Fire tube boiler—a device that passes hot combustion gases through the tubes to heat water on the shell side, p. 298. Igniter—a device (similar to a spark plug)
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290 Chapter 14 Boilers Objectives After completing this chapter , you will be able to: 14.1 Identify the components of boilers and the purpose of boilers in the process industries. (NAPTA Boilers 1, 3-5*)˜p. 292 14.2 Identify the common types of boilers and their applications. (NAPTA Boilers 2)˜p. 296 14.3 Explain the operating principles of boilers. (NAPTA Boilers 6)˜p. 298 14.4 Identify potential problems associated with boilers. (NAPTA Boilers˜11) ˜p. 302 14.5 Describe safety and environmental hazards associated with boilers. (NAPTA Boilers 7, 10)˜p. 303 14.6 Describe the process technician™s role in safe boiler operation, maintenance, and operator qualification. (NAPTA Boilers 8, 9)˜p. 304 14.7 Identify typical procedures associated with boilers. (NAPTA Boilers˜7) ˜p. 304 * North American Process Technology Alliance (NAPTA) developed curriculum to ensure that Process Technology courses will produce knowledgeable graduates to become entry level employees in process technology. Objectives from that curriculum are named here in abbreviated form. For example, fi(NAPTA Boilers 2)fl means that this chapter™s objective relates to objective 2 of the NAPTA curriculum about boilers). Key Terms Air registers Šdevices that control the flow of air to the burners to maintain the correct fuel-to-air ratio and to reduce smoke, soot, or N O x (nitrogen oxide) and CO (carbon monoxide) formation, p. 297. Blowdown Šthe process of taking water out of a boiler to reduce the concentration level of impurities. There are two types of blowdown, continuous and intermittent, p. 293. Burners Šdevices that introduce, distribute, mix, and burn a fuel (e.g., natural gas, fuel oil, or coal) for heat, p. 297. M14_NAPT1262_02_SE_C14.indd 290 29/06/2018 13:50
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Boilers 291CoagulationŠa method for concentrating and removing suspended solids in boiler feedwater by adding chemicals to the water, which causes the impurities to cling together, p. 301.CondensateŠcondensed steam, which often is recycled back to the boiler , p. 294.DamperŠa movable plate that regulates the flow of air or flue gases in boilers ,˜ p. 297.DeaerationŠremoval of air or other gases from boiler feedwater by increasing the temperature using steam and stripping out the gases , p. 301.DemineralizationŠa process that uses ion exchange to remove mineral salts; also known as deionization. The water produced is referred to as deionized water , p. 301.Desuperheated steamŠsuperheated steam from which some heat has been removed by the reintroduction of water. It is used in processes that cannot tolerate the higher steam temperatures , p. 300.DesuperheaterŠa system that controls the temperature of steam leaving a boiler by using water injection through a control valve , p. 297.DowncomersŠtubes that transfer water from the steam drum to the mud drum , p.˜297.Draft fanŠa fan used to control draft in a boiler , p. 297.EconomizerŠthe section of a boiler used to preheat feedwater before it enters the main boiler system, p. 293.FiltrationŠthe process of removing particles from water or some other fluid by passing it through porous media , p. 301.FireboxŠthe area of a boiler where the burners are located and where radiant heat transfer occurs, p. 296.Fire tube boilerŠa device that passes hot combustion gases through the tubes to heat water on the shell side, p. 298.IgniterŠa device (similar to a spark plug) that automatically ignites the flammable air and fuel mixture at the tip of the burner , p. 292.ImpellerŠa fixed, vaned device that causes the air/fuel mixture to swirl above the burner; different from an impeller in a turbine or pump , p. 292.Knockout potsŠdevices designed to remove liquids and condensate from the fuel gas before it is sent to the burners , p. 292.Mud drumŠthe lower drum in a boiler; also called the water drum; serves as a settling point for solids in the boiler feedwater, p. 293.PilotŠan initiating device used to ignite the burner fuel, p. 297.Premix burnerŠa device that mixes fuel gas with air before either enters the burner tip, p. 297.Radiant tubesŠtubes containing boiler feedwater that are heated by radiant heat from the burners and boiled to form steam that is returned to the steam drum ,˜p.˜297.Raw gas burnerŠa burner in which gas has not been premixed with air , p. 297.Refractory liningŠa bricklike form of insulation used to reflect heat back into the box and protect the structural steel in the boiler , p. 296.Reverse osmosisŠmethod for processing water by forcing it through a membrane through which salts and impurities cannot pass (purified bottled water is produced this way), p. 301.Riser tubesŠtubes that allow water or steam from the lower drum to move to the upper drum , p. 297.Saturated steamŠsteam in equilibrium with water (e.g., steam that holds all of the moisture it can without condensation occurring) , p. 300.SofteningŠthe treatment of water that removes dissolved mineral salts such as calcium and magnesium, known as hardness in boiler feedwater , p. 301.SpidersŠdevices with a spiderlike shape that are used to inject fuel into a boiler,˜p.˜292.SpudsŠdevices used to inject fuel into a boiler, p. 292.M14_NAPT1262_02_SE_C14.indd 29129/06/2018 13:50
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292 Chapter 14 Stack Šan opening at the top of the boiler that is used to remove flue gas, p. 297. Steam drum Šthe top drum of a boiler where all of the generated steam is collected before entering the distribution system, p. 293. Steam trap Ša device used to remove condensate or liquid from steam systems , p. 294 . Superheated steam Šsteam that has been heated to a very high temperature so that a majority of the moisture content has been removed (also called dry steam ), p. 300. Superheater Štubes located near the boiler outlet that increase (superheat) the temperature of the steam, p. 297. Waste heat boiler Ša device that uses waste heat from a process to produce steam,˜ p.˜297. Water tube boiler Ša type of boiler that contains water-filled tubes that allow water to circulate through a heated firebox, p. 296. 14. 1 Introduction Steam has many applications and a long history in the process industries. Steam provides efficient heat transfer and contains a high amount of latent heat. It is used to heat and cool process fluids, power and purge equipment, fight fires, facilitate distillation, and induce other physical and chemical reactions. Boilers are an important source of energy in the process industries because they supply steam to operate process equipment and produce the steam used throughout the process facility. Examples of process equipment that uses steam includes turbines, reactors, distilla – tion columns, stripper columns, and heat exchangers. General Components of Boilers Boilers are devices in which water is boiled and converted into steam under controlled con – ditions. Boiler components can vary, but the most common components include a firebox, burners, drums, tubes, an economizer, a steam distribution system, and a boiler feedwater system. Firebox Like other process furnace and direct-fired heater fireboxes, boiler fireboxes have a refractory lining, burners, a convection-type section, a radiant section, fans, air flow control, a stack, and dampers. The boiler firebox is insulated to reduce the loss of heat and enhance the heat energy being transferred to the boiler™s internal components. Burners Burners inject air and fuel through a distribution system that mixes them in proper concen – trations so combustion can occur. Most boilers use natural gas, fuel oil, or coal burners to provide heat to the boiler. The key components of natural gas burners include pilots, impellers, spuds, spiders, and igniters. Knockout pots remove liquids and condensate from the fuel gas before it is sent to˜the burners. Damp – ers regulate the flow of air to the burner. Impellers in boilers are not the same as impellers in turbines or pumps. Impellers are fixed, vaned devices that cause the air/fuel mixture to swirl above the burner. Spuds or spiders (devices with a spiderlike shape) are used to inject fuel into the boiler (Figure 14.1). Igniters automatically ignite the flammable air and fuel mixture at the tip of the burner. Knockout pots devices designed to remove liquids and condensate from the fuel gas before it is sent to the burners. Impeller a fixed, vaned device that causes the air/fuel mixture to swirl above the burner; different from an impeller in a turbine or pump. Spiders devices with a spiderlike shape that are used to inject fuel into a boiler. Igniter a device (similar to a spark plug) that automatically ignites the flammable air and fuel mixture at the tip of the burner. Spuds devices used to inject fuel into a boiler. Did You Know? Low N O x formation burners use nozzles, called spuds , to inject fuel into a boiler. Spuds promote better combus – tion than traditional burners. M14_NAPT1262_02_SE_C14.indd 292 29/06/2018 13:50
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Boilers 293 Drums The drums that comprise a water tube boiler resemble a large water distribution header connected by a complex network of tubes. The mud drum is the lower drum in a boiler. The steam drum is the upper drum of a boiler where all of the generated steam is collected. The mud drum and water tubes are filled completely with water, while the steam drum is only partially full. Maintaining this vapor space in the upper drum allows the saturated steam to collect and pass out of the header. Feedwater to the boiler is treated to achieve the required chemical composition. Water lost in the boiler is replaced through a makeup water line. Sediment accumulates in the bottom of the mud drum and is removed through blowdown. Blowdown is the process of removing small amounts of water from the boiler to reduce the concentration of impurities. Blowdown can be either continuous or intermittent. Continuous blowdown is the constant removal of a small quantity of boiler water from the steam drum to remove suspended solids and salts that could concentrate in the steam drum. Intermittent (or bottoms ) blowdown is the occasional opening of a valve on the bottom of the mud drum to remove solids that have settled. Economizer The economizer is the section of a boiler used to preheat feedwater before it enters the main boiler system. Preheating the water increases boiler system efficiency. This heat exchanger transfers heat from the stack gases to the incoming feedwater. The economizer is usually located close to the stack gas outlet of the boiler. Economizers can be supported from overhead or from the ground. The feedwater line that serves the boiler is piped into and travels through the economizer. No additional feedwater control valves or stack gas dampers are required. An economizer is similar to the convection section in a direct-fired heater. Both oper – ate under the energy-saving concept of recovering some of the heat from the hot flue gases before they are lost out of the stack. The typical improvement in efficiency of a boiler with an economizer is 2 to 4 percent. Steam Distribution System The steam distribution system consists of valves, fittings, piping, and connections suitable for the pressure of the steam being transported. Steam exits the boiler at sufficient pressure required for the process unit or for electrical generation. For example, when steam is used to drive steam turbine generators to produce electricity, the steam must be produced at a much higher pressure than that required for process steam. The steam pressure can then Mud drum the lower drum in a boiler; serves as a settling point for solids in the boiler feedwater. Steam drum the top drum of a boiler where all of the generated steam is collected before entering the distribution system. Blowdown the process of taking water out of a boiler to reduce the concentration level of impurities. There are two types of blowdown, continuous and intermittent. Economizer the section of a boiler used to preheat feedwater before it enters the main boiler system. Figure 14.1 Fuel injection devices. A. Spud. B. Spider. Main gas spuds (6) Pilot gas spud B. A. M14_NAPT1262_02_SE_C14.indd 293 29/06/2018 13:50
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294 Chapter 14 be reduced for the turbines that drive process pumps and compressors that require lower pressure steam. Most steam used in a process facility is ultimately condensed to water. Condensate is typically returned through condensate return systems and reused as boiler feedwater. This saves the plant considerable money because the condensate, which has already been treated with chemicals, reduces the cost of boiler feedwater treatment. A facility™s steam system is usually composed of two or three piping systems with different pressure steam headers. There are different methods of maintaining several steam headers operating at different pressures. One method is through a letdown valve or a reducing station in which a pressure control valve is used. In this type of arrange – ment, a pressure drop across the valve equalizes with the pressure contained in the lower pressure steam header. In many instances, the steam header itself has another letdown valve that repeats the same function, supplying steam to yet another, lower pressure steam header. Another method of supplying steam to headers of different pressures is by piping the exhaust of steam turbines into headers. For example, a steam turbine that operates at 1,500˜psig pressure might exhaust into a 550 psig steam header. The turbine that uses the 550˜psig steam then might exhaust into a 50 psig header. A letdown valve or reducing station (see Figure 14.2) offers the advantage of maintaining a steam header at an interim pressure that might not be suitable for all steam turbines.Condensate condensed steam, which often is recycled back to the boiler. Figure 14.2 A. Diagram of a letdown station. B. Letdown station with pressure-reducing valve, safety valves, and˜separator. Pressure reducing valveIsolatingvalveIsolatingvalveSafetyvalveCondensateStrainerSeparatorHighpressuresteam˜owLowpressureB.A.TurbineTurbineRV12PCPC44451500 PSIG550 PSIG50 PSIGSome process plants are supported by a cogeneration plant, in which electricity and steam are generated in the same unit. Steam is brought to a very high temperature and pres -sure. This high-pressure steam is used to turn a turbine that turns an electrical generator. The exhaust from the turbine goes into a lower pressure steam header. In this way, a cogen – eration plant supplies the process facility with both electricity and steam. An important component used in steam systems is a steam trap. Steam traps are used to remove condensate or liquid from steam systems. There are several steam trap designs. The most common, however, are the mechanical trap (Figure 14.3) and the thermostatic trap (Figure 14.4). See also Figures 4.18 and 4.19. Mechanical traps operate based on the density difference between steam and conden -sate. Internal floats attached to mechanical linkages are found inside most mechanical traps. As the condensate levels rise, the linkage causes the valve to open. As the levels drop, the valve closes. Inverted bucket and float traps are both common examples of mechanical traps. Steam trap a device used to remove condensate or liquid from steam systems. M14_NAPT1262_02_SE_C14.indd 29429/06/2018 13:50
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Boilers 295Thermostatic traps operate on the principle of temperature change (steam is hotter than condensate). These traps contain valves that are opened or closed by thermal expansion and contraction. Bimetallic and bellows traps are examples of temperature-operated traps. Condensate will cause the bellows to collapse or bimetallic element to move, releasing the condensate. When the condensate is removed and steam is present, the bellows expands or the bimetallic element moves the opposite way, and the steam trap closes. Boiler Feedwater SystemThe boiler feedwater supply is a critical part of steam generation. There must always be as many pounds of water entering the system as there are pounds of steam leaving. The water used in steam generation must be free of contaminants such as minerals and dissolved impurities that can damage the system or affect its operation. Suspended materials such as silt and oil create scale and sludge, and must be filtered out. Dissolved gases such as carbon dioxide and oxygen cause boiler corrosion and must be removed by deaeration and other methods of treatment. Because dissolved minerals cause scale, corrosion, and turbine blade deposits, boiler feedwater must be treated with lime or soda ash to precipitate these min – erals from the water. Recirculated condensate must be deaerated to remove dissolved gases. Depending on the individual characteristics of the raw water, boiler feedwater can be treated by clarification, sedimentation, filtration, ion exchange, deaeration, membrane processes, or a combination of these methods. Boiler feedwater treatment is discussed in greater detail in the following section. Figure 14.3 Examples of different designs for mechanical steam traps. A. Inverted bucket trap. B. Float trap. Inverted bucketA.B.OutletVentInletInletOutletValve headThermostaticair vent Ball ˜oatValve seatFigure 14.4 Example of a type of thermostatic steam trap (Bellows). M14_NAPT1262_02_SE_C14.indd 29529/06/2018 13:50
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Boilers 297Figure 14.6 Diagram of a waste heat boiler. Hot ˜ue gas inlet Bare tubesMud drum Steamdrum External insulation Steam outletSuper-heated steam outletMist padassemblyMist padassemblyFeedwater inletDowncomerpipesBare tubesBoiler casingRefractory liningBoiler casingRefractory liningCooler ˜ue gas outlet Located in the firebox area of the boiler are radiant tubes. Radiant tubes and riser tubes both contain boiler feedwater that is heated by radiant heat from the burners and boiled to form steam that is returned to the steam drum. Burners are devices that introduce, distribute, mix, and burn a fuel (e.g., natural gas, fuel oil, or coal) for heat. Pilots are used to light burners. A burner can be a premix burner (fuel gas and air mixed before either enters the burner tip), a raw gas burner (gas not pre -mixed with air), or a combination.Air registers control the flow of air to the burners to maintain the correct fuel-to-air ratio and to reduce smoke, soot, or NOx (nitrogen oxide) and CO (carbon monoxide) formation. A draft fan is used to control draft in a boiler. Depending on the design of the boiler, the draft fan either forces the air through the boiler ( forced draft ) or pulls the air through the boiler (induced draft). In some boilers, a combination of the two is used (balanced draft).Water tube boilers have a stack at the top of the boiler to remove flue gas. Contained within the stack is a damper for regulating the flow of flue gases. A steam drum at the top of the boiler is where all generated steam gathers before exiting the boiler. Depending on the boiler design, water enters the steam drum or the mud drum from the economizer. At the bottom of the boiler is the mud drum, where sediment accumulates. This sedi -ment is removed by intermittent blowdown, which is done manually. Downcomers are tubes that transfer water from the steam drum to the mud drum. As cooler water descends from the steam drum and flows through the downcom ers, it picks up heat from the firebox and replenishes the water supply to the mud drum. Riser tubes allow water or steam from the lower drum to move to the upper drum. The superheater is a set of tubes located toward the boiler outlet that increases (super -heats) the temperature of the steam flow. The steam drum usually is connected to the super -heater through a coil or pipe. A desuperheater is a temperature control point at the outlet of the boiler steam flow˜that maintains a specific steam temperature by using boiler feedwater injection through a control valve. The purpose of the desuperheater is to lower the temperature of the steam. Waste Heat Boilers Waste heat boilers use excess or waste heat from a process to produce steam. Waste heat boilers have two functions: to produce steam and to provide cooling for a process in order for it to proceed or to recover heat that would otherwise be released to the atmosphere, losing a tremendous amount of usable energy. Figure 14.6 shows a waste heat boiler that˜can be used to recover waste heat energy and cool the flue gas stream from a turbine exhaust. Radiant tubes tubes containing boiler feedwater that are heated by radiant heat from the burners and boiled to form steam that is returned to the steam drum. Burners devices that introduce, distribute, mix, and burn a fuel (e.g., natural gas, fuel oil, or coal) for heat. Pilot an initiating device used to ignite the burner fuel. Premix burner a device that mixes fuel gas with air before either enters the burner tip.Raw gas burner a burner in which gas has not been premixed with air. Air registers devices that control the flow of air to the burners to maintain the correct fuel-to-air ratio and to reduce smoke, soot, or NOx (nitrogen oxide) and CO (carbon monoxide) formation. Draft fan a fan used to control draft in a boiler. Stack an opening at the top of the boiler that is used to remove flue gas. Damper a movable plate that regulates the flow of air or flue gases in boilers.Downcomers tubes that transfer water from the steam drum to the mud drum.Riser tubes tubes that allow water or steam from the lower drum to move to the upper drum. Superheater tubes located near the boiler outlet that increase (superheat) the temperature of the steam. Desuperheater a system that controls the temperature of steam leaving a boiler by using water injection through a control valve. Waste heat boiler a device that uses waste heat from a process to produce steam. M14_NAPT1262_02_SE_C14.indd 29729/06/2018 13:50
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298 Chapter 14 Waste heat boilers improve efficiency and save money by allowing steam to be produced through the use of waste gases. Use of waste gases as a heat source reduces the amount of money spent on burner fuels and reduces environmental impact. Because of the duty required of waste heat boilers, construction is usually thick-walled and designed to withstand high pressures and temperatures. Waste heat boilers usually are single-pass, floating-head type heat exchangers that experience a considerable amount of expansion and contraction of the tubesheet. In many furnaces, the waste heat boiler is on the outlet of the furnace. This design recov – ers heat by generating steam and thus cooling the flue gas stream exiting the furnace stack. Because of this, waste heat boilers are sometimes referred to as steam generators . Fire Tube Boilers Fire tube boilers pass hot combustion gases through the tubes to heat water on the shell side of the boiler. In this type of boiler, combustion gases are directed through the tubes while water is directed through the shell. As the water begins to boil, steam is formed. This steam is directed out of the boiler to other parts of the process, and makeup water is added to compensate for the fluid loss. In this type of system, the water level within the shell must always be maintained so that the tubes are covered. Otherwise, the tubes could overheat and become damaged. Figure 14.7 shows examples of fire tube boilers. Fire tube boiler a device that passes hot combustion gases through the tubes to heat water on the shell side. Figure 14.7 Fire tube boilers. CREDIT: A. Jay Petersen/Shutterstock. B. By Milen Mkv/Shutterstock. A. B. Did You Know? Steam locomotives and home water heaters are both examples of fire tube boilers. Safety valves Firebox Grate Secondary air Primary air Cylinders Piston Boiler Superheated steam Smokebox Superheater header Saturated steam CREDIT: Pearson Education, Inc. 14.3 Principles of Operation of Boilers Boilers use a combination of radiation, convection, and conduction to convert heat energy into steam energy. Proper boiler operation depends on controlling many variables, includ – ing boiler feedwater quality, water flow and level in the boiler, furnace temperatures and pressures, burner efficiency, and air flow. M14_NAPT1262_02_SE_C14.indd 298 29/06/2018 13:50
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Boilers 299To illustrate how boilers work, consider the simple boiler shown in Figure 14.8. Simple boilers consist of a heat source, a water drum, a water inlet, and a steam outlet. In this type of boiler, the water drum is partially filled with water and then heat is applied. Steam forms after the water is heated sufficiently. As the steam leaves the vessel, it is captured and sent to other parts of the process (e.g., used to turn a steam turbine, or sent to a heat exchanger to heat a process fluid). Makeup water is then added to the drum to compensate for the liquid lost as steam.Boilers use the principle of differential density when it comes to fluid circulation. For boilers to work properly, they must have adequate amounts of heat and water flow. Factors that affect boiler operation include pressure, temperature, water level, and differences in water density. As fluid is heated, the molecules expand and the fluid becomes less dense. When cooler, denser water is added to hot water, convective currents are created that facili – tate water circulation and mixing. Water Circulation The circulation of boiler water (shown in Figure 14.9) is based on the principle of convection. A fluid that is heated expands and becomes less dense, moving upward through heavier, denser fluid. Convection and conduction transfer heat through pipe walls and water cur -rents, resulting in unequal densities. Cold water flows through the downcomer to the bottom of the mud drum and then flows upward through the riser (water wall tubes) as it is heated. In a water tube boiler, circulation occurs because the temperature of the fluid in the down -comer is always lower than the temperature in the boiler and generating (riser) tubes. Steam bubbles are formed as the liquid temperature continues to increase. These˜bubbles˜increase Figure 14.8 Simple boiler. Fuel inBurner Water in Drum Steam outSteamturbine Figure 14.9 Water circulation. Riser Downcomer Steam drumSteam outletFeed waterMud drumM14_NAPT1262_02_SE_C14.indd 29929/06/2018 13:50
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300 Chapter 14 the circulation as they move up the riser tubes. The pressure builds as the water vapor col -lects in the upper drum. Each time the water passes through the tubes, it picks up more heat energy. As the pressure increases, the boiling point of the water increases. When the target pressure is achieved, steam is delivered to the steam header. To maintain this pressure, makeup water must be added, heat must be continually applied, and circulation must be controlled. In a fire tube boiler, the water level in the boiler shell must be maintained above the tubes to prevent overheating of the tubes. Superheated Steam Saturated steam is steam in equilibrium with water (e.g., steam that holds all of the moisture it can hold and still remain a vapor). Saturated steam can be used to purge process equip – ment or perform other functions, or it can be superheated. As long as the steam and water are in contact with each other, the steam is in a saturated condition. Saturated steam cannot absorb additional water vapor, but the boiler can continue to add heat energy to it. Steam that continues to take on heat energy or get hotter is known as superheated steam. Superheated steam, which is produced downstream of the steam drum (typically in the firebox), is steam that has been heated to a temperature above its saturated temperature. Superheated steam is typically 200 to 300 degrees F (93 to 149 degrees C) hotter than satu – rated steam. Typical uses for superheated steam include: Ł Driving turbinesŁ Catalytic crackingŁ Product stripping Ł Maintenance of steam pressures and temperatures over long distances Ł Producing steam for systems that require dry, moisture-free steam. Desuperheated Steam Superheated steam might not be the best choice for heat transfer in some heat exchangers because the amount of energy given up by superheated steam is relatively small compared to the energy given up by saturated steam. Also, some facility processes cannot tolerate the high temperatures of superheated steam. The process of cooling the superheated steam is called desuperheating. Desuperheated steam is superheated steam from which some heat has been removed by the reintroduction of boiler feedwater. Typically, desuperheating does not occur at the boiler but at specific points in the process where boiler feedwater is injected into superheated steam. Boiler FeedwaterBoiler feedwater levels and flows are critical to proper boiler operation. If feedwater flow is reduced and the water level decreases to the point where the boiler runs dry, the tubes will overheat and fail. If the boiler water level becomes too high, excess water will be carried over into the steam distribution system. This negatively affects process facility steam consumers and can damage turbines and other equipment.During the boiling process, most suspended solids stay in the water section of the drum while steam is sent to the distribution system. Suspended solids are removed from the steam drum by sending a small amount of the feedwater, called continuous blowdown, to a blowdown tank. This continuous blowdown is usually released to a waste water treat – ment processing unit. Boilers utilize both continuous and intermittent blowdown systems to remove suspended solids from the steam drum and solids that have settled from the mud drum. Blowdowns limit the scale buildup that can negatively affect turbine blades and superheater tubes. Saturated steam steam in equilibrium with water (e.g., steam that holds all of the moisture it can without condensation occurring).Superheated steam steam that has been heated to a very high temperature so that a majority of the moisture content has been removed (also called dry steam ).Desuperheated steam superheated steam from which some heat has been removed by the reintroduction of water. It is used in processes that cannot tolerate the higher steam temperatures. M14_NAPT1262_02_SE_C14.indd 30029/06/2018 13:50
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