GP 44-80 Design Guidelines for Relief Disposal Systems

Document No.GP 44-80 Applicability Group Date 31 March 2006 Guidance on Practice for Relief Disposal Systems GP 44-80 BP GROUP ENGINEERING TECHNICAL PRACTICES 31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems Foreword This is the first issue of Engineering Technical Practice (ETP) BP GP 44-80. This Guidance on Practice (GP) is based on parts of heritage documents from the merged BP companies as follows: BP RPSE RP 44-3 Design Guidelines for Relief Disposal Systems. Amoco A PC-PRD-00-E Process Control-Pressure-Relief Devices-Device Selection and Systems Design Specification. A PC-PRD-00-G Process Control-Pressure-Relief Devices Guide. ARCO Engineering Standards Std 350 Spring Opposed Pressure Relief Valves. Std 356 Low Pressure Tank Protective Devices. Copyright Copyright  2002,  2006, BP Group. BPAll rights reserved. Group. The information All rights reserved.contained The in this document is subject to the terms and conditions of the agreement or contract under which information the document wascontained inrecipient’s supplied to the this document is subject organization. toinformation None of the the terms and contained in this conditions ofdocument shall be disclosed the agreement outside the or contract recipient’s under which ownthe organization document was supplied to the recipient’s organization. None of without the prior written permission of Director of Engineering, BP Group, unless the terms of such agreement or contract expressly allow. the information contained in this document shall be disclosed outside the recipient’s own organization without the prior written permission of Manager, Standards, BP Group, unless the terms of such agreement or contract expressly allow. Page 2 of 62 31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems Table of Contents Page Foreword ........................................................................................................................................ 2 Introduction ..................................................................................................................................... 5 1. Scope .................................................................................................................................... 6 2. Normative references............................................................................................................. 6 3. Terms and definitions............................................................................................................. 7 3.1. General ....................................................................................................................... 7 3.2. Terms ......................................................................................................................... 7 3.3. Definitions ................................................................................................................... 8 4. Symbols and abbreviations .................................................................................................... 9 5. Choice of disposal systems.................................................................................................... 9 6. Atmospheric discharge ........................................................................................................ 10 6.1. Scope ....................................................................................................................... 10 6.2. General ..................................................................................................................... 11 6.3. Non-hazardous discharge ......................................................................................... 12 6.4. Flammable and toxic discharge in atmospheric vents ............................................... 12 6.5. Blowdown drums discharging to atmosphere ............................................................ 18 7. Closed systems ................................................................................................................... 18 7.1. General ..................................................................................................................... 18 7.2. Pipe & header sizing and layout ................................................................................ 19 7.3. Special relief arrangements ...................................................................................... 21 7.4. Winterisation ............................................................................................................. 22 8. Flare system design ............................................................................................................. 22 8.1. General ..................................................................................................................... 22 8.2. Component parts of the systems............................................................................... 22 8.3. Design considerations ............................................................................................... 23 8.4. Engineering diagrams ............................................................................................... 25 8.5. Flare types ................................................................................................................ 25 8.6. Smokeless flaring...................................................................................................... 26 8.7. Sizing of relief and flare systems............................................................................... 27 8.8. Siting......................................................................................................................... 28 8.9. Elevated flares .......................................................................................................... 33 8.10. Enclosed ground flares ............................................................................................. 33 8.11. Minimum heat content of flare gas ............................................................................ 33 8.12. Ignition systems ........................................................................................................ 33 8.13. Flashback prevention ................................................................................................ 34 8.14. Noise levels .............................................................................................................. 36 8.15. Flare sparing philosophy ........................................................................................... 36 Page 3 of 62 ........................................................................ Buoyancy seals (molecular seals) ....................... 47 12........ Purge control ............................31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 9..................... 51 15... 52 16. Control and instrumentation ....................................................... 46 12............. 47 12.............................................................. 39 10..................... 50 14...............................................................4................... Primary knockout drum (Onshore) ............................................1.........................................................................................5....................................................... 62 Page 4 of 62 ........................... Cold service .....................................................................................................3......................... 50 15..................... 45 11.................................1. Testing ..............................................................................................................5.............. 42 10.............................................................. Gas purge ...........................................1........4........... 53 16................................................................................................................... 39 10.......... 51 15.......................................4....................3..................... 48 12............... 59 Annex C-2 (Normative) Blowdown system assessment .................................. Liquid removal (Offshore)................ 39 9............................ Husa’s correction formulae ........................................................................................................ Oxygen monitoring ....................................2............ 46 12...............................................................................................2............................... Design and construction.......................................................................... 49 13......... 36 9....................... Flame arresters ..........................................................2...................... 44 10.........1..................................................................................................2..... General .................................. Routing ..... 37 9.......... Unit knockout drum (Onshore) ... 44 11...... 61 Bibliography ...................... 48 12....... 45 12................................................................................................................................................................. 43 10...................2....................................................................................................................................... Flare smoke control ................................... Flare and relief line headers and piping .................3............... Closed vent system..........1.............................. 54 Annex A (Normative) H.... Design criteria ....................................................................................................................................................... Liquid removal ............................................................... Blowdown system liquid handling .......1..........................................................................................................................................................................................................................6.. Flare purging and sealing .. Liquid seals ................................................................................................... Requirements for instrumentation ........................................... Spares . Vent systems ..................................... Blowdown system ......... 53 17... 39 10......................... Burn-back .................. 53 16................................................ 60 Annex C-3 (Normative) Relief system studies and documentation ..... 58 Annex C-1 (Normative) Atmospheric relief chart ...........................2.............................................. Flow measurement ...... Flare gas recovery systems ....................................................W.............................. 36 9............................................................ 56 Annex B (Informative) Flare system training information ................................................................................... Efflux velocity accelerators (Velocity seals) ........... 45 11............................................ specific relief disposal systems. statutory and local regulations must be complied with. users are urged to inform BP of their experiences in all aspects of its application. This GP refers to national and international standards that are widely accepted.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems Introduction This Guidance on Practice (GP) provides guidance on relief disposal systems that are within its stated scope and is for use in determining the need for. In any case. Codes and standards of the country in which the equipment is manufactured and/or operated should be considered and may be accepted if they can be used to achieve an equivalent safe technical result. Page 5 of 62 . For this reason. The value of this GP to its users is significantly enhanced by their regular participation in its improvement and updating. and design of. Crude oil and gas gathering centres f. Offshore installations e. and Installation of Pressure-Relieving Devices in Refineries). BP gHSSEr BP Getting Health. any of these publications do not apply. This GP should also be used in conjunction with sound engineering judgement and with full consideration of country. Selection. a piping system with associated control valves or manual valves and a discharge system. This system terminates in one or more disposal systems such as a flare. state. Page 6 of 62 . above ground. or sub-sea g.e. Terminals d. 2. Pipelines: buried. Security and Environment Right. which specifies relief device requirements and the calculation of relief loads. However.. Storage installations h. Steam generating plant and ancillary equipment It shall be used in conjunction with GP 44-70. Floating production systems i. no treatment (i. through reference in this text. and local rules and regulations. Well Pads and Production Facilities j. In addition. In principle a relief disposal system includes individual pressure relief devices. a scrubber or absorber in which a component(s) of the relief stream is removed before venting. constitute requirements of this technical practice. Normative references The following normative documents contain requirements that. or blowdown systems in which non-volatile liquids are removed. The mechanical design of flare tips and stacks is covered in GP 22-20.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 1. or revisions of. Scope This GP specifies general requirements for designing relief disposal systems based on the engineering principles set out in ISO 23251 or API RP 521 (Guide for Pressure-Relieving and Depressuring Systems) and ISO 4126 or API RP 520 (Sizing. GP 22-20 Guidance on Practices for Design of Flares (API 537). vented directly to the atmosphere). parties to agreements based on this technical practice are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. in which the fluids are combusted. subsequent amendments to. GP 14-01 Guidance on Practice for Noise Control. the latest edition of the normative document referred to applies. This GP provides a level of safety acceptable to BP in the design and operation of the following installations: a. Chemical plants c. For undated references. For dated references. Safety. Refineries b. Blowdown drums are prohibited in services which handle heavier-than-air hydrocarbon vapour or light hydrocarbon liquids (gasoline and lighter). relief disposal system shall be designed in compliance with the requirements of the insurance covering the plant or installation. Petrochemical and Natural Gas Industries – Pressure- Relieving and Depressuring Systems. American Petroleum Institute (API) API RP 520 Recommended Practice for Sizing. GP 30-81 Guidance on Practice for SIS – Operations and Maintenance. GP 24-03 Guidance on Practice for Inherently Safer Design Concept Selection. is used if BP does not wish a design to proceed unless certain features have been agreed in writing with a contractor or supplier. API Std 537 Flare Details for General Refinery and Petrochemical Service.2. GP 44-70 Guidance on Practice for Overpressure Protection Systems. GP 44-60 Guidance on Practice for API RP 500 Area Classification. In this GP the term ‘approve’. General a. ‘shall’ and ‘must’. Part 4: Pilot operated safety devices. have specific meanings. GP 31-01 Guidance on Practice for Analyser Systems. For the purposes of this GP. Part 6: Application. when used in the context of actions by BP or others. GP 30-76 Guidance on Practice for Safety Instrumented Systems (SIS) – Development of the Process Requirements Specification. as applied to BP. Selection.Hazard & Operability Studies- Training. This does not imply that all details of a document have been considered by BP and does not affect the design responsibilities of the contractor or supplier. Terms “Will” . ‘may’. GP 30-80 Guidance on Practice for SIS – Implementation of Process Requirements. ISO 23251 Petroleum. Throughout this document. GP 48-02 Guidance on Practice for HAZOP . the words ‘will’.1. Part 7: Common data.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems GIS 22-201 Guidance on Industry Standard for Procurement of Flares to API 537.used normally in connection with an action by BP. Terms and definitions 3. selection and installation of bursting disc safety devices. API RP 521 Recommended Practice for Pressure-Relieving and Depressuring Systems. Part 5: Controlled safety pressure relief systems. ‘should’. API Std 526 Flanged Steel Pressure Relief Valves. GP 44-65 Guidance on Practice for IP 15 Area Classification. the following terms and definitions apply: 3. GP 76-01 Guidance on Practice for HSSE in Design and Loss Prevention. GP 44-10 Guidance on Practice for Plant Layout. rather than by a contractor or supplier. GP 44-30 Guidance on Practice for Event Modelling and Risk Based Evaluation. Page 7 of 62 . 3. b. and Installation of Pressure-Relieving Devices in Refineries. Part 2: Bursting disc safety devices. International Organisation for Standardisation (ISO) ISO 4126 Safety devices for protection against excessive pressure Part 1: Safety valves. Flare vendor A company that undertakes the design. The following additional technical definitions also apply: Coanda flare A flare burner designed to employ the aerodynamic effect in which moving fluids follow a curved or inclined surface over which they flow. guyed. Combustion support The addition of fuel gas to the effluent to be flared for any of the following reasons: a. and sometimes the erection of a flare. Flares of this type generally use steam or pressure to achieve smokeless operation. but may also refer to a ground multi-burner flare or a burn pit. c. igniters. To maintain an adequate slot velocity in a flare tip using the Coanda effect. To prevent air infiltration.used if alternatives are equally acceptable. b. e. Flare tip The part of flare in which fuel and air are mixed at velocities. To increase fuel concentration in order to make the effluent flammable. and concentration required to establish and maintain proper ignition and stable combustion. “Shall” . To avoid burn-back in the flare tip or flame lick outside the flare tip or a lazy flame situation which could damage an adjacent flare tip.is used if a provision is mandatory. containing a flare tip. To increase air turbulence and allow the flare to burn smokeless during periods of flare relief.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems “May” . and miscellaneous auxiliaries. also known as “assist gas”. Flare A general term used to designate a device or system used to safely dispose of relief gas in an environmentally compliant manner through the use of combustion. To increase the volume of the effluent in order to increase flare tip velocity. pilot burners. Ground flare Any non-elevated flare. 3. “Must” . or structure-supported). turbulence. service pipes. terminating in one or more flares. or manually operated valves. supply.is used if a provision is preferred. control valves. other pressure relief devices.3.is used only when a provision is a statutory requirement. Definitions The technical terms used in this GP have the meanings as defined in ISO 23251 or API RP 521. Elevated flare An elevated stack. d. f. Flare system The whole closed disposal system for fluids discharged from pressure relief valves. (self-supported. smoke-suppressing devices. It is also referred as flare burner. Page 8 of 62 . “Should” . It is normally an enclosed flare. Smokeless Without emitting ‘dark smoke’ as defined in the UK Clean Air Act 1956 Section 34(2) or as defined in EPA CFR 60. Purge rate The rate of flow of an inert or combustible gas required to prevent the oxygen concentration exceeding a specified level at a specified location in the flare stack or supply ducting. Molecular seals A gravity seal using the difference in molecular weight between air and the purge gas being used. but only for initial installation). this can be either higher or lower than the emergency flare load. Depending on the design philosophy. Choice of disposal systems The selection of a disposal method is subject to many factors that may be specific to a particular location or an individual unit. ISO 23251 or API RP 521 outline the Page 9 of 62 . Self-erecting flare Flare that is dismountable and can be erected without the use of cranes (may need cranes. and main process activities. the following symbols and abbreviations apply: CONCAWE Conservation of Clean Air and Water . Symbols and abbreviations For the purpose of this GP. when oxygen ingress is undesirable.Europe LNG Liquefied natural gas LPG Liquefied petroleum gas 5.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems Maximum flaring rate The maximum rate of flow to the flare calculated in accordance with the specified blowdown and relief philosophy for the plant. Operational flaring load Flare load results from process venting. normal safety valve leakage. Safe location A safe location is one that would not cause significant deleterious impact or harm to personnel. Emergency depressuring load Load resulting from blowdown of the facility to zero pressure inside the piping and process vessels. environment or property. pressure relief devices (relief from upset and/or emergency blowdown system is activated). Emergency flaring load Load results from emergency flare. Flare tips using molecular seals are susceptible to burn back inside the flare tip. 4. Dispersion analyses and/or consequence analyses may be required to validate the suitability of the discharge location.18 (c)(1) which basically does not allow the emission of ‘visible’ smoke during flaring. Atmosphere. and temperature. See GP 76-01. depending on the fluid properties.1. 3.g. These analyses determine the suitability for atmospheric relief and. offshore. Any deviation from the atmospheric venting requirements of this GP or any new atmospheric blowdown stack installation requires approval from the BP Group Director of Engineering.3 of GP 44-70. the choice of pressure relief discharge location shall generally be in the following order of preference. the location and elevation at the point of discharge to assure that allowable thresholds for thermal radiation. 2. Atmospheric venting for flammable and/or toxic gases in BP shall be eliminated or severely curtailed when practicable. In remote or offshore locations where there can be fewer potential sources of ignition. c. an analysis of vapour cloud dispersion. in accordance with clause 7. at the earliest stage of process design in order to implement the most cost effective solutions and to minimise effects on the environment. Atmospheric discharge 6. Environmental considerations of releases need to be discussed thoroughly with the appropriate Regulatory Authorities. Closed system. subject to the requirements of clause 7. Direct atmospheric releases shall be made within the limitations of environmental regulations and corporate gHSSEr guidelines. blast overpressure. The safe disposal approach requires an analysis for possible consequences including thermal radiation levels from atmospheric vents which could ignite. When Page 10 of 62 . depending upon the release. Scope Atmospheric discharge is the release of vapours and gases from pressure-relieving and depressuring devices to the atmosphere. subject to the detailed limitations of this GP: 1. molecular weight. In general. d. subject to the requirements of clauses. dependability and generally lower capital cost. a. including any toxic products either present or formed from ignition of the vent discharge. if feasible. including but not limited to vapour cloud explosions and flash fires. Normal venting of flammable and toxic materials arising from controlled process variations and sustained discharges for plant operability shall usually be taken to a closed system. 6. and toxicity/flammability are not exceeded. If permitted by local statutory regulations. Other parts of the process plant or system. Atmospheric discharge. Constraints on atmospheric relief may be imposed by compact installations. a. The system design approach shall include provisions for the disposal of warm or cold fluids to a closed collection system with associated vapours or directly vented gases passing to a vapour disposal system (such as a flare) or being directly vented to atmosphere. offers advantages over alternative methods of disposal because of its inherent simplicity. e. if permitted. for relief of other than non-hazardous fluids. the magnitude and frequency of relief discharge should be reduced by using pressure-limiting instrumentation. without compromising personnel safety or equipment/ plant integrity. b. b. such flammable and toxic discharges may be to atmosphere provided dispersion analysis and consequence analyses do not indicate significant impact to personnel safety or equipment / plant integrity and subject to BP approval. and an analysis of other possible consequences that may occur.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems general principles and design approach for determining the most suitable type of disposal system. To adopt a conservative interpretation of API recommended practices involves limited cost where safety relief devices discharge to atmosphere. and is acceptable to the local authorities. The general criteria for this were 1-2% H2S content. Proposed venting of anything other than steam. However. provided such a reduction does not increase the Page 11 of 62 . the outlet end of the tail pipe may be reduced in diameter to improve dispersion. each individual discharge line should have at least the same bore as the outlet from the pressure relief device. c.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems performing these analyses. General a. Noise limits shall be maintained in normally manned areas to meet the Special Limits given in GP 14-01. prevailing meteorological conditions shall be considered to ensure personnel safety. If pressure relief devices discharge to atmosphere. c. In the past. If atmospheric relief discharge is permitted within this GP. Under normal operation there shall be no continuous venting of raw (unburned) hydrocarbons or toxic vapours to the atmosphere. b. provided it was below 1-2% H2S content. shall be specified or approved by BP for each project. no additional closed system need be provided for such discharges. General discussion of atmospheric discharge from pressure relief devices in processing installations is given in GP 44-10. below a molecular weight of 72. air. BP philosophy was modified accordingly. h. the flare system being sized on total discharge from the 'largest relieving unit'. If required by a regulating authority. d. f. This led to development of the principle that emergency hydrocarbon relief streams could be taken to atmosphere wherever this could be regarded as 'safe'. meets BP corporate guidelines. condensability (associated with a generally-accepted increased molecular weight of 100) and restrictions on location and velocity of discharge. corrosive and/or otherwise hazardous relief streams venting directly to atmosphere.2. Dispersion analyses and consequence analyses shall be performed for all flammable. The definitions and calculation methods used to justify this general criterion. or nitrogen shall be evaluated on a case-by-case basis with due consideration of regulatory and BP corporate guidelines. the increased use of air-cooling and much larger process units. 6. toxic exposure. flammable and toxic discharges shall comply with the requirements of clause 6. which may dictate modified distances. If acceptable dispersion cannot be attained with this tail pipe bore. or an explosive hazard. Other relief streams were taken to flare. g. e. This restriction does not apply to analyser sample streams (GP 31-01) with very small venting rates and purge gas systems required to prevent air ingress and designed to keep atmospheric vent systems safe. toxic. Atmospheric relief shall present no unacceptable secondary hazard such as increased risk of fire. Noise levels from operational vent systems and relief devices that discharge directly to atmosphere shall be analyzed to assure compliance with allowable levels. an integrity assessment analysis study shall be made or approved by BP to assess the estimated frequency and duration of atmospheric emergency relief streams covered by clause 7. allowed per applicable national codes or standards. With development of the 'integrated facility design’ approach. but considerable cost (both capital and operating) where relief streams discharge to flare. These general criteria have since been replaced by the requirement for a rigorous dispersion analysis and consequence analysis using BP approved methods. and permitted by statutory regulations. Credit for the use of automatic pressure-limiting instrumentation should be taken.4. emergency pressure relief to atmosphere was accepted by BP. (See GP 14-01).6 of GP 44-70. as justified by the study. where not covered by this GP. Non-hazardous discharge a.h above. Flammable and toxic discharge in atmospheric vents If atmospheric discharge from relief devices has been permitted and approved by BP. 3. 2. the following criteria shall be met for flammable and/or toxic relief streams as defined by BP: 1. b) The use of pilot-assisted relief valves may be necessary (in services in which these types of valves can be used) to achieve adequate jet velocities. any flammable or toxic atmospheric discharges from pressure relief devices shall comply with GP 44-70 and guidelines in this clause. 6. The discharge velocity shall be sufficient to reduce the concentration of flammable material at a suitable distance downstream of the point of discharge to below the lower flammable limit.4. The decision to intermittently discharge flammable and toxic materials to the atmosphere requires careful attention to ensure that disposal can be accomplished without creating a potential hazard or causing other problems.4. Note that analyses may be required to validate the suitability of the discharge location for gases that may pose asphyxiation or thermal burn hazards. ISO 23251 or API RP 521 covers in greater detail these associated potential hazards or problems. Safety relief devices venting steam. as approved by BP. exposure of personnel to toxic materials. and air pollution.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems relief device back pressure to where the pressure relief device does not operate stably or where the relief rate is reduced below that calculated or required by the process.5. process and breathing air intakes. instrument/plant air. every pressure relief device should have its own discharge line where practical. General a. The duration of atmospheric relief discharge should be limited by the use of pressure- limiting instrumentation. in addition. 6. To maximise dilution in atmospheric discharge. buildings. Dispersion analysis and consequence analyses shall be performed to confirm that the discharge does not represent a significant impact to personnel safety. but not such a high outlet velocity that a build-up of static electricity could occur. b. excessive noise levels. 6. However.4. ignition of relief streams at the point of emission. a) The additional effect of wind-assisted dispersion between the jet and any source of ignition may be taken into consideration subject to BP approval. the discharge line to atmosphere shall have a 10 mm (3/8 in) diameter drain hole at its lowest point.1.4.1. c. maintenance. Air or steam used during start-up operations. reseating relief valves. environment or to equipment/ plant integrity per clause 6.d below. such as the formation of flammable mixtures at grade level or at elevated structures. or other non- flammable and non-toxic gases shall be discharged to the atmosphere through a tail pipe terminating at a suitable and safe location. c) The distance downstream is set by plant layout and environmental considerations. When the discharge is not flammable or toxic. See clause 6. or operator intervention. or decoking and regeneration purposes may be discharged to the atmosphere and not to a closed disposal system. Page 12 of 62 . The discharge pipe shall be adequately supported and sized for rated capacity of the relief device per clause 6.2.3. b. nitrogen vapours. a) Criteria for assessing condensability require specific calculations for each case. If multiple pressure relief devices are fitted on a system. the set pressures should be staggered to assist in maintaining a high discharge velocity and to minimise chatter in the case of relief valves. equipment or structure within a horizontal radius of 30 m (100 ft) measured from the point of discharge. While not normally required. d. parallel relief devices. 1. Therefore. There shall be no direct flame impingement or unacceptable radiation levels at operating positions (i. c) For higher molecular weights a more detailed analysis is required. involving the examination of vapour cooling rates and dew point conditions at the specified minimum ambient temperature for the site. flammable vapours should discharge at a point not less than 30 m (100 ft) measured in a straight line from any permanent source of ignition. 2. (Typical assumption is that flammable releases can ignite). a hydrocarbon vapour of average molecular weight 100 or less should not generally condense under typical discharge conditions unless the minimum expected ambient temperature allows condensation to occur. Vapour from LPG or other low boiling point material storage vessels remote from a process area shall discharge to atmosphere not less than 3 m (10 ft) above the pressure relief devices and not less than 3 m (10 ft) above any platform within a 15 m (50 ft) radius. pressure relief devices on flammable services shall discharge at a point not less than 3 m (10 ft) above any platform. The possibility of relief discharge ignition coincident with the presence of an operator in the vicinity shall be considered and. but may need to have their distance increased pending national codes and standards or HSSE guidelines for exposure limits. as appropriate.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 4. consideration should be given to performing a quantified risk analysis to assess the likelihood of relief discharge ignition. There shall be no significant condensation of flammable or toxic material released from the discharge pipe at the minimum expected ambient temperature. if necessary. Flammable and toxic vapours shall discharge at a level not less than 25 m (82 ft) above grade or any main operating floor. However the European Pressure Equipment Directive (PED) does not allow any pressure relief valve to have a set pressure above the vessel design pressure unless the pressure relief valve is tested at that condition. 5.e. e. The calculation methods used to justify these general criteria shall be subject to approval by BP. f. Relief devices in toxic service are subject to these same minimum distances. 4. as measured from the discharge of a low boiling point pressure relief device. in the event a flammable. c. If equipment is protected by multiple. 3. There shall also be no permanent source of ignition within the radii specified in GP 44-60 or GP 44-65. Page 13 of 62 . See clause 8. specific means for operator protection or escape shall be provided as approved by BP. b) As a preliminary guide.4 for thermal exposure limits. if ignition would create a higher risk level. The following are minimum spacing requirements for atmospheric venting and may need to be increased for adequate dispersion. In process installation. most pressure vessel design codes require the maximum set pressure to be not more than 5% above design pressure for the second and later devices. atmospheric release would ignite). In process installations.8. it may be necessary to provide an additional margin between operating and design pressures to permit adequate staggering of set pressures. but should also consider the potential size of the release and radius of impact. and (c) the possible presence of an operator at a point close enough to be significantly affected. and shall be considered only if there is no practical alternative and if approved by BP. extinguishing connections shall be inert gas instead of steam. Chlorofluorocarbons (CFCs) and Halons are now generally accepted as being significant man-made contributors to the depletion of the ozone layer. If the relief system is capable of creating significant system backpressure. 90/2. k. Page 14 of 62 . failure or opening can occur if the outlet pressure exceeds the inlet pressure thereby allowing air to enter the equipment. and should be consulted if a new use for a Halon is being considered. 1. Some pilot operated pressure relief valves may open if the outlet pressure exceeds the inlet pressure. only for use in extinguishing any residual burning. 2. This shall be by hand control from grade level using double block-and-bleed valves. shall be provided in the vent line. or be located to discharge away from any operating platform. effective prevention/mitigation measures shall be provided. for example. as required. higher discharge rates than normally expected may flow through this drain. left permanently open. connected to the vent after the relief device. This is approximately one-tenth the lowest flammable limit concentration for many hydrocarbons. (b) the possible ignition of this discharge. may well be considered to be remote. g. steam or inert gas connections may be provided for atmospheric relief streams at ambient temperatures or above. j. If the discharge is flammable or toxic. hydrogen sulphide vapours can cause unconsciousness and fatality within seconds following exposure to a concentration above 1 000 ppm. the discharge line to atmosphere shall have a 10 mm (3/8 in) drain at its lowest point. the potential for formation of a flammable atmosphere inside the equipment shall be evaluated and. Extra attention is required when relief gas may contain vapours that are dangerous at extremely low concentrations. Backflow preventers can be installed to minimize the potential for opening. reasonable steps shall be taken to minimise the release of the vaporising liquid to atmosphere. For some pressure relief devices. When specified by BP. As required. The vent line drain hole shall be fitted with a short line to a safe location. In this eventuality. The implications of this philosophy for the BP Group are contained in Safety Guidance Note No.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems The combination of (a) a process upset causing a flammable discharge. Selection of the pressure relief device shall consider the potential effects (if possible) for vacuum formation in the protected equipment. The use of Halons (bromo. but may be ineffective at high pressure differentials. h. the drain shall be piped to a safe location and may contain a locked-open isolation valve in an easily accessible location. This document outlines the approach that should be taken when selecting an extinguishant for various applications. A drain hole. Double block and bleed valves are used to prevent undetected steam or inert gas leakage to atmosphere. 3. If air entry is possible. Such locations shall be subject to approval by BP. i.or chlorofluorocarbons) and other vaporising liquids shall be avoided if possible. 1. 2. When installed for discharge temperatures below 0°C (32°F). Some rupture disks require vacuum supports to prevent disk damage or failure when the outlet pressure exceeds the inlet pressure. if more expensive. or shutdown operations. See ISO 23251 or API RP 521 for additional guidance. In general.4. If these vessels are provided with pressure relief devices that discharge directly to atmosphere. Liquid overfill potential for vessels with pressure relief devices discharging directly to atmosphere a.3. If the vent gas exit velocity is too low. Mist emission Guidance in this clause should only be used for preliminary estimates and does not replace the requirement for a dispersion analysis and consequence analysis discussed in clause 6. Mists can result from condensation occurring after a vapour only release. 6. Many process vessels have a liquid level present during normal. and/or toxic liquid may be discharged. See ISO 23251 or API RP 521. b. then overfilling of the vessel with liquid shall be considered to be a credible event. the flammability envelope for potential mists that may be formed can be taken as the same as the stack vapour emission envelope.54  10 4    (from ISO 23251 or API RP 521)  a  Where Re = Reynolds number calculated at the vent outlet ρg = Density of the gas at vent outlet ρa = Density of the air b. 3. start-up. Other criteria for atmospheric relief specified elsewhere in this GP must also be met. Relief valves designed to relieve vessel vapours shall have the valve inlet connected to the vessel vapour space.4.5. 2. Note that in cases in which fire is the only credible scenario involving vapour relief and if the equipment is normally liquid-full. Vapour emission Guidance in this clause should only be used for preliminary estimates and does not replace the mandatory requirement for dispersion and consequence analyses in clause 6. Released vapours or gases can be diluted below the lower flammable limit by entraining air if the Reynolds number at the vent outlet meets or exceeds the following criterion:  g   Re  1. the relief device should be Page 15 of 62 . 1. Relief valves do not always relieve at full capacity and reduced flow rates with lower dispersion factors shall be considered in the required dispersion studies.4.4.4.4. 6. c. 6. Unlike vapour or mist discharges. air entrainment is limited and the released vapours will be wind dominated. hazardous. the use of an inert gas or steam is a practical. a. potentially forming flammable or explosive mixtures some distance from the emission source. provided the Reynolds number in clause 6. Appropriate design interventions shall be implemented to ensure liquid is not released from the vessel. a discharge of liquid can settle out to grade creating a hazardous environment. b.4. a. for further details and proposed methods of dealing with potential mists.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems In many applications. alternative that should be thoroughly evaluated.2.5.2 above is met or exceeded. The following minimum safeguards listed in this clause against overfilling the vessel and potentially relieving liquid to the atmosphere are required and shall apply to services in which a flammable. Level instruments used for safeguards against overfilling shall use separate process taps.. A Hazard Evaluation shall be done using the techniques described in GP 30-76 to determine if additional protective measures should be implemented.) and shall be proven in use for the specific process applications. the minimum level safeguards are: 1. If the time between when a critical high level alarm occurs and when the vessel becomes liquid full is between 15 and 30 minutes. At least two independent and diverse liquid level measurements shall be provided. Level instruments used for safeguards against overfilling shall use diverse technologies (e. An additional independent critical high level alarm shall be provided. Page 16 of 62 . All safeguards listed above (c) for over 30 minute reaction times and the addition of a Safety Integrity Level rated Safety Instrumented System that activates on High-High level to prevent further accumulation of liquid or shut down the equipment. If the time between when a critical high level alarm occurs and when the vessel becomes liquid full is less than 15 minutes the minimum required safeguards are either: 1. but criteria for their use as surrogate level indicating devices need to be developed.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems installed at the high-point of the equipment or at a location that would promote vapour- liquid disengagement. Tower differential pressure devices may be acceptable. 3. Operations actions to prevent further accumulation of liquid level in response to the critical high level alarm shall be clearly defined in operating manuals and training materials and shall be reviewed with operating crews at regular intervals. or 2. indicated level is 100% or higher). d. f. In cases where a closed relief disposal system is not considered adequate for a liquid overfill case. Measured level ranges for systems with atmospheric relief shall be sufficient to indicate actual level during all operations including start-up and shutdown. If the time between when a critical high level alarm occurs and when the vessel becomes liquid full is 30 minutes or more. The required integrity level shall be determined from a Hazard Evaluation using the techniques described in GP 30-76. 2. c.g. No credit for Operator response to alarms shall be taken in this evaluation. 2. the instrumentation criteria above (clauses c-e) covering overfill protection shall be applied. displacer and float. etc. displacer and differential pressure. h. e. but shall discharge to a safe closed system or back to a safe location within the process. The operator actions shall be simple and effective in reducing levels. g. All safeguards listed in (c) above for over 30 minute reaction times. differential pressure and radar. (e. The required integrity level shall be determined from a Hazard Evaluation using the techniques described in GP 30-76. Equipment shall not continue to be operated if the level indications and alarms are not functional or if the indicated level measurements are above the high range of the measurements. Addition of a Safety Integrity Level (SIL) rated Safety Instrumented System (SIS) that will activate on High-High level to prevent further accumulation of liquid or shut down the equipment.g. 2. based upon normal liquid inflow or production rates with no liquid removal. the minimum level safeguards are: 1. It should be noted that this system may have to be SIL 3. 4. The following criteria shall be met when installing the level measurements and alarms mentioned above: 1. The pressure relief valve shall not discharge to atmosphere. 5. buildings. process and building air intakes. Also. elevated platforms. Industrial Hygiene specialists should review potential exposures to benzene and other carcinogens.) while still flammable or toxic. unless special fluid properties or conditions apply. This means that the lowest expected relief rate into the system from any single device should be considered. EPA SCREEN grid or local equivalent). modelling shall cover the rated relief capacity of device and a representative low-flow case. note that lower venting rates than the design basis (design basis is usually the highest rate) shall be considered. In most cases PHAST or CIRRUS should be satisfactory. 1. process and breathing air intakes. 6. As a minimum. and model a range of cases. b. Potential ignition radiation impacts on personnel and equipment shall be considered. Lower velocities from the stack tend to reduce the dispersion and can increase the potential for the cloud to reach grade or other potentially vulnerable locations (buildings. The model should be based on a matrix of all relief cases.S. For vents and blowdown systems to which more than one device discharges.4. e. ERPG2 or other threshold as required by local regulation) at ground level and working platforms. This behaviour shall also be included in operator training and operating procedures. either connecting relief devices to closed systems or risk reduction measures such as installation of high-integrity protective systems (HIPS) per GP 30-76 shall be considered. and shutdown operations shall be considered in setting alarms and trip points. The following criteria shall be followed: a. A range of atmospheric conditions should be considered (such as using the U. acceptable concentrations shall be 50% of the lower flammability limit (LFL) at ground level. f. The range of at least one of the two primary level measurements shall be such that it indicates a valid level reading at the high critical alarm point and any shutdown or interlock points. and at potential sources of ignition. Acceptable concentrations shall also be below the short term exposure limit (STEL. Operating characteristics of the level measurement during off-design.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 3. If dispersion and consequence analyses indicate that flammable or toxic thresholds may be exceeded. c. 2. such as 25% of rated capacity or failure of the device at normal operating pressure. For example a differential pressure or displacer level measurement reads low compared to actual level if the fluid specific gravity is less than the design gravity. d. the lowest density material expected to be in the vessel shall by used for instrument calibration of overfill protective devices. Toxic impacts outside the plant and process boundary may also need to be considered. but an additional transmitter is preferred. Page 17 of 62 . dispersion modelling and hazard consequence analysis shall be conducted for atmospheric relief devices and blowdown systems in flammable and/or toxic venting services. depending on which are the most sensitive. etc. start-up. This can mean that the indicated level cannot reach 100% even if the actual level is well above the measured range. 5. 3. working platforms. g. For determining consequences. Float type level switches may be used for high critical alarms. Dispersion and consequence modelling Other than for thermal relief (intended to intermittently relieve a very small volume of fluid (not sustained relief)). dependent on plant location and circumstances. 4. A model appropriate to the situation shall be chosen. Horizontal vent or exhaust pipes (often cut at a 45 degree angle) may be used on BP tank and sphere installations. dump tanks. f. Consideration shall be given to nests. The minimum expected vacuum 2. to avoid any violent vaporisation or the possible formation of solids. etc. Blowdown drums discharging to atmosphere See clause 15. Relief streams that do not satisfy the requirements for atmospheric relief as given in clause 6. b. subject to BP approval. e. Partial vacuum and equipped with a vacuum relief device but without a gas repressuring line. in remote or offshore locations where there are fewer potential sources of ignition. c. Closed relief systems are most frequently flare systems incorporating knockout drums if necessary. the use of either an inert or fuel gas pressurization line with a vapour recovery system is preferred to allowing air ingress into or venting from a tank in hydrocarbon service. Fluids returned to other parts of the process shall be compatible in composition and temperature. or other foreign material that could potentially plug these vents. but may in some cases be absorbers. A combination breather – vacuum vent satisfies both the pressure and vacuum relief requirement for one level of pressure and vacuum protection in a single device. d. Existing tanks installation should strive to achieve this level of protection. Inlet and outlet piping and pipe fittings shall be included in the relief device capacity calculations. or 3.4. Venting from tanks and spheres a. Closed systems 7.1.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 6. 7. Relief streams that satisfy atmospheric relief requirements but where regulating authorities prohibit atmospheric venting. Pressure vacuum vents should be installed on hydrocarbon storage tanks in lieu of open vents. however note the exclusion of viscous oil tanks in API 2000. such flammable discharges might be to atmosphere. General a. Partial vacuum and equipped with a vacuum relief device along with a gas repressuring line. 2.6.5. The following relief streams shall be taken to a closed system: 1. However. however.4. Closed systems shall be subject to design and installation approvals by BP. New construction tanks and low-pressure spheres shall have a minimum of two means of protection from overpressure and/or vacuum conditions when this is a potential process risk. If practical. a gas repressuring line does not circumvent the requirement for vacuum relief protection due to reliability considerations. debris. d. Venting for storage and low pressure spheres or tanks generally shall be in accordance with API 2000 requirements. Inlet and outlet piping on pressure relief and vacuum relief devices for atmospheric or low pressure equipment should be kept to a minimum. c. Normal venting of flammable and toxic materials arising from controlled process variations and sustained discharges for plant operability shall be taken to a closed system. scrubbers. Spheres and other LPG vessels should be (in order of preference) designed for: 1. quench towers. Page 18 of 62 . b. 6. 6. The following are some relief event examples: 1. e. f. utility rearrangement or closed system modifications. 7.g. A failure of the whole or part of any instrument system.1. (See clause 10.e. independent of instrumentation. When any addition or modification is being made to an existing closed relief system protective instrumentation. This may be for economic or environmental reasons. affecting a number of relieving points simultaneously. etc.. plant-wide or unit-wide loss of: electric power. but not both unless both are required to prevent overpressure. plus the emergency load arising from the most severe single event. If a small proportion of low-pressure vessels share a common closed relief system with higher-pressure vessels. The emergency depressuring load shall be added to the worst relief load which could be the cause of the need for depressuring. but they shall meet the requirements of this GP unless otherwise approved by BP. or other limited condition. cooling water. g.2. Temporary flaring during equipment start-up and ramp up. Vessels protected by both depressuring valves and pressure relief valves should use the larger of the depressuring load or the pressure relief load from the fire. leading to the ingress of air into the system. A failure of a utility section (e. it may be economical to up-rate the lower-pressure vessels rather than size the collecting system for the lowest back-pressure or use two separate systems. analyser vents). 8. It may be necessary to provide either a liquid seal drum or an emergency supply of non- condensable purge gas to avoid this condition (clause 10. the feasibility of installing a flare gas recovery system shall be investigated. liquid seal drum or rupture disk).. This effect can also occur during normal operation from rapid cooling of an uninsulated. Page 19 of 62 . h. Closed relief systems shall be sized on the basis of the normal venting (i. If a flare gas recovery system is installed.g. or shutdown or gas venting from a drying unit. A failure affecting a single equipment item together with its related effects. An overall utility failure or event affecting a number of relieving points simultaneously (e.). 7..k).2 for liquid seal requirements).31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems e. A fire affecting the whole of the largest single section of plant that can be readily isolated by fire-fighting personnel and their equipment. (i. shall use the principles of reliability analysis as outlined in GP 44-70 clause 7. PSV leaking seals.. taking account of interaction. instrument air. The resulting design measures may not necessarily be identical to those for new designs. Attention is drawn to the possibility of condensation in a closed relief system immediately following pressure relief device operation.e. 2. If specified by BP. a free path to flare (e. shall be provided to allow for failure of the recovery system. but acceptable closed system capacity shall be maintained during all phases of the modification. turbine start-up. A failure of any flare gas recovery plant associated with the closed system. Pipe & header sizing and layout a. 3. 5. fuel gas. steam. Any such modifications need to be phased in with operating requirements. a fire.g..) 4.4.g. Knockout drums shall be provided if necessary to prevent liquid being carried over to the disposal system (see clause 9). compressor venting at start-up. loss of electric power to single motor control centre). solar heated header by a rainstorm. For multiphase compressible flow. which reflects BP experience of increased roughness in relief system piping: 1. h. or larger than.8 Mach Number if practical. good engineering judgement shall be used in accordance with ISO 23251 or API RP 521 generally. multiphase fluids.006 in or 0. Higher Mach numbers are allowed if an analysis is performed to ensure acoustically induced vibration fatigue failures will not occur and that the piping and pressure relief devices are adequately supported/ braced for the reaction forces caused by venting. e. Laterals from the relief device outlet flange to the main header shall use the rated capacity flow from the pressure relief valve except for modulating pilot operated relief valves where the required relief load can be used.001 5 ft) for heavily corroded piping. Relief headers shall be self-draining toward their respective “knockout” drums or receivers and all relief sub-headers shall be self-draining toward the main headers. and/or fluids above the thermodynamic critical point) shall evaluate the potential to reach sonic velocity (i. the homogeneous equilibrium model or equivalent that considers potential for choking (i. Main headers. Relief system piping shall be sized using the following pipe roughness.15 mm (0. In all cases. seal drum.e. choking conditions) in the piping.046 mm (0. however it is possible for pressure losses to be increased by as much as 50- 80%. b. The isothermal flow model should be used to determine the hydraulic profile for gas/vapour flow (See ISO 23251 or API RP 521).046 mm (0.018 in or 0. Laterals from a device that absolutely must be located below the header shall rise continuously to the top of the header entry point and means shall be provided to both prevent and confirm no condensate or liquid accumulation in the device discharge piping. j. 3. In determining the above.. and flare installation.The evaluation of pressure drops in piping handling compressible fluids (gases.. c. i. and flare stacks should be sized for the required relief load.001 8 in)) is generally not considered to be sufficiently conservative basis for the sizing of relief headers and relief valve discharge lines. The use of an equivalent roughness of 0. Unless certain measures are undertaken. the size of the pressure relief device outlet flange. The effect of using a higher pipe roughness varies with the system concerned.000 15 ft) for stainless steel and other alloy piping in normal service.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems For Exploration and Production flares the depressuring load can be very large. it is possible that either as a transient or even longer.000 15 ft (0. d. The hydraulic profile of closed disposal systems shall be verified and calculations shall be updated as required to reflect the current piping arrangements. Outlet relief device piping shall be sized to limit the maximum velocity to 0. 0.46 mm (0. g. reaching sonic velocity) should be used to determine the hydraulic profile (See ISO 23251 or API RP 521). knockout drums. 0. f. and GP 44-70 in particular. Discharge piping shall be self-draining from the pressure relief device to the relief header. the discharge pipe size should be as large as. 2. flare headers. design of the closed pressure relief system shall meet requirements for air quality and for the release of combustion products to a safe location. 0. vapours.001 8 in or 0.000 5 ft) for carbon steel piping in normal service. relief and blowdown loads could be superimposed if necessary to prevent overpressure. Locating a pressure relief device below a header in closed systems should be avoided. The header upstream of the flare shall similarly drain back to the drum.e. approaching or exceeding the other individual or group loads. If practical. Discharge piping from operational vents should be insulated for a certain length if external ice formation is to be prevented in a specified plot area to avoid falling ice hazards for Page 20 of 62 . In general. and these shall be subject to approval by BP.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems personnel.3.4 governs) 1–2 BP to approve greater than 2 Flare (separate line should be considered for concentrations exceeding 10%) 7.3.3. Special relief arrangements 7. this instrumentation shall be evaluated and designed in accordance with GP 30-76 and GP 30-80. Corrosive relief Specific arrangements shall be made for corrosive relief streams. Hydrogen sulphide relief Since hydrogen sulphide is a highly toxic material that is frequently encountered. Low-temperature relief Pressure relief header systems for refrigerated storage should be designed to:  Provide a high-toughness material in the flare header. so that the probability of opening any atmospheric vent is greatly reduced. Credit taken in this way for the operation of pressure-limiting instrumentation is subject to approval by BP for each project.  Not have low points in which liquid could accumulate to prevent adequate relief flow. appropriately SIS rated.4. Page 21 of 62 . % by Volume H2S Disposal means less than 1 Atmosphere (Clause 6. estimates shall be made of the possible incidence of pressure relief devices lifting simultaneously due to the failure of pressure-limiting instrumentation or devices in response to estimated demand rates. Materials shall be subject to BP approval. l. If credit is taken for the operation of pressure-limiting instrumentation in the sizing of closed relief systems. in accordance with the general principles of this GP and the GP 30 series covering SIS systems as listed in clause 2. 7. general practice rules for selection of disposal means have been established and should be applied as follows. in which vessels may be fitted with pressure-limiting instrumentation.2. Note that all discharges shall comply with clause 6. and regarded as an integral part of the pressure-relieving system. and the economics of segregating such discharges shall be evaluated. m.3.1.3. 7. discharge piping from operational vents in hot service (above 65°C or 150°F) shall be insulated or barricaded if personnel could be exposed. shall be sized on the general basis of ISO 23251 or API RP 521 which states the maximum load as the sum of the loads of the individual devices connected to it which are assumed to be relieving under the governing emergency condition. The criteria for sizing a closed relief system can be refined using Quantified Risk Assessment principles as detailed in GP 50-01 and GP 50-02 such that the design flow rate is not likely to be exceeded within a period to be determined for each case. k. Discharge of corrosive substances normally involve special materials of construction. Failures have occurred where steels appropriate for the temperature of gas entering the header was not used (auto-refrigeration effect). n.  Provide a very low pressure drop to flare. A closed system receiving multiple relief streams. The number ‘assumed to be relieving’ simultaneously may be reduced if credit for the operation of pressure-limiting instrumentation is taken. Similarly. 31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems a. Low-temperature relief of fluids shall be segregated from other streams that are wet, to avoid freezing of lines. For other than atmospheric discharge, a separate knockout drum and closed vent system shall be provided using materials of construction specifically selected for low-temperature service. b. If appropriate, consideration shall be given to the provision of methanol injection facilities to prevent hydrate formation. 7.4. Winterisation a. If the danger of freezing of vent lines or pressure relief devices exists, heat tracing or other positive precautions shall be taken. b. If any overpressure protection is by pressure-limiting instrumentation that relies on heat tracing, the heat tracing shall be included in reliability considerations if necessary. c. If liquid seals are being considered, winterization or freeze protection of these seals shall be carefully considered during the closed system design phase. 8. Flare system design 8.1. General Flare systems are used to convert flammable, toxic, or corrosive vapours to less objectionable compounds by combustion. The type of flare as well as any design features required will be based on many factors such as the characteristic of the flare gas, namely, composition, quantity, pressure level; economics, including both initial investment and operating costs; availability of space; and public relations. a. Flare systems and all associated components (headers, knockout drums, etc.) that are described in ISO 23251 or API RP 521 shall be designed to meet the minimum requirements of this document and shall comply with this GP as well as other referenced documents. b. Refer to GP 22-20 for further details on flare: mechanical design, operation, maintenance issues and use of the flare as a disposal device. 8.2. Component parts of the systems The flare system may comprise some or all of the following: a. Lateral discharge lines from individual fluid discharge devices. b. Relief headers connecting the lateral discharge lines together. c. Flare header, to which relief headers from different units are connected, and which leads to: 1. Knockout drum(s). 2. Quench drum(s). 3. Liquid seal drum(s). 4. Flare, consisting of: flare tip or flare burners, flare stack (if elevated) or enclosure (if ground flare), stack support system, continuous pilots, pilot igniters and piping. d. Ignition system. e. Flame supervision (i.e. monitoring and pilot ignition). f. Flashback prevention. g. Purge system. Page 22 of 62 31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems h. Isolation system. i. Smoke suppression control system. j. Gas sampling system. k. Oxygen analyzer. l. Flow, temperature and level measurements and alarms. m. Pump out facilities for drums. n. Fire protection. o. Insulation. p. Heating and heat tracing. q. Cold liquid/vapour vaporization and heating system. r. Flare gas recovery system. 8.3. Design considerations a. In normal operation, continuous hydrocarbon flaring shall be minimized as much as practical. b. The following points shall be given specific attention in the overall design of the flare system: 1. The safety and well being of all personnel in the vicinity (both on-site and off-site) under all conditions of flare operation. This includes start-up, purging, operational and emergency flaring, shutdown, inspection, and maintenance of all or parts of the system. 2. The protection of plant and equipment in the vicinity of the flare system under all conditions including surface protection of the flare system itself per GP 22-20. 3. The protection of the flare system from damage by external events, e.g. fires. 4. The inherent safety of the flare system itself especially in respect of the following: a) Flammable or explosive mixtures Blockages or flow restrictions b) Chemical reactions Toxic components c) Corrosion, erosion & hydrogen embrittlement Mechanical damage d) Flare flame stability Security of ignition e) Security of pilots Change over to another flare 5. The flow rate, composition, molecular weight, temperature, frequency and duration of process streams discharging into the flare system at any one time, and any inherent restrictions imposed, e.g. allowable back pressure, solids deposition. Particular attention should be paid to depressuring flow rates especially if depressuring is activated because of a fire or due to utilities failure that might cause all depressuring valves to open simultaneously (if all depressuring valves are designed to fail open). 6. BP will approve or provide design data such as flare gas composition, molecular weight, flow rates and utility services (e.g., steam, electric power, instrument air, fuel gas, etc.) available. 7. Materials of construction for flare systems should be selected to be suitable for operation at the minimum temperature of the system, allowing for any auto- refrigeration from depressuring. For more information on material selection for the flare refer to GP 22-20. Page 23 of 62 31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 8. Required life of flare system components. 9. The philosophy adopted for inspection and maintenance of the flare system and the impact of these requirements on plant operation (flare system sparing requirements). See GP Group 32 practices. 10. Meteorological and any other relevant environmental conditions pertaining to the site. 11. Any national and local regulations, particularly concerning smokeless burning, flare visibility, pollution, and noise restrictions. 12. The need for segregation or isolation of relief headers for reasons of temperature (See clause 9.3), toxicity, corrosive materials, etc. a) Segregation is particularly required to prevent freezing of water wet streams, solidification of viscous materials, or reactions which could lead to plugging of lines. b) If it can be proven that no wet (non-dehydrated) relief streams are present or feasible (such as in an LNG import terminal), then no cold or wet collection/flare system is necessary although a dry collection/flare would still be needed. 13. Handling systems for the safe disposal of condensed hydrocarbons and sour water from both knockout and seal drums. 14. A secure supply of seal fluid to the seal drum with provision to prevent overfill to the flare header and knockout drum(s). 15. Plot space/layout considerations. 16. The requirement for a highly reliable cold liquid/vapour vaporisation and heating system in situations where a cold flare cannot be justified. 17. Two or more elevated flares are allowed to be open to a common flare header only if each flare is equipped with a liquid seal drum or other system to prevent the chimney effect whereby gas flows out one flare causes a down draft in the other flare allowing air to be drawn in. c. The following is a checklist of possible hazards that should be considered in the design of flare systems. 1. Flammable/Explosive mixtures in the flare system which could result from air entering the system by any of the following mechanisms: a) Down draft due to buoyancy effects, loss of purge gas flow, failure of the purge reduction (molecular seal). b) Condensation or cooling of vapours in the flare system (i.e. plant shutdown) can cause air to be sucked in at the flare tip or through open vents or drains. This can be a very serious problem since the capacity of the flare pipework to absorb heat can lead to a very large and rapid contraction in volume. c) A rain shower suddenly cooling off flare system pipework and drums that had been exposed to the sun can cause air to be sucked into the system. d) Buoyancy of light gases can create sub-atmospheric pressure in the low level flare pipework. The resultant pressure differential may induce air to enter the system through any openings, vents, drains, flanges, etc. e) Vacuum systems connected to the flare can cause air to be sucked in. Special high integrity segregation mechanisms are required to prevent this. f) Process air may enter the flare due to loss of control in oxidation plants or uncontrolled air purging. Page 24 of 62 4. hydraulic surge of liquid slugs.5. etc. condensate in flare lines or molecular seals. b. hydrates. seals and flares. drains. e) Valves incorrectly closed or failing closed. If this cannot be prevented. Liquid accumulations in relief and flare systems: Liquid relief to the flare collection system should be avoided. d) Liquids trapped through faulty drains. including knockout drums. process line diagrams (PFDs. corrosion products. venting of high temperature gases into the flare system. Mechanical damages. purging arrangements. careful consideration shall be given to potential problems associated with liquid disposal from pressure relief devices and liquid de-inventorying into the flare systems. c) Solids carried forward from the plants. isometric diagram). 6. Chemical reactions within the flare system. 3.3.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 2. impact. line sizing calculations and material specifications shall be subject to BP approval before mechanical design is finalized. acetylides. 5. catalyst. 8. 3. steam condensing. and level control failure. burn-back at the flare tip. flame lick. etc. and should preferably be coupled to the main flare gas stream near the flare tip to minimise exposure of the main flare pipe work to the corrosive effects of H2S. 2. Flare types The type of combustion device or flare to be used will be specified by BP or shall be proposed by the flare vendor for approval by BP. Careful consideration should be given to the disposal of foul liquid effluents from flare seals. hydrates. propulsion of solid ice-slugs. peroxides.4. low temperature discharges or auto- refrigeration are all potential sources of vapour line blockage or restriction. Appreciable quantities of liquid discharged to the flare during an emergency vapour release could cause slug flow in the horizontal lines and the entrance to the knockout receivers causing possible mechanical damage. Page 25 of 62 . waxes. Use GP 22-20 for additional details on flare type descriptions. Engineering line diagrams (e. pyrophoric scale. bad design. freezing in the flare tip during low steam flow under winter conditions. Sizing of the system is probably carried out by a main contractor or BP using BP (GN 44-001) as reference. Nature. and quantity of relief. 8. Toxic components: Streams containing more than 10% volume H2S or other highly toxic material should be run in a separate line to the flare. Blockages/flow restrictions a) Freezing of liquid seals.g. polymers etc. Space available. low temperature embrittlement through auto-refrigeration. See clause 9. P&IDs). low ambient temperatures. external fire damage. frequency. See clause 9 for additional design considerations. Engineering diagrams shall separately show the whole flare system from the downstream flange of the pressure relief valve or liquid drain valve. The main contractor must obtain BP approval before information is passed to the flare vendors for mechanical design. Engineering diagrams a. liquids disposal. basing the final flare selection on: 1. b) Polymerisation products. Effect on surrounding plants and neighbourhood. i. outward for external mixing. Slots may be fixed or variable. 8. entraining air up to twenty times its own volume and introducing oxygen and turbulence required for complete combustion. Environmental requirements regarding smoke. and maintenance of a smoke suppression system when utilized on flaring devices.Steam may be used in Coanda or other flare tips to draw in air for mixing with gas in single large units with steam flowing outwardly. high pressure gas or air flowing from a narrow slot follows the profile of a curved surface. Smokeless operation is normally the overriding requirement when designing a burner for a flare system. radiation.e.This method can utilizes the aero-dynamic skin-adhesion effect known as the Coanda effect. The design shall provide smokeless flaring for: 1.e. pollution.6.ISO 23251 or API RP 521 provides preliminary steam injection design rates for flared gas to promote smokeless burning. . burners should be used in groups operated in sequence. or with multiple units in a single tip. noise. a. or the flare vendor shall propose and submit to BP for approval. either by liquid seals of increasing head. and emission of light. Ultimately. The most credible flaring scenario or 10-20% of the maximum flaring capacity. with internal injectors. Requirement (b) may be achieved by any of the following methods:  Premixing Air with Fuel In this method. with external injector. the flow rates for both smokeless and non-smokeless flaring.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 4. If the calorific value of the vented gases is not adequate to fulfil condition (a) above. Slots should be wide enough not to get blocked by impurities in the smoke suppressing media. BP will specify. but requires adequate gas pressure and has a poor turndown ratio.e. All cases of operational flaring. i.The Coanda slot may be facing inwards for internal mixing. an incinerator should be used. the mechanism should be robust and well protected against ambient conditions. To improve the latter. gas jets are used in Bunsen type burners to inspirate air and mix it with the fuel. Provision for smokeless flaring shall be made to comply with any national or local regulations applicable to the site. Smokeless flaring Refer to GP 22-20 for mechanical details on design. . operation. vendors should be consulted for steam rate requirements for their specific tip design. With variable slots.  Providing a Highly Turbulent Condition within the Flame Page 26 of 62 . and (b) an adequate supply of air mixed sufficiently with the fuel. or by automatic valves backed by liquid seals. or may be linear. i. a controlled release of fluid to the flare system for a continuous period exceeding 30 minutes. in which steam. with steam flowing inwardly in each unit. This type of tip may be used in ground flares. .  Inspirating Additional Air into the Combustion Zone . 2. To achieve smokeless combustion: (a) a minimum critical combustion temperature must be maintained. The latter may be achieved either by the discharge of multiple steam jets into the combustion zone which also inspirates air thereto. b. Steam. but upstream of the flow control valve. with steam traps. Note that supplemental gas is not required for low flow rates corresponding to flare header continuous purges involving inert gas.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems . opening equipment- depressuring valves) and other operations in which fluids are discharged into the flare system shall be taken into account in the design. API Std 526 provides typical limits for various valve types. or by causing the turbulence by steam or air jets. initial commissioning. Both these methods may be combined in one tip. b. Steam flow may be either automatically or manually controlled in relation to the gas flow or by the visible characteristics of the flame. This requires special attention for relief valves set in the 2-5 bar (ga) (30-75 psig) range. the following points shall be observed: 1. Sizing of relief and flare systems a. However. The back pressure limitation on relief valves should be noted. or water may be used for smoke suppression in flaring. The system shall be designed to provide dry steam at the flare tip with the steam pipework suitably insulated. 2.Highly turbulent conditions within the flame required for smokeless combustion may be achieved as a by-product of inspiration of air as in Coanda effect flares. . c. air.g. the vapour loads to which the flare system can be subjected as a result of maintenance operations (i. a minimum flow of steam as specified by the vendor shall be maintained by a bypass round the steam control valve. or by a high velocity steam jet centrally placed in the tip which entrains air and creates enough turbulence to attain efficient mixing of fuel and air. 3. 8. Steam lines should be suitably filtered as close to the flare base as practicable. Depending upon local regulatory requirements. sizes. If using steam for smoke suppression. Maintenance and other operations in which inert gas is discharged into the flare system can cause the flare pilots to be extinguished unless supplemental fuel gas is added to ensure the heat value of the flare gas is a minimum of 11 200 kJ/Nm3 (300 Btu/SCF). Expansion loops shall be installed in the steam riser as required. Many pilot operated relief valves have backpressure limits equivalent to the flange rating. Absolute back pressure limits as specified by the vendor for the specific type of pressure relief valve shall not be exceeded. high-pressure gas. shut-down. most conventional and balanced bellows spring-loaded pressure relief valves have backpressure limits considerably less than the flange rating. As well as overpressure situations. materials of construction and temperature ranges.7. d. start-up.e. Drainage. 6. the requirements for smokeless flaring may be relaxed by agreement with BP for periods of non-normal operation e. At an early stage it is Page 27 of 62 . In order to cool the pipework at the tip. the back pressure on a balanced relief valve cannot be more than 50-60% of the set pressure in gauge unit. shall be provided at the low points and the steam lines shall be frost protected. 5. 4. The capacity and conditions for which the flare system is designed shall be based on GP 44-70. The system shall be provided to avoid steam condensation introduced to the flare tip resulting in extinguishing the pilots or mechanical damage. Except for special low-pressure valves.. 7. In general. etc.2 for relief and flare header sizing requirements. piping losses may be calculated using data from any recognised source. in many cases the published data is thought to significantly underestimate losses through tees. For these calculations. Identification of equipment and systems tied into the flare system and their pressure limitations. liquid seal (if any). A pipe roughness consistent with the pipe material and operating conditions. See GP 44-10 for additional information and spacing requirements. Flow of Fluids Though Valves. This includes.S. General principles No detailed rules can be given regarding the location of flares. entrance losses.8. 8. Miller .8. The final piping configuration including fittings.5 Mach No. j. is generally accepted as a maximum for pipe flares. Siting 8.2 Mach No. and piping is normally dictated by the back pressure limitation on critical relief valves. dryers. and shall be subject to approval by BP. agitator seal vent. depressuring and process venting conditions. the following general principles should be applied: Page 28 of 62 . 3. following ISO 23251 or API RP 521 guidance would be sufficient which allows credit for some favourable instrumentation response during plant wide failure scenarios depending upon their reliability.g. etc. High pressure flares are available where the flame is stable even at 1.BHRA Fluid Engineering. 2. All potential relief. See clause 7. However. However.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems worthwhile checking the size of relief lines required and the cost of increasing vessel design pressure to reduce the flare main size required. In many cases. Very low pressure equipment such as atmospheric storage tanks should not normally be connected to a flare system because of the back pressure effects or potential for reverse flow.0 Mach No. This type of analysis is considered to be valid for a plant wide upset event providing it is reviewed and approved by BP. The total allowable pressure loss through the flare system including stack. but if this velocity is exceeded then experience of satisfactory operation of the design should be examined. as each installation has its own specific characteristics. noise. Dynamic analysis of events occurring during a plant wide failure scenario may indicate reduced overall loads based upon the sequence of the relief loadings considered. vessels and tanks that float on the header through an open pipe. The basis and methods to be used for determining system pressure losses shall be submitted to BP for approval. centrifuge seal vents. e. the data of Internal Flow Systems edited by D. Crane Technical Paper No. 410. other seal vents. h. The velocity shall be chosen to satisfy requirements for flame stability. i. For emergency flaring 0. Fittings and Pipe. It is prohibited to tie-in bleeds from double-block-and-bleed assemblies into flare systems. resulting in the risk of flame extinction of pipe flares. f. Above that figure. e. the flame could become unstable and lifts off. Evaluation of flare system back pressure shall consider: 1.1.. Pressure drop limitations may dictate the flare stack diameter. pump seal vents. and dispersion. published by Gulf Publishing or VDI Waermeatlas can be used. d. but is not limited to analyzer vents. g. c. The flare vendor shall submit the calculated flare tip velocity to BP for approval. knockout drums. The latest designs of pipe flare tips permit smokeless flaring at velocities above 0. 31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems a. and buildings. Most flare vendors have developed proprietary programs that are empirically based for their specific flare tips and the fraction of heat radiated in these models may not Page 29 of 62 . Height of flares The height of a flare shall be determined by the following considerations: a. f. An adequate dispersion of flammable and/or toxic gases. Potential gas release from a plant and the possibility of the flare as an ignition source should also be considered in flare siting. d. a higher molecular weight vapour at a lower velocity may reach grade level. Unless a shutdown of all flares is an operational requirement. process and breathing air intakes or platforms while flammable).. c. It is recommended. that the estimated average droplet size and average wind speed should be used in such calculations due to the improbability of the worst conditions occurring in combination. 8. 3.4.8. an improbably large area would result. BP approved method may be used to estimate preliminary thermal radiation levels at grade level.2. In very exceptional circumstances burning droplets of liquid could be discharged from the tip of a flare. Calculation methods for flare thermal radiation a. Any local or national height restrictions. CIRRUS.8. for aircraft movements. consideration should be given to possible future expansion requirements into what will become the sterile area. A flare should be as close as possible to the unit or units it serves. Acceptable concentrations shall be based on the period over which the conditions leading to the release can be sustained and the health hazard which they represent. such that their concentration does not cause any significant impact to personnel or property and are in accordance with any local regulations. However. When performing the dispersion analysis. control system response. the position of one flare in relation to another should be selected so that either can be maintained during the other’s operation. b. Prevailing wind direction should be taken into account in siting the flare to minimise environmental effects if possible. 2. Calculations of ground level concentrations shall be submitted for BP’s approval. therefore.3.g. The area which could be affected by the burning droplets would depend upon the size of the droplets and the wind conditions. ISO 23251 or API RP 521 or another similar.1). e. not only should the flare design flow rate be considered but also other scenarios representing other conditions that may increase the potential severity of impact (e. The maximum allowable thermal radiation levels as specified in 8. c. 8. 1. PHAST or another BP approved dispersion model shall be used to determine concentration versus distance and possible consequences if the flare is extinguished. e.8. Consideration shall also be given to a potential “flame out” event and the impact on plant/flare siting.g. The siting should take account of the likely route for the flare line (see clause 11. buildings. b. combustion device locations. The possibility of burning droplets being emitted from the flare tip should be taken into account in the siting. If the least favourable extremes of droplet size and wind speed are combined to calculate the extent of the possible area which could be affected by the burning droplets. even with the flare extinguished. 4. etc. elevated platforms. Operational blowdown . a 3. In general. vendor thermal radiation calculations.) shall be considered in relation to.2 kW/m2 (max.7 kW/m2 (escape time to safe haven) (1 500 Btu/ft2h) 4. etc.8. The flare vendor/contractor shall indicate the basis for the calculations including flame emissivity and shall supply calculated results for flame length. 60-second peak exposure . results. 4. and any other relevant documentation shall be provided to verify compliance to the following flame radiation limits. Plant or process areas containing high thermal radiation levels (fired heaters. 20-second peak exposure . 1. and shall be additive to. are: Base of flare boom Nearest edge of platform Helideck Crane cabs Monkey board (drilling derrick) Drillers pipe rack Radio mast (includes fittings) Environmental conditions that should be used in the thermal radiation calculations are:  No wind  32 to 50 km/h (20 to 30 mph) wind Water curtains or thermal radiation shield may be considered for reducing thermal radiation from flaring. 8.6 kW/m2 (offsite public location. c. but it is significantly inaccurate at fewer than 2 flame lengths. outside (500 Btu/ft2h) plant boundary where public can be present) 2. NOTES Page 30 of 62 . flame shape and emission. Thermal radiation levels a.2 kW/m2 (1 000 Btu/ft2h) threshold should be used where personnel can be normally working and 4. b.4. Continuous full shift exposure . The flare vendor thermal radiation model is generally preferred. assumptions.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems be interchangeable with that used in ISO and API. Higher wind speed can be evaluated as required but the user should recognize that there is a better cooling at higher wind speed that would mitigate the heat radiation. the expected thermal radiation rates from both operational and emergency flaring events. 6. The duration of and additive effect from radiation of any other elevated flare(s) located on the site which would flare simultaneously with the flare under design shall also be considered.3 kW/m2 (escape time to safe haven) (2 000 Btu/ft2h) b. If possible. 30 minutes) (1 000 Btu/ft2h) 3.7 kW/m2 (1 500 Btu/ft2h) should be used where personnel are not normally working. particularly offshore. The maximum permissible design level of radiation for exposure of personnel at maximum emergency flaring shall be based on the following: 1. 3. Positions critical to Flare Radiation Calculations. ISO 23251 or API RP 521 methods can be used for initial rough calculations. exothermic reactors. BP will specify the locations or positions where flare radiation calculations are required and the applicable environmental and operating conditions. 73 (231) 10 0. solar radiation can be excluded. For flaring where the peak radiation load is intermittent. 3. The requirements for any shielding system and the type of system to be employed shall be agreed with BP at an early stage.6 kW/m2 (500 Btu/ft2h) (continuous full shift) value and the peak value with all others. The figures given assume at least single-layer whole-body working clothing and hard hat. Bracknell. for an air mass appropriate to a suburban environment. latitude 60°). dependent on latitude.88 (279) 0.98 (311) 0. e. the flare structure.00 (317) 0. The main problem with exceeding the full shift or blowdown exposure levels can be both heat exhaustion and overt burns. ref D/Met 01/21/1/2/L.44 (139) These figures are taken from data supplied by the Meteorological Office. 1.96 (304) 0. the mean is for the daylight period only.01 (320) 0. If necessary. should be made when determining permissible flare radiation. LAT) solar time. The average radiation is the arithmetic mean of the monthly average irradiance. 2. For offshore flares it may not be possible to satisfy some of the requirements.g.54 (171) 60 0. The average value should be used in conjunction with the 1. The data are derived from latitude averages of the correlation of sunshine and irradiation (i.e. Solar Radiation Table Latitude Peak radiation Average radiation degrees kW/m2 (Btu/ft2h) kW/m2 (Btu/ft2h) 0 0. For flaring where the peak radiation load is continuous. Each monthly value used for the particular latitude refers to the 15th day in each month. England.99 (314) 0. solar radiation should be included. 2. An appropriate allowance. The data refer to the global irradiance received on a horizontal surface.73 (231) 30 1.69 (219) 40 1. Metal surfaces irradiated at any of the time/level ratios given may produce burns on contact with bare skin. Access to some areas may therefore have to be restricted. Linear interpolation between latitudes can be used. these design levels may be achieved by the use of displacement or shielding. It should be possible for any vital work in these areas to be carried out under specified and controlled conditions.00 (317) 0. except when the day length is less than 8 hours (only Nov-Jan. The peak radiation is the maximum of the monthly peak irradiance received at 1200 (Local Apparent Time. The following values should be used for the solar radiation allowance unless specific measured values are available for the site.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 1. the Angstrom relation) and should be considered to be only rough approximations to the actual values at specific sites. for the period 0800-1600 LAT.74 (235) 20 1. Page 31 of 62 .63 (200) 50 0. the bridge for a linked flare and the drilling tower. b. from the following aspects: a) High temperature from radiation.8. 3. between exposed and non-exposed surfaces. On towers or other elevated structures where rapid escape is not possible. The radius of this zone shall be defined by the larger of the distances calculated as follows: 1. c. Page 32 of 62 .2 kW/m2 (1 000 Btu/ft2h) for more than 30 minutes. c) Corrosive action of pollutants. Emergency (for periods up to 60 seconds) The distance from the flare tip beyond which the thermal radiation level does not exceed 4. d. ladders shall be provided on the side away from the flare. 2. (usually at ground level. so that the tower or structure can provide some degree of shielding if necessary. To minimise the risk of injury to personnel through thermal radiation or related heat exhaustion.4 shall be designated a Restricted Access Zone. 2.8. using the same design level above. Equipment may be located within a restricted access zone provided that: 1. Shielding shall be provided if specified by BP. 6. 5. 3. 8.7 kW/m2 (1 500 Btu/ft2h) at maximum flaring rate and a wind speed determined by local environmental conditions. The effect of flaring on equipment in the vicinity shall be considered. Blowdown (for periods up to 30 minutes) The distance from the flare tip beyond which the thermal radiation level will not exceed 3. all access requirements shall be considered. d) Possibility of burning of un-ignited droplets. It is designed such that it will not be damaged by the highest levels of thermal radiation to which it could be exposed. Operational (for periods of one shift or more) The distance from the flare tip beyond which the thermal radiation level does not exceed 1. Restricted access zone (Sterilisation zone) a. A Restricted Access Zone shall be specified around the flare tip unless otherwise approved by BP. It is possible to carry out emergency maintenance without risk of injury from thermal radiation to personnel (wearing protective clothing or using radiation shields if necessary). 7. A maximum ground level radiation will be specified by BP.5. either where access across a restricted access zone without shielding is required or where the ground covering may be ignited. e. At places where it may be possible for personnel to enter this zone. grass or peat. but also possibly via elevated structures). the volumetric zone around the flare flame within which the radiation may exceed the levels specified in 8.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 4. The equipment requires no regular operator attention or maintenance whilst the flare is in operation. The contribution from solar radiation shall be taken into account unless otherwise specified by BP. In tower-supported multiple flare systems. b) Large temperature gradients.g.6 kW/m2 (500 Btu/ft2h) at maximum operational flaring rate and a wind speed determined by local environmental conditions. e) Effect of hot gases. access shall be restricted by warning notices located in prominent positions. Elevated flares that use steam to control smoking are the most common form of smokeless flare tip although air can also be used to minimize smoke formation. Volume 1 and Chapter 13. Elevated flares The simplest type of elevated flare is commonly referred to as a utility or pipe flare consisting of a pipe fitted with a flame retention device for flame stability and a pilot for gas ignition. Refer to GP 22-20 for further details on design. The flare tip or burners shall be provided with pilot burners capable of igniting flare gas under all relevant flow conditions and ambient conditions. d. Concentrations below 9 300 kJ/m3 (250 Btu/ft3) require the addition of fuel gas for complete combustion. Refer to GP 22-20 for further details on design. 8. b. b. Page 33 of 62 .. high turndown rates). the minimum heat content of flare gas should be 11 250 kJ/m3 (300 Btu/ft3) to ensure a high combustion efficiency for the flare.12. High pressure flares do not require steam or utility air to promote smokeless combustion.11. Ignition systems Refer to GP 22-20 for details on flare ignition system design. multipoint flares typically burn ‘smokeless’ in most services having the ability to readily entrain air into the gas being burned without the use of steam. Care must be taken when specifying the minimum turndown for each stage to avoid possible burn back inside the flare piping that may occur if the flare load drops below the minimum. These flares can offer the advantages of hiding flames. c. a.10.5 Mach No. With their small nozzles. Minimum heat content of flare gas Per the US Environmental Protection Agency (EPA) document AP 42.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 8. and maintenance of elevated flares. operation. selection. It may also be termed as a multipoint flare. a.9. 8. Multipoint ground flares can be staged to allow a wide range in flare loads (i. Dispersion and consequence analyses shall be performed to evaluate possible impacts due to release of unburned flare gases in the event of ground flare ignition system failure. 8. The exit area of enclosed ground flares should be such as to provide adequate dispersion of all combustion products exiting the ground flare. General a. but use energy of the flare gas itself. High pressure flares can operate at a velocity of 1. A significant disadvantage of ground flares is the flare gas is released near grade-level in the event of a flare ignition system failure. assist gas or other fluids to enhance air entrainment. 8. monitoring combustion emissions and lowering noise. and maintenance issues. These flares typically have velocity limits of about 0.0 Mach No.12.5. If plot space is available.e. and maintenance of enclosed ground flares. operation. operation. a fenced ground flare can be designed to have a very large capacity.1. but require equipment connected to the flare system to be able to withstand potentially high back pressure. Enclosed ground flares An enclosed ground flare represents any non-elevated flare that is normally enclosed in a shell or fenced (solid wall to block thermal radiation) area. The vendor shall confirm the pilot gas molecular weight and calorific value range their flare pilots and pilot gas ignition system will work satisfactorily without adjustment to the air and gas flows.1). This can also occur if the flare header is suddenly cooled by a passing rainstorm. The following methods may be used. or some other reliable sources. Automatic back-up gas supplies should be used if necessary to achieve an acceptable overall reliability. The choice of method shall be subject to approval by BP.2). Liquid seals (clause 10. A check valve shall be installed on the air line and pilot gas line if a flame front generator pilot ignition system is being used. with two filters in parallel or a dual filter with adequate valve arrangement to allow cleaning/replacing filters. e. The pilot burners shall be ignited by a reliable ignition system capable of operating under all relevant ambient conditions.13.5). d. a LPG vaporizer. This shall be by a top mounted branch. The pilot gas main is preferably supplied by a natural gas pipeline. 2. g. molecular seals – see clause 10.e. The pressure-reducing valve shall be of a self-operating type. b. either singly or in combination: 1. there is a significant probability of large amounts of air being sucked back. Page 34 of 62 .31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems b. This prevents reverse flow of pilot gas into the pilot gas or air into the pilot gas when the flame front generator is blocked.2. 8. Gas purge (clause 10. The filter elements shall have a mesh size of approximately 0. If condensable materials are being flared. The use of a flame arrester shall be considered only in cases in which none of the above methods are suitable and subject to the restrictions of clause 10. c. d. Typical purge rates to prevent air in filtration on the cases can be more than an order of magnitude greater than the gas purge used for air infiltration due to wind effect. 3. Efflux velocity accelerators (clause 10. The filter and piping and fittings downstream from filters shall be in type 321 or 347 stainless steel to avoid blockages by products of corrosion. Pilot gas supply a. both gas purge and liquid seals should be installed. The above methods are primarily intended to prevent diffusion of air into the flare stack. The pilot gas supply shall be from a high-reliability source approved by BP. c. Flashback prevention a. As automatic ignition system is the primary means of ignition. If flare gas recovery is used. b.5 mm (0.3) are not sufficient by themselves to prevent flashback. Note that gas seals (i. f. A reliable method of flashback prevention shall be incorporated into the flare system design. placed downstream of the filters. The pilot gas supply should be taken directly from the plant fuel gas main if available. 8. A flare gun is not an independent manual back up. To check the pressure drop through the filter a differential pressure gauge shall be fitted across it or possible a bypass across filter should be installed to allow maintenance while the flare is in operation.020 in).12. c. an independent manual back up is recommended.4. which may lead to overheating and loss of mechanical integrity. fairly steady combustion may occur within the flare stack. (This still applies even if the flare gases have been premixed with air upstream of the flare tip).  Relief valves are removed for servicing.11) at all phases of operation. The best location for the addition of inert gas is as close to the flare tip as possible compatible with good mixing of gases before burning at the tip. Method of Calculating the Flash Back Velocity:  A method of calculating the flash back velocities for some gases commonly occurring in flare systems when mixed with nitrogen. Velocity accelerators and inert gas addition may be used in combination. (It may be possible to reduce or even prevent condensation by heating and insulating the flare line. (To achieve such internal combustion for long enough to overheat the flare stack in this way would require the in-leakage of sufficient air. the gas mixture may become non-combustible due to the excess inert gas present. The practicability of using inert gas to reduce the flash back velocity would further depend upon the availability of a very high integrity source of inert gas at the site. The main advantages of using inert gas is that a properly designed system gives protection against flash back through air ingress from any source. The flare flame does not travel back into the flare stack if the efflux velocity of the flare gas exceeds the flash back velocity. the pilot and main flames can be extinguished. such measures may be expensive to install and difficult to maintain in a reliable condition). the efflux velocity may be increased by the addition of purge gas. 1959. has been developed by Van Krevelin and Chermin and reported in the transactions of the Seventh International Symposium on Combustion.  Lighter-than-air gases particularly hydrogen are being flared. the mixture will be ignited by the pilot burners at the flare tip. or both.  Condensation or rapid cooling can occur within the flare system.  Two or more flares are open to a common header without liquid seals in between. If the flash back velocity of the mixture exceeds the efflux velocity. Ensure the flare gas heat value is above the minimum allowed (clause 8.  Air or oxygen is used in processes connected to the flare system. Unburned toxic and/or strong smelling components may escape to atmosphere and possibly cause a nuisance. pages 358-368. To achieve this essential condition. however. Flare stacks have been ruptured by such explosions. carbon dioxide. This can occur if insufficient purge gas is used to mitigate air in filtration due to wind effect). the flame will burn back into the flare stack and an explosion is likely to result. Some of the conditions conducive to the formation of flammable mixtures within the flare system are if:  Vacuum systems are linked to the flare. in sufficient quantity at an economic price. Otherwise. Page 35 of 62 . The major disadvantage is that at low flare gas flow rates.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems If air (or oxygen) enters a flare system and forms a flammable mixture of gases within the system. If the efflux velocity is very close to the flash back velocity. by use of a velocity accelerator or the flash back velocity may be reduced by the addition of inert gases to the flammable mixture. 15. If actual sparing is required. unless otherwise approved in writing by BP. such discharge should meet GP 44-70 for atmospheric discharge. Therefore. Unit knockout drum (Onshore) a. maintenance and inspection can be performed during normal shutdown periods. etc. 9. inspection. Flare vendors. A lower noise limit may be specified by BP to be applied in a particular case. and if this would in any case involve shutdown of one unit only. shall provide information on the noise emission from the flare at maximum emergency flow and at the maximum smokeless flaring rate. tips and flare auxiliaries. the quieter the flare. in their quotations.1. 8. b. Liquid removal 9. the most economical way of providing this may be the use of a common structure supporting multiple risers with flare tips that can be individually lowered and serviced while the remaining tips continue to operate. Adequate provision shall be made to enable the full specified range of continuous and intermittent flaring operations to be sustained during this period. Sparing of flares shall be considered to allow for maintenance. may be considered an acceptable risk. Measurements shall be made according to CONCAWE report 2/79. in general. the lower the ratio of steam to flared gas. Flare sparing philosophy a. the flare lines may be configured to allow one of the flares to be taken out of service. b. Page 36 of 62 . This type of flare can be designed to allow for the maintenance. In this instance back-up shall be provided in the form of atmospheric discharge. then some form of sparing may be specified by BP. Noise levels a. Breakdown may be considered unlikely to occur. The main contributor to noise in a smokeless flare is the steam jet noise. 8. A unit knockout drum or receiver shall be provided in all cases in which significant quantities of liquid can be relieved from the facility or process and where the flare is located. ground flare.g. If a flare system serves more than one unit that can function independently. i. If two or more flares are available. the noise level at positions normally accessible to personnel should not exceed 80 dB(A). having an additional spare flare that can replace the primary plant/process flare during maintenance. inspection or breakdown. If flares serve multiple units or plants. If a flare serves one unit only. 1. a knockout drum should normally be provided within the battery limit of each unit or plant. e.14. Special arrangements for inspection and maintenance of the supporting structure are required. An excess inert gas flow of twenty five percent above the calculated value should provide an ample margin of safety to compensate for measuring errors and minor flow disturbances. The time between overhauls will be specified by BP. proximity to local residents. The noise emission data shall be provided as a test report containing the sound-power levels in octave bands from 31 Hz to 8 kHz. and breakdown.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems  This method may be used to calculate the inert gas flow corresponding to the peak flash back velocity of the gas mixture. inspection and replacement of risers.e. The flare shall be sited such that at the maximum emergency flow rate. offshore platform. Piping that can be in liquid or multiphase service should be tied into the flare header using 45 degree tees pointed towards the direction of flow as opposed to 90 degree tees if practical. should use long radius elbows that are braced and supported for slug impacts. API RP 521.024 in) at the maximum emergency gas flow to the flare. the flare header design should consider the potential for and mitigation of slug flow.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 2. b. The drum shall be located as close as practical to the flare taking account of access requirements and the possible use of a liquid seal drum which shall be located downstream of the knockout drum. A primary knockout drum shall be provided for each flare system. The unit knockout drum shall comply with primary knockout drum requirements detailed in clause 9. If vaporisation facilities are relied upon to prevent metal Page 37 of 62 . b.e. If liquid carry-over from the unit knockout drum to the primary knockout drum is possible. and above 150 µm (0. 3. In exceptional cases. Knockout drum sizing calculation methods in ISO 23251. 2. Vaporisation facilities shall be provided for liquid disposal. which may require separate knockout drums to maintain segregation between ‘cold’ and ‘wet’ streams. The choice between a horizontal and a vertical drum should be made on economic considerations. if necessary. for flares that are capable of burning larger sized droplets. and the necessary slope of the flare header. Drums shall be designed for full vacuum and a maximum allowable working pressure of at least 3. 1. The knockout drum shall be sized to remove liquid droplets above 600 µm (0. A knockout receiver or drum shall be sized to separate and collect all disengaged liquid particles per the following criteria: 1. Primary knockout drum (Onshore) a. 3. the pump trip-in level) and the maximum level allowable in the drum taking into account any : a) Simultaneous requirements for vapour/liquid separation. c. if required. or equivalent shall be used.2. The essential purpose of this drum is the removal of the bulk of the liquids in the gas stream and to prevent liquid carry-over to the flare. A minimum corrosion allowance of 3 mm (1/8 in) shall be provided on knockout drums.3. (50 psig). subject to BP approval.5 barg. Use of 90 degree elbows should be minimized or. the liquid storage required. Attention is drawn to clause 9. 1. 9.2. b) Flashing of the relieved liquid at the knockout drum pressure.006 in) from the gas flow equivalent to the maximum smokeless capacity of the flare. d. This capacity shall be provided between the maximum normal liquid level (i. 2. Liquid storage capacity of the knockout drum shall allow for a minimum of 20 to 30 minutes hold-up at maximum liquid in-flow to the drum or free space greater than the maximum possible quantity of liquid that can be discharged into the drum. a waiver of these requirements may be accepted. 2. taking into account the vapour flow rate. The knockout drum shall meet the minimum design requirements of ISO 23251 or API RP 521 and shall also comply with GP 46-01. The weight of liquid that can be in the piping when relieving shall be considered when specifying the pipe supports. Personnel protection shall be provided if metal temperatures can exceed 65°C (150°F) while relieving.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems temperatures falling below their minimum design temperature. the use of a demister pad to limit the size of the drum shall be avoided. Introduction of liquids with a temperature in excess of 93°C (200°F) into knockout drums containing water or light liquids shall be avoided to prevent the potential for ‘steam’ or vapour explosions. Page 38 of 62 . 6. If appropriate. If closed drain liquids are sent to a knockout drum. 9. 10. 4. Instrumentation and control systems for the drum shall be in accordance with clause 12. and cleaning if the associated plants cannot be shut down and proposed methods shall be submitted for BP approval. long-term reliability of the vaporisation facility shall be assured. the drum shall be sized to accommodate both the maximum expected fluid levels from its sloped piping as well as the maximum liquid possible from the closed drain vessel(s). inspection. Facilities shall be provided for isolation. The design of the knockout drum or receiver shall also meet all of the following criteria: 1. b) Particular attention should be paid to prevent creation of a hazard due to the release to atmosphere of flammable or toxic materials from drain points. Adequate winterisation shall be provided for the drum as approved by BP if necessary. e. materials suitable for the low temperature service shall be used. 3.6. maintenance. The knockout drum shall be provided with automatic hydrocarbon liquid removal unless otherwise specified by BP. routing liquids to a suitably instrumented and protected alternative.  If there is any risk of the materials freezing. Knockout drum pump(s) designed and installed shall be capable of emptying the drum from its highest level shutdown to the drum normal operating level or low level shutdown in a maximum of 2 hours. particular care should be taken in the design and operation of any drain points. Due to the potential knockout drum size increase in this circumstance. the drain should be routed to a closed system. a) The disposal route and facilities for these liquids shall be approved by BP. Since the liquid in the KO drum may be toxic or flammable. 8. Specific attention shall be given to the requirements of inspection. 7. therefore relief piping and headers shall always be free draining towards their associated knockout drum. The split entry configuration of inlet piping to the knockout drum should be avoided unless uniform flow distribution can be assured. these may be automatic or manual. maintenance. or have toxic or flammable material dissolved in it. Condensed liquids in the relief or flare header systems shall drain to and collect in the knockout drum or receiver.  If there is any risk of toxic materials being released. and cleaning of the drum. a second valve in series is required as a minimum. separate facilities for water or heavy hydrocarbon removal shall also be provided. Otherwise. Because of the potential for blockage from scale or waxy deposits. See ISO 23251 or API RP 521 for additional details. 2. low-pressure vessel should be considered. internal elbow or baffle to direct liquid away from the knockout drum outlet. 11. venting and purging. The knockout drum inlet should be equipped with a diverter plate. 5. The minimising of possible liquid relief to the flare system should be a normal feature of any design. b.3.4. c. d. The flare vendor’s proposals shall be submitted for BP approval. The liquid removal facilities should be designed to remove entrained droplets (which may carryover as burning hydrocarbons) from the gas flow and provide sufficient liquid hold- up capacity to collect any surges of liquid. c. Cold service a.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 9. Page 39 of 62 . Appropriately rated SIL instrumentation should be installed on equipment trip systems taking into account the undesirability of discharging liquids into the flare system. 10.4. 9. the dry gas collection system terminates in a ‘dry gas’ knockout receiver. Attention shall also be given to the presence of other materials that freeze or are highly viscous at temperatures above 0°C (32°F). e.3. The situation is most likely to occur in plants handling liquefied gases or gas streams at high pressure. a continuous purge system shall be used.1. and vapour/liquid mixtures which contain components likely to condense. Cold-liquid collection drums may require vaporisation facilities. separate systems shall be provided for cold streams with segregation maintained until the streams become compatible. Specific attention shall be given to liquid removal facilities in flare systems that dispose of both ‘cold’ and ‘wet’ streams. a. gases. or freeze at 0°C (32°F) or above should be considered for routing to a separate relief/flare header system. Devices which provide warning (and if necessary execute shutdown action) shall be fitted to relief valves which can discharge liquids to flare. Similar to the wet gas system. The system should make use of indirect heating to avoid the possible contact of cold fluid with steam condensate in the event of tube rupture in the heat exchanger. should be routed to the dry gas header system. The relief of liquid propane and butane frequently results in a cold two phase discharge. form hydrates. Unless the disposal system is specifically designed to handle low temperature fluids. Vapours.3. and vapour/liquid mixtures which expand isenthalpically through the relieving devices to a downstream temperature of less than 0°C (32°F) not containing any components likely to freeze. Vapours. it is necessary to provide a heating system to vaporise any liquid and then superheat the cold vapour before it enters the main flare system. Liquid removal (Offshore) a. Suggested heating media are methanol or glycol. or on mixing with a stream containing free or dissolved water. See GP 30-76 and GP 44-70. The hold-up capacity should be based on the longest estimated time required to isolate the incoming flow. a ‘cold’ stream is defined as a stream at a temperature below 0°C (32°F) which could cause freezing of water in a knockout drum. Maximum use should be made of surge capacity within the process area to accommodate liquid relief. but others may be considered. Gas purge Unless otherwise specified by BP. An inert gas (e. b. If practical.3 and 7. gases. In this context. fuel gas or natural gas from a reliable source shall be continuously introduced as a purge into vapour disposal systems. Attention is drawn to the special sealing provisions for cold service in clause 7. clauses 7. Flare purging and sealing 10. d.g. nitrogen). The purge gas supply shall be from a high-reliability source approved by BP. Prevent burn-back inside the flare tip.c). The methods given for calculating the required quantity of purge gas are based on a sufficiently high efflux velocity to prevent the oxygen concentration 8 m (26 ft) from the top of the stack becoming more than half the lower flammable limit. see 10. Automatic back-up supplies should be used if necessary to achieve an acceptable overall reliability. 2. the maximum oxygen content shall not exceed the following: a) Maximum oxygen of 5 volume percent for flare gases with a MW greater than 6 but not exceeding 8. e.1. This has the advantage of significantly reducing the risk of a damaging explosion in the event of an unforeseen occurrence such as a suck-back. For a given relief composition there is a maximum required purge rate which can be significantly smaller than the purge rates required by the Husa formulae. The purge system shall be designed so that loss of a single purge gas source or injection point does not allow hazardous conditions to occur. it should be noted that one volume of fuel gas produces about ten volumes of inert gas in an inert gas generator. the inert gas generator must be purchased and a back-up supply must be provided. specific attention shall be given to any potential consequences of releasing unburned toxic materials to the atmosphere. and 2. 2. This maximum purge rate is calculated by establishing the required purge rate to balance flash-back and efflux velocities for a variety of relief flow rates. 3. The use of site generated nitrogen in this application could require evaluating the potential for oxygen entry into the inert gas stream from the package generator. since it must be inherently less reliable than its feed gas supply. If either the purge gas or flare gas is low molecular weight (e. The larger of the two flow rates shall be used. With an inert gas purge. Page 40 of 62 . For purge gas mixtures that are heavier than air. d. However. however BP preference is to use inert gas if economically viable. 1. In choosing between a fuel gas and an inert gas purge.. (See clause 10. This is intended to prevent flash-back down the stack. c. A minimum purge gas velocity or flow rate shall be maintained at the flare tip to: 1. Lower molecular weight gases require larger quantities of purge gas to achieve similar safe conditions within the stack.g. When deciding to use an inert purge. containing high concentrations of hydrogen). The purge rate shall be such that the oxygen content in the flare gases 8 m (25 ft) down from the top of the flare shall be less than 6%. If an inert gas is used the flash-back velocity (the speed with which a flame travels through the mixture) is significantly reduced. Locations where prevailing winds often exceed this wind speed may need somewhat higher purge rates.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 1. 2. b. The minimum purge rate to minimize air ingress due to wind effects shall be calculated using Husa’s correlation formulae in Annex A. the object is to ensure that the efflux velocity is always greater than the flash-back velocity. Note the Husa correlation applies where typical wind speeds do not exceed about 13. 1.1. Minimize air ingress due to wind effects. The choice should primarily be evaluated on cost.j.4 m/s (30 MPH). b is very small. This may result in burning inside the tip. then the purge rate need be based only on that required to prevent air ingress due to wind effects. This is not necessarily a dangerous condition. The required purge rate using a gas mixture heavier than air shall be calculated using nitrogen parameters in the Husa equation in Annex A.4. the flare tip). Generally. With a liquid seal..2). This purge gas supply does not need to be continuous. depending on final facility layout. resulting in higher tip temperatures and shorter tip life. For minimum purging. The continuous purge gas used during normal operation can be tied into either near the knockout drum and/or at the flare header extremities. For flammable purge gases heavier than air. Emergency purge gas shall be provided to the flare system. to check if the safe oxygen levels specified in 10. BP will specify if it is necessary to maintain the flare alight. however the reliability of these systems should be considered before installation. even though in such cases the minimum purge referred to in 10. 2.1. 1. If not.e. Purge gas connections shall be provided in every disposal system at the header extremities. If the tip is refractory lined. if appropriate. 2. Vendors shall specifically identify their minimum purge rate to avoid internal tip combustion on the flare data sheet and confirm the Husa calculation using nitrogen eliminates burning inside the tip or flame extinguishment. temperature. These connections are required to ensure the system is air-free before introduction of hydrocarbon. j. This flow velocity shall be verified based on the selected proprietary flare tip or molecular seal design. c) Maximum oxygen of 3 volume percent for flare gases with a MW ≤ 4. A calculation methodology is described in GN 44-002. l.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems b) Maximum oxygen of 4 volume percent for flare gases with a MW greater than 4 but not exceeding 6. alternatives such as additional tip cooling or upgraded flare tip metallurgy should be considered. or flame extinguishment. h. and stack. high initial purge flow may be required to air-free the entire flare system. continuous purge gas is introduced near the knockout drum as it is the closest to the primary source of air ingress (i. but rather should be automatically initiated and controlled by pressure.c are maintained. the minimum purge rate could theoretically be achieved with very low flow rates. f. g. The use of increased purge gas versus alternative tip cooling or metallurgy must be evaluated against local and national regulations as well as total life cycle costs considering the increased purge gas rates. knockout drum.1. a liquid seal drum may be used (see clause 10.  No unintended flows are initiated because of increased pressure differential. When recommissioning an air-filled flare header network. Alternatively. the stack may be equipped with an oxygen monitoring system as described in 12. or a combination of both. i. k. the relief disposal piping system upstream of the seal is subject to vacuum conditions when a hot relief flow stops and cools. provided that:  The equipment is designed for the maximum vacuum conditions that can occur. 1. a supplemental. The flare vendor shall specify the minimum purge required to prevent burn-back inside non-refractory lined flare tips. Page 41 of 62 . to prevent the formation of a vacuum in the flare header or stack due to vapour condensing or gas contraction as the system cools following a release or due to cooling of uninsulated metal pipework during a rainstorm. If liquid seals are impractical. The traditional style of a single dip-leg with a serrated end was satisfactory when there was always an appreciable flow of gas to the flare tip. The minimum submergence depth of the inlet downcomer shall be 10 cm (4 in) per ISO 23251 or API RP 521. the volume of liquid in the seal drum above the level of the top of the submerged weir shall be sufficient to fill the 3 m (10 ft) vacuum leg. A liquid seal is only one of a number of methods of preventing the ingress of air into a flare system via the flare tip due to thermal contraction. b.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 10. then suitable adjustments should be made to the water seal or the seal liquid.1.3 barg (–4 psig) of vacuum protection. Uses of seals a.2. See ‘Liquid Seal Drums’ in ISO 23251 or API RP 521 for further details. Liquid seals should be incorporated in relief disposal systems as close as practical to elevated flares.2. b. depending on the anticipated temperature of vapours. 2. It is normally utilised in conjunction with continuous gas purging. One large. or to increase turndown and burning efficiency. glycol or other suitable material shall be used. These valves are used to accommodate increasing flow rates. Water seals are normally used if either the ambient temperature or the temperature of the relief streams cannot fall below 0°C (32°F). The vertical leg of the flare header inlet shall form a vacuum leg of adequate length above the liquid level in the drum for the maximum vacuum expected in the header due to cooling and/or condensing of hot vapours. The fluid shall be compatible with all the fluids that can enter the flare header.g. To guard against freezing in cold weather the seals shall be part of the winterisation program and shall be fitted with automatic heating. If such chemicals could be relieved. Hence.3. Liquid seals may not prevent flashback in cases in which a large volume of gas is relieved. Liquid seals 10. Incidents have shown the flame can propagate back through a continuous flow of bubbles. 1. either pure or with water. as specified by BP. For cold service. and either to differentiate between smokeless and non-smokeless flaring. for flare gas recovery systems). (See ISO 23251 or API RP 521). If more than one flare is connected to a relief header and automatic pressure-actuated valves are used. The reduction of leakage and the addition of flare gas recovery systems have significantly changed this. Some chemicals can raise the freezing point of water above 0°C (32°F). either electrical or steam coil. Types of liquid seals a.2. a full capacity back-up route to the flare shall be provided via a liquid seal or another BP approved alternative which ensures an ‘open’ relief route. however. a continuous purge or other method should be used to ensure the flare header is air-free. non-serrated dip-leg invariably leads to flow pulsations which are Page 42 of 62 .. 10. Liquid seals may be employed for prevention of air ingress into the flare header network due to most thermal contraction events and for diversion of vapour flows (e.2. It shall be at least 3 m (10 ft) high which corresponds to about –0. Liquid seal design a. c.2. another system is required which provides both a guaranteed emergency relief route and a guaranteed protection against air ingress to the flare and header systems. 10. 8 to 2. Make-up lines shall be sized to replace the seal within 10 minutes. b. A more effective system is based on separate dip-legs of different sizes. d. 2. Prevention of displacement of seal liquid. This latter option should be provided for seal systems containing anti- freeze. There are two main types of gas seals: the labyrinth type and the flow restriction type. H2S.e. The dip-leg should be surrounded by an anti-splashing perforated baffle sheath. b. but may be used subject to BP approval. Alternatively. Details of the dip leg design shall be submitted for BP approval. If in exceptional circumstances it is intended to use either type of seal.2. 4. Maintaining the correct seal liquid level. the free area for the gas flow above the liquid should equal at least 3 times the inlet pipe cross-section area.h may be under manual control.5 in) diameter holes on 75 mm (3 in) diagonal centres. 3.g. over the operating pressure range. antifreeze systems may be used. h. Buoyancy seals (molecular seals) a. The design shall be capable of flowing all quantities from maximum emergency flow down to 1/3 000th of that flow without causing flow pulsations that cause nuisance. A minimum pressure of 3. it is impossible to quantify the benefit and hence no reduction in purge flow should be used. If water is used for the seal. Equipment shall be provided to maintain the design seal level. To prevent surges of gas flow to the flare. The maximum depth to which the inlet pipe may be submerged shall be based on the maximum exit back pressure allowable in the relief or flare header. In this case. The design of the seal system shall provide for: 1. Page 43 of 62 . f. the requirements of 10. Neither gas seals of the labyrinth type.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems seen at the flare as flame pulses and can cause noise complaints of a rumbling type noise. g. a recirculation system may be provided with capacity to allow for make-up and for checking the liquid inventory. e. To provide enough circumference for placement of serrations and reduce the gas velocity the dip pipe diameter may have to be increased. the design of the disposal system for excess water shall take into account the likely contamination with relieved materials. static liquid seals). e. Recent designs have shown a need to provide pulse free flaring from about 300 000 kg/hr (660 000 lb/hr) (for the maximum emergency case) to about 100 kg/hr (220 lb/hr) (for the normal leakage case). Prevention of hydrocarbon build up. 10. The flare header shall slope from the top of the vacuum leg back to the off-site knockout drum.5 barg (50 psig) shall be used for the design of the seal drum. If make-up requirements are not significant (i. nor seals of the flow restriction type are recommended. The diameter of the baffle sheath should be 1.3.3. so that each release route allows a progressively larger flow without any noticeable pulsation. with 13 mm (0. i. sometimes with side slots (see ISO 23251 or API RP 521). The flow range of maximum to 1/3 000th may be too small a range. These make the flare more noticeable and defeat any attempt at maintaining a controlled steam flow to keep the flare smokeless.0 times the diameter of the dip leg. c. Continuous purging of seal water shall be considered to prevent build up of H2S and CO2. Provision shall be made for periodically checking and maintaining their condition. and therefore such seals are not recommended. A flame arrester should be considered only when there is no other viable or economic alternative.e. the purpose of which is to reflect back the atmospheric ingress turbulence. 10. Efflux velocity accelerators (Velocity seals) Flashback from the flare flame into a flare stack does not occur if the efflux velocity of the flare gases always exceeds the flashback velocity (i. or with refractory. normal.5. It shall be possible to maintain or replace a flame arrester without shutting the plant down. In the labyrinth type of seal when using purge gas lighter-than air.atmospheric pressure at the top of the seal. exemplified by National Airoil's Fluidic Seal. Their use shall be subject to BP approval. These are not very commonly used. a. or other accumulation cannot occur during any phase of operation (start-up. Page 44 of 62 . and the labyrinth prevents atmospheric ingress. dust. The possibility of ignition occurring below the velocity accelerator from any conceivable cause. If an efflux velocity accelerator is used. There are no credible cases during any phase of operation (start-up. consists of a flow restriction in the form of a series of stepped cone sections of changing diameter. emergency. which prevents air from entering the flare stack. shutdown. materials liable to polymerisation. also referred to as an inverted gas seal is known under the trade name of John Zink Molecular seal. especially at high discharge rates. carry-over. air entry can no longer be precluded and a risk of an explosion exists. d. Flame arresters Flame arresters shall be used only in clean systems in which plugging. scale build-up. etc. Deceleration or reverse flow due to condensation of vapour or gas cooling and contraction within the flare system. Purge gas heavier-than-air 'floods' the seal.) and if there are no practical alternatives. Though called seals. etc. when the volumetric condensation or cooling rate of vapour in the relief system exceeds the purge rate plus the incoming gas volume. or other failure so that flashback becomes possible. emergency. f. the buoyancy of the purge gas creates a zone of greater-than. Possible back mixing and deceleration effects due to wind. dislodged from the flare tip. only reduce it. normal. etc. etc. mechanical failure. the following points shall be considered: a. b. labyrinth gas seals require drains. flame speed). Their disadvantage comes from the fact that they can easily become blocked by dust. The efflux velocity may be increased by the use of an orifice plate with a single or multiple orifices.4. c.) in which flow could be reduced by human error. These can block with ice or carbon. The increased back pressure imposed by the orifice plate.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems The labyrinth type. The possibility of the orifice plate becoming blocked or corroded in service. b. or Flaregas 'Flarex'. if any. 10. They are both installed immediately below the flare tip. The flow restriction type. corrosion products. e. shutdown. c. Flame arresters are another way of preventing flashback. However. neither stop the reverse flow completely. Due to the ingress of rain water and the possibility of condensation. but could be effective against flash back. Flare headers should not normally be insulated due to corrosion under insulation concern. Design and construction a.2. dump valves. If this is not possible.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 11. or other common cause failure shall be included in the main flare header lateral and header design along with pressure relief devices that open at the same time. and other devices that open to flare in the event of instrument air or electric failure shall be included in the main flare header design. other process plants. depressurisation valves. These pots shall also be routinely checked for and drained of any accumulated liquid. 3.g. provisions shall be installed and maintained to prevent liquid accumulation (e. Any relief or flare system liquid carryover or condensed liquids shall drain to and be collected in its associated knockout drum which shall be sized per clause 9. bellows should only be used when essential. The route should avoid areas of high fire risk..1. Flare and relief line headers and piping 11. Individual relieving devices in closed systems shall be located above the header if practical. etc).1.. Both relief headers and flare lines should slope all the way towards their respective knockout drums at a minimum 1 in 400 slope. Thermal movement of flare lines shall preferably be accommodated by providing flexibility in the piping layout or alternatively by expansion loops. depressurisation valves. c. The main flare header shall be sized for the worst case originating from a fire or a common mode failure (i. f.e. Sliding expansion joints shall not be used. e. 2. automatic pump-out facilities and frost protection if required. the routing and fire protection of the line supports shall be proposed for BP approval. Liquid accumulation in flare header low points can result in slug flow and liquid hammer effects causing possible header or elbow failure and shall be avoided. When flaring streams likely to contain H2S or water vapours. A flare line should be routed to avoid areas of high fire risk or otherwise hazardous areas. Horizontal sections of line to accommodate possible flow in either direction are not acceptable. Any use of piping bellows shall be subject to approval by BP. 11. loss of cooling media. drip pan elbows draining to a safe location). a neighbouring unit had to be shutdown for safety reasons. whether in the unit of origin or in another unit. Each individual unit flare header shall be designed to handle the worst single relief rate or the largest combined rate from the pressure relief devices within that unit as a result of any common event. This involved dumping to the flare through a flare line passing through the unit on fire. b. If this is not practicable. Vent valves. b. loss of instrument air. during a process unit fire.g. power failure. e. electric failure. Page 45 of 62 . If relief devices are located where liquid can accumulate in their discharge lines. d. The pots shall be fitted with a level gauge. Routing a. dump valves. drainage pots shall be provided at low points. Vent valves. and other devices that open to flare in the event of instrument air failure. An incident is known in which. 1. The line was damaged by the fire and fed it with additional material. shall be designed for this condition. In many cases. i. Butterfly valves shall not be used because of uncertainty in failure position (i. m. particularly foundations and supports.. isolating block valves with flushing connections. locking devices (car seals or chain locks for example) which can be locked open and spectacle blinds upstream. 12. may stick closed or fail in a closed position). If using automatic controls. ice formation or auto refrigeration cooling in cold service. shall be provided in sub-headers at the unit battery limits.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems g. l. acoustic fatigue due to high velocity flow. pilot flame detection and instrumentation design. manual actuation of the steam system upon visual detection of smoke is adequate to minimize smoking. 12. shall be provided to enable all parts of the relief system to be purged and steamed out. H2S) that may be vented into the flare network and migrate back into interconnected piping. 1. Excessive steam flow is not only costly but also increases flare noise. Large spade or spacer blinds can be difficult to identify externally in a congested process plant and may require additional operational controls. and fire protection.e. 4. If several units are connected to one flare system. If it is required. Isolating block valves shall be provided with: position indicators. Relief devices and header material of construction selection shall consider potential corrosive materials and other materials that can cause degradation (e. and not simply the maximum specified relief temperatures. operation.1. blanked. the higher quality material shall be used for at least 10 m (33 ft) upstream of the change in the process conditions. h.g. two-phase flow. Control and instrumentation Refer to GP 22-20 for details on flare controls. all components. if any. Pipe stressing and anchor and support design shall allow for thermal expansion or contraction. Consideration shall be given to the need for hydrotesting after construction. j. In order to avoid expensive over design. 3. selection. k. drain branch shall be provided upstream of the block valve to facilitate the draining and purging of the isolated branch. unless otherwise specified by BP. 2. If specified by BP.. in view of possible backflow. Flare smoke control a. If headers of different materials of construction are connected together. A valved. b. three main types of control systems that may be used are: Page 46 of 62 . See GP 44-70. Purge gas connections. and maintenance issues. slugs of liquid. including vents and drains. Gate valves shall be installed in a horizontal position so that the gates cannot fall into the closed position should they become detached from the stem. Rising stem gate valves are preferred. smokeless flares using steam or other pressurized fluids for smoke suppression should be equipped with either manual or automatic control systems which will apportion the suppressant to the flare gas to produce clean burning without excess flow. These shall be connected to the fuel gas system or nitrogen supply as specified by BP. the flare header mechanical design should be based on a realistic evaluation of the maximum temperatures and durations of each relief situation. Controls shall be matched to flaring conditions in the process to conserve smoke suppression fluids while preventing smoke from being formed during a flare event. The preferred method of control is by flare gas flow. Signals from the monitor shall operate an actuated control valve via appropriate converters which shall be adjustable up to the required flow range and down to zero flow under normal conditions. d. Purge control Continuous purge flow rates for the flare system shall be monitored and alarmed. Smoke control using system 12. burn-back detection shall be provided by one or more thermocouples in thermopockets whose location inside the flare tip shall be subject to approval by BP. Burn-back Burn-back detection is not normally required. The thermocouples shall be wired to control room alarms through a temperature switch adjustable for a temperature range appropriate for the tip. If system 12. 12. sun/cloud) may be required. adequate purging) should be used. f.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 1. Based on measurement of the flow rate of the flare gas through an ultrasonic mass flow meter.1. 2. compensation for ambient variations (night/day. The disadvantages are that it requires a very precise aiming that can easily be disturbed. Either refractory lining or burn-back prevention methods (i. it shall contain facilities for on-stream inspection and maintenance of all the important parts of the system and where specified by BP. or by paralleled high level radiation sensors spaced around the stack just below the tip. If burn-back in the tip can occur. Appropriate control algorithms shall be developed to automatically position valves and controls for smoke suppression.e. it also has a fast response.3. Ground mounted optical flare infrared radiation sensor. g. 1. 2.1 above depends on the fact that radiation from a smoking flame is greater than that from a smokeless one.b. For both types of radiant heat measurement. The telescope shall be of waterproof design and allow regular cleaning of the lenses. Page 47 of 62 . The optical monitor shall be a rugged telescope with a restricted field of view. 3. equipped with a photocell sensitive to near infra-red radiation. c. As the flare operates over a very wide range of flow rates. the flow-measuring device shall not obstruct the line or reduce its capacity.1. 12. the system shall include a density-measuring device to provide correction using more suppressant for heavier hydrocarbon gases. and is not sufficiently selective to permit its use for multi-burner installations.3 is proposed. Manual control to allow direct operator intervention shall be provided. High level flare radiation sensor. The advantage of an optical monitor is that it is located at ground level and therefore can be checked and maintained at any time.2. Measuring the radiant-heat energy from a portion of the flame may be achieved either by an optical monitor located at ground level a moderate distance from the flare stack base and trained on the base region of the flame. Density measurement and compensation need only be considered if flared gas accounting is necessary.b. h. e. The oxygen analysing installation should be suitable for mounting in an outdoor. A portion of the sample gas shall be taken through a regulating needle valve to an oxygen analyser and exhausted to atmosphere. c. below the tip exit. d. Flow measurement should be by use of an ultrasonic mass flow meter unless an alternative is approved by BP. If an alternative type of flare meter is used which is installed in the flare gas stream. such as a reliable. Under these conditions it must be assured that no air ingress occurs in the vent/relief system potentially forming an explosive mixture. The oxygen sampling probe shall be located 8 m (25 ft) or 15 diameters. accounting). The very wide range of the flow rates between a purge and a full emergency release presents a difficult problem for the instrumentation. c. fitted upstream with liquid knockout pot. If installed. and installed per manufacturer’s recommendations. 2. Flow measurement of the flare gas may be required for three reasons: 1.O. etc. If ultrasonic mass flow meters are used.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 12. Information (e. particularly considering potential flare radiation impacts. b. The sample gas shall be withdrawn by a diaphragm type vacuum pump. 1. 12. they shall be mounted in the flare line downstream of the off-site knockout drum. An additional problem is contaminants that are often present in the gas. flaring rates). the typical alarm point for the oxygen monitor should be about 1% oxygen in flare gas. The oxygen sensor and its electronic support package shall be capable of operating in the expected temperature ambient range. Page 48 of 62 . exposed location at the base of the stack or on the flare K. If located in an area where radiation level may exceed 4. Local and control room indications and alarms shall be provided as specified by BP. This is required to avoid a fluctuating pressure in the sampling line. Oxygen monitoring is not generally required if there are alternate means of assuring the system is air-free. loss management. whichever is the smaller. The primary purpose of oxygen monitoring equipment is to ensure that if a minimum purge rate is used an explosive atmosphere does not result. continuous purge. Particular care should be taken when using “light” purge gas coupled with taller stacks. due to changes in the pressure drop through the stack induced by changes in the flow rates. drum. Regulatory reporting purposes (i. e.e.4. It may also highlight the spurious ingress of oxygen due to operating deviations.g. Flow measurement a. the measurement device should be inserted via a seal housing and isolating valve. d. providing the capability of withdrawing the instrument during flare operation.73 kW/m2 (1 500 Btu/ft2h) it shall be provided with suitable shielding. See GP 30-10 and the GP 64 series on flow measurement.5. Oxygen monitoring a. The probe piping shall be in accordance with vendor requirements and shall be resistant to fouling or contamination by CO2 or other waste gases in the flare stream. b. Control of the flow rate of the smoke suppressant. 2. and returned to the stack above the sample point. such as contraction of flare stack gas during rapid cooling or a process shutdown.. 3. 7. For each liquid seal: a) Level indication. c) Low flow alarm. 5. For purge gas: a) Flow indication. b) Flow control. For each knockout drum: a) Level indication. c) Pressure control valve. d) Pressure indicator. b) High oxygen contents alarm (if oxygen monitor installed). e) Upstream pressure gauge.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 12. For flare stack internal atmosphere: a) Oxygen contents indication (if oxygen monitor installed).6. d) Liquid temperature indication. c) Level switches or transmitters (if automatic operation of pumps is used). b) High level alarm (This alarm shall be a critical alarm). For steam to flare stack (if installed): a) Flow control: automatic and/or manual. For pilot gas: a) Flow indication. b) Flow indication and recording. b) Pressure control valve. Requirements for instrumentation a. For air to pilots (if a separate source): a) Flow indication. d) Level gauge. f) Adequate instrumentation for liquid sump tank and liquid overhead drum if fitted. Page 49 of 62 . 2. c) High level alarm. e) Temperature control (by steam or electricity). e) Pilot flame failure (alarm). c) Low flow alarm. b) Low level alarm. d) Indication of back-up supply in operation. 3. b) Low flow alarm. f) Temperature indicators/alarms for liquid phase (as appropriate) 4. 6. Normal requirements for flare system instrumentation are as follows: 1. 2. Testing The decision on the acceptability of pneumatic testing should be taken at an early stage in the design and cannot be left until the line is constructed. commissioning and on-site performance testing of the flare system will be specified by BP. listing the precommissioning and commissioning activities based on GP 32-20. c) High/low temperature alarm (either or both as appropriate). If reduced flow testing is proposed by the flare supplier. d. 14. e. One set of spares to cover the first overhaul. b) Temperature indication. Supplementary (optional) requirements are as follows: 1. One set of spares for the smoke-suppressant apportioning instrumentation. The flare vendor shall produce documentation for BP approval. One of each type of the equipment forming part of the ignition panel. Spares a. This shortens the tip Page 50 of 62 . Apart from procurement difficulties. 6. They should consider the following as a minimum: 1. they shall clearly demonstrate scale-up factors to validate the full flow flare capacity. 2. burning in the tip and higher metal temperature may result. For flare gas from knockout drum to flare stack: a) Flow indication and recording. On many construction sites there is a great reluctance to carry out such testing. f. Other flare rates such as smokeless flare capacity or steam/assist gas flow rates shall be demonstrated by the vendor either in their facility or on site. For steam to flare stack: closed circuit television monitoring of flare tip. Unless otherwise specified by BP. Spares lists shall be compiled by the flare vendor and submitted for BP approval. cold testing and static testing of the flare system in accordance with GP 32-10 and GP 32-20. Replacement for all the gaskets for the joints that have to be broken during construction or after testing. 4. c. Maximum use of computer simulation and modelling should be performed to minimize actual flare testing. One complete pilot burner. consideration must be given to selecting welding consumables with good fracture toughness to guard against brittle fracture and give additional confidence in the safety of the line during test. 3. 5. b. One complete set of spare thermocouples. full or maximum flow testing using air and measuring the associated pressure drop shall be carried out by the flare vendor in their fabrication facility. Any further requirements of the flare vendor for the attendance of specialist operators and service staff during the precommissioning. The flare vendor shall carry out the flushing. using testing methods proposed by the vendor and approved by BP. If purge is at the minimum rate and only sufficient to prevent air ingress at the tip. 13. a.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems b. providing any special equipment required for this testing. removal of relevant relief devices from the blowdown system. platforms. etc.5). Benzene. buildings. Discharging flammable liquids into the sewer system or venting from the stack in the event of an overfill could lead to a vapour cloud explosion or other significant hazards. options such as elimination of relevant relief cases (HIPS. stacks and blowdown drums or receivers open or otherwise vented to atmosphere) in hydrocarbon and/or toxic service are not allowed unless all of the following criteria are met: a. b.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems life. ignition sources. etc. potential liquid carryover. Have a liquid residence time of at least 30 minutes or free space greater than the maximum possible quantity of liquid that can be discharged into the drum after high level alarm (based on highest anticipated liquid relief load). meet current API RP 521 or ISO 23251 design criteria for flare knockout drums with the additional measures noted as follows. Flammable gas concentrations shall be below 50% of the Lower Flammability Limit (LFL) or below the Short-Term Exposure Limit (STEL) at grade.). The blowdown drum shall: 1. 3. This shall include consideration of all relief scenarios and modelling a range of cases. while still flammable. flammable liquids with a flash point < 37°C (100°F) are prohibited from atmospheric blowdown systems. Page 51 of 62 . usually greater than the cost of replacing a tip.4. Because of the high potential for generating a heavy vapour cloud. b. Atmospheric blowdown systems (i. or sources of ignition. Dispersion modelling with subsequent hazard consequence analysis shall be performed to ensure that residual vapours vented from the blowdown stack do not pose flammable.1. 4. 15.. and shall be able to empty out within 2 hours of high level alarm (unless the free space is greater than the maximum possible quantity of liquid that can be discharged into the drum). potential odour. buildings. or elimination of the blowdown system shall be considered. d. c. but may produce significant energy saving. If dispersion and consequence analyses indicate that flammable or toxic levels may be exceeded. H2S. or other hazardous conditions (see clause 6. Lower stack velocities tend to reduce dispersion and can increase the potential for the cloud to reach grade. potential ground level emissions. The design of the atmospheric blowdown drum shall. process and breathing air intakes. redesign of equipment. The design review of hydrocarbon blowdown stacks to atmosphere shall be consistent with GP 48-02 and shall consider personnel health and safety. platforms. as a minimum. 1. The above spare components should be evaluated on economic grounds. noise. toxic. process and breathing air intakes. Design criteria Hydrocarbon blowdown systems discharging directly to atmosphere shall be in compliance with HSSE design and loss prevention requirements per GP 76-01. and thermal radiation. including an allowance for change-out time. depending on which are the most sensitive. and other toxic gases or carcinogens shall be controlled at levels below those specified in the BP gHSSEr guidelines and/or applicable local and national standards for hazardous or toxic materials. 2.e. Lower venting rates than the design basis (design basis is usually the highest rate) must be considered. a) The residence time shall consider the maximum level of slop liquid in the vessel that can be expected. Blowdown system 15. maximum of 20 micron (0. If quench systems are used. 2.2. but in no case greater than 65°C (150°F) before discharge to any ‘open’ system such as an oily water sewer. g. An appropriate and reliable continuous inert purge gas shall be in place at a rate determined by the Husa correlation to prevent air intrusion. b) Design of level instrumentation should include functionality for ease of on-line maintenance and may therefore require redundancy. elbows. 1. The reliability (SIL level) of the high level trip function shall be determined per GP 30-76. h.e. a) There shall be a procedure in place for operator response to prevent overfilling under all foreseeable conditions. Inlet and outlet piping shall be designed to provide adequate vapour-liquid disengagement. including start-up. process systems connected to a vent stack are susceptible to overpressure of only a few pounds and may be very susceptible to vacuum conditions. f. In general. shutdown and other non- routine operations. i. 15. may accomplish this). 2. they must comply with clause 10. possible consequences from failure of the quench medium. shall be considered.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems b) No credit shall be allowed for pump out systems in determining the residence time.. Flame arresters in vent stacks (not a ‘clean’ service) are prohibited because of plugging potential. resulting in release of excessive vapours to atmosphere and/or hot liquids to the sewer. 1. Consideration shall be given to potential ignition of vapours from the vent stack. Not be used for temporary storage of liquids generated from maintenance or other activities. An overflow via a gooseneck to a suitably designed closed oily water sewer can be installed as a backup. such as baffles. it should not directly contact liquid that is > 93°C (200°F) to prevent ‘steam explosion’ events that can lead to overpressure of the vessel and/or entrainment/discharge of hydrocarbon liquid to the environment. Steam is not an effective purge for prevention of air infiltration because it can condense. Be designed to knock out liquid droplets such that any remaining droplets act as vapour (i.4. e. In most piping configurations this would require installation of internal diverter plates. Normal design is to provide an adequate purge rate to prevent ‘flashback’ into the vent system. If water is used as the quench medium. Have a continuous level indication and high level alarm with an independent high level trip. 3. 4. Whenever flame arresters are used. Methods to extinguish the flame shall be determined. j. Quench designs should take into account the need for good contact between the quench stream and the hot vapours being cooled/condensed (internals. Liquid in the blowdown system shall be cooled to less than its flash point. Blowdown system liquid handling The preferred disposal method for liquids is via pump to a closed slops system.000 8 in) liquid particle size). or other deflecting devices which shall be properly maintained. 2. Thermal radiation and possible impacts on nearby areas where personnel and equipment can be located shall be evaluated. Page 52 of 62 . or +/–1% of temperature being measured. and to provide the necessary residence time and minimum temperature as required by regulations. and local regulations. Emergency vent to atmosphere may be used if it meets applicable country. b. the incinerator shall be designed to reduce VOC emissions by 99%. 3. Page 53 of 62 . If vent is to an incinerator. e.1. 2. d. d. c. Incinerator shall be equipped with continuous recording temperature monitoring devices. 4.4 shall be met. Verify performance during start-up. Piping components for vent lines shall be designed and installed as process piping. minimum. including start-up and normal operation. Systems should have provisions to automatically start/stop the pumps via level control. If venting directly to atmosphere. all criteria in clause 6. of VOC emission vented. Combustion chambers. whichever is greater. Closed vent system a. 16. Winterisation protection should be provided if the gooseneck contents can freeze at minimum ambient temperatures. state. 4. Vent systems 16. Gooseneck seal depth should take into account the maximum stack back-pressure at blowdown conditions. Monitor temperature in combustion zone. Vapour recovery system. General a. Vents shall be provided to safely dispose of hydrocarbon vapours from equipment into one of the following closed systems: 1. Sump vent stack. Flow indicator shall be installed on the vent system. 3. Vents from process systems handling streams containing greater than 10 ppmw benzene on an annual average may need to be controlled by a device such as carbon canister or an incinerator. 16. Incinerator/thermo oxidizer. Measure temperature immediately before and after catalyst bed (if there is a catalyst bed in the incinerator).31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems provided it can be demonstrated that the closed sewer system can adequately handle the materials (and the above restrictions on temperature and flash point are met). Have an accuracy of +/–1/2°C (+/–1°F). Tie-in of a vent to the flare system shall consider the impact of higher flare header pressures under upset conditions on upstream vent process vessel pressures. If vent is to a vapour recovery system then system shall be designed to recover 95%. 2. Process gas line (either fuel gas or wet gas). b. c. which shall: 1.2. minimum. 5. Flare system. 6. 2. and/or process controls reduce the volumes of flare/vent gases? b) Can users be found for recovered low pressure gas? c) Can recovered low pressure gas be transferred to another facility for consumption or recovery? d) Can eductor compression technology be provided using high pressure gas to compress low pressure gas (may be applicable in oil/gas production facilities)? e) Can compressor(s) be used for compression regarding utilities. the recovery system shall not be used. Environmental Cost Factors: What effects would eventual use of the flare gas recovery system have on capital. Page 54 of 62 . Reputation issues include: public and government interest. Further process design aspects when using flare gas recovery can be found in ISO 23251 or API RP 521.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 17. and weight limitations? f) Variability of flow and value of available volumes for recovery versus cost of system? g) Purity of gas and is a gas cleanup system required? 4. Technical Feasibility: On a case by case basis. Impact on Safety: Would implementation of flare gas recovery potentially have a significant negative impact on safety? If yes. export. space.. operational.g. or re- injection. Flare gas recovery issues are closely related to BP’s HSSE policy that specifies “No accidents. 3. is the goal to recover part or all vent gases technically feasible? If not. 5. Reputation Issues: Even though reputation issues cannot readily be quantified.g. The eventual use of flare gas recovery may have effects on all three principles of this policy. eliminate source as first option) or use flare gas/vent gas recovery system and use recovered gas for internal use. Flare gas recovery should be evaluated on a project specific basis. Eliminate routine flaring/venting (e. and maintenance costs? Justification for use of the recovery system shall include costs for each category. The effects of flare visual impacts. etc. operational practices. they should be considered in the evaluation process. Good Engineering Practice: Some topics in a good engineering practice review should include: a) Can process design improvements. b. and use of inert gas blanketing instead of fuel gas. odour nuisance of flare plume. alternative technical solutions should be considered.. no harm to people. and no damage to the environment”. These systems collect and recover low pressure gases from the flare header system. noise impacts at sensitive times (evenings. are some issues that need to be considered in this group. along with any third party impact. 2. balancing fuel gas systems. 3. nights). such as reduced leakage in pressure reducing valves. maximising product recovery and minimising flaring environmental impact. c. Flare gas recovery systems a. Basic issues associated with decisions to use or not use a flare gas recovery system should include: 1. Recapture fugitive gases associated with loading/unloading facilities. The safety implications for any project utilizing this option shall be evaluated. use of high integrity blowdown compressors). The following are some examples related to flaring reduction/elimination: 1. Consider options to prevent the occurrence of non routine and emergency flaring/venting (e. Some vendors have developed and are offering a flare ignition system without use of conventional combustion based pilot design or conversely have a need to continuously operate these pilots when they are provided. 1. Page 55 of 62 . e. the flare stack still needs to have the continuous purge gas flow specified in clause 10. the system is a proprietary design and patented. in most applications.1.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems 4. For safety reasons. The choice of equipment is dependent on the process design and installation specifics and needs to be assessed individually in each case. may be below minimum rates for flare tip design. If not required from a regulatory standpoint. however. Two basic types of flare gas recovery systems are currently being used in flare gas recovery and compression: the ejector system and compressor based system. An isolation valve closes the gas flow to flare and diverts flow to the flare gas recovery system. Recover low pressure vent gases produced by gas blanketing operations. The system includes all required components. The flare gas recovery system is typically installed downstream of the flare knockout drum. During flare gas recovery operation. Various vendors are currently offering flare gas recovery system/equipment as a complete system (kit or skid based). many regulations require flare pilots to be left in service (i. In some cases. a rupture disk shall be installed in parallel with the isolation valve. f. Flare gas recovery equipment capacity is typically selected to handle normal or routine gas flows to flare with spare capacity to manage smaller releases from blowdown valves/pressure relief valves. During larger releases. the flare stack will be left without flow.. resulting in tip mechanical damage and short tip life. Most flares are designed for the anticipated maximum flow rates. it requires a high rate (due to a low overall efficiency) of high pressure motive gas that in many cases limits its application. Low flaring rates. typical or routine flaring rates are substantially lower. The ejector based system offers very high availability and is virtually maintenance free.11). the recovery system is isolated from the flare system and full flow is diverted to flare where it needs to be ignited and burned (see clause 8. continuous ignition source). 2. the decision to shut down the pilots shall be evaluated by performing a risk analysis and evaluation of pilot ignition system availability and reliability. however. If flare gas recovery is implemented. Therefore.e. The compressor based system is superior from an energy efficiency viewpoint and provides larger system capacity. d. Another important economic and safety benefit from having flare gas recovered and not continuously flared is the expected life of the flare tips and pilots is increased. 7 to 8.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems Annex A (Normative) H.067 (No wind) Nitrogen: K = +1. volume percent Ci = Volume fraction of component i Ki = Constant for component i (see above) These equations can be simplified using the standard criteria of limiting the oxygen concentration to 6 volume % 25 ft (7. If the purge gas mixture is heavier than air.651 C4+: K = –6.46 ln    Ci K i  y  O2  i where:  Q = purge gas rate. m y = column depth at which the oxygen concentration (O2) is to be predicted.328 Nitrogen: K = +1.65  Q  201.9 m/s) Ethane: K = –1.651 CO2: K = –2.9  n 0. volume percent Ci = Volume fraction of component i Ki = Constant for component i.783 Helium: K = +5.067 Propane: K = –2. m3/hr D = flare stack diameter.07068D 3.707 (Wind 15 to 20 MPH or 6.W.1.66 D 3.c): Page 56 of 62 . In metric units: 1  20.586 Note that steam or other condensable is not a suitable purge gas.9  n 0.62 m) down the flare stack (note that lower oxygen concentrations should be used for certain compounds such as hydrogen – see clause 10. Husa’s correction formulae The Husa correlation shall be used to calculate minimum purge gas flow rates for gases lighter than air. The Husa correlation may be expressed either as: 1  20. 46 ln    Ci K i  y  O2  i  where: Q = purge gas rate.078 Methane: K = +2. ft O2 = oxygen concentration. SCFH D = flare stack diameter. in y = column depth at which the oxygen concentration (O2) is to be predicted. Typical values for Ki are: Hydrogen: K = +5. m O2 = oxygen concentration. a purge rate based on nitrogen shall be used.65  Q  0. in K = constant (see above) In metric: Q = 0.0004044 D3. Locations where prevailing winds can often exceed this wind speed may need somewhat higher purge rates.065 (29–MW i)) Values of Ki for gases heavier than air are determined from an amended Husa correlation where (MWi–29)) is substituted for MWi as follows: Ki = exp (0.46 K where: Q = purge gas rate.46 K where: Q = purge gas rate. Note: It should be recognised that the Husa correlation was derived under calm or no wind conditions and is generally applicable for wind speeds up to about 13 m/s (30 MPH).0004044 D3.065 (MW i–29)) where MWi = molecular weight of the ith component of n components. Page 57 of 62 . m K = constant (see above) Values of Ki for gases lighter than air are determined from Ki = exp (0. SCFH D = flare stack diameter.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems Q = 0. m3/hr D = flare stack diameter. Note. click on the “Next” button as some material may still continue on that topic. Simply select “Module 7 (Flare)” in the following website: http://amposs408/cmas/process_operations/. training material and an assessment tool on a variety of process areas including Flare Systems.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems Annex B (Informative) Flare system training information BP has a website containing general process background. A pop-up screen will ask for login information. but no login is required. Page 58 of 62 . merely click the topic of interest in the table of contents (left hand side) to proceed. though some screens will indicate “End of Page”. photos. 31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems Annex C-1 (Normative) Atmospheric relief chart Page 59 of 62 . 31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems Annex C-2 (Normative) Blowdown system assessment Page 60 of 62 . 31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems Annex C-3 (Normative) Relief system studies and documentation Page 61 of 62 . 90/2: The use of Halons in Firefighting (Feb 1990).BHRA Fluid Engineering. 410 [4] Internal Flow Systems edited by D. Fittings and Pipe. England. Miller . Bracknell.S. published by Group Safety Centre (now Corporate Safety Services) Page 62 of 62 . published by Gulf Publishing or VDI Waermeatlas [5] GN 44-001 Relief System Design Guidelines [6] Safety Guidance Note No.31 March 2006 GP 44-80 Guidance on Practice for Relief Disposal Systems Bibliography [1] Van Krevelin and Chermin and reported in the transactions of the Seventh International Symposium on Combustion. 1959 [2] Meteorological Office. Crane Technical Paper No. ref D/Met 01/21/1/2/L [3] Flow of Fluids Though Valves.