Study Material NPTI- Boiler

STUDY MATERIAL (BOILER)

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National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06)

©NPTI (NR) PGDC 10th Batch 2005



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.

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1 Chimney.

2 ID Fan.

3 ESP.

4 FD Fan.

5 PA Fan.

6 SCAPH.

7 Air Pre Heater.

8 Economizer.

9 Feed Water Line.

10 Primary SH(LTSH).

11 Final SH.

12 Platent SH.

13 Extended Steam Wall.

14 Reheater.

15 Super Heated Steam.

16 Cold Reheat Line.

17 Hot Reheat Line.

18 Boiler Drum.

19 Down Commer.

20 BR Header.

21 Furnace.

22 Burner.

23 Wind Box.

24 Hot PA Header.

25 Cold PA Header.

26 Coal Mill.

27 Coal Crusher.

28 Seal Air Fan.

29 RC Burner.

30 PC Pipes.

31 Water Platent.

32 HP Turbine.

33 IP Turbine.

34 LP Turbine.

35 Condenser.

36 Ejactor.

37 Condensate Pump.

38 Gland Steam Cooler 1,2.

39 LP Heaters.

40 Deareator.

41 Boiler Feed Pump.

42 HP Heaters.

43 Makeup Pump.

44 Circulating Water Pump.

45 Water Treatement Plant.

46 Control Structure.

47 Generator.

48 Hydrogen Plant.

49 Main Transformer.

50 Aux. Transformer.

51 Air Circuit Breaker.

52 Cooling Towers.

53 CT Pump.

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Definition: BOILER Steam Generator is a complex integration of furnace, superheater, reheater, boiler or evaporator, economizer and airpreheater along with auxiliaries such as pulvarizers, burners fans, stockers, dust collectors and precipitators, ash handling equipment, 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. However the term “boiler“ is traditionally used to mean the whole steam generator. History of Boilers Early steam boilers consisted of little more than kettles filled with water and were heated on the bottom, similar to those shown in Figures a and b. Boilers of the early 1700s still used the kettle principle, but burned the fuel in an enclosed furnace to direct more heat to the boiler kettle.

Haycock Boiler,

33385

Early Steam Boiler A

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33386

Early Steam Boiler B

Boiler Efficiency In the mid 1700s, 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. To improve boiler efficiency, an integral furnace was developed with the fuel actually burned in a container enclosed within the water vessel (Figure 3). A smoke flue wound through the water from the combustion chamber to the atmosphere much like a coil in a still. 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.

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33387

Early Fire tube Boiler

Boiler Safety As the demand for power increased, many gas tubes that increased the heating surface replaced the single flue. More water was subjected to the heat from the flue gases. While this fire tube design was popular until about 1870, it was also dangerous. Many disastrous explosions resulted from the direct heating of the pressure shell that contained large amounts of water at saturation temperature. 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. This design would limit the consequences of a pressure-part rupture.

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Benefits While several water tubes boiler designs were patented between the late 1700s and the mid 1800s (Figure 4), it was not until 1856 that a significant breakthrough occurred. The design incorporated inclined water tubes connecting water spaces at the front and rear of the furnace with a steam space above (Figure 5). It provided a better water circulation and more heating surface than other designs, along with the reduced steam explosion hazard.

42 in.

64 in. Water-tube boiler of small tubes connected at one end to a reservoir. John Stevens, 1803.

46 in.

First water-tube boiler. Built and Patented by William Blakely in 1766.

Water-tube boiler with tubes connecting water chamber below and steam chamber above. John Cox Stevens, 1805. 33388

Early Water tube Boilers

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Inclined water tubes connecting front and rear water spaces complete circuit with steam space above. Stephen Wilcox, 1856. 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. Also during this time, feedback control was used to control the level in the boiler by regulating the water to the boiler. 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, furnace draft, and feedwater, combustion, and steam temperature. During the 1950s burner control systems were developed to start and stop burners and to include flame safety systems. In the 1960s control switched from predominantly pneumatic analog control to predominantly solidstate, discrete element, electronic analog control. 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. Beginning in the 1970s fuel economics have influenced changes in boiler control. The high price of fuel has allowed a greater degree of control sophistication than could be justified in 1970. In addition, the development of microprocessor control has caused an advantageous transition to the greater precision of digital control.

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Types of Boilers Fire tube Boilers

In Fire tube Boilers the hot flue gas products of combustion flow through boiler tubes surrounded by water. The heat transferred through the walls of the tubes to the surrounding water generates steam. The flue gases are cooled as they flow through the tubes, transferring their heat to the water. 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. To increase the heat transfer area and improve the heat transfer coefficient, longitudinal tubes were installed in the pressure vessel and flue gases were passed through the tubes. Other fire tube boilers include the Locomotive-type boiler and the Scotch Marine Boiler. The Scotch Marine Boiler is designed with the combustion chamber as a long cylinder, jacketed by a larger cylinder fitted with several passes of fire tubes. Today, 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. In the Dry back Fire tube Boiler in which the combustion chamber is lined with high temperature insulating material. 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. 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. Early water tube boilers were shown in Figures 4 and 5 of this module. Today a typical water tube boiler has a single burner with up to approximately 125,000 pounds per hour steam flow but is available in sizes up to several million pounds per hour with more than one burner.

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Drum Circulating Water Downcomer Boiler Riser

Superheater

Mud Drum Economizer

Feedwater Riser Burners

Steam Coil Air Heater

Air Air Heater

To Stack Gas Outlet

Gas

Air

Forced Draft Fan

Induced Draft Fan

33390

Modern Day Boiler

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Drum

Boilers operating below critical pressure are usually fitted with a steam drum, (Figure 7). In the steam drum, saturated steam is separated from a recalculating steam/water mixture. The recalculation flow is from the steam drum via downcomer tubes to either the mud drum or the water wall header, and from there through riser tubes back to the steam drum. Most boilers rely on natural convection for this flow, because of heat absorption by the risers from the furnace. Some larger boilers use low head, high working pressure pumps to provide positive circulation. The steam rises up through separation devices in the drum and exits to one or more superheating passes through the furnace. The water from the steam/water mixture is then recalculated together with the makeup feedwater to downcomer circuits. Water treatment chemicals may be added to the steam drum and feedwater may be discharged, or "blown down" from the mud drum, to reduce dissolved and undissolved solids in the boiler water. 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. Steam Outlets

Secondary Scrubber Primary Scrubber

Cyclone

Blowdown Feedwater Inlet

Chemical Feed

Upcomer/Riser

33391

Steam Drum: Figure 7

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Mud Drum The mud drum is completely filled with water and is the low velocity point of the circulating water. Unresolved solids that develop in the boiler gravitate to the bottom of the mud drum and can be drawn off. Riser Heat collecting surfaces constructed from tubing and conveying boiler circulating water upwards to the steam drum are generally called risers. The risers may originate from either the water wall header at the base of the furnace, or from the mud drum. Boiler circulating water absorbs primarily radiant energy from the furnace fireball while resident in risers jacketing the furnace. These heat absorption surfaces called water walls are fed from the water wall header at the base of the furnace. 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 downcomers are not heated and are located outside of the furnace cavity.. Superheater The superheater is a flue gas to steam heat exchanger. 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, turbulence, and concentration to maintain ignition and combustion of the fuel within the furnace. Economizer Feedwater from the condensate-feedwater system enters the economizer located in the furnace flue gas ductwork. Waste heat from the flue gas is absorbed by the feedwater in order to improve efficiency. 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. 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.

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Steam Coil Air Heater The steam coil air heater is a tublar steam to air heat exchanger in which auxiliary steam charges the coil. Combustion air flows across the tubes in order to provide a minimum combustion air temperature. 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. Forced Draft Fan The forced draft fan supplies low head air necessary for fuel combustion, and secondarily to make up for air heater leakage and for some seal-air requirements. Induced Draft Fan The induced draft fan used in a balanced draft furnace exhausts combustion products from the furnace. The induced draft fan creates a sufficient draft to establish a slight negative pressure in the furnace.

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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, is heated, is converted to steam, and exits the system in the form of steam. 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 combustion converts the chemical energy of the fuel to heat or to thermal energy. Heat Transfer

In most tube steam generators, the radiant section of the furnace is lined with a heat transfer surface of boiler circulating water tubes (water wall or mud drum risers.) The tubes receive radiant heat from the fireball and transfer it to the steam/water system. The water wall and the boiler circulating water heat transfer surfaces cool combustion flue gases. Flue gases exiting the furnace also transfer heat to the working fluid by conduction as they pass through the various heat transfer surfaces. 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 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. 15




Feedwater Supply System Figure 8 illustrates the basic feedwater supply system. The function of the feedwater supply system is to continuously supply water to the boiler through piping to the steam drum.

Intermediate Pressure Turbine

To Process Header Intermediate Steam to Process Drive to BFP

Lo Pressure Turbine

Lo Pressure Steam to Process Condenser

Superheater

Deaerating Heater

Steam Drum

Superheat Spray Valve

Makeup Softened Condensate Storage Tank

Economizer 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. The feedwater deaerator supplies the suction side of the boiler feedwater pump(s). These high-pressure pumps supply the economizer from the deaerator. Boiler feedwater from the economizer enters the steam drum, and the boiler circulating water system. Saturated steam from the drum passes through the superheater and is discharged to the process as high pressure superheated steam. The steam may be used to supply power turbines or manufacturing processes. Some of the energy in the high-pressure steam may be partially expended by the process of power turbines. The resulting intermediate pressure steam is used for feedwater heating, processes using low-pressure steam, or low-pressure power turbines. Steam pressure letdown and attemperation ©NPTI (NR) PGDC 10th Batch 2005 16



stations may provide supplements to the intermediate pressure steam loads. Excess high-pressure steam, resulting from sudden load drops may also be routed through letdown stations (sometimes referred to as steam bypasses) to the low-pressure steam header. Low-pressure steam is phased back to liquid by the steam condenser and is stored in the condensate storage tank. 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. The discharge side of the pump is provided with a recirculation line back to the reservoir-feeding pump, or occasionally backs to the suction side of the pump. A mechanical pressure relief valve, a shutoff valve and orifice, or modulating control valve may feed the recirculation line. 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 heat exchanger may be either air to steam or water to steam. Feedwater Heaters

Feedwater heaters are used to heat the boiler feedwater so that less fuel is required to generate steam. The heaters may be classified either as low pressure prior to the deaerator or high pressure after the boiler feedwater pumps. Heating the feedwater is also necessary for the process of deaeration.

Deaerator Heater and Storage Tank The deaerator is used to eliminate air, oxygen, CO2, and other gases from the boiler feedwater. These gases are removed by vigorous boiling and venting the gases to atmosphere. If CO2 were allowed to remain in the water, the heat exchangers and condensate return piping would become corroded. If oxygen were allowed to enter the boiler, serious corrosion could occur. Boiler Feed Pump

The boiler feed pump is used to supply high-pressure boiler feedwater to the drum. Boiler feed pumps operates in a constant speed or variable speed manner. A variable speed motor, a magnetic or hydraulic coupling, or a steam turbine can drive a variable speed pump’s speed. A recirculation line is open at low flow to keep the pump from cavitating and overheating. Economizer

The economizer is used to recover heat from the flue gas to the boiler feedwater. The flue gas exits the boiler and enters the economizer where it transfers heat to the boiler feedwater. The flue gas temperature decreases, and the boiler feedwater temperature increases. 17




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. Boiler blowdown can be continuous and is used to remove impurities in the boiler water. There are various methods for the internal treatment of boiler water. A blanket recommendation of any one method is not realistic. The type of treatment to be used in a particular boiler should be based on the raw water supply, the percent of make-up required, the nature of condensate returns, and other factors. Boilers require high-purity feedwater. Careful monitoring of boiler feedwater as well as condensate chemistries is crucial to boiler operations. The monitoring of PH, specification conductivities, and dissolved oxygen (DO) are required for high-purity feedwater maintenance.

Boiler Feedwater (BFW) Supply Flash Evaporator Sea Water- Feedwater conditioning removes dissolved salt and mineral solids that tend to form ions in solution. One method that removes dissolved salts and minerals is flash evaporation. The flash evaporator operates with its flash chamber under partial vacuum. Water that enters the chamber is preheated sufficiently to cause water to flash into a vapor upon entering the chamber. 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. Osmosis is based upon the principle that when a semi permeable membrane separates two solutions of different concentrations, solvent (water) will be transported from the dilute to the more concentrated side. 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). If the solution is salt water and a membrane is chosen that is permeable to water but not to salt, water will flow to the unpressurized side. The result will be a solution that is more dilute than the original and a solution that is more concentrated than the original.

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National Power Training Institute (Northern Region) PGDC in TPPE – 10th Batch (2005-06) Pressure Applied

Semi-Permeable Membrane Water

Water Dilute Solution

Concentrated

Dilute Solution Concentrated Reverse Osmosis

Osmosis

33393

Reverse Osmosis Figure 9 Chemical Treatment of BFW

Supplementing internal boiler water treatment is possible by injecting chemicals through the chemical feed line into the steam drum. 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. The continuous blowdown and chemical feed lines are separated so that the injected chemicals do not flow directly to the blowdown line. Oxygen Scavengers control corrosion by dissolved oxygen (DO). 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. Sulfite, which is a reducing agent, functions strictly through reaction with DO. Hydrazine also functions as an oxygen scavenger. Hydroquinone is an oxygen scavenger that has been applied in blends with hydrazine to catalyze its reaction with DO, but hydroquinone may also be used as a hydrazine substitute. Other chemicals such as carbodihydrazide decompose at feedwater temperatures and form hydrazine as a by-product. Organic oxygen scavengers, such as erythorbic acid and diethyl hydroxylamine, are also available. Corrosion Inhibitors- Hydrazine provides corrosion protection through the formation of magnetite film on

steel and through the formation of cupric oxide on copper alloys. Boiler Blow down Methods Continuous- During normal operation, feedwater is constantly added to the drum as steam is removed. The impurities in the feedwater and the impurities separated from the steam will remain in the boiler water. If the impurities are not removed, these impurities will become more concentrated and eventually deposit on internal tube surfaces. The formation of scale on tube surfaces reduces heat transfer and can lead to overheating and possible tube failures. Dissolved oxygen, organic-chemicalbreakdown products, acids, and excess caustic can cause corrosion of the boiler. Contaminants that

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can form deposits on boiler surfaces include calcium, magnesium, iron, silica or silicates, phosphates, sulfates, oils, and organic elements. Boiler water solids are maintained at recommended limits by the continuous blowdown line. The line is positioned internally, along the length of the drum, in a zone where solids tend to collect. A calibrated flow control valve regulates the amount of blowdown to the drain system, based on water solids concentration and feedwater flow. This process tends to remove the most contaminated water in the system and replace it with fresh feedwater. Intermittent blowdown is performed by periodically opening a blow off valve that is connected to the

lowest part of the mud drum. 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 intermittent blowdown may be operated anytime depending on the concentration of impurities in the boiler water.

Boiling Process and Steam Generation The process of boiling water to make steam is a familiar phenomenon. As heat is added to water, the temperature of the water increases. When the water temperature reaches the boiling point, or saturation temperature, some of the water begins to vaporize to steam. Saturated Water and Saturated Steam

When water just begins to boil, it is called saturated water. As more heat is added (at constant pressure), the fluid temperature will remain at the saturation temperature until all of the water is converted to steam. Once the conversion from water to steam is complete (but before the temperature is raised above saturation temperature) the fluid is called saturated steam. The speed of conversion depends on the rate of heat that is being added. It must be remembered that heat and temperature are not the same thing. A considerable amount of heat is added to the fluid while its temperature remains constant at the boiling point saturation temperature. Although the temperature remains constant, the heat being applied is not lost or wasted. It is being utilized to convert water into steam. The heat input or enthalpy necessary to convert saturated water to saturated steam is called the heat of vaporization. The conversion of water to steam requires much more energy beyond that required to reach the boiling point.

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o Temp ( F)

Superheated Steam

Saturated Water Constant Pressure

470

Saturated Steam

500 psi

450

Enthalpy (BTU/lb)

1205 33394

Boiling Process and Steam Generation Figure 10 Boiling Point- The term boiling point is most frequently used to identify conditions at atmospheric

pressure (29.92 inches of mercury.) For instance, the boiling point of water at atmospheric pressure is 212 degrees Fahrenheit, however, pressure increases when steam is generated in a closed vessel. The boiling point is actually a function of pressure and increases as pressure increases, as illustrated in Figures 11 and 12. At higher pressures, more heat energy is required to raise the fluid temperature to the boiling point. Enthalpy- The amount of heat energy contained in the fluid is termed enthalpy and is measured in BTUs/lb. 800

o Temp ( F)

700

2500 2000

600

1500 1000

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

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1200 1100 1000

o Temp ( F)

900 800 700 600 500

Length of line = h fg

Saturated Curve

Boiling Points

2000 psi 500 psi

400

200 psi 100 psi

300 400

500

600 700 800 900 1000 1100

1200 1300 1400 1500 33396

Saturation Curve–Boiling Point–Pressure Relationship Figure 12 Heat of Vaporization- The points at which all of the water has been converted to steam are indicated by

the saturated steam line (Figure 12). 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. For example, water begins to boil at about 470 degrees Fahrenheit when the pressure is 500 pounds per square inch. The enthalpy at this point is about 450 Btu per pound. As more heat is added (at constant pressure), the enthalpy increases and more water is converted to steam. The temperature remains constant until all the water has been converted to saturated steam. This point would be at the same pressure and temperature (500 pounds per square inch, 470 degrees Fahrenheit), but the enthalpy would be increased to 1205 Btu per pound. The heat of vaporization would then be the enthalpy of the saturated steam minus the enthalpy of the saturated water or 1205 - 450 = 755 Btu per pound. Steam Quality- The measure of how far the conversion from saturated water to saturated steam has

progressed is called quality and is shown in Figure 13. Quality is the percent by weight of vapor in a steam/water mixture. As more water is converted to steam, quality increases. Water on the saturated water line has a quality of 0%. 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%.

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1200 1100 1000 Superheated Steam

o Temp ( F)

900 800 700

Saturated Water 600 (0% Quality)

Constant Pressure Line

500 20

400

40

60

80

Saturated Steam (100% Quality)

300 400

500

600 700 800 900 1000 1100

Enthalpy (BTU/lb)

1200 1300 1400 1500 33397

Steam Quality Figure 13 Superheated Steam

If still more heat is added to saturated steam, the temperature will again begin to rise. This is shown by the dotted lines to the right of the saturated steam lines in Figure 10. The fluid in this area is said to be superheated steam. The super heaters derive their name from their function of heating steam above the saturation curve. Steam is sometimes referred to as having a number of degrees of superheat. 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. One is nucleate boiling. The second is film boiling. Nucleate boiling is preferred over film boiling. Nucleate Boiling- As a water-cooled tube is heated, steam bubbles form at the tube's inner surface.

The steam bubbles condense quickly in the main stream, giving up their heat to raise the temperature of the water. Normally these bubbles diffuse well and mix with the water in the center of the tube as shown in Figure 14. This process is referred to as nucleate boiling and promotes two benefits: (1) it heats the fluid inside the tube to saturation, and (2) it maintains tube metal temperature at saturation keeping the tube cool.

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Tube Wall

Steam Bubbles (Mixing) 33398

Nucleate Boiling Film Boiling- With high heat input levels and high steam quality, the nucleate boiling process breaks down. 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. This condition is known as film boiling (Figure 15). Little heat will be transferred from the tube metal through the film to the water in the center of the tube. The tube metal temperature will rapidly increase, resulting in a failure. Tube Wall

Steam Film (No Mixing)

33399

Film Boiling Figure 15 24




Departure From Nucleate Boiling- The point at which nucleate boiling stops and film boiling begins is

Increased Metal Temperature

determined by the heat input and steam quality. The point is termed Departure from Nucleate Boiling or DNB. Metal temperatures are shown in Figure 16 as a function of steam quality for several heat input levels. As the curves illustrate, only moderate heat inputs can be tolerated at high quality levels (Area B); however, much higher heat input levels can be tolerated at lower qualities (Area A). This means that high heat inputs, which result in higher levels of circulation and steam generation, can be used at low quality levels.

Superheated Steam

Subcooled Water

nput eat I H h Hig

Low

Heat In

put

Area A Area B 0%

Quality

100% 33400

DNB Metal Curve Figure 16 Research into DNB has found several parameters that affect DNB: A.

High fluid velocities decrease the occurrence of DNB at a given fluid quality.

B.

Fluid quality has a great effect on DNB. Lower qualities afford greater margins of safety and reduce the possibilities of the occurrence of DNB.

C.

Wall construction or location of heat flux also affects DNB. Heating a wall from one side could allow a steam film to form on the heated side of the tube, causing overheats.

D.

Tube type has a major impact on the prevention of DNB. 25




E.

Research and experience has shown that DNB is more likely to occur at operating pressures above 2000 psig.

F.

Higher heat flux also increases the possibility of DNB caused by the higher qualities generated.

Natural Circulation

Modern water tube boilers were developed from the early fire tube designs. Modern water tube boilers not only have a larger surface area available for heat transfer, but by proper design, a natural circulation effect is created with water continuously moving within the boiler tubes to remove and replace the generated steam. In a natural circulation system, circulation increases with increased heat input until a point of maximum fluid flow is reached.

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. Steam is significantly less dense than water. Water is supplied from a drum to the furnace wall tubes through 26




down comers. The down comers are not heated. As the unit is fired, a steam/water mixture is generated in the furnace wall tubes. 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. 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. As more heat is added to the furnace tubes, the quality of the fluid increases. Because the density difference becomes greater, more pumping power is available from the natural circulation effect. Up to a point, circulation will naturally increase with increased heat input and provide more flow to keep furnace tubes cooled as more steam is generated. Beyond a certain level, friction in the tubes overcomes the difference in density and circulation is reduced with additional heat input as shown in Figure 18. Natural circulation boilers are designed to operate in the left region of the curve so that circulation increases with heat input.

Possible Overheat

Inc reasing Flow

Normal Operation

Increasing Heat Input

33402

Effect of Flow versus Heat Input Figure 18

Natural circulation also provides an additional benefit. It partially compensates for normal imbalances in the heat input to the furnace. As shown in the left portion of Figure 17, if one tube receives more heat than adjacent tubes, it will generate more steam with a lower density and thus will receive more flow to keep it cool. Within normal limits, tubes exposed to higher heat levels will receive more cooling flow. If the heat imbalance becomes too great, flow will be reduced and the tube will overheat. Drum boilers operate in the area on or under the saturation curve. Steam quality leaving the riser tubes and entering the steam drum is usually 5 to 30%, depending on the boiler load and pressure. 27




This means that of the water that flowed down the down comers, between 5 and 30% will be converted to steam by the time it reaches the top of the furnace. Staying at low quality levels is necessary to protect the tubes from overheating failures caused by the nature of the boiling process. Steam Drum Internals

In modern drum boilers (Figure 6 of this module), the separation of steam from the steam/water mixture generated in the furnace usually takes place in two steps. Primary separation removes nearly all of the water from the mixture, so that in effect, no steam is recirculated to the boiler water; however, the steam may still contain solid contaminants that must be removed or reduced in amount before the steam is sufficiently pure for use. This step is called secondary separation or steam scrubbing. When wide load fluctuations and variations in water quality are suspected, secondary scrubbers may also be installed to provide nearly perfect steam separation. Cyclone Steam Separators- Primary steam separation is accomplished with cyclone steam separators.

The cyclones, essentially cylindrical in form, are arranged internally along the length of the drum. The steam/water mixture enters the cyclone steam separator tangentially. Centrifugal force throws the more dense water to the outside of the cylinder where it forms a layer against the cylinder wall. The less dense steam moves to the core of the cylinder and moves upward. The water flows down the cylinder wall and is discharged from the cyclone through an annulus located below the water level. The separated water returns to the boiler cycle virtually free of steam bubbles, thus providing a maximum available head for producing flow through natural circulation. Primary Scrubbers- The upward rising steam from the cyclones passes through the primary scrubbers

at the top of the cyclones for secondary steam separation. After primary separation, the steam may still contain dissolved solids suspended in tiny water droplets. These water droplets that contain solids are removed from the steam as it passes through the corrugated plate elements of the primary scrubber. Secondary Scrubbers- Further steam scrubbing of any trace amounts of water contaminants in the

steam are achieved by the secondary scrubbers. Secondary scrubbers are corrugated plates that are located at the top of the steam drum, and provide a large surface to intercept water particles as the steam weaves through the closely fitted plates. Steam velocity through the corrugated plate assembly is very low, so that re-entrainment of water is avoided. The collected water is drained from the bottom of the scrubber assembly to the water below.

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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).

Steam Coil Air Heater Stack Forced Draft Air

Scrubber

Air Heater

Vent Gas Gas Supply Dryer

Precipitator (Optional)

Windbox

Induced Draft Fan

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. The stack and fans control air flow. The differential pressure required for air flow is produced by a combination of the stack and fans. In Figure 19, air flows from the forced draft fan through a steam coil air heater into the boiler. Combustion products exit the boiler and flow through an induced draft fan to the scrubber and the stack. Specifically, this section will discuss: •

The function and components of the boiler forced draft system.

•

The function and components of the boiler balanced draft system.

•

The function and components of the air flow system.

Forced Draft and Natural Draft Systems

A forced draft system or a pressure fired boiler, operates with the air and combustion products that are maintained above atmospheric pressure. 29




Forced Draft Fan

The forced draft fan provides sufficient pressure to force the air and flue gas through the system. Natural Draft

Natural draft occurs as a result of the stack effect. Hot air or hot gases rise through vertical ducts. 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. 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. A pressure and draft profile of a forced draft system with an air preheater is shown in Figure of balanced draft, Profile of Pressure and Draft of a Pressure-Fired Boiler (Typical-Includes Air Preheater), shows in the figure. 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. At 70% boiler load the draft losses are approximately 50% of the full load draft losses. Balanced Draft Systems

Balanced draft systems have a forced draft fan at the system inlet, and an induced draft fan near the system outlet (Figure20, Balanced Draft Boiler (With Air Preheater), 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. The air preheater does not change the controlled furnace draft set point. Induced Draft Fan

The induced draft fan takes suction at the flue gas exit. The induced draft fan reduces the furnace pressure and ensures that it is negative.

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. 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).The air preheater does add additional draft losses to the combustion air and flue gas sides of the boiler. 30



Pressure


D A C

(+) B 0 Balance Point

(-)

F.D. Fan Out

Air Preheater Inlet

Windbox

High Load

Furnace

-

Boiler Outlet

Air Preheater Outlet

I.D. Fan Outlet

(100%)

Reduced Load -

(70%)

Stack

A B

Change Balance by Increasing Induced and Reducing Forced

C

Change Balance by Decreasing Induced and Increasing Forced

D 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. The negative furnace pressure condition is cooler and cleaner, which results in a better environment for the furnace housing, the auxiliary equipment, and the operating personnel.

Air Flow System

Air flow requirements of the combustion control system require that fans operate at different pressures and volume discharge rates. To meet these requirements, 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.

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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. The constant speed fan is attractive because of its low initial cost; however, a constant speed fan consumes significantly more energy than a forced draft fan at low boiler rates. Variable Speed Fan

The variable speed fan is attractive because it reduces energy at reduced flow rates. 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.

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COMPONENTS OF BOILER FUEL SYSTEMS: FUNCTION AND BASIC OPERATION

This section will discuss the boiler fuel oil and fuel gas systems. Specifically, this section will discuss: •

The function and components of the fuel gas system.

•

The function and components of the fuel oil system.

Fuel Gas System

The most common gaseous fuel is natural gas. 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.

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. It is usually necessary to heat Number 6 fuel oil so that it can be pumped through the system. It is normally not necessary to heat Number 2 fuel oil. Fuel oil is pumped through the fuel oil system, and the boiler control system regulates the BTU input by a control valve. The amount of heat (BTU) liberated per unit quantity of gas or oil is called the HigherHeat Value (HHV). The HHV for a fuel, typically in units of BTU per pound, can be found in the Combustion Engineering Fuel Burning and Steam Generation Handbook.

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GLOSSARY

Air

The mixture of oxygen, nitrogen, and other gases that with varying amounts of water vapour, forms the atmosphere of the earth.

Air heater

Heat transfer apparatus through which air passes and is heated by a medium of higher temperature, such as the products of combustion (flue gases) or steam.

Air preheater

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.

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).

Blowdown

Removal of a portion of boiler water to reduce chemical concentration, or to discharge sludge.

Boiler

A closed vessel in which water is heated, steam is generated, steam is superheated, or any combination thereof, under pressure or vacuum by heat from combustible fuels in a self- contained or attached furnace.

Boiler feed pump

A pump that is used by the boiler system to supply high pressure boiler feedwater to the steam drum.

Boiling point

The temperature that a liquid changes to vapor for a given pressure.

Burner

A device or group of devices for the introduction of fuel and air into a furnace at the required velocities, turbulence, and concentration to maintain ignition and combustion of the fuel within the furnace.

BTU

British Thermal Unit.

Circulation

The movement of water and steam within a steam generating unit.

Condenser

A heat exchange device that is used to condense steam. A condenser is used to condense the exhaust steam from turbines.

Constant speed A fan with a single-speed motor that uses damper control or inlet vane control to Fan vary air flow. Continuous blowdown

The continuous blowing down of water.

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Control

Any manual or automatic device for the regulation of a machine to keep it at normal operation.If automatic, the device is motivated by variations in temperature, pressure, water level, time, light, or other influences.

Corrosion

The wasting away of metals caused by a chemical action that is usually caused by the presence of O2, CO2, or an acid.

Deaerator

A device that is used to remove air and gases from boiler feedwater prior to its introduction into a boiler.

DNB

Departure from Nucleate Boiling: The point at which nucleate boiling stops and film boiling begins.

Downcomer

The lines that carry cold water from the steam drum to the mud drum of the furnace for heating.

Draft

The difference between atmospheric pressure and some lower pressure existing in the furnace or gas passages of a steam generating unit.

Drum

A cylindrical shell closed at both ends designed to withstand internal pressure.

Economizer

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. Heat input comes from the hot flue gases passing the tubes (containing feedwater) just before going to the air heater.

Efficiency

The ratio of the output to the input.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.

Enthalpy

The amount of heat energy that is contained in a fluid or gas in BTU/lb.

Feedwater

Water that is introduced into a boiler during operation. It includes make-up and return condensate.

Feed water heater

A tube-shell heat exchanger used to heat boiler feedwater.Specified as low and high pressure heaters that use main turbine extraction steam to heat water.

Film boiling

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.

Flash evaporator A device that is used with boilers operating at 1000 gal or over to purify boiler water by an evaporation technique.

Forced draft

The furnace draft that is caused by forcing air into the furnace with a fan. ©NPTI (NR) PGDC 10th Batch 2005 35


Forced draft fan


A fan that is used to force a flow of air into the boiler furnace.

Fuel

A substance that contains combustible materials that are used for generating heat.

Fuel oil equivalent barrels

Unit of energy management equivalent to energy in a barrel of oil, or 6,300,000 BTU.

Furnace

Combustion chamber of a boiler.

Heat exchanger A device for transferring heat energy from one medium to another. Heat of vaporization

The heat input or enthalpy necessary to convert saturated water to saturated steam.

HHV

Higher Heating Value. The number of heat units that are liberated per unit of quantity of fuel burned.

Induced draft

The furnace draft produced by drawing the flue gases out of the furnace by an induced draft fan. The furnace pressure is usually controlled at less than atmospheric pressure with the fan.

Induced draft fan

A fan that is used to produce a flow of air through the furnace by creating a lower pressure. An induced draft fan is commonly used to aid the exhaust of flue gases.

Intermittent blowdown

The blowing down of boiler water at intervals to remove suspended solids. Intermittent blowdown is achieved by periodically opening a blowoff valve located at the bottom of the mud drum.

Makeup

Water that is added to replace losses, blowdown, etc.

Mud drum

Lowest boiler drum in which suspended solids collect.

Natural circulation The circulation of water in a boiler that is caused by differences in density. Natural draft

A furnace draft that is caused solely by the effect of the stack.

Nucleate boiling

The process of steam bubbles at a tube's inner surface condensing and giving up their heat to raise the temperature of the water.

Oxygen scavengers

Chemicals that are used to control boiler corrosion.

PMC

Process Measurement & Control. 36


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.

Quality is the percent by weight of vapor in a steam/water mixture. The reintroduction of part of the flowing fluid to repeat the cycle of circulation.

Reverse osmosis A procedure that is used to remove dissolved salts and minerals in a solution. Risers

The tubes that are heated through which the steam/water mixture moves from the lower or mud drum to the boiler drum.

Saturated steam Steam that is at the boiling point temperature that corresponds to a particular pressure, without any water present. Saturated water

Water at the temperature of its boiling point.

Scrubber

An apparatus for the removal of solids from gases by entrainment in water.

Specific volume The volume that a pound of steam or water occupies at a given pressure or temperature. Stack

The flue, vent or passage through which smoke or heated air, etc. escapes.

Stack effect

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.

Steam coil air heater

A tubular recuperative air heater in which gas flows vertically through tubes. Air flows horizontally across the tubes in order to provide heat transfer.

Steam drum internals

All apparatus within a drum.

Steam quality

The percent by weight of vapor in a steam and water mixture.

Superheated steam

Steam that is at a higher temperature than its saturation temperature.

Superheater

A device that is used to raise the temperature of steam above its saturation temperature.

Variable speed fan

A fan with a variable speed drive that is used to change speed to vary air flow.

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Study Material NPTI- Boiler

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