Study Material NPTI- Boiler

STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) 1 ©NPTI (NR) PGDC 10th Batch 2005 STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Thermal Power Plant In a thermal power plant, steam is produced and used to spin a turbine that operates a generator. Shown here is a diagram of a conventional thermal power plant, which uses coal, oil, or natural gas as fuel to boil water to produce the steam. The electricity generated at the plant is sent to consumers through high-voltage power lines. 2 ©NPTI (NR) PGDC 10th Batch 2005 STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) 1 Chimney. 6 SCAPH. 11 Final SH. 16 Cold Reheat Line. 21 Furnace. 26 Coal Mill. 31 Water Platent. 36 Ejactor. 41 Boiler Feed Pump. 46 Control Structure. 51 Air Circuit Breaker. 2 ID Fan. 7 Air Pre Heater. 12 Platent SH. 17 Hot Reheat Line. 22 Burner. 27 Coal Crusher. 32 HP Turbine. 37 Condensate Pump. 42 HP Heaters. 47 Generator. 52 Cooling Towers. 3 ESP. 8 Economizer. 13 Extended Steam Wall. 18 Boiler Drum. 23 Wind Box. 28 Seal Air Fan. 33 IP Turbine. 38 Gland Steam Cooler 1,2. 43 Makeup Pump. 48 Hydrogen Plant. 53 CT Pump. 4 FD Fan. 9 Feed Water Line. 14 Reheater. 19 Down Commer. 24 Hot PA Header. 29 RC Burner. 34 LP Turbine. 39 LP Heaters. 44 Circulating Water Pump. 49 Main Transformer. 5 PA Fan. 10 Primary SH(LTSH). 15 Super Heated Steam. 20 BR Header. 25 Cold PA Header. 30 PC Pipes. 35 Condenser. 40 Deareator. 45 Water Treatement Plant. 50 Aux. Transformer. 3 ©NPTI (NR) PGDC 10th Batch 2005 STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) 4 ©NPTI (NR) PGDC 10th Batch 2005 . History of Boilers Early steam boilers consisted of little more than kettles filled with water and were heated on the bottom. and chimney or stack. The boiler (or evaporator) is that part of steam generator where phase changes (or boiling) occurs from liquid (water) to vapour (steam). essentially at constant temperature and pressure. burners fans. economizer and airpreheater along with auxiliaries such as pulvarizers. Boilers of the early 1700s still used the kettle principle. stockers. superheater. 33385 Early Steam Boiler A 5 ©NPTI (NR) PGDC 10th Batch 2005 . but burned the fuel in an enclosed furnace to direct more heat to the boiler kettle. similar to those shown in Figures a and b. However the term “boiler“ is traditionally used to mean the whole steam generator. dust collectors and precipitators. boiler or evaporator.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Definition: BOILER Steam Generator is a complex integration of furnace. ash handling equipment. Haycock Boiler. reheater. To prevent a deficiency of combustion air. a bellows was used to force air to the combustion zone and gases through the flue in what was the first application of forced draft.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) 33386 Early Steam Boiler B Boiler Efficiency In the mid 1700s. an integral furnace was developed with the fuel actually burned in a container enclosed within the water vessel (Figure 3). boiler designers noted that nearly half of the heat from the fire was lost because of short contact time between the hot gases and the boiling heating surface. 6 ©NPTI (NR) PGDC 10th Batch 2005 . A smoke flue wound through the water from the combustion chamber to the atmosphere much like a coil in a still. To improve boiler efficiency. While this fire tube design was popular until about 1870. 7 ©NPTI (NR) PGDC 10th Batch 2005 . many gas tubes that increased the heating surface replaced the single flue. More water was subjected to the heat from the flue gases. Many disastrous explosions resulted from the direct heating of the pressure shell that contained large amounts of water at saturation temperature. it was also dangerous. This design would limit the consequences of a pressure-part rupture.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) 33387 Early Fire tube Boiler Boiler Safety As the demand for power increased. Boiler designers recognized that one way to overcome the deficiencies of the fire tube boilers was to develop a water tube design in which the heating surface consist of water-filled tubes. 64 in. The design incorporated inclined water tubes connecting water spaces at the front and rear of the furnace with a steam space above (Figure 5). John Stevens. Built and Patented by William Blakely in 1766. 42 in. First water-tube boiler. Water-tube boiler with tubes connecting water chamber below and steam chamber above. 1805. along with the reduced steam explosion hazard. John Cox Stevens.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Benefits While several water tubes boiler designs were patented between the late 1700s and the mid 1800s (Figure 4). 1803. it was not until 1856 that a significant breakthrough occurred. 46 in. It provided a better water circulation and more heating surface than other designs. Water-tube boiler of small tubes connected at one end to a reservoir. 33388 Early Water tube Boilers 8 ©NPTI (NR) PGDC 10th Batch 2005 . and steam temperature. 1856. electronic analog control. In addition. Beginning in the 1970s fuel economics have influenced changes in boiler control. During the 1950s burner control systems were developed to start and stop burners and to include flame safety systems.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Inclined water tubes connecting front and rear water spaces complete circuit with steam space above. combustion. Stephen Wilcox. furnace draft. feedback control was used to control the level in the boiler by regulating the water to the boiler. In the 1960s control switched from predominantly pneumatic analog control to predominantly solidstate. Also during this time. 33389 Early Inclined Water tube Boiler Boiler Control Boiler control began in the late 1700s with the introduction of the "flyball" governor for speed control of the first rotative steam engines. Between 1950 and 1970 not much money was invested into boiler control development because of the continual reduction in fuel prices relative to the cost of boilers and boiler accessories. Using automatic draft regulation also used feedback control to control steam pressure. There were no further advances in boiler control until the early 1900s when integrated systems were designed to control steam pressure. discrete element. the development of microprocessor control has caused an advantageous transition to the greater precision of digital control. 9 ©NPTI (NR) PGDC 10th Batch 2005 . The high price of fuel has allowed a greater degree of control sophistication than could be justified in 1970. and feedwater. Early fire tube boilers consisted of a spherical or cylindrical pressure vessel mounted over the fire with flame and hot gases around the boiler shell. In the water tube boiler heating the riser tubes with the hot flue gases causes the water to circulate and steam to be released in the boiler drum. In the Dry back Fire tube Boiler in which the combustion chamber is lined with high temperature insulating material. Today a typical water tube boiler has a single burner with up to approximately 125. the most common fire tube boilers are similar to the Scotch marine boiler and Wetback Fire tube Boiler in which the combustion chamber is water-jacketed. Early water tube boilers were shown in Figures 4 and 5 of this module. The heat transferred through the walls of the tubes to the surrounding water generates steam. transferring their heat to the water.000 pounds per hour steam flow but is available in sizes up to several million pounds per hour with more than one burner. Other fire tube boilers include the Locomotive-type boiler and the Scotch Marine Boiler. Today. jacketed by a larger cylinder fitted with several passes of fire tubes. 10 ©NPTI (NR) PGDC 10th Batch 2005 . To increase the heat transfer area and improve the heat transfer coefficient. Water tube Boilers Water tube Boiler's design features one or more relatively small drums with many tubes in which the steam/water mixture circulates. longitudinal tubes were installed in the pressure vessel and flue gases were passed through the tubes. The flue gases are cooled as they flow through the tubes. The Scotch Marine Boiler is designed with the combustion chamber as a long cylinder.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Types of Boilers Fire tube Boilers In Fire tube Boilers the hot flue gas products of combustion flow through boiler tubes surrounded by water. STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Drum Circulating Water Downcomer Boiler Riser Mud Drum Economizer Feedwater Riser Burners Superheater Steam Coil Air Heater To Stack Gas Outlet Air Air Heater Gas Air Induced Draft Fan Forced Draft Fan 33390 Modern Day Boiler 11 ©NPTI (NR) PGDC 10th Batch 2005 . The primary purpose of the steam drums however is to provide a free controllable surface for separation of steam from water and housing for any mechanical separating devices. (Figure 7). Some larger boilers use low head. because of heat absorption by the risers from the furnace. high working pressure pumps to provide positive circulation. The water from the steam/water mixture is then recalculated together with the makeup feedwater to downcomer circuits. The steam rises up through separation devices in the drum and exits to one or more superheating passes through the furnace. to reduce dissolved and undissolved solids in the boiler water. Most boilers rely on natural convection for this flow. The recalculation flow is from the steam drum via downcomer tubes to either the mud drum or the water wall header.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Drum Boilers operating below critical pressure are usually fitted with a steam drum. Steam Outlets Secondary Scrubber Primary Scrubber Cyclone Blowdown Feedwater Inlet Chemical Feed Upcomer/Riser 33391 Steam Drum: Figure 7 12 ©NPTI (NR) PGDC 10th Batch 2005 . Water treatment chemicals may be added to the steam drum and feedwater may be discharged. saturated steam is separated from a recalculating steam/water mixture. or "blown down" from the mud drum. and from there through riser tubes back to the steam drum. In the steam drum. Riser Heat collecting surfaces constructed from tubing and conveying boiler circulating water upwards to the steam drum are generally called risers. The downcomers are not heated and are located outside of the furnace cavity. Downcomer Water is carried down from the boiler drum to the mud drum or to the water wall feedwater header through tubes called downcomers. The heat is transferred to the air from the flue gas through a regenerative heat-transfer surface in a rotor that turns continuously through the gas and airstreams. turbulence. These heat absorption surfaces called water walls are fed from the water wall header at the base of the furnace.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Mud Drum The mud drum is completely filled with water and is the low velocity point of the circulating water. Waste heat from the flue gas is absorbed by the feedwater in order to improve efficiency. Unresolved solids that develop in the boiler gravitate to the bottom of the mud drum and can be drawn off. The risers may originate from either the water wall header at the base of the furnace. Air Heater The steam-generator air heater improves boiler efficiency by transferring heat to incoming combustion air from the flue gases before they pass to the atmosphere. 13 ©NPTI (NR) PGDC 10th Batch 2005 . Economizer Feedwater from the condensate-feedwater system enters the economizer located in the furnace flue gas ductwork. Boiler circulating water absorbs primarily radiant energy from the furnace fireball while resident in risers jacketing the furnace. Heat from the flue gases is added to the saturated steam from the drum. Burner The burner is used to introduce fuel and air to the furnace at the required velocities. and concentration to maintain ignition and combustion of the fuel within the furnace. or from the mud drum.. Superheater The superheater is a flue gas to steam heat exchanger. The steam coil air heater is normally in service only during boiler startup. or possibly low load conditions when the regenerative air heater cannot provide sufficient heat to the combustion air. and secondarily to make up for air heater leakage and for some seal-air requirements. The induced draft fan creates a sufficient draft to establish a slight negative pressure in the furnace. Combustion air flows across the tubes in order to provide a minimum combustion air temperature. Forced Draft Fan The forced draft fan supplies low head air necessary for fuel combustion. 14 ©NPTI (NR) PGDC 10th Batch 2005 .STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Steam Coil Air Heater The steam coil air heater is a tublar steam to air heat exchanger in which auxiliary steam charges the coil. Induced Draft Fan The induced draft fan used in a balanced draft furnace exhausts combustion products from the furnace. is converted to steam. The water wall and the boiler circulating water heat transfer surfaces cool combustion flue gases.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Systems Functional Overview A boiler is composed of two separate systems: the steam-water system and the draft system Steam-Water System Water enters the steam/water system. The combustion converts the chemical energy of the fuel to heat or to thermal energy. Draft System The draft system supplies the heat that is necessary to boil the water. Fuel and air enter the draft system and are mixed and ignited in a furnace. The economizer transfers heat from the hot flue gases to the boiler feedwater. COMPONENTS OF BOILER FLUID CIRCULATION SYSTEMS: FUNCTIONS AND BASIC OPERATION The fluid circulation systems of a boiler are the following: • Feedwater supply system • Feedwater conditioning and boiler blowdown • Boiling process and steam generation This section will discuss the components of these systems and the functions of these components. and exits the system in the form of steam.) The tubes receive radiant heat from the fireball and transfer it to the steam/water system. Additional heat is recovered from the flue gases by use of the combustion air preheater The combustion air preheater transfers heat from the hot flue gases to the combustion air. the radiant section of the furnace is lined with a heat transfer surface of boiler circulating water tubes (water wall or mud drum risers. is heated. Heat Transfer In most tube steam generators. Flue gases exiting the furnace also transfer heat to the working fluid by conduction as they pass through the various heat transfer surfaces. 15 ©NPTI (NR) PGDC 10th Batch 2005 . Saturated steam from the drum passes through the superheater and is discharged to the process as high pressure superheated steam. Steam pressure letdown and attemperation ©NPTI (NR) PGDC 10th Batch 2005 16 . The steam may be used to supply power turbines or manufacturing processes. These high-pressure pumps supply the economizer from the deaerator. The resulting intermediate pressure steam is used for feedwater heating. The function of the feedwater supply system is to continuously supply water to the boiler through piping to the steam drum. or low-pressure power turbines. The feedwater deaerator supplies the suction side of the boiler feedwater pump(s).STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Feedwater Supply System Figure 8 illustrates the basic feedwater supply system. Some of the energy in the high-pressure steam may be partially expended by the process of power turbines. Boiler feedwater from the economizer enters the steam drum. and the boiler circulating water system. Intermediate Pressure Turbine Lo Pressure Turbine To Process Header Intermediate Steam to Process Drive to BFP Lo Pressure Steam to Process Condenser Makeup Softened Superheater Steam Drum Economizer Deaerating Heater Superheat Spray Valve Condensate Storage Tank Condensate Feed Tank Feedwater Heater Boiler Feed Pump Deaerator Storage Tank Drive from Turbine 33392 Basic Feedwater Supply System Figure 8 Relatively cool water leaves the condensate storage tank and enters the deaerating heater and is deposited into a deaerator storage tank. processes using low-pressure steam. Deaerator Heater and Storage Tank The deaerator is used to eliminate air. The flue gas temperature decreases. Excess high-pressure steam. a shutoff valve and orifice. or occasionally backs to the suction side of the pump. Boiler Feed Pump The boiler feed pump is used to supply high-pressure boiler feedwater to the drum. A variable speed motor. The heaters may be classified either as low pressure prior to the deaerator or high pressure after the boiler feedwater pumps. CO2. 17 ©NPTI (NR) PGDC 10th Batch 2005 . The heat exchanger may be either air to steam or water to steam. A mechanical pressure relief valve. The flue gas exits the boiler and enters the economizer where it transfers heat to the boiler feedwater. a magnetic or hydraulic coupling.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) stations may provide supplements to the intermediate pressure steam loads. or modulating control valve may feed the recirculation line. or a steam turbine can drive a variable speed pump’s speed. and the boiler feedwater temperature increases. Most pumps in an industrial or utility environment will have some form of working fluid recirculation to prevent damage at "deadhead" or low flow conditions. Boiler feed pumps operates in a constant speed or variable speed manner. Low-pressure steam is phased back to liquid by the steam condenser and is stored in the condensate storage tank. If oxygen were allowed to enter the boiler. Economizer The economizer is used to recover heat from the flue gas to the boiler feedwater. the heat exchangers and condensate return piping would become corroded. These gases are removed by vigorous boiling and venting the gases to atmosphere. oxygen. and other gases from the boiler feedwater. If CO2 were allowed to remain in the water. A recirculation line is open at low flow to keep the pump from cavitating and overheating. Heating the feedwater is also necessary for the process of deaeration. resulting from sudden load drops may also be routed through letdown stations (sometimes referred to as steam bypasses) to the low-pressure steam header. serious corrosion could occur. Condenser The steam condenser in the feedwater supply system is a heat exchanger used to transfer sufficient heat from the low pressure steam to condense it back to its liquid phase. The discharge side of the pump is provided with a recirculation line back to the reservoir-feeding pump. Feedwater Heaters Feedwater heaters are used to heat the boiler feedwater so that less fuel is required to generate steam. A blanket recommendation of any one method is not realistic.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Feedwater Conditioning and Boiler Blowdown Feedwater conditioning and boiler blowdown are used to maintain a proper boiler chemical balance. Feedwater conditioning includes flash evaporation and reverse osmosis of sea water and the use of oxygen scavengers and corrosion inhibitors. Reverse osmosis is based upon the principle that if pressure is applied to the more concentrated side the solvent will flow in the reverse direction (Figure 9). water will flow to the unpressurized side. The vapor is condensed to form condensate and the precipitated solids removed and disposed of. Reverse Osmosis of Sea Water.Another procedure that is used to remove dissolved salts and minerals is reverse osmosis. One method that removes dissolved salts and minerals is flash evaporation. If the solution is salt water and a membrane is chosen that is permeable to water but not to salt. and dissolved oxygen (DO) are required for high-purity feedwater maintenance. The type of treatment to be used in a particular boiler should be based on the raw water supply. and other factors. The flash evaporator operates with its flash chamber under partial vacuum. Boiler blowdown can be continuous and is used to remove impurities in the boiler water. The monitoring of PH. the percent of make-up required. There are various methods for the internal treatment of boiler water.Feedwater conditioning removes dissolved salt and mineral solids that tend to form ions in solution. Osmosis is based upon the principle that when a semi permeable membrane separates two solutions of different concentrations. Boiler Feedwater (BFW) Supply Flash Evaporator Sea Water. Water that enters the chamber is preheated sufficiently to cause water to flash into a vapor upon entering the chamber. The result will be a solution that is more dilute than the original and a solution that is more concentrated than the original. Boilers require high-purity feedwater. the nature of condensate returns. specification conductivities. Careful monitoring of boiler feedwater as well as condensate chemistries is crucial to boiler operations. 18 ©NPTI (NR) PGDC 10th Batch 2005 . solvent (water) will be transported from the dilute to the more concentrated side. Dissolved oxygen. acids. Contaminants that 19 ©NPTI (NR) PGDC 10th Batch 2005 . Boiler Blow down Methods Continuous. functions strictly through reaction with DO. Other chemicals such as carbodihydrazide decompose at feedwater temperatures and form hydrazine as a by-product. which is a reducing agent. Corrosion Inhibitors. are also available. The impurities in the feedwater and the impurities separated from the steam will remain in the boiler water. Hydrazine also functions as an oxygen scavenger. Sulfite. The chemical feed line discharges into a turbulent zone of the drum for thorough mixing with the boiler water before the mixture enters the downcomers. these impurities will become more concentrated and eventually deposit on internal tube surfaces. such as erythorbic acid and diethyl hydroxylamine. feedwater is constantly added to the drum as steam is removed. The continuous blowdown and chemical feed lines are separated so that the injected chemicals do not flow directly to the blowdown line. If the impurities are not removed. Hydroquinone is an oxygen scavenger that has been applied in blends with hydrazine to catalyze its reaction with DO. and excess caustic can cause corrosion of the boiler.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Pressure Applied Semi-Permeable Membrane Water Dilute Solution Concentrated Water Dilute Solution Concentrated Reverse Osmosis Reverse Osmosis Figure 9 33393 Osmosis Chemical Treatment of BFW Supplementing internal boiler water treatment is possible by injecting chemicals through the chemical feed line into the steam drum.Hydrazine provides corrosion protection through the formation of magnetite film on steel and through the formation of cupric oxide on copper alloys.During normal operation. Corrosion by DO is more critical in the feedwater system because corrosion rates increase with temperature. Sodium sulfite and sodium sulfite that are catalyzed with cobalt have been applied in low-pressure cycles. Oxygen Scavengers control corrosion by dissolved oxygen (DO). but hydroquinone may also be used as a hydrazine substitute. organic-chemicalbreakdown products. The formation of scale on tube surfaces reduces heat transfer and can lead to overheating and possible tube failures. Organic oxygen scavengers. The speed of conversion depends on the rate of heat that is being added. The conversion of water to steam requires much more energy beyond that required to reach the boiling point. The intermittent blowdown may be operated anytime depending on the concentration of impurities in the boiler water. It must be remembered that heat and temperature are not the same thing. the temperature of the water increases. Although the temperature remains constant. Intermittent blowdown is performed by periodically opening a blow off valve that is connected to the lowest part of the mud drum.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) can form deposits on boiler surfaces include calcium. and organic elements. it is called saturated water. When the water temperature reaches the boiling point. As more heat is added (at constant pressure). magnesium. oils. in a zone where solids tend to collect. Once the conversion from water to steam is complete (but before the temperature is raised above saturation temperature) the fluid is called saturated steam. Boiler water solids are maintained at recommended limits by the continuous blowdown line. Boiling Process and Steam Generation The process of boiling water to make steam is a familiar phenomenon. along the length of the drum. sulfates. A calibrated flow control valve regulates the amount of blowdown to the drain system. 20 ©NPTI (NR) PGDC 10th Batch 2005 . As heat is added to water. iron. phosphates. A considerable amount of heat is added to the fluid while its temperature remains constant at the boiling point saturation temperature. It is being utilized to convert water into steam. some of the water begins to vaporize to steam. This process tends to remove the most contaminated water in the system and replace it with fresh feedwater. The heat input or enthalpy necessary to convert saturated water to saturated steam is called the heat of vaporization. or saturation temperature. based on water solids concentration and feedwater flow. the fluid temperature will remain at the saturation temperature until all of the water is converted to steam. Saturated Water and Saturated Steam When water just begins to boil. The primary purpose of the intermittent blowdown is to remove undissolved solids that collect at the low velocity point of the boiler circulating water. The line is positioned internally. silica or silicates. the heat being applied is not lost or wasted. The amount of heat energy contained in the fluid is termed enthalpy and is measured in BTUs/lb.) For instance.The term boiling point is most frequently used to identify conditions at atmospheric pressure (29. 800 700 o Temp ( F) 2500 2000 1500 1000 600 500 400 300 500 psi 160 psi 400 500 600 700 800 900 1000 Enthalpy (BTU/lb) of Saturated Water 33395 Boiling Point–Pressure Relationship Figure 11 21 ©NPTI (NR) PGDC 10th Batch 2005 . pressure increases when steam is generated in a closed vessel. as illustrated in Figures 11 and 12.92 inches of mercury. more heat energy is required to raise the fluid temperature to the boiling point. The boiling point is actually a function of pressure and increases as pressure increases. At higher pressures. the boiling point of water at atmospheric pressure is 212 degrees Fahrenheit. Enthalpy.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Superheated Steam o Temp ( F) Saturated Water Constant Pressure 500 psi Saturated Steam 470 450 Enthalpy (BTU/lb) 1205 33394 Boiling Process and Steam Generation Figure 10 Boiling Point. however. The points at which all of the water has been converted to steam are indicated by the saturated steam line (Figure 12).The measure of how far the conversion from saturated water to saturated steam has progressed is called quality and is shown in Figure 13. The enthalpy at this point is about 450 Btu per pound. 470 degrees Fahrenheit). but the enthalpy would be increased to 1205 Btu per pound. quality increases. For example. The heat of vaporization would then be the enthalpy of the saturated steam minus the enthalpy of the saturated water or 1205 . Quality is the percent by weight of vapor in a steam/water mixture. 22 ©NPTI (NR) PGDC 10th Batch 2005 .450 = 755 Btu per pound. The temperature remains constant until all the water has been converted to saturated steam. As more heat is added (at constant pressure). This point would be at the same pressure and temperature (500 pounds per square inch. Water on the saturated water line has a quality of 0%. As more water is converted to steam. water begins to boil at about 470 degrees Fahrenheit when the pressure is 500 pounds per square inch. The heat input or enthalpy necessary to convert saturated water to saturated steam is termed the heat of vaporization and is indicated for a given temperature by the horizontal constant pressure lines. Steam Quality. Superheated and saturated steams have a quality of 100%. Water that has been heated to saturation and has sufficient additional heat added to convert half of it to steam has a quality of 50%.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) 1200 1100 1000 900 o Temp ( F) 800 700 600 500 400 300 400 500 Saturated Curve 2000 psi 500 psi 200 psi 100 psi Length of line = h fg Boiling Points 600 700 800 900 1000 1100 1200 1300 1400 1500 33396 Saturation Curve–Boiling Point–Pressure Relationship Figure 12 Heat of Vaporization. the enthalpy increases and more water is converted to steam. Nucleate Boiling. This process is referred to as nucleate boiling and promotes two benefits: (1) it heats the fluid inside the tube to saturation. One is nucleate boiling. The super heaters derive their name from their function of heating steam above the saturation curve. the temperature will again begin to rise. The number of degrees of superheat describes how far the steam has been heated above the saturation curve. Boiling Process Two types of boiling processes exist. Nucleate boiling is preferred over film boiling. 23 ©NPTI (NR) PGDC 10th Batch 2005 .STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) 1200 1100 1000 900 o Temp ( F) 800 700 Saturated Water 600 (0% Quality) 500 400 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 33397 Superheated Steam Constant Pressure Line 40 60 80 20 Saturated Steam (100% Quality) Enthalpy (BTU/lb) Steam Quality Figure 13 Superheated Steam If still more heat is added to saturated steam.As a water-cooled tube is heated. The fluid in this area is said to be superheated steam. and (2) it maintains tube metal temperature at saturation keeping the tube cool. Steam is sometimes referred to as having a number of degrees of superheat. giving up their heat to raise the temperature of the water. steam bubbles form at the tube's inner surface. Normally these bubbles diffuse well and mix with the water in the center of the tube as shown in Figure 14. The steam bubbles condense quickly in the main stream. The second is film boiling. This is shown by the dotted lines to the right of the saturated steam lines in Figure 10. the nucleate boiling process breaks down. Tube Wall Steam Film (No Mixing) 33399 Film Boiling Figure 15 24 ©NPTI (NR) PGDC 10th Batch 2005 .STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Tube Wall Steam Bubbles (Mixing) 33398 Nucleate Boiling Film Boiling. This condition is known as film boiling (Figure 15). The tube metal temperature will rapidly increase. Little heat will be transferred from the tube metal through the film to the water in the center of the tube.With high heat input levels and high steam quality. The bubbles of steam forming on the hot tube surface will begin to interfere with the flow of water to the surface and the bubbles of steam eventually coalesce to form a film of superheated steam over part or all of the tube surface. resulting in a failure. The point at which nucleate boiling stops and film boiling begins is determined by the heat input and steam quality. however. Lower qualities afford greater margins of safety and reduce the possibilities of the occurrence of DNB. much higher heat input levels can be tolerated at lower qualities (Area A). C. Metal temperatures are shown in Figure 16 as a function of steam quality for several heat input levels. D. Fluid quality has a great effect on DNB. 25 ©NPTI (NR) PGDC 10th Batch 2005 . can be used at low quality levels. which result in higher levels of circulation and steam generation.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Departure From Nucleate Boiling. Tube type has a major impact on the prevention of DNB. Wall construction or location of heat flux also affects DNB. High fluid velocities decrease the occurrence of DNB at a given fluid quality. As the curves illustrate. Increased Metal Temperature Subcooled Water Superheated Steam ut t Inp Hea h Hig Low Heat In put Area A Area B 0% 100% 33400 Quality DNB Metal Curve Figure 16 Research into DNB has found several parameters that affect DNB: A. This means that high heat inputs. causing overheats. only moderate heat inputs can be tolerated at high quality levels (Area B). The point is termed Departure from Nucleate Boiling or DNB. B. Heating a wall from one side could allow a steam film to form on the heated side of the tube. STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) E. circulation increases with increased heat input until a point of maximum fluid flow is reached. In a natural circulation system. Natural Circulation Modern water tube boilers were developed from the early fire tube designs. Water is supplied from a drum to the furnace wall tubes through 26 ©NPTI (NR) PGDC 10th Batch 2005 . Higher heat flux also increases the possibility of DNB caused by the higher qualities generated. Steam is significantly less dense than water. a natural circulation effect is created with water continuously moving within the boiler tubes to remove and replace the generated steam. Research and experience has shown that DNB is more likely to occur at operating pressures above 2000 psig. Modern water tube boilers not only have a larger surface area available for heat transfer. but by proper design. F. Steam Outlet Steam Drum Water / Steam Downcomer (Unheated) FurnaceWalls (Heated) Furnace Wall Supplies 33401 Natural Circulation Figure 17 Natural circulation is based on the difference in density between water and steam. Beyond a certain level. Within normal limits. more pumping power is available from the natural circulation effect. As shown in the left portion of Figure 17. The steam/water mixture in the wall tubes is less dense than the water in the down comers and forced up the steam drum by the heavier water as shown in Figure 17. tubes exposed to higher heat levels will receive more cooling flow. if one tube receives more heat than adjacent tubes. If the heat imbalance becomes too great. Because the density difference becomes greater. It partially compensates for normal imbalances in the heat input to the furnace. As more heat is added to the furnace tubes.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) down comers. flow will be reduced and the tube will overheat. Steam quality leaving the riser tubes and entering the steam drum is usually 5 to 30%. circulation will naturally increase with increased heat input and provide more flow to keep furnace tubes cooled as more steam is generated. friction in the tubes overcomes the difference in density and circulation is reduced with additional heat input as shown in Figure 18. The down comers are not heated. 27 ©NPTI (NR) PGDC 10th Batch 2005 . the quality of the fluid increases. Up to a point. As the unit is fired. depending on the boiler load and pressure. a steam/water mixture is generated in the furnace wall tubes. The process continuously repeats with a steam/water mixture being generated in the furnace tubes and being replaced with heavier water in the down comers. Normal Operation Possible Overheat Inc reasing Flow Increasing Heat Input 33402 Effect of Flow versus Heat Input Figure 18 Natural circulation also provides an additional benefit. Drum boilers operate in the area on or under the saturation curve. it will generate more steam with a lower density and thus will receive more flow to keep it cool. Natural circulation boilers are designed to operate in the left region of the curve so that circulation increases with heat input. no steam is recirculated to the boiler water. This step is called secondary separation or steam scrubbing. however. The less dense steam moves to the core of the cylinder and moves upward. The cyclones. The collected water is drained from the bottom of the scrubber assembly to the water below. essentially cylindrical in form. are arranged internally along the length of the drum. Steam Drum Internals In modern drum boilers (Figure 6 of this module). The water flows down the cylinder wall and is discharged from the cyclone through an annulus located below the water level. the steam may still contain solid contaminants that must be removed or reduced in amount before the steam is sufficiently pure for use.Primary steam separation is accomplished with cyclone steam separators. When wide load fluctuations and variations in water quality are suspected. and provide a large surface to intercept water particles as the steam weaves through the closely fitted plates. so that in effect. Centrifugal force throws the more dense water to the outside of the cylinder where it forms a layer against the cylinder wall. After primary separation. secondary scrubbers may also be installed to provide nearly perfect steam separation. the separation of steam from the steam/water mixture generated in the furnace usually takes place in two steps.Further steam scrubbing of any trace amounts of water contaminants in the steam are achieved by the secondary scrubbers. between 5 and 30% will be converted to steam by the time it reaches the top of the furnace. Primary separation removes nearly all of the water from the mixture. the steam may still contain dissolved solids suspended in tiny water droplets. The steam/water mixture enters the cyclone steam separator tangentially. These water droplets that contain solids are removed from the steam as it passes through the corrugated plate elements of the primary scrubber. Primary Scrubbers. thus providing a maximum available head for producing flow through natural circulation. Staying at low quality levels is necessary to protect the tubes from overheating failures caused by the nature of the boiling process. so that re-entrainment of water is avoided. Secondary Scrubbers.The upward rising steam from the cyclones passes through the primary scrubbers at the top of the cyclones for secondary steam separation. Steam velocity through the corrugated plate assembly is very low. 28 ©NPTI (NR) PGDC 10th Batch 2005 .STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) This means that of the water that flowed down the down comers. Secondary scrubbers are corrugated plates that are located at the top of the steam drum. Cyclone Steam Separators. The separated water returns to the boiler cycle virtually free of steam bubbles. operates with the air and combustion products that are maintained above atmospheric pressure. The differential pressure required for air flow is produced by a combination of the stack and fans. Combustion products exit the boiler and flow through an induced draft fan to the scrubber and the stack. The stack and fans control air flow. The function and components of the air flow system. The function and components of the boiler balanced draft system. In Figure 19. this section will discuss: • • • The function and components of the boiler forced draft system.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) COMPONENTS OF THE VARIOUS BOILER AIR AND DRAFT SYSTEMS: FUNCTIONS AND BASIC OPERATION This section will discuss the functions and components of the various boiler air and draft systems (Figure 19). Specifically. Forced Draft and Natural Draft Systems A forced draft system or a pressure fired boiler. Steam Coil Air Heater Stack Forced Draft Air Vent Gas Gas Supply Dryer Air Heater Precipitator (Optional) Induced Draft Fan Scrubber Windbox Boiler/ Furnace 33681 Boiler Air and Draft System: Figure 19 The function of the air and draft system is to provide an adequate flow of air and combustion gases for the complete combustion. air flows from the forced draft fan through a steam coil air heater into the boiler. 29 ©NPTI (NR) PGDC 10th Batch 2005 . The induced draft fan reduces the furnace pressure and ensures that it is negative. The pressure is slightly negative for all boiler loads and is not affected by the addition of an air preheater (Figure 20 of this module). At 70% boiler load the draft losses are approximately 50% of the full load draft losses. Furnace Air Pressure and Boiler Load Additional draft losses cause the forced draft system to operate at higher pressure at all loads and to be under positive pressure except at very low loads. The air preheater does not change the controlled furnace draft set point. Balanced Draft Boiler (With Air Preheater). 30 ©NPTI (NR) PGDC 10th Batch 2005 . Balanced Draft Systems Balanced draft systems have a forced draft fan at the system inlet. Profile of Pressure and Draft of a Pressure-Fired Boiler (Typical-Includes Air Preheater). Pressure and Draft Profile A pressure and draft profile of a forced draft system is shown in Figure 20. The negative pressure at the right side of the profile is caused by the natural draft of the stack. Air Preheater The air preheater is used for flue gas heat recovery and adds additional draft losses to both the combustion air and the flue gas sides of the boiler. Natural Draft Natural draft occurs as a result of the stack effect. Pressure and Draft Profile A pressure and draft profile for the balanced draft system is shown in Figure Profile of Pressure and Draft of a Balanced Draft Boiler The forced draft fan and the induced draft fan work to maintain the balance point or pressure in the furnace. and an induced draft fan near the system outlet (Figure20. shows in the figure. Hot air or hot gases rise through vertical ducts. Induced Draft Fan The induced draft fan takes suction at the flue gas exit. A pressure and draft profile of a forced draft system with an air preheater is shown in Figure of balanced draft.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Forced Draft Fan The forced draft fan provides sufficient pressure to force the air and flue gas through the system.The air preheater does add additional draft losses to the combustion air and flue gas sides of the boiler. Hot flue gases. that have a lower density than the outside air rise through vertical ducts and create a suction that causes combustion air to flow through the boiler. which results in a better environment for the furnace housing. 31 ©NPTI (NR) PGDC 10th Batch 2005 . the auxiliary equipment. Fan Outlet A B C D Stack High Load - (100%) (70%) Reduced Load - Change Balance by Increasing Induced and Reducing Forced Change Balance by Decreasing Induced and Increasing Forced 33404 Pressure and Draft Profile–Balanced Draft System–Includes Air Preheater Figure 20 Benefit of Balanced Draft System The benefit of balanced draft system is that negative operating pressures cause any leakage to be cool air leaking into the furnace rather than hot combustion gases leaking out. some means of varying the fan output is required such as a constant speed fan with damper or inlet vane control or a variable speed fan. The negative furnace pressure condition is cooler and cleaner.D. and the operating personnel.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Pressure D A C B (+) 0 Balance Point (-) F.D. Air Flow System Air flow requirements of the combustion control system require that fans operate at different pressures and volume discharge rates. Fan Out Air Preheater Inlet Windbox Furnace Boiler Outlet Air Preheater Outlet I. To meet these requirements. STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Constant Speed Fan A constant speed fan used with damper control or inlet vane control introduces sufficient variable resistance in the system to alter the fan output as required. Variable Speed Fan The variable speed fan is attractive because it reduces energy at reduced flow rates. however. 32 ©NPTI (NR) PGDC 10th Batch 2005 . The variable speed fan significantly improves fan efficiency during periods when the boiler is operating at less than its maximum load. Even so. variable speed fans require a higher initial cost than constant speed fans that may not be offset by lower power requirements. The constant speed fan is attractive because of its low initial cost. a constant speed fan consumes significantly more energy than a forced draft fan at low boiler rates. Fuel Oil System The most common oils used for boiler fuel are the lightweight Number 2 fuel oil and the Number 6 grade of heavy residual fuel oil. Fuel Gas System The most common gaseous fuel is natural gas.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) COMPONENTS OF BOILER FUEL SYSTEMS: FUNCTION AND BASIC OPERATION This section will discuss the boiler fuel oil and fuel gas systems. 33 ©NPTI (NR) PGDC 10th Batch 2005 . The amount of heat (BTU) liberated per unit quantity of gas or oil is called the HigherHeat Value (HHV). and the boiler control system regulates the BTU input by a control valve. Fuel oil is pumped through the fuel oil system. can be found in the Combustion Engineering Fuel Burning and Steam Generation Handbook. The HHV for a fuel. typically in units of BTU per pound. this section will discuss: • • The function and components of the fuel gas system. Waste gas or gas produced as a process byproduct can also be used. Natural gas is transported through pipelines and is delivered by the suppliers as it is used. It is normally not necessary to heat Number 2 fuel oil. Specifically. The function and components of the fuel oil system. It is usually necessary to heat Number 6 fuel oil so that it can be pumped through the system. steam is generated. Heat transfer apparatus through which air passes and is heated by a medium of higher temperature. Air heater Air preheater Balanced draft A system of furnace pressure control in which the inlet combustion air flow and the outlet flue gas flow is controlled to maintain the furnace pressure at a fixed value (typically slightly below atmosphere). such as the products of combustion (flue gases) or steam. A closed vessel in which water is heated. British Thermal Unit. Continuous blowdown The continuous blowing down of water. Removal of a portion of boiler water to reduce chemical concentration. under pressure or vacuum by heat from combustible fuels in a self. or any combination thereof. steam is superheated. and concentration to maintain ignition and combustion of the fuel within the furnace. turbulence. The movement of water and steam within a steam generating unit. nitrogen. A heat exchange device that is used to condense steam. and other gases that with varying amounts of water vapour. A pump that is used by the boiler system to supply high pressure boiler feedwater to the steam drum. forms the atmosphere of the earth. A device or group of devices for the introduction of fuel and air into a furnace at the required velocities. 34 ©NPTI (NR) PGDC 10th Batch 2005 .STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) GLOSSARY Air The mixture of oxygen. A condenser is used to condense the exhaust steam from turbines. or to discharge sludge. Blowdown Boiler Boiler feed pump Boiling point Burner BTU Circulation Condenser Constant speed A fan with a single-speed motor that uses damper control or inlet vane control to Fan vary air flow.contained or attached furnace. A steam coil or similar heat exchange device used to keep the temperature of the flue gas above its dew point to minimize corrosion to the air heater. The temperature that a liquid changes to vapor for a given pressure. An energy saving heat exchanger that is commonly used on all large boilers to provide additional heat to the feedwater prior to the water entering the boiler. time. Forced draft The furnace draft that is caused by forcing air into the furnace with a fan. The lines that carry cold water from the steam drum to the mud drum of the furnace for heating. Heat input comes from the hot flue gases passing the tubes (containing feedwater) just before going to the air heater. ©NPTI (NR) PGDC 10th Batch 2005 35 . Water that is introduced into a boiler during operation. pressure. or other influences. It includes make-up and return condensate. A device that is used to remove air and gases from boiler feedwater prior to its introduction into a boiler.The efficiency of a steam-generating unit is the ratio of the heat absorbed by water and steam to the heat in the fuel fired. or an acid.If automatic. light. Deaerator DNB Downcomer Draft Drum Economizer Efficiency Enthalpy Feedwater Feed water heater Film boiling Flash evaporator A device that is used with boilers operating at 1000 gal or over to purify boiler water by an evaporation technique. The amount of heat energy that is contained in a fluid or gas in BTU/lb. CO2.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Control Any manual or automatic device for the regulation of a machine to keep it at normal operation.Specified as low and high pressure heaters that use main turbine extraction steam to heat water. The difference between atmospheric pressure and some lower pressure existing in the furnace or gas passages of a steam generating unit. the device is motivated by variations in temperature. The ratio of the output to the input. Corrosion The wasting away of metals caused by a chemical action that is usually caused by the presence of O2. A condition where steam forming on a hot tube's surface begins to interfere with the flow of water to the surface by coalescing to form a film of superheated steam over part or all of the tube surface. A tube-shell heat exchanger used to heat boiler feedwater. water level. Departure from Nucleate Boiling: The point at which nucleate boiling stops and film boiling begins. A cylindrical shell closed at both ends designed to withstand internal pressure. The process of steam bubbles at a tube's inner surface condensing and giving up their heat to raise the temperature of the water. The blowing down of boiler water at intervals to remove suspended solids. etc. The furnace pressure is usually controlled at less than atmospheric pressure with the fan. 36 ©NPTI (NR) PGDC 10th Batch 2005 . A fan that is used to produce a flow of air through the furnace by creating a lower pressure.000 BTU. Lowest boiler drum in which suspended solids collect.STUDY MATERIAL (BOILER) National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Forced draft fan Fuel Fuel oil equivalent barrels Furnace A fan that is used to force a flow of air into the boiler furnace. Oxygen scavengers PMC Process Measurement & Control. Combustion chamber of a boiler. A substance that contains combustible materials that are used for generating heat.300. Chemicals that are used to control boiler corrosion. blowdown. An induced draft fan is commonly used to aid the exhaust of flue gases. Heat exchanger A device for transferring heat energy from one medium to another. Heat of vaporization HHV The heat input or enthalpy necessary to convert saturated water to saturated steam. or 6. Natural draft Nucleate boiling A furnace draft that is caused solely by the effect of the stack. Intermittent blowdown is achieved by periodically opening a blowoff valve located at the bottom of the mud drum. The number of heat units that are liberated per unit of quantity of fuel burned. Water that is added to replace losses. Higher Heating Value. The furnace draft produced by drawing the flue gases out of the furnace by an induced draft fan. Unit of energy management equivalent to energy in a barrel of oil. Induced draft Induced draft fan Intermittent blowdown Makeup Mud drum Natural circulation The circulation of water in a boiler that is caused by differences in density. Steam that is at a higher temperature than its saturation temperature. A fan with a variable speed drive that is used to change speed to vary air flow. Quality is the percent by weight of vapor in a steam/water mixture. Specific volume The volume that a pound of steam or water occupies at a given pressure or temperature. escapes. Variable speed fan 37 ©NPTI (NR) PGDC 10th Batch 2005 . without any water present. vent or passage through which smoke or heated air. Reverse osmosis A procedure that is used to remove dissolved salts and minerals in a solution. Steam coil air heater Steam drum internals Steam quality Superheated steam Superheater The percent by weight of vapor in a steam and water mixture. Saturated water Scrubber Water at the temperature of its boiling point. etc. All apparatus within a drum. Risers The tubes that are heated through which the steam/water mixture moves from the lower or mud drum to the boiler drum. A tubular recuperative air heater in which gas flows vertically through tubes.Pressurized furnace Quality Recirculation National Power Training Institute STUDY MATERIAL (BOILER) (Northern Region) PGDC in TPPE – 10th Batch (2005-06) A furnace that is operated above the atmospheric pressure by using just a forced draft fan. An apparatus for the removal of solids from gases by entrainment in water. Stack Stack effect The flue. Air flows horizontally across the tubes in order to provide heat transfer. The movement of hot flue gases out of the stack caused by differences in density between these flue gases and the cooler air surrounding the stack. Saturated steam Steam that is at the boiling point temperature that corresponds to a particular pressure. The reintroduction of part of the flowing fluid to repeat the cycle of circulation. A device that is used to raise the temperature of steam above its saturation temperature.