the size of the hazardous area. wall or partition? Separating your hazards in this manner will contribute to a safer workplace. smoking materials etc. Tell workers. and they include sparks from electrical equipment or welding and cutting tools. will it be contained and prevented from spreading to other parts of the working area? Use of lidded containers and spillage catchment trays. The IP code is for use .Hazardous Area Classification for Flammable Gases and Vapours. open flames from heating equipment. the method of extinguishing obviously based on the materials stored. and others who need to know. S Separation: are flammable substances stored and used well away from other processes and general storage areas? Can they be separated by a physical barrier. such as vessel or line rupture are not considered by an area classification study. or by more quantitative methods that require a more detailed knowledge of the plant.ess. Storage rooms and storage facilities may be of a design by which fire detection and suppression system are installed. and the zone number. reporting on the levels could automatically exhaust and ventilate the storage facility. though there is no actual release. The VICES information is extracted form an HSE publication 'Safe Working With Flammable Substances' candidates will find this an ideal source of further information. These may arise from constant activities. inside process equipment may be a hazardous area if both gas/vapour and air are present. The IGE code addresses specifically transmission. Various sources have tried to place time limits on to these zones. rather than gas utilisation plant. If this aspect is important. these values are the most appropriate but for the majority of situations. Zone 1: An area in which an explosive gas atmosphere is likely to occur in normal operation. if it occurs. The alternative of specifying the extent of zones more conservatively is not generally recommended. although the use of fuels handled . Area classification is a method of analysing and classifying the environment where explosive gas atmospheres may occur.53 .by refinery and petrochemical type operations.Zoning. and illogicalities in respect of control over health effects from vapours assumed to be present. distribution and storage facilities for natural gas. The most common values used are:    Zone 0: Explosive atmosphere for more than 1000h/yr Zone 1: Explosive atmosphere for more than 10. The zone definitions take no account of the consequences of a release. Hazardous areas are classified into zones based on an assessment of the frequency of the occurrence and duration of an explosive gas atmosphere. As an example: A proposal was made to zone an aircraft hanger as Zone 1. and mobile equipment that creates an ignition risk. but some of the information will be relevant to larger scale users. When the hazardous areas of a plant have been classified. Hazardous areas are defined in DSEAR as "any place in which an explosive atmosphere may occur in quantities such as to require special precautions to protect the safety of workers". but none has been officially adopted. as given in BS EN 60079 -10 1 . it may be addressed by upgrading the specification of equipment or controls over activities allowed within the zone. a purely qualitative approach is adequate. In this context. sometimes referred to as 'safe areas'. 3. as follows:    Zone 0: An area in which an explosive gas atmosphere is present continuously or for long periods. will only exist for a short time. but less than 1000 h/yr Zone 2: Explosive atmosphere for less than 10h/yr. The main purpose is to facilitate the proper selection and installation of apparatus to be used safely in that environment. but still sufficiently likely as to require controls over ignition sources. taking into account the properties of the flammable materials that will be present. the remainder will be defined as nonhazardous. DSEAR specifically extends the original scope of this analysis to take into account non-electrical sources of ignition. as it leads to more difficulties with equipment selection. the practical consequences could usefully be discussed during site inspection. Where people wish to quantify the zone definitions. installation and use of apparatus. Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation and. Where occupiers choose to define extensive areas as Zone 1. 'special precautions' is best taken as relating to the construction. while standards for non-electrical equipment are only just becoming available from CEN. The results of this work should be documented in Hazardous Area Classification data sheets. Instead. Prevailing operating temperatures and pressures. and the letter giving the type of protection are listed below. Standards set out different protection concepts. A hazardous area extent and classification study involves due consideration and documentation of the following:        The flammable materials that may be present. and also of equipment. the IGE code gives a methodology for natural gas. DSEAR sets out the link between zones. and the equipment that may be installed in that zone. the standard current in mid 2003. The probability of each release scenario. Similarly. Dispersion of released vapours to below flammable limits. and special instructions issued for the rare event of using more volatile fuels. The source of potential releases and how they can form explosive atmospheres. It proved difficult to obtain a floor-cleaning machine certified for Zone 1 areas. The IP code gives a methodology for estimating release rates from small diameter holes with pressurised sources.54 . though the floor needed sweeping regularly. in the main. It tabulates values for an LPG mixture. and shows how both the buoyancy and momentum of the release influence the extent of a zone. relating the leak rate to the hole-size and the operating pressure. Most of the electrical standards have been developed over many years and are now set at international level. natural gas and refinery hydrogen for pressures up to 100barg. degree and availability of ventilation (forced and natural). The equipment categories are defined by the ATEX equipment directive. Zone 0 Zone 1 Zone 2 Category 1 Category 2 Category 3 'ia' intrinsically safe EN 50020. There are different technical means (protection concepts) of building equipment to the different categories. The option of writing out an exception to normal instructions to allow a non Ex-protected machine to be used regularly is not recommended. quite large diameter deliberate vents.EN 50021 . The physical properties and characteristics of each of the flammable materials. The tables of dispersion distances to the zone boundary address. Presence. gasoline. set out in UK law as the Equipment and Protective Systems for Use in Potentially Explosive Atmospheres Regulations 1996. with further subdivisions for some types of equipment according to gas group and temperature classification. These.Selection of Equipment. 3. There is. in practice. This applies to new or newly-modified installations. little overlap between the codes. supported by appropriate reference drawings showing the extent of the zones around (including above and below where appropriate) the plant item. a more realistic assessment of the zones is needed.above their flash point would be a rare event. These factors enable appropriate selection of zone type and zone extent. The DSEAR ACOP describes the provisions concerning existing equipment. 2002 'd' .Flameproof enclosure Electrical Type 'n' . Special protection 'Table 1' Correct selection of electrical equipment for hazardous areas requires the following information: · Classification of the hazardous area (as in zones shown in the table above).Powder filling EN 50017. 1998 'e' . the material that gives the highest classification dictates the overall area classification.Ignition Temperature of gas or vature.Encapsulation EN 50028.EN 50018 2000 Ex s . Consideration should be shown for flammable material that may be generated due to interaction between chemical species. 2002 'm' . · Temperature class or ignition temperature of the gas or vapour involved according to the table below: Temperature Classification Maximum Surface Tempera. °C T1 450 >450 T2 300 >300 T3 200 >200 T4 135 >135 T5 100 >100 T6 85 >85 'Table 2' If several different flammable materials may be present within a particular area.Increased safety EN 50019.Oil immersion EN 50015.Pressurised EN 50016 2002 'q' . 1987 's' . 2000 'ib' .Special protection if specifically certified for Zone 0 1999 Non electrical EN 13463-1. The IP code considers specifically the issue of hydrogen-containing process streams as commonly found on refinery plants.Intrinsic safety EN 50020. °C pour. 1998 'o' . 2001 'p' . . tanker loading/unloading. Hot process vessels. Impact sparks. A range of petrochemical and refinery processes use direct-fired heaters. e. Space-heating equipment. Precautions to control the risk from pyrophoric scale usually associated with formation of ferrous sulphide inside process equipment. Stray currents from electrical equipment. unless specially designed or modified are likely to contain a range of potential ignition sources. e.g. Friction heating or sparks. Controls over the use of normal vehicles. Vehicles. Elimination of surfaces above auto-ignition temperatures of flammable materials being handled/stored. Controls over activities that create intermittent hazardous areas. limitations on the power input to fibre optic systems. New mechanical equipment will need to be selected in the same way.Ignition Sources . Electrical equipment and lights.Identification and Control. but which are still within a risk area). Direct-fired space and process heating. Clearly. Hot surfaces. Correct selection of vehicles/internal combustion engines that have to work in the zoned areas. Electrostatic discharge sparks.g. if the fuel supply to the heater or the pipework carrying the process . Cutting and welding flames. Ignition sources may be:                    Flames. Mechanical machinery. Spontaneous heating. steam crackers for ethylene production. Direct Fired Heaters. Sources of ignition should be effectively controlled in all hazardous areas by a combination of design measures and systems of work:            Using electrical equipment and instrumentation classified for the zone in which it is located. Electromagnetic radiation of different wavelengths.55 . Correct selection of equipment to avoid high-intensity electromagnetic radiation sources. Earthing of all plant/equipment. Provision of lightning protection. Lightning strikes. Hot Oil Systems and Processes Operating Above Auto-Ignition Temperatures. Sparks from electrical equipment.3. consideration needs to be made of smoking outside what might be defined as work premises. avoidance of high intensity lasers or sources of infra-red radiation. Heated process vessels such as dryers and furnaces. Control of maintenance activities that may cause sparks/hot surfaces/naked flames through a Permit to Work System. e. Use of cigarettes/matches etc.g. Prohibition of smoking/use of matches/lighters (whilst smoking in work premises is now illegal. Standard EN 1755 1 sets out the requirements for diesel-powered ATEX category 2 or 3 lift trucks. an unprotected type has to be extensively rebuilt. 3.Vehicles. In these circumstances. These will include    electrical circuits. Consequently. together with a means of rapid detection and isolation of any pipes that do fail. but with the suggestion they may be useful in cases of remote risk. Any such processes should be specifically identified in a safety case. either directly at the flames. Again. about vehicles with gas detection systems.fluid leaks close to the furnace. the inlet and exhaust of any internal combustion engine. particularly with floating roof tanks. and areas where they must be excluded. this may be acceptable. At present these are sold without any claim for ATEX compliance. and it is unlikely that such an engine could be built economically. area classification is not a suitable means of controlling the ignition risks. Further guidance can be found in BS 6651:1999 1 .57 . Site rules should be clear where normal road vehicles may be taken. No specification is available for vehicles with spark ignition engines. designed to shut the engine and isolate other sources of ignition in the event of a gas release. Other processes (such as hot oil heating circuits) may handle products above their auto-ignition temperature. and other moving parts. with the objective of clarifying when storage areas should be classified as zone 2.Lightning Protection.56 . Ignitions caused by lightning cannot be eliminated entirely. The conclusions from this exercise will be made available in due course. and the same considerations apply as with fired heaters. any leak must be expected to find a source of ignition. perhaps with limited use of a vehicle.(Code of practice for protection of structures against lightning). In these circumstances. 3. overheating brakes. measures to mitigate the consequences of a fire should be provided. In some stores. where vapour is usually present around the rim seal. many employers are likely to try and justify not zoning storage compounds. expensive and for many applications. Discussions have been held with the British Chemical Distributors and Traders Association. or by a surface heated by a flame. . electrostatic build up. Electric-powered vehicles can also be built using a combination of this standard and the normal electrical standards. hazardous area classification. The consequences of the failure of a pipe carrying process materials within the furnace should be considered in any HAZOP study. Vehicles certified to ATEX requirements are. and appropriate selection of ATEX equipment is not suitable as a basis of safety for preventing fire and explosion risks. Discussions are also ongoing. Protection against lightning involves installation of a surge protection device between each nonearth bonded core of the cable and the local structure. however. Instead. where lift trucks handle flammable liquids or gases in containers. safety should be achieved by a combination of a high standard of integrity of fuel and process pipelines. Most normal vehicles contain a wide range of ignition sources. and commissioning checks to ensure the ventilation achieves the design aim. or a formal permit to work system. Safe systems of work are needed to ensure safety where such 'transient' zones exist. and what assessment has been made to ensure it remains safe for use? Is there a reference to the impact upon extent and classification of hazardous areas in the section describing plant modification. the sources of release and consequently the location and extent of hazardous areas have been minimised? Do any zone 2 areas extend to places where the occupier has inadequate control over activities that could create an ignition source. is there evidence that by design and operation controls. 1 and 2. passive items like new walls and buildings can influence this if they obstruct natural ventilation of adjacent plant Have all ignition sources been considered? A check list is provided in the DSEAR ACOP on control and mitigation measures. an assessment is needed of the risk that an ignition within a storage compound will produce a major accident. including raw materials. e. Have all flammable substances present have been considered during area classification. either directly or because a fire or explosion spreads to involve other materials. Many sites will have operations of filling and emptying road tankers with flammable materials. Hazardous areas may be considered to exist during the transfer operation.For the purposes of COMAH. proper controls and plant isolation may allow the normal zones to be suspended. these will involve written instructions. they need not all be provided in the report.59 . Where specialist vehicles (e. intermediates and by-products. Does the report identify old electrical equipment still in service in a hazardous area. it is more appropriate to provide controls to prevent the spread. Basic concepts and methodology).Factors to be Considered During an on Site Inspection. Explosion prevention and protection. Locations where a large release is possible and the extent of hazardous areas has been minimised by the use of mechanical ventilation should be identified. Typically. Have appropriate standards been used for selection of equipment in hazardous areas? Existing plant will not meet the formula in DSEAR. gas turbine power generation units.g. but should not be present once the transfer is complete. Some reference to design codes. or is there any suggestion that the zone boundaries have been arbitrarily adjusted to avoid this? Has ignition-protected electrical equipment been installed and maintained by suitably- . The consequences of a loss of power to the system should be included in any section looking at other consequences of power loss. rather than simply apply more conservative zoning and more restrictive rules on the equipment used in the store. Controls will be needed to prevent or minimise the release of gas or vapour but controls over ignition sources are also needed. but those examples relevant to the representative set of major accidents upon which the ALARP demonstration is based must be included. cranes) are needed during maintenance operations. compressor houses.g. final product and effluents? Commonly these will be grouped for the purposes of any area classification study. but older standards distinguished between electrical equipment suitable for zones 0. If this is possible.    If there are any large areas of zone 1 on the drawings. 3. 3.       Is a full set of plans identifying hazardous areas available? For a large site. should be provided.58 . as specified in DSEAR schedule 1. and BS EN 1127 part 1 1 (Explosive atmospheres.Factors for Assessor of a Safety Case to Consider. Ignition due to a hot surface is possible. Zoning as described above may be applied. 21 or 22. The issues about representative samples of dust. 3. described in HSG 103 2 . A dust explosion could then be an initiator of a major accident.  trained staff? Are the risks from static discharges controlled properly? Earthing of plant.60 .Dust Explosions. for many years a small-scale screening test has been used. The inside of different parts of the plant may need to be zoned as 20. There is no legally defined test for an explosible dust. which is an adaptation of the IEC equivalent. and take this as the worst case. In general. portable gas detectors. it is common to test material that passes a 63-micron sieve. and the extent of any zone 21 or 22 outside the containment system should be minimal or nonexistent. For COMAH sites with toxic dusts. Classification of dusts relating to autoignition and minimum ignition current is undertaken similarly to gases/vapours. The zone numbers used are 20. but the temperature required to ignite a dust layer depends on layer thickness and contact time. releases into the general atmosphere should be prevented. corresponding to 0. particle size. However. in most cases occupiers will need to carry out testing of the product for its explosive properties. replacing 'gas atmosphere' with 'dust/air mixtures'. but if the particle size distribution varies. dusts with a particle size greater than 500 µm are unlikely to cause an explosion. but involves additional complications. many toxic materials are handled in fine powder form. Measures to prevent major accidents should address all potential initiators. typical precautions include the use of supplementary ventilation. 21 and 22. The COMAH Regulations do not apply to any material if the only risk created is that of a dust explosion. Where toxic dusts are processed. The explosibility of dusts is dependent upon a number of factors:    chemical composition. and other factors that might cause the results to vary are also discussed in this guidance. For most chemical products. DSEAR requires that hazardous area classification for flammable dusts should be undertaken in the same manner as that for flammable gases and vapours. other precautions are described in the references. if substantial quantities were held for extended periods . drums and tankers is the most basic requirement. oxygen concentration. depending on the conditions at particular locations. A dust explosion involving a non-toxic dust like polyethylene would not result in a major accident as defined in the regulations. and a serious dust explosion could cause a major accident. and inerting of sections of plant.1 and 2 used for gases/vapours The only relevant standard to help people zone their plant is BS EN 50281 part 3. Where toxic dusts are handled. However. unless it also led to loss of containment of a COMAH substance. Companies able to undertake such testing are listed in the IChemE's book on the prevention of dust explosions. the vertical tube test. the most likely hazard would arise in drying processes. What control measures over ignition sources are adopted in hazardous areas during maintenance? Where ignition sources must be introduced. 2002 1 . it is preferable to test dust taken from the process. Equipment built to such a harmonised standard may assume automatic conformity with those essential safety requirements of relevant directives that are covered by the standard.Status of Guidance. 3. information about selection and maintenance. Existing codes of practice provide information with respect to good practice for hazardous area classification. electrostatic risks from clothing. Typical uses are within storage tanks where a material may be above its . but was published in the UK as PD CLC/TR 50404:2003 Electrostatics. The most recent general source of advice was drafted by a European standards working group. 3. It contains much useful advice about limiting pumping speeds. A list of ATEX harmonised standards can be checked on the EU web site . The standards detailing selection of appropriate electrical apparatus have been updated to take into consideration ventilation effects. because they contain some useful information not duplicated by the PD. substitution of flammable material with non-flammable. The partial or complete substitution of the air or flammable atmosphere by an inert gas is a very effective method of explosion prevention. Section 9 provides guidance on lightning protection of structures with inherent explosive risks.Electrostatic Ignition Risks.e. The two parts of the older BS 5958: 1991 1 Code of Practice for the control of undesirable static electricity remain current.62 .hot enough to start self-heating or smouldering combustion.Inerting. BS 7430:1998 1 Code of practice for earthing. 3. and many detailed operations.61 . Part 2: 1991 Recommendations for particular industrial situations       BS EN 50281 1 . Code of practice for the avoidance of hazards due to static electricity.63 . The EPS regulations describe the conformity assessment procedures that apply to different types of equipment. Lightning protection. General principles. The different parts of this standard set out requirements for construction of equipment for use in atmospheres containing explosive dusts. and BS EN 50281-3: 2002 1 covers the classification of areas where combustible dusts are or may be present. BS 6651:1999 1 . Code of practice for protection of structures against lightning. Inerting is normally only considered when the flammable or explosive hazard cannot be eliminated by other means i. European equipment standards may become 'harmonised' when a reference to them is published in the Official Journal of the European Community. Section 23 provides guidance on lightning protection. The two parts are: Part 1: 1991 General considerations. British Standards Institution. British Standards Institution. adjustment of process conditions to ensure substances are below flammable limits. Oxygen. Gases that can be used for inerting include:      Nitrogen. The fuel must be within its range of flammability. Inert gases may be generated on site. Appendix E . appropriate methods should be employed to determine the flammable limits for mixtures of materials. i. a slight positive pressure should be maintained within the enclosed plant to reduce the possibility of air ingress. This technique can also be employed to provide protection to interconnected plant by inerting plant items downstream of the explosion. Argon. determination of . since the potential for further dilution would bring the material within its flammable range. three key 'ingredients' are required simultaneously. Carbon Dioxide.Flammable Limits. To produce an explosion. Inerting is applicable to enclosed plant. The final measure that can be adopted involves reducing the oxygen levels necessary to sustain combustion. a formal management control system in the form of a Permit to Work should be in place so that appropriate precautions and control measures can be implemented.64 . but below the upper flammable limit. If the fuel cannot be eliminated or minimised. The Permit to Work system is covered separately.e. A typical method is cited in BS 5345 : Part 1: 1989. In those events where people are required to enter a confined space. however. Helium. This in turn triggers injection of an explosion suppressant such as chlorobromomethane or carbon dioxide into the path of the advancing flame front. steps must be implemented to eliminate or minimise the source of ignition (see the Technical Measures documentation on Hazardous Area Classification). Ignition. This means that it is perfectly acceptable to have an environment with the flammable material above the upper flammable limit provided appropriate control protocols are in place. The practice of inerting is also employed in explosion suppression systems. A major risk associated with the use of inerting is that of asphyxiation. and within reactor systems when excursions into flammable atmospheres may occur. or via bulk storage of cylinder facilities. The flammable limits for individual materials with air are readily available in standard references.flashpoint. at a concentration above the lower flammable limit. Flue gases. 3. where typically a quick-acting pressure switch responds to the initial comparatively slow increase in pressure due to initiation of explosion conditions. In most inerting systems. This can be achieved by pressurising / purging with an inert gas such as nitrogen. Inert gases are also used to transfer flammable liquids under pressure. particularly in confined spaces. These are:    Fuel. Where flammable dusts are handled in an atmosphere containing flammable gas or vapour. since plant that is substantially open to atmosphere cannot be effectively inerted because the prevailing oxygen concentration is likely to vary. flammable limits is difficult and use of inerting should be considered wherever possible. in the absence of which many potential hazards could be realised. and the system should be regularly inspected and maintained. the maintenance of an inert atmosphere is a safety-critical measure.65 . .Reliability / Back-up / Proof Testing. Control systems based upon the use of 'explosimeters' and oxygen analysers should protect against asphyxiation if entering such areas. The presence of inert atmospheres should always be taken into consideration during operational or maintenance activities. In many applications. since potential hazards could arise from:   asphyxiation or loss of inert atmosphere. Inert gases are often used to purge tanks and vessels which normally contain flammable substances prior to maintenance. Consideration should also be given to the reliability of the control systems employed for operation and changeover. and controlled by a Permit to Work system. A major consideration when designing plant to be protected by inerting is the need for continuous monitoring of oxygen and flammable gas or vapour concentrations. can be facilitated by the use of a hazardous storage cabinet specifically designed for the purpose 'Image 1' Safe Systems of Work are the complete systems which create frameworks by which procedures. 3. Consideration should be given to the possibility of failure of the inert gas supply and the acceptable unavailability. This will involve calculations to determine the rate of leakage/replacement in all process conditions encountered to find the worst case that must be considered. Back-up facilities may be via alternative bulk storage or cylinder provision. Back up of supplies with alarm systems to bring about operator intervention or automatic changeover should be provided as required to meet the required availability determined. commissioning or decommissioning. Reliability of the supply of inert gas is therefore of vital importance. in particular those which are 'pre-packed' in small containers.Operating / Maintenance Procedures. Smaller-scale bulk storage of flammable materials. Maintenance activities should only be undertaken by suitably-trained and authorised personnel. 3.66 . if possible. Naked flames must not be used in an area where flammable solvents are in use. If possible. To keep the risk as low as possible. a COSHH assessment and a risk assessment.methods. a means of increasing the air flow in the case of a significant spillage. Include in the procedures where necessary the use of permits to work.67 . the volume of solvent must be kept to a working minimum both for storage and in use in laboratories. . A well-documented and functional safe system of work or its component parts not only constitutes a fundamental aspect of fire prevention. External doors should be left open during pouring operations to aid the flow of air. should be constructed so as not to produce sparks whilst in operation. must be completed. In fire prevention and other disciplines of health and safety. Feed-back any information on weaknesses or failures in the system. Rectify these by modifying the system. Safe operating procedures detail how a piece of equipment should be correctly (safely) used. in control measures of a risk assessment. develop a safe system of work. formalise these systems of work into procedures. a room preferably away from the main building. with relevance to DSEAR. disconnected or removed to prevent sparks when solvents are being poured. we can follow a sequence or process to fulfil the requirements of a described safe system of work:          Make a risk assessment. but also demonstrates best practice at times of audit or inspection. exhaust fan motors etc. Lifting equipment to be kept lubricated to prevent sparks. such procedures are often derived from outcomes of risk assessments and can also appear in reference. Bulk storage of flammable solvents is to be in a suitable enclosed lockable area. the floor of the room in use should be in the form of a well. how methods of work should be undertaken and how a process should be operated. The area is to be marked with a specified "EX" sign at the point of entry. only drawing extra stocks from suppliers on a "call off" basis. All electrical equipment . Keep monitoring. The quantity of solvent to be kept to a minimum working level. 3. sufficient to contain a major spillage of solvent.Safe Operating Procedures. Electrical items which are not in use must be switched off and the plug disconnected at the socket.lights. The following is an actual safe operating procedure issued by an educational establishment and details the decanting of flammable solvents for use in a laboratory. Sources of ignition must be identified and. In cases of extreme hazard. Should hazards remain. where possible. maintenance schedules etc are operated. Monitor the observance of all parts of the procedure. There should be adequate ventilation for normal working where pouring of the solvent is carried out and. There is no one singular definition of what constitutes a safe system of work. Before any new work is started involving flammable liquids. Determine what can be done to remove the identified hazards and do it. The dispensing must only be done by trained solvent store operatives and no-one else. Solvents may be used in large quantities in several departments and this is unlikely to change. where necessary. which should have a vented screw cap and which must be emptied into the correct waste drum in the solvent store. Maintenance schedules are not applicable except for periodic testing of portable electrical equipment.Explosion Mitigation. a stores request form must be completed and presented to the storekeeper (and. the doors must be labelled with a flammable solvent sign. Waste solvent is as flammable as pure solvent and should not be left in the open laboratory. use a laboratory trolley to transport them. In either case. Trolleys . Remember the manual handling regulations. 3. Other solvents may be in "pre-packed" Winchesters or metal cans. When collecting the solvents. Waste solvent must be disposed of correctly into the appropriate container. Storage in the laboratory the Winchester of flammable solvent must be stored in a metal solvent cabinet or in a vented cupboard below a fume cupboard.5 litre glass bottles (Winchesters) for use in the laboratories. This includes the transfer from one container to another and the storage container returned immediately to its cabinet. This includes waste solvent. Keeping the bottles in a carrier whilst on the trolley will prevent them knocking against each other. and if several Winchesters are to be collected. The solvent may be transferred to a 500 ml reagent bottle for use in the lab. It is an offence to keep more than 50 litres of flammable solvent in total in a single laboratory. Solvents must not be stored with incompatible materials such as concentrated nitric acid (oxidising agent). reducing the risk of breakage. a Winchester carrier or appropriate bottle carrier must be used to transport the solvents to the laboratory.any stores lifting equipment and bottle carriers should only be used if they are in good condition. the explosion risk may be reduced. the relevant customs and excise book signed). The screw tops on the bottles must not be tightened fully so that vapours may escape as the solvent reaches room temperature. Work with flammable solvents under normal circumstances must be carried out in a fume cupboard away from possible sources of ignition. so the flammability risk will remain but with constant vigilance on the part of the users. Flammable solvents must never be poured down sinks. Operators should demonstrate that appropriate measures are in place either to prevent . (These metal cans are usually 5 litres and may be heavy enough to fall under the manual handling regulations). They are dispensed into 2. Explosion relief. To obtain the solvents. (The amount of vapour will be minimal and should not cause an increased risk). The solvent can then be collected at a time specified by the storekeeper from the designated collection point. Passenger lifts must not be used to convey the solvents between floors.Certain solvents are supplied in 200 litre drums. Goods lifts only should be used. General principles. Any container of flammable solvent larger than 500 ml must be stored in the appropriate cabinet. Winchesters of solvent must be put away at night and not stored in or on the workbench.68 . Under no circumstances should the bottles be carried by the neck or cradled in your arms. A secondary explosion or continuing fire is much more likely. Measures to prevent major accidents should address all potential initiators. Alternative mitigation measures are available.with shorter hazard ranges as a result. including explosion suppression. unburnt substance in addition to combustion products. solvent evaporating processes/ovens and plants handling explosible dusts. A dust explosion involving a non-toxic dust like polyethylene would not result in a major accident as defined in the regulations. These factors will have a significant impact on plant layout. pressure effects. which contain plant that gives rise to risk of a rapid and substantial release of gas inside the building. and may well disturb dust deposits in the area. and a serious dust explosion could cause a major accident. it is worth noting that even finely-powdered solids are significantly less easily dispersed than are gases and volatile liquids . The reports on the Manro (1982) and Chemstar (1981) incidents show the possibilities with distillation plant.explosions from taking place. unless it also led to loss of containment of a COMAH substance. will usually release burning and unburnt dust. design of plant and supporting structures and explosion relief routes. However. however. The adoption of measures such as these is likely to be necessary where the process of venting could itself lead to the release of sufficient toxic material to create a major accident. The advent of improved gas control systems means that in many cases. In assessing the risks from an explosion. and relief designed to protect plant from some other cause of overpressure. A European harmonised standard for solvent evaporating ovens prefers alternative precautions for many applications. The COMAH Regulations do not apply to a material if the only risk created is that of a dust explosion. operators should consider flame propagation. but continuing burning is only likely if there is a continuing source of release. explosion relief is not now fitted to combustion plant. Operators need to draw a clear distinction between pressure relief designed to protect against an explosion. Historically. or mechanical moving parts in contact with the dust often create ignition risks that cannot be eliminated completely. It may also be provided on buildings. but certainly not in the vicinity of regularly-occupied areas or plant that would be easily damaged. many toxic materials are handled in fine powder form. A dust explosion could then be an initiator of a major accident. or building the plant strong enough to withstand the anticipated explosion pressures. or to protect against/minimise the effects of explosions. but in the dust-handling industries explosion relief remains a widely-used mitigation measure. . particularly where an explosion is likely to result in emission of toxic material. explosion relief has been used as a mitigation measure in three main types of plant: large-scale gas-fired combustion plant. Explosion prevention is always preferable to explosion protection. much larger sizes of vent panels or doors are needed in order to work successfully and duct work downstream from the relief panel needs careful design to avoid throttling the flow and preventing the relief acting fast enough. The material released from an explosion relief vent typically includes quantities of the original. Relief points from explosion protection devices should normally be located outside the building containing the plant. Explosions are rapid events and consequently:    protective devices have to work very rapidly. perhaps because nearby pipework has been damaged. However. This is because it is often impossible to prevent the formation of dense dust clouds inside the process. The dust itself. but inadequate explosion relief was not the prime problem in this case. recoil forces and the materials that would be released. Compressor houses are an example. A dust explosion. An explosion of gas or vapour will release hot combustion products. Sizing of Explosion Panels. as prEN 1449.69 . A harmonised standard for the design and testing of explosion vent panels and doors is under preparation within CEN TC 305. exclusion of oxygen by use of inert gases. the location of the ignition source. explosion flames can accelerate to detonation. Normally.g. Explosion panels for gases and vapours. but they should be suitable for the purpose. In extreme cases. 3. In particular. including:         the explosion properties of the gas or vapour. the possibility of pressure piling needs to be considered. and be tested and certified by a Notified Body. the size of the vessel to be vented. Elimination of ignition sources. much higher ultimate pressures can result. any initial or induced turbulence in the system.The ACOPs and Guidance to DSEAR discusses the alternatives for explosion prevention and mitigation measures. If an explosion that starts at one location inside a plant causes the explosive mixture ahead of the flame front to be compressed. generating very high pressures and explosion relief is unlikely to be a suitable method of protection in this case. by excluding air. Explosion suppression. High-speed isolation. . the opening pressure of the vent panel/door and its inertia. As such. the geometry of the hazardous region. Existing panels do not need to be replaced. but the precautions to prevent an explosion inside the process plant are not the same. In particular. e. new panels or doors must comply fully with the essential health and safety requirements. Inerting. risks may remain in the ALARP region. The ALARP demonstration should then include an assessment of the following options for mitigation:     Containment (explosion-resistant construction). the strength of the plant. This hierarchy relates to intentional or unintentional releases. or purging with air before start up of combustion plant. Having implemented these precautions. Explosion pressure relief (venting). Explosion vent panels and doors are considered as autonomous protective systems within the meaning of the ATEX equipment regulations. Segregation (keeping catalysts or pyrophoric materials apart from other products). Monitoring and detection of smouldering particles with automatic quench systems (specific to dust explosions). the following options should be considered to prevent an explosion inside the plant: Substitution of combustible materials:    Control of concentration. the influence of any vent ducts needed. they provide a hierarchy of controls. The size of vent area required for effective control depends upon a number of factors. and this has not changed as a result of DSEAR. and much of the advice is contained in the other sources quoted. KST is defined as the maximum rate of pressure rise measured in a 1m3 vessel. The dust cloud must be in contact with an ignition source of sufficient energy to cause an ignition.72 . A vent sizing routine for buildings is given in the NFPA 68 code. COMAH removed licensing for petroleum stores at COMAH sites. and plants will be found which have used all these design equations. Checking of calculations is best done by the computer expert system DUST EXPERT. it is usually calculated from measurements in smaller test apparatus. and DSEAR removed licensing for all drum stores.Explosion Vent Relief Sizing Panels for Dusts & Powders.Dust Explosions (Especially in Powder Transfer & Dryers). KST and P max figures for the dust itself. Some equipment suppliers use the German VDI 3673 guide as a basis for the design of explosion relief vents. 3. the missing value of C1 = 0. Basic input data required is the strength of the process equipment.80% of explosible dusts fall in this .045 is given in the NFPA code.70 . and the opening pressure of the vent panel or door. the equipment volume. This is acceptable. Stores designed for storage of petroleum liquids under earlier legislation normally had heavy concrete structures. and this standard should be adopted when it becomes available. More recently. work within CEN has produced a rationalisation and simplification of the design equations recommended. developed by HSE. and repeated in the IChemE Dust Explosion book. A dust explosion can take place only if a number of conditions are simultaneously satisfied:      The dust must be explosible (refer to table on dust explosion classes).71 . The dust cloud must have a concentration within the explosible range. The dust must have a particle size distribution that will allow the propagation of flame.3. A considerable amount of experimental data have been used to develop empirical design equations. or via the HSE but without the explosibility constant for organic vapours. each with a restricted range of applicability.Explosion Relief from Buildings. Many different equations have been published. There is insufficient justification to seek structural alterations to buildings. Dust may be grouped into dust explosion classes as determined. These groupings are as follows: ST0  Not explosible. using standard test apparatus. but these can still be very destructive . The atmosphere into which the dust is dispersed as a cloud or suspension must contain sufficient oxidant to support combustion. 3. which will act as explosion relief if a vapour cloud ignited within the building. if any petroleum-type stores are found on COMAH sites. HSE's traditional advice for buildings storing flammable liquids has been to ask for a lightweight roof. ST1 KST less than 200  Slow explosion. and available to relevant process safety specialists. to provide maximum protection for the stored product from a fire in the vicinity. 74 . These hazards should be identified and documented. this group includes mainly metal dusts. The assessment is specifically concerned with the physical properties of the products. a systematic approach to the identification of hazards relating to the nature of the materials has been followed. the energy release comes from decomposition not combustion). boiling gas/liquid mixtures) and sizing routines for gas and dust explosions are not appropriate. When unstable substances are in use. The operator should have shown due consideration of these hazards and taken appropriate measures to provide pressure relief. toxicity. environmental problems.g. with subsequent evidence of implementation of control measures. in which the rate of generation of heat is greater than the available cooling capacity of the system.e. ST3 KST>300  Very high speed explosion.73 . explosion relief is an inappropriate mitigation measure. Relevant statutory provisions.Exothermic Reactions. Hazards that merit consideration include:      explosibility. Various testing strategies and experimental methods are commonly available for determination of thermal decomposition hazards. the operator should demonstrate that at the research stage of the product. Dangerous Substances and Explosive Atmospheres Regulations 2002 Electricity at Work Regulations 1989 . Hazards from exothermic reactions occur in the event of thermal runaway of the reaction mixture. Pressure relief needs to take into account the nature of the reaction mixture involved.Unstable Substances. (e. 3. flammability. and possible by-products. viscosity.group ST2 200<KST=<300  Medium speed explosion. thermal and pressure conditions. If products show properties that indicate they can explode in the solid phase (i. 3. Major Incidents . displaced the platform's condensate for transport to the coast. The discovery of a small gas leak was not unusual and no cause for concern. This permit disappeared and was not found. The on-duty engineer filled out a permit which stated that Pump A was not ready and must not be switched on under any circumstances. As the entire power supply of the offshore construction work depended on this pump. Two condensate pumps. as they were sorted by location. The manager assumed from the existing documents that it would be safe to start Pump A. The valve was in a different location from the pump and therefore the permits were stored in different boxes. the fire-fighting system was under manual control on the evening of July 6: Piper Alpha procedures required manual control of the pumps whenever divers were in the water (as they were for approximately 12 hours a day during summer) regardless of their location. the engineer neglected to inform him of the condition of Pump A. Coincidentally there was another permit issued for the general overhaul of Pump A that had not yet begun. Like many other offshore platforms. produced an overpressure which the loosely fitted metal disc did not withstand.m.m. Because the work could not be completed by 6:00 p. Some witnesses to the events question the official timeline.. the platform was operating as normal.m. otherwise the power supply would fail completely.  9:55 p. analysis of events can only suggest a possible chain of events based on known facts. and many of those involved died. and the night shift started with 62 men running Piper Alpha. The day shift ended. and while this work disrupted the normal routine. Because the platform was completely destroyed. but not the other permit stating that the pump must not be started under any circumstances due to the missing safety valve. drawing the attention of several men and triggering six . driven by both diesel and electric pumps (the latter were disabled by the initial explosions).Piper Alpha Timeline of the incident A new gas pipeline was built in the weeks before the 6th July explosion. Condensate Pump A was switched on.0 .  7:00 p. A search was made through the documents to determine whether Condensate Pump A could be started.m. Pump A's pressure safety valve (PSV #504) was removed for routine maintenance. Gas audibly leaked out at high pressure. Gas flowed into the pump. the manager had only a few minutes to bring the pump back online. and because of the missing safety valve.The Contribution of Typical Mechanical & Systems Failures to Major Accidents. The pump's fortnightly overhaul was planned but had not started.m. designated A and B. On the morning of July 6. The open condensate pipe was temporarily sealed with a blind flange (flat metal disc). However. None of those present was aware that a vital part of the machine had been removed. Instead he placed the permit in the control centre and left.  9:45 p. Condensate (natural gas liquids NGL) Pump B stopped suddenly and could not be restarted.m. to prevent divers from being sucked in with the sea water (fire pumps on other platforms were switched to manual control only if the divers were close to the inlet). particularly as the metal disc replacing the safety valve was several metres above ground level and obscured by machinery. The missing valve was not noticed by anyone. The diesel pumps were designed to suck in large amounts of sea water for fire fighting. As he found the on-duty custodian busy. The permit for the overhaul was found.  6:00 p. Piper Alpha had an automatic fire-fighting system. the pumps had an automatic control to start them in case of fire.  12:00 p.4. the blind flange remained in place.  9:52 p.m. It was only after this second explosion that the Claymore stopped pumping oil.m. They were never seen again.m. However. rescue and accommodation vessel. which immediately ignited. The second gas line ruptured. The Tharos.5 x 1. the firewalls were designed to resist fire rather than withstand explosions. but before anyone could act. but the fire prevented them from doing so. Wind. the results of which warned of the dangers of these gas lines. the platform's destruction was assured. Also. Emergency procedures instructed personnel to make their way to lifeboat stations. The Claymore continued pumping until the second explosion because the manager had no permission from the Occidental control centre to shut down. which began to melt the surrounding machinery and steelwork.  10:20 p. the gas ignited and exploded. a large semi-submersible fire fighting. The fire would have burnt out were it not being fed with oil from both Tartan and the Claymore platforms. Gas lines of 140 to 146 cm in diameter ran to Piper Alpha. the resulting back pressure forcing fresh fuel out of ruptured pipework on Piper. A massive fireball 150 metres in diameter engulfed Piper Alpha. drew alongside Piper Alpha. As the crisis mounted. The control room was abandoned. The Tharos was driven off by the fearsome heat. with smoke beginning to penetrate the personnel block. fire and smoke prevented helicopter landings and no further instructions were given. and the platform's organisation disintegrated. Piper Alpha's design made no allowances for the destruction of the control room. as its manager had been directed by his superior. Tartan's gas line (pressurised to 120 Atmospheres) melted and burst.m.  10:50 p. Claymore and Tartan were not switched off with the first emergency call. with substantial financial consequences. the platform would then have been isolated from the flow of oil and gas and the fire contained. closing huge valves in the sea lines and ceasing all oil and gas production. spilling millions of litres of gas into the conflagration. creating another fire.  10:04 p. Due to their length and diameter it would have taken several hours to reduce their pressure. two men donned protective gear in an attempt to reach the diesel pumping machinery below decks and activate the firefighting system. killing two crewmen on a fast rescue boat launched from the standby vessel Sandhaven and the six Piper Alpha crewmen they had rescued from the water. Theoretically. The Tharos used its water cannons where it could. releasing 15-30 tons of gas every second. so that it would not have been possible to fight a fire fuelled by them. It would have taken several days to restart production after a stop. The first explosion broke the firewall and dislodged panels around Module (B). Personnel still left .5 metre panels bolted together. Huge flames shot over 300 ft (90 m) in the air. One of the flying panels ruptured a small condensate pipe.m. Although the management admitted how devastating a gas explosion would be. Instead the men moved to the fireproofed accommodation block beneath the helicopter deck to await further instructions. directly into the heart of the fire. No attempt was made to use loudspeakers or to order an evacuation. blowing through the firewall made up of 2. Two years earlier Occidental management ordered a study. which were not designed to withstand explosions. The custodian pressed the emergency stop button. but it was restricted because the cannons were so powerful they would injure or kill anyone hit by the water.gas alarms including the high level gas alarm. the connecting pipeline to Tartan continued to pump.  10:30 p. From that moment on. because the platform was originally built for oil. The reason for this procedure was the exorbitant cost of such a shut down. public address. which was fabricated from 18mm steel plate and weighed 20 tons. smoke-filled accommodation block or leaping from the deck some 200 ft (60 m) into the North Sea. slipped into the sea. his fire-fighting monitors did not function properly. At 07:00 hours the following morning there was a violent explosion and subsequent fire. were inaccessible and seem to have been damaged from the beginning. July 7 The entire platform had gone. the plant was held in stand by condition with no fresh feed. The main problem was that most of the personnel who had the authority to order evacuation had been killed when the first explosion destroyed the control room. which fuelled an extremely intense fire under the deck of Piper Alpha. There is controversy about whether there was sufficient time for more effective emergency evacuation. but waited for orders from OIM to fight the fire. 165 died and 59 survived. and to destroy the control room and the radio room in the early stages of the accident. general alarm. The fire fighting equipment on board could not be operated because the diesel pumps. and even if it had been ordered. THE ACCIDENT AND THE FAILURE PATH The accident started with a process disturbance. From then on until the time of the incident. followed by a flange leak that caused a vapour release.  11:20 p. and fire detection and protection systems also failed shortly after the first explosions. The generation and utilities Module (D).m. was ineffective almost from the beginning. When the master of one of the vessels on-site decided to assume the role of onscene-commander (OSC). The explosion centred on a low pressure (LP) separator vessel.m. Electric power generation.  12:45 a. Another contributing factor was that the nearby connected platforms Tartan and Claymore continued to pump gas and oil to Piper Alpha until its pipeline ruptured in the heat in the second explosion.alive were either desperately sheltering in the scorched.. That fire impinged on a gas riser from another platform. Piper Alpha was eventually lost in a sequence of structural failures. During the recommissioning a plant trip occurred. The explosion could be heard and felt up to 30km away. Many evacuation routes were blocked and the life boats. and died during the accident. Two men from the Standby Vessel Sandhaven were also killed. were mostly inaccessible.1 . The layout of the topside allowed the fire to propagate quickly from production modules B and C to critical centres. The superintendent of the platform (Offshore Installation Manager or OIM) panicked. all in the same location.  11:50 p. could not have been fully carried out given the location of the living quarters. 4. On Saturday 21st March 1987 the hydrocracker unit was being recommissioned following a routine shut down. even though they could see that Piper Alpha was burning. V306. which included the fireproofed accommodation block. At the time of the disaster 224 people were on the platform. .m. This was a consequence of the platform design. Evacuation was not ordered. This was thought to be a spurious trip and the plant operators started to bring up the unit to normal operating conditions. including the absence of blast walls. emergency shutdown. which had been put on manual mode.Major Incidents . The largest part of the platform followed it. Their operations crews did not believe they had authority to shut off production. The pipeline connecting Piper Alpha to the Claymore Platform burst. Fire boats were at hand.Grangemouth. Module (A) was all that remained of Piper Alpha. and the ineffectiveness of the safety equipment. the layout of the topside. Several explosions followed and severed a petroleum line causing a pool fire. Plant Modification / Change Procedures: HAZOP Alarms / Trips / Interlocks: level measurement The LP Separator pressure relief had been sized for a fire case. This action allowed the remaining liquid in the HP separator to drain away and for high pressure gas to break through into the LP separator. The seat of the fire was located in a raw materials warehouse at Allied Colloids site in Low Moor. The control valve did not close automatically because the low low level trip on the HP separator had been disconnected several years earlier. It is thought that a steam condensate line was responsible for heating a number of AZDN kegs. AZDN in contact with SPS is likely to have been ignited by an impact. The warehouse itself had two rooms allocated for the storage of oxidising and flammable products known as No. 2 oxystore. relief valves There were no means of isolating the HP separator following an incident in the LP separator. On the morning of the incident steam heated blowers in the warehouse had been turned on to dry out moisture. It was determined later that the AZDN powder probably mixed with unintended spills of SPS and other oxidising products. which were stored at height in the No. As the pressure relief on the LP separator had been designed for a fire relief case.Allied Colloids. Bradford. Relief Systems / Vent Systems: sizing of vents. Before the employee could douse the SPS with water the vapour plume ignited and became a jet flame of about 300 mm in length. The operators did not trust the main level control reading and referred to a chart recorder for a back up level reading.1 and No. 2 oxystore had steam heating as it was originally designed to store frost sensitive products. Failures in technical measures:            Operators did not trust the reading given by the float gauge in the HP separator and the nucleonic level indication was misread due to an offset on the chart recorder. Isolation: emergency isolation 4. Within a few seconds the jet flame became a flash fire which was transmitted all around the room. 2 oxystores. The heating effect caused two or three of the AZDN kegs to rupture and spill white powder all over the floor. There was an offset on this chart recorder which led them to assume that the level in the HP separator was normal.The investigation of the accident suggested that an air operated control valve on the high pressure (HP) separator had been opened and closed on manual control at least three times.2 . possibly from a lid and associated metal ring closure from one of the damaged AZDN kegs falling . Liquid level in the LP separator fell and the valve was opened. not gas breakthrough the vessel subsequently exploded. A passing employee thought that the powder was smoke and raised the alarm. While waiting for confirmation from the appropriate vacuum cleaner manufacturer an employee noticed a plume of smoke/vapour and a hissing noise coming from a bag of SPS that was located underneath the AZDN kegs. not a gas breakthrough from the HP separator. It was determined that no immediate hazard was present and the AZDN data sheet was referred to before a clean up plan was devised. No. Maintenance Procedures: fault reporting systems Control Systems: sensors Operating Procedures: provision of comprehensive written operating procedures The low low level trip on the HP separator had been disconnected several years earlier when a plant modification had been made. Emergency Response / Spill Control: Fire fighting Secondary Containment: bunds. This is used in forms of machinery. barriers Question 1. Stabilising to safe condition. heat or other ignition source. Considerable environmental damage to the Aire and Calder rivers resulted from the firewater run off. Active / Passive Fire Protection The fire brigade and police should have been informed immediately the first incident had been discovered. and in thermobaric weapons. True/False (HP) Answer 1: True Response 1: Jump 1: Next page Answer 2: False Response 2: Jump 2: This page Question 2. fire fighting Significant environmental damage was caused to the Aire and Calder rivers by the firewater run-off. Plant Modification / Change Procedures: Decommissioning procedures The oxystores and warehouse were not fitted with adequate smoke detection and fire fighting facilities. spark. Incompatible substances Raw Materials Control / Sampling: Safety management systems Failure of the steam heating system or operator error meant that heating was applied in No. Emergency Response / Spill Control: Site emergency plan. Multiple Choice (HP) Answer 1: Complete combustion . Failings in technical measures:             AZDN kegs were stored in the same section of the warehouse as SPS and other oxidising substances. The fire was contained that day and the fire brigade was not stood down until 18 days later due to risk of re-ignition during clean up.onto a bag or the floor. The fire spread throughout the warehouse and smoke was blown towards nearby motorways. catchpots. such as internal combustion engines. 2 oxystore as well as in the main warehouse. after being wrongly classified in the documentation. Spontaneous combustion can begin without any flame. This often occurs as a fire. As it was there was a 50 minute delay before the fire occurred and the emergency services informed. Segregation of Hazardous Materials: Warehouse storage. _____ is a form of combustion in which large amounts of heat and light energy are released. Response 1: Jump 1: This page Answer 2: Slower combustion Response 2: Jump 2: This page Answer 3: Rapid combustion Response 3: Jump 3: Next page Answer 4: Incomplete combustion Response 4: Jump 4: This page .