18 августа, 2013 
admin Subcooled FW enters the drum, mixes with the circulating boiler water, and attains saturation temperature instantly, as the boiling, circulating water is several times the incoming water flow. This circulating water picks up its latent heat progressively from the hot flue gases to form steam as it goes around the evaporator circuits several times. This steam is continuously separated in the drum by the steam separators. There is a balance between the incoming feed water (FW) and the outgoing steam when the system is properly functioning. Circulation ratio is the water in circulation divided by the steam flow. In other words it is the number of times the water has to go around the various evaporator circuits before it is all converted into steam.
Latent heat is added to the circulating water at constant pressure and constant temperature. There is no circulation in SC boilers as it is a forced flow arrangement. In once-through (OT) subcritical boilers also there is no circulation. To take advantage of the relatively low boiling temperature of water (critical temperature is 374.1°C), the hottest portion of the boiler, namely, the furnace, is encased in tubes carrying boiling, circulating water. The screen, division wall, boiler bank (BB), and EVAP tubes also form parts of the circulating system. It therefore follows that the most important use of circulating water is extracting high amounts of heat, particularly in the furnace, to keep the tubes cool. This is only possible as long as the steam bubble formation on the inside of tubes does not give way to a film of steam. In other words the departure from nucleate boiling (DNB) does not set in.
It is important to remember that a boiler is not designed for circulation, but for cooling the gases with ECON, evaporator, SH, and RH surfaces. It is then checked for circulation. Adequacy of circulation to prevent DNB is vital in all conditions of operation—at all loads with all fuels and combinations. It means that the velocities of steam-water mixture at all points are high enough to keep the tubes wet with no DNB. This is the essence of circulation requirement, and circulation check should be performed to verify that this condition is fully met. Usually, changes to the supply and riser tube geometry that feed and collect the water-steam mixtures, respectively, in the various circuits are needed to remedy the deficiencies. At times, other measures such as fitting of ferrules, using ribbed/rifled tubes, and so on may also be needed.
Flow in Vertical and Horizontal Tubes
Nucleate and film boiling has been briefly covered in Section 2.2. Figures 12.19 and 12.20 illustrate the flow in vertical and horizontal tubes.
As the water is heated in a vertical tube, a progressively increasing amount of steam is formed as shown in Figure 2.19 from tubes A to E. Water entering the heated tube undergoes these stages in sequence. These stages are not distinct, but blend into each other smoothly.
1. 
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FIGURE 2.20 Flow in horizontal tubes. |
FIGURE 2.19 Flow in vertical tubes. |
Tube A is bubble flow with low velocity and a few steam bubbles in a predominant water flow.
2. Tube B is emulsion flow where the steam bubbles increase and hence produce froth.
3. Tube C is slug flow with fine bubbles coalescing to form big bubbles almost filling the bore of the tube.
4. Tube D is wet wall flow where the steam fills the tube with an annular film of water cooling the tube.
5. Tube E is dry wall flow where the water film is replaced by a thin steam film that has poor cooling ability.
In a horizontal tube the flow patterns are different. Owing to the density difference, all the steam bubbles migrate to the top of the tube and slide along the tube wall.
1. At higher velocities (Tube A) of >1 m/s, the steam bubbles join together and move along with water, resembling wet water flow.
2. At low velocities (Tube B) of <0.5 m/s, the flow is even more asymmetrical and unstable. The water and steam flow separately in the tube.
Departure from Nucleate Boiling
As stated earlier, in the flow of a water-steam mixture, it should be ensured that the wet wall flow is always maintained and the dry wall flow is never allowed to set in, as the cooling provided by the steam film is not adequate to prevent tube overheating and puncture. The point at which this happens is the DNB, which is illustrated in Figure 2.21. It is in the transition zone that the excursion of metal temperature is the highest and, hence the tube failures occur.
The factors promoting DNB are
1. High heat flux
2. Low water velocity
3. High pressure
High heat flux (>250,000 kcal/m2 h or —92,000 Btu/ft2 h) cannot be avoided, particularly in the burner zone. There is a limit to increasing the velocities at (higher) pressures >150 bar when the circulation ratios are on the lower side.
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Steam-water |
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FIGURE 2.21 FIGURE 2.22 Departure from nucleate boiling. Rifled/ribbed tube. |
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Heat flux % Steam FIGURE 2.23 Effect of ribbed tube on permissible steam quality to avoid DNB. |
Ribbed or rifled tubes (Figure 2.22) are helpful in delaying the onset of DNB when compared to smooth tubes, as they offer more wetted surface for adherence of water film. The permissible steam by weight percentage (%SBW) for the same heat flux is raised from a range of 20-40% level to a range of 70-90% level by the use of ribbed tubes. Since they are expensive, they are employed around the burner zone and mainly in high pressure and SC boilers. Figure 2.23 illustrates the effect of ribbed tubing on steam by volume percentage (%SBV).
To maintain wet wall flow or nucleate boiling under all conditions, the following criteria must be satisfied for each circuit. A circuit is a set of heated tubes of similar shape and heat input that allow upward flow of water.
1. Exit quality. SBW at the top of any circuit should be less than a specified limit depending on the drum pressure and the location of burners—whether at the top or bottom—to prevent film boiling at the top of the circuit.
2. Minimum velocity. Water velocity at the commencement of the circuit should exceed a specified limit, depending on the inclination of the tubes to prevent the steam bubbles from adhering to the tube walls, causing overheating, and also to prevent sludge accumulation.
3. Saturated water head (SWH). The SWH, the ratio of pressure loss (including static head) to the pressure produced by a column of saturated water of the same height, is required to be at a certain specified minimum to prevent flow reversal.
The usual remedy for meeting this requirement is to increase the water flow to the defaulting circuit.
For drum pressures up to 100 bar, the SBW at the top of a circuit governs the circulation; that is, if the SBW limits are met at the top of the circuit, satisfactory conditions would result at the lower levels. But at higher pressures a more detailed analysis of the heat flux at all levels would be required to check whether safe conditions prevail over the entire tube length.
The exit quality is limited to —55-85% steam by volume (SBV) to stay within wet wall flow or nucleate boiling. The SBV varies with the pressure, and the approximate limit for conventional boilers is as shown in Table 2.7.
The equivalent SBW for the same SBV increases with pressure as shown in Figure 2.24. It follows that the number of times the water has to complete the circuit reduces with the
TABLE 2.7
Percentage of Steam by Volume (SBV) at Various Pressures
Drum pressure (bar) 20 40 70 100 140 180
SBV (%) 80 75 70 6 7 65 55
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Drum pressure (ata) 20 40 60 80 100 120 140 160 180 200
Drum pressure (psig) FIGURE 2.24 Relationship between SBV and SBW. |
Typical Circulation Ratios for Various Drum Pressures
TOC o "1-5" h z Drum pressure (bar) 20 40 60 80 100 120 140 160 180
Bidrum boilers 55 40 30 20
Single-drum boilers 18 15 13 11 10 8 7
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TABLE 2.9 Typical Minimum Water Velocities in Circulation
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Drum pressure (bar) Drum pressure (bar)
FIGURE 2.25 FIGURE 2.26
Typical circulation ratios for single and bidrum boilers. Minimum percentage of SWH at various
Pressures.
Increasing drum pressure. The circulation ratio falls with pressure (Table 2.8). Typically, for conventional boiler, the circulation ratios are as shown in Table 2.8. They vary with the type of boiler as the geometry varies (Figure 2.25). Certain process boilers with horizontal evaporator banks have lower ratios.
The minimum velocities at the entry to the circuits required for maintaining a good turbulent wet wall flow that prevents steam bubbles from sticking to the walls are shown in Table 2.9.
Percentage Saturated Water Head
The ratio of the anticipated density and pressure drop characteristics of the steam-water mixture in the riser tubes to the static saturated water column is the percentage saturated water head (%SWH). There are minimum values established from experience. When %SWH falls below these levels, flow reversals can occur between tubes of the same circuit, leading to stagnation and eventual tube failures. The typical minimum %SWH ranges from —50% to 70%, increasing with the pressure as shown in Table 2.10 (Figure 2.26).
TABLE 2.10
Typical Minimum Percent SWH versus Drum Pressure
Drum pressure (bar) 35 70 105 140 175 200
%SWH 51 56 60 63 65 66
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Back-pressure valve
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FIGURE 2.27
Types of circulation.
There are four basic types of circulation and several variations that are possible in boilers:
1. Natural circulation
2. Assisted circulation
3. Forced circulation
4. Combined circulation
These types are illustrated in Figure 2.27. It is very important to realize that regardless of the type of circulation the heat transfer from the gases remains the same and nucleate boiling regime pervades at all times.
It follows generally that
• Almost all the industrial boilers work at subcritical conditions and adopt natural circulation.
• Forced circulation is employed mostly in waste heat boilers to ensure adequate flow at all conditions with
— Low heat flux
— Rapidly fluctuating heating/cooling
— Horizontal evaporators
• Once-through or forced, assisted, and combined circulations are found only in utility boilers. There can be a few variations, depending on whether the operation is at subcritical condition, and fixed or variable SOP, which are described later on.
Natural circulation is the most common type adopted for subcritical pressures because of its
• Simplicity; no pump or associated equipment
• Self-limiting characteristics
For drum pressures up to 211 bar (3000 psia), where the density differential between water and steam is still —2.5 times, natural circulation has been employed. For higher pressures operation becomes increasingly more expensive because of larger downcomers and risers. Natural circulation relies only on the density difference between the saturated water in the downcomer and the steam-water mixture in the heated tubes.
In Figure 2.28, natural circulation is shown simplistically for a single circuit. The height of the tube is H. The downcomer is unheated and the riser is heated. The thermosyphonic head or circulation that drives the flow is H(pd — pr) and is proportional to
1. The height of column
2. The difference in densities between the downcomer and riser (pd and pr)
It follows that
• Taller boilers and higher pressures go together. The minimum drum centers are in package boilers, and they are rarely <3 m to provide sufficient thermosyphonic head.
• To enhance the density in downcomers there should be no steam bubbles trapped in the downcomer water after steam-water separation in the steam drum, and it is preferable to use subcooled water. Hence, the addition of ECON water to the steam drum in preference to the water drum.
• The separation of steam from water must be very efficient. Steam-free water under all conditions should be made available by the steam separators.
• The large bore downcomers have to be generously sized to keep the water velocities low so that steam bubbles are not carried along. Vortex breakers in the drum at the entry to the downcomers are provided for the same reason.
As the riser tubes are heated, there is more steam formation resulting in lowering of density in risers and increasing of differential density. The steam so formed rushes upward to the drum and is replaced by cooler and denser water at the bottom. A vigorous circulation is thus set up. The circulating head is more than the friction and shock losses. More heat results in more velocity and hence greater cooling of the tubes. Thus, there is no fear of overheating of the tube in this self-limiting system, which is a great advantage of natural circulation. In the rising part of the natural circulation curve, the friction and impact losses in the risers are less than thermo-syphonic head (Figure 2.29). At a certain point there is more steam in the riser tubes and the friction exceeds the driving force, thus reducing the circulation, as depicted in the drooping part of the curve in Figure 2.29. Natural circulation boilers always operate in the rising part of the curve.
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Water |
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FIGURE 2.28 Typical natural circulation circuit. |
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Water+ Steam |
Steam generated
FIGURE 2.29
Self-limiting characteristic of natural circulation.
Assisted Circulation or Full Load Recirculation
For pressures of 150 bar and above, mainly for large utility boilers, it is beneficial to supplement the thermosyphonic head by circulating pumps. Additional cost of pumps, ferrules, and associated equipment can be compensated:
• The smaller tubes that can be employed result in lowered tube weights due to reduced tube thicknesses.
• The savings in the downcomer and riser tubes arise from lower sizes and reduced lengths.
In addition, there is a greater flexibility in the disposition of surfaces as the flow can be controlled in tubes. The additional pumping power of the circulating pumps is substantially recovered in the resulting heating of circulating water due to friction in the ferrules. At pressures close to 200 bar, this arrangement provides a good security against circulation lapses. Several large boilers are operating satisfactorily on this principle. Not all boilermakers and customers favor this over the natural circulation, which is simple, time tested, and devoid of any pumps and drives (and hence fractionally more efficient).
Forced Circulation and Once-Through Forced Circulation
In the SC stage, all the water entering the boiler progressively turns into steam without having to go through an evaporator cycle as there is no latent heat to be absorbed. There is no circulation, and the circulation ratio is 1. There are no drums in SC boilers as there is no separation of steam and water. The OT journey of water through the furnace tubes is made possible by the pump pressure.
The ECON and SH in natural circulation boilers are always in forced flow condition. Even the furnace tubes are in forced circulation, but the furnace tubes form part of the ECON. Compared to the furnace tubes carrying saturated boiling water
of natural circulation boilers, which are all at the same relatively low temperature, do not experience temperature differential. In the OT boiler
• The tubes attain a much higher temperature and require alloy tubing.
• The adjacent tubes are at different temperatures.
• Greater temperature differential can exist during start-up and upset conditions.
At start-ups and low loads the flow is too inadequate to keep the tubes cool. This is resolved by means of a turbine bypass system with which a minimum required flow can be provided.
In the waste heat boilers mainly operating in the subcritical regime, forced flow is adopted for reasons explained earlier, with a low circulation ratio of ~3.
Forced circulation or OT is adopted for both SC and subcritical boilers in Benson design, also called universal pressure (UP) boilers in the United States, where vertical ribbed tubes are employed in furnace instead of spiral tube circuits.
The pump power is substantially returned to water in the form of heat as there are ferrules installed at the entry to the tubes for flow equalization.
Combined Circulation
This is a variation of the forced circulation principle, where circulating pumps are provided in addition, to deal with start-up and low loads. During start-ups the water-steam mixture is heated in furnace to —90%, dryness and the water is separated in separator vessels to be circulated in the boiler again with the help of the circulating pump. The back-pressure valve is kept closed during this period. On attaining the specified load of —60%, when the pumping is no longer required for tube protection, the valve is opened and the circulating pump is closed to run the boiler in forced circulation mode. This type of circulation is adopted for SC OT boilers. This system lends itself to variable pressure operation. Also a dual pressure operation is possible where the furnace is at SC pressure and SH at subcritical pressure.
Limits for Natural Circulation
The limits for natural circulation are set primarily by the ability of water in the evaporator circuits to keep the surfaces wet at all loads. In other words, the onset of film boiling on reaching DNB should be avoided. The reduced flow through the evaporator at higher pressure, due to the reduced density differential between water and steam, leads to a possible DNB. This can be avoided solely by increased circulation. This is achieved by increasing the downcomers and risers.
The broad limits for the natural and forced circulation boilers are listed in Table 2.11, where the operation is at fixed SOP in subcritical condition. For sliding or variable
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TABLE 2.11 Approximate Limits of Natural and Forced Circulation Boilers in Fixed Pressure Subcritical Operation
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A Drum or separating vessel. |
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(a) FIGURE 2.30 Evaporation processes with (a) fixed and (b) variable SOPs. |
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Pressure employed in modern utility boilers, other advanced systems of circulation are briefly described in the ensuing paragraphs.
The various evaporation processes with fixed pressure operation are depicted on the left sides of Figures 2.30a and 2.30b. The flow through the evaporator, as a multiple of the steam generated at that load, is different for different processes. Likewise, the various evaporation processes with variable pressure operations are depicted on the right side. In the lower range of evaporation a recirculation is superimposed by means of recirculation pumps to increase the mass flow velocities to avoid DNB. For high-capacity utility boilers, variable SOP is increasingly preferred, together with SC pressure, for better cycle efficiency and superior load dynamics. This is described in Chapter 9 in more detail. No details of the evaporation process are given here; only figures are included to provide a simplified view.
Circulation Systems for Subcritical and Supercritical Pressures
From the early twentieth century, efforts have made to operate steam cycles at very high pressures, preferably at SC conditions to gain advantage in cycle efficiency and to avoid recurring tube failures in evaporator system. Benson and La Mont, both engineers from Central Europe, patented their slightly different systems with Siemens and Sulzer, respectively. The Benson system was suitable for all pressures, whereas the La Mont was more suitable for subcritical pressure. Over the years, changes were made to the original principles by the main licensors and the licensed boilermakers to adapt to the needs of the power generation cycles. The Sulzer design is currently in Alstom’s possession.
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FIGURE 2.31 Subcritical pressure evaporator systems. |
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FIGURE 2.32 Supercritical pressure evaporator systems. |
The most salient features of the various evaporator systems are depicted in Figures 2.31 and 2.32.
Similar to the fans that set air and gas circuit into motion, the boiler feed pumps (BFPs) activate the water and steam circuit. Both are rotating devices. Together with fans the BFPs constitute important auxiliary features vital for both running and safety.
In smaller industrial plants the BFPs are included in the boiler supply, but in larger plants, where the HP heaters are introduced in the system, the BFPs must be located in the turbine hall and are treated as part of the turbine island. The following guidelines are important in the sizing and selection of BFPs.
• Boiler feed pumps can feed a single boiler or a battery of boilers.
• At least two BFPs are required with independent sources of drive power.
• However, certain countries permit a single BFP operation, provided the following requirements are met.
— On failure of the source of energy to BFP, the drive firing of boiler is interrupted.
— Steam pressure and water supply must be continuously and automatically controlled. Reliability of the FW controller is very important, and evidence of its reliable operation has to be demonstrated.
— There should be a reliable controlling of firing in the boiler. After interruption of fire, the stored heat in the furnace and boiler passes should not produce excess evaporation that can cause damage. This is particularly important in boilers with large amounts of refractory, such as the CFBCs with hot cyclones or boilers with refractory furnaces.
— The boiler should be equipped with an automatic trip system to cut off both the fuel and air when the water level in the steam drum drops below the trip level.
• With two BFPs there should be a reliable mechanism to instantly start the standby pump when the running pump fails.
• Boiler feed pumps can be driven by either electric motors or turbines. Considering cycle and plant efficiency, turbines are preferred, as the energy conversion from steam to electric power is not involved. The convenience of installation and lower O&M of motors against the higher expense, complexity, and O&M of turbines have to be carefully evaluated when a particular system permits both.
• In process plants, where steam availability is assured and there is use for LP steam exhausted by the turbines, turbo drives are overwhelmingly preferred to motors.
If the electric motors alone are used, care should be taken to employ independent feeders to assure power all the time. In utilities it is normal to have turbine drives for BFPs based on cycle efficiency considerations. Besides, they are more amenable to speed variation, which can make sliding pressure operation possible.
• Boiler feed pumps consume a lot of power, and in a utility they rank as the highest power consumers. It is therefore necessary to size them carefully to save on the auxiliary power.
• The design capacity of the pumping installation should equal the total evaporation of the boilers fed by the pump, with an added allowance for continuous blowdown
As applicable, together with any additional uses such as the desuperheater spray water.
• A safety margin of usually 10% is added to the pump capacity to arrive at the test block margins in industrial plants. This depends on the class of pump that defines the tolerances on head, capacity, and power. This can also be dictated by the client or consultant based on past practice.
• The design pressure of a BFP is the sum of the following losses added to the highest safety valve lift pressure (SVLP) with a margin of 2.5%.
— Loss through ECON and connections
— Loss through fully open feed control valve
— Loss through feed piping and valves
— Loss through feed heaters
— Static head
• The operating and design temperature of the pump is the same as that of the upstream deaerator.
• Net positive suction head (NPSH) is a very important consideration as the BFPs operate at relatively high temperature and speed. Flooded suction is necessary. The minimum required NPSH is a function of pump design at the operating speed determined by the pumpmaker. The suction losses should be added, and there must be adequate margin in the available head.
• A continuously drooping characteristic should be opted so that the parallel running of pumps can take place without the fear of hunting. The no-load pump pressure (NLPP) or the head generated at zero flow should preferably be limited to 110% of the design pressure. Higher the NLPP, more expensive are the feed piping and valves.
• An automatic recirculating valve, to protect the pump working against closed discharge valve, is necessary to prevent the overheating of the pump due to the idle churning of water. Below a predetermined flow of —10-15%, the valve opens and discharges the water to the deaerator or sometimes to the pump suction.
• Pumps must be hydraulically balanced by a balance disk or other means. The leak from the balance chamber is usually piped to the pump suction.
• There are mainly two designs in the BFP—radially split or ring section pumps and axially split pumps. Radial pumps come with (1) individual ring sections, consisting of impeller and casing sections, which are bolted together and (2) the impeller and casing that are inserted from one end in an external barrel. They are called ring section and barrel casing pumps, respectively.
• Radial ring section pumps are popular due to simplicity and low cost and smaller boilers. For maintenance and access purposes the suction and discharge pipes should be removed from ring sections. This drawback is eliminated with barrel
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Suction Impeller Head insulating Stay bolt Discharge Balance
FIGURE 2.33 Cross-sectional view of a typical 10-stage radial ring section centrifugal pump. |
Casing pumps, which are used for high-pressure and large-capacity boilers. Pump internals can be withdrawn from one end of the barrel without touching the piping.
• Axially split pumps also offer the same advantage of access to pump internals without disturbance to piping, as the pump casing is made in upper and lower halves and bolted together. The disadvantages are that the casings are heavy and each pump requires a pair of castings, making the pumps expensive. The pump efficiency is decided essentially by the impellers, which are common for all designs of casings.
A typical pump calculation for an industrial boiler is included in Appendix A. A typical ring section pump is depicted in Figure 2.33.
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