Process technology equipment and systems chapter 9, 10 & 11, Boilers, Furnaces & Instruments
Boilers OBJECTIVES After studying this chapter, the student will be able to: • • • • • • • • • Describe the basics of boiler operation Describe a fire-tube boiler Describe the main components of a water-tube boiler and explain how it operates List some boiler operating problems Distinguish between superheated and desuperheated steam Describe the primary responsibilities of a boiler technician Describe an inverted bucket steam trap Describe a float steam trap Describe the bellows thermostatic steam trap 213 Chapter ● Boilers Key Terms Bellows trap—a thermostatic steam trap that operates by opening or closing a bellows as the temperature changes; this movement opens and closes a valve Boiler load—plant demand for steam Burner—used to evenly distribute air and fuel vapors over an ignition source and into a boiler firebox Damper—a device used to regulate airflow Desuperheating—a process applied to remove heat from superheated steam Downcomers—the inlet tubes from the upper to lower drum of a water-tube boiler; these tubes contain hot water Economizer—a section of a fired boiler used to heat feedwater before it enters the steam drums Fire-tube boiler—a type of boiler that passes hot gases through tubes to heat and vaporize water Flame impingement—frequent or sustained contact between flames and tubes in fire-tube boilers and furnaces Float steam trap—a steam trap that operates with a float that opens a valve as the condensate level rises Inverted bucket steam trap—a mechanical steam trap that operates with an inverted bucket inside a casing; effective on condensate and noncondensing vapors Mud drum—the lower drum of a water-tube boiler Risers—the tubes from the lower drum to the upper drum of a water-tube boiler; these tubes contain steam and water Spuds—gas-filled sections in a boiler-fuel gas burner Steam-generating drum—a large upper drum partially filled with feedwater This drum is the central component of a boiler It is connected to the lower mud drum by the downcomer and riser tubes and receives steam from the steam-generating tubes Steam trap—a device used to separate condensate from steam and return it to the boiler to be converted to steam Superheated steam—steam that is heated to a higher temperature Thermostatic steam trap—a type of steam trap that is controlled by temperature changes Water hammer—a condition in a boiler in which slugs of condensate (water) flowing with steam damage equipment Water-tube boiler—a type of boiler that passes water-filled tubes through a heated firebox 214 Water-Tube Boilers Boiler Applications and Basic Operation Steam generators or, as they are commonly called, boilers, are used by industrial manufacturers to produce steam Steam is used to operate steam turbines, distillation systems, and reaction systems They can be used for such processes as laminating, vulcanizing, extrusion, firefighting, and flare systems; and to provide cooling or heating to process equipment Boilers use a combination of radiant, conductive, and convective heat transfer methods to change water to steam A simple boiler consists of a heat source, water-containing drum, water inlet, and steam outlet (Figure 9.1) As heat is added to the drum, the temperature increases until the water boils As the steam rises, it is captured in a line and sent on for further processing Factors that affect boiler operation are density differences for internal circulation, pressure, temperature, and water level Fire-Tube Boilers A more complicated boiler is the fire-tube boiler, which resembles a modified shell-and-tube heat exchanger This type of boiler is composed of a shell and a series of tubes designed to transfer heat from the fire-tubes and into boiler feedwater Combustion gases exit through a chamber similar to an exchanger head and pass safely out of the boiler The water level in the boiler shell is maintained above the tubes to protect them from overheating The term fire-tube denotes that the heat source is from within the tubes A fire-tube boiler (Figures 9.2 and 9.3) consists of a boiler shell with feed inlet and outlet connections, fire-tubes, a combustion tube, burner, feedwater inlet, steam outlet, combustion gas exhaust port, and tube sheets Water-Tube Boilers The most common type of large commercial boiler is a water-tube boiler (Figure 9.4) A water-tube boiler consists of an upper and lower drum Steam Figure 9.1 Simple Boiler Drum Water 215 Chapter ● Boilers Figure 9.2 Fire-Tube Boiler Operation Combustion Gases Steam Water In Tube Sheet Hot Gas Chamber Burner Baffle Hot Gas Chamber Natural Gas (Heated Tubes Submerged in Water) Figure 9.3 Fire-Tube Boiler connected by tubes The lower drum and water-tubes are filled completely with water, whereas the upper drum is only partially full This arrangement allows steam to pass through mechanical separators in the upper drum, flow to a superheater section, and then exit the boiler As heat is applied to the boiler firebox, water flows from the upper drum through downcomers into the lower drum Tubes, called risers, cause water and steam to flow into the upper drum because of density differences Boiler water circulation operates under the principle of differential density When a fluid is heated, it expands and becomes less dense Cooler water flows from the upper—or steam—drum through the downcomers to the 216 Main Components Figure 9.4 Water-Tube Boiler Boiler Stack Desuperheated Steam Superheated Steam Economizer Section Downcomer Heat Riser Water In Boiler Furnace Water mud drum (the lower drum) and then rises as some steam is generated Circulation continues, and makeup water is added to the upper drum to replace the steam that is generated Water circulation continues in a water-tube boiler because steam bubbles in the lower drum move up the riser tubes and cause water density to decrease The cooler water in the downcomer flows into the mud drum The riser and steam-generating tubes are physically located near the burners Steam moves up the riser and steam-generating tubes and into the upper steam-generating drum Steam generation causes pressure to rise When the target pressure is achieved, the boiler is “placed on the line.” Pressure is maintained by adding makeup water and continuously applying heat Main Components Furnace The water-tube boiler firebox (that is, the furnace) is designed to reduce the loss of heat and enhance the heat energy being applied to the boiler’s internal components Boiler furnaces have a refractory lining, burners, c onvection-type section, radiant section, fans, oxygen control, stack, damper, and many other components associated with fired heaters 217 Chapter ● Boilers Tubes Boilers contain several types of tubes Steam-generating tubes are attached to the upper and lower drums Flow goes through the firebox and back up to the upper steam drum Downcomer tubes are warm-water tubes connecting the upper and lower drums Risers are hot-water tubes between the upper and lower drums A water makeup line flows into the upper drum Steam is removed from the upper steam-generating drum and heated to the desired temperature in superheater tubes Superheated steam temperature can be increased as it re-enters the furnace Some processes cannot handle high temperatures, so the superheated steam is cooled off This process is called desuperheating Drums The drums inside a boiler furnace are pressure cylinders connected by a complex network of tubes The drums are classified as the upper (steam) drum and the lower (mud) drum The steam drum contains a water-steam interface The upper drum contains the feedwater inlet distributor, a blowdown header, and water separation equipment The lower mud drum is always full of liquid Gas and Oil Burners Most boilers use natural gas or atomized fuel oil burners to provide heat to the furnace Burners inject air and fuel through a distribution system that mixes them into the correct concentrations so combustion can occur easily Some large boilers, primarily in electrical generating plants, burn coal The key components of the combustion apparatus (Figure 9.5) include the following: • Dampers that regulate air into the burner • Air ducts with fixed blades that create a swirling effect as air enters the furnace • Components called spuds that distribute fuel gas • An igniter that works like a spark plug to ignite the flammable mixture Flame detection instruments shut off fuel gas if the flame goes out; and factory mutual valves (FM valves) shut off fuel gas when potentially dangerous Figure 9.5 Natural Gas Burner Dampers Spuds Impeller Ignitor 218 Boiler Functions situations arise such as low drum level, flame failure, and the like Most plant boilers use forced-draft fans to supply combustion air Economizer Section The economizer section (see Figure 9.4) is used to increase boiler efficiency by preheating the water as it enters the system This section is a series of headers and tubes located between the firebox and the stack Temperatures are typically lower in the economizer section than in the rest of the system, but the hot flue gases moving out of the firebox and into the stack still have enough heat to offset energy costs The economizer section in a boiler is very similar to the convection section in a fired heater system Both operate under the energy-saving concept of using the hot flue gases before they are lost out the stack Boiler Functions When a boiler is being started up, the following process occurs The furnace, which contains cool water in drums and tubes, starts to heat up When the burners are lit, hot combustion gases begin to flow over the generating tubes, riser tubes, downcomer tubes, and drums Radiant, convective, and conductive heat transfer begin to take place Hot gases flow out of the firebox, into the economizer section, and out the stack Water temperature increases at programmed rates Pressure begins to increase Steam may initially be vented to the atmosphere As the temperature of the water inside the generating and riser tubes increases, the density of the water decreases and initial circulation is established Bubbles begin to form and rise in the water, increasing circulation and pressure (Figure 9.6) This circulation rate can easily reach million pounds per hour At this point, approximately 65,000 pounds per hour of steam is being produced Figure 9.6 Steam and Water Drum Circulation Steam Water Tube Water In Water Radiant Heat Downcomer Riser Mud Drum 219 Chapter ● Boilers Each time the water passes through the tubes, it picks up more heat energy When the pressure increases to slightly above the system pressure, steam will flow through the nonreturn valve into the system Boiler load is a term used to describe the plants demand for steam Steam As long as steam and water are in contact with each other, the steam is saturated This saturated condition means that for every temperature of water, a corresponding pressure of steam exists The pressure on the water sets the temperature as long as the steam and water are in contact Basic boiler design removes the steam from the upper steam water drum and heats it up at essentially the same pressure This process is referred to as superheating Some plant processes cannot tolerate high temperatures The process of cooling the superheated steam is referred to as desuperheating During the desuperheating process, part of the superheated steam is returned to the steam drum The cooler liquid in the steam drum removes heat from the superheated stream and allows it to be used in specific plant processes Boiler Operation Starting up a boiler requires the following steps: Fill the steam drum with water to the normal level Start the fan Purge the furnace Check furnace for percentage of flammables Light the burners Bring the boiler up to pressure Place the boiler online Each of these steps requires the operator to perform a number of tasks These tasks vary from site to site, and you will spend many hours training for your specific procedure before being allowed to operate the boiler Because each site is different, it is difficult to identify every task a boiler operator has The most common operator responsibilities are related to the prevention of typical boiler problems Typical boiler problems include tube rupture, soot buildup in superheater and economizer tubes, loss of water flow, flame impingement (frequent or sustained contact between flame and tubes), scale, impurities in steam or water, flame failure, and improper water level It is usually the operator’s responsibility to control water and steam flow rates and temperatures and water level in the boiler The operator also checks for smoke and checks burner and flame pattern The operator maintains good housekeeping and unit logs and checks fuel pressure and temperature and oxygen level Finally, the operator monitors the pressure of 220 Steam Systems the firebox and drum; the temperatures in the firebox, stack, superheater, and desuperheater temperatures; and ensures fan operation Steam Systems Steam is used in a variety of applications in industrial manufacturing environments There is a considerable cost incurred in the treatment and production of steam, so steam reclamation is an important and common feature at most companies that use steam in their processes As steam flows from the boiler to the plant, it begins to cool As it cools, condensate is formed Condensate can cause many of serious problems as it flows with the steam Slugs of water can damage equipment and lead to a condition known as water hammer Devices known as steam traps are used to remove condensate Steam traps are grouped into two categories: mechanical and thermostatic Mechanical steam traps include inverted buckets and floats Thermostatic traps include bellows-type traps A steam system that includes a steam trap is shown in Figure 9.7 Inverted Bucket Steam Trap The inverted bucket steam trap (Figure 9.8) is a simple mechanical device used to remove condensate from steam and return it to a condensate header The condensate header runs back to the boiler, where the Steam Load Check Valve Stack TR PIC Ti LIC Pi FIC PIC Mud Drum r Rise Economizer Section Deaerator ncom LP Steam er PR Dow Treated Water SteamSteam Generating Drum Super-heated Burners Fan Steam-Generating Tubes Fan Steam FIC Pump Fuel Oil Tank FIC Heat Exchanger Pump Figure 9.7 Steam System 221 Chapter ● Boilers Figure 9.8 Inverted Bucket Steam Trap Outlet Cap Valve Air Vent Steam Bucket Condensate Bucket Weight Inlet clean condensate is converted to steam Inverted bucket traps can handle condensate, air, and other noncondensable gases such as nitrogen and oxygen During operation, the steam enters the bottom of the trap via the inlet and fills the inverted bucket An air vent is located on the top of the bucket Gases escape through this hole and into the outlet line The outlet valve is also located on the top of the inverted bucket The position of the bucket determines whether the valve is open or shut When the bucket is in the lower position, the valve is open When the bucket is in the upper position, the valve is closed Condensate in the steam drops to the bottom of the inverted bucket, and gases escape out the air vent When the body of the bucket trap is full of condensate, the inverted bucket rests on the bottom The outlet valve on the top of the inverted bucket is in the open position As steam fills the inverted bucket, the bucket rises and the valve closes Float Steam Trap Another type of mechanical steam trap is a float Float-type traps have a float that rests on the top of the condensate (Figure 9.9) A rod to the outlet valve attaches the float The position of the float determines the position of the valve As the level in the trap increases, the float lifts, allowing condensate to flow Float steam traps feature the following components: • Body • Inlet and outlet • Bonnet • Float • Rod • Valve 222 ... distillation systems, and reaction systems They can be used for such processes as laminating, vulcanizing, extrusion, firefighting, and flare systems; and to provide cooling or heating to process equipment. .. water and steam flow rates and temperatures and water level in the boiler The operator also checks for smoke and checks burner and flame pattern The operator maintains good housekeeping and unit... temperature, and pressure control that regulates fluid feed rates in and out of the process furnace Furnace hi/lo alarms—alarm warnings that warn when the process flow is off specification and prevent equipment