Boiler feedwater circulation

An operator working at the main boiler would open the boiler s blowdown valve. Some lime-softened water, about 10 percent of the boiler s total feedwater must be drained to maintain the total dis-solved-solids (TDS) content of the boiler s circulating water below a safe maximum. 

S. Use 12Cr cases and impellers for main boiler feedwater above 250°F (120 C). Use carbon steel for circulating boiler feedwater. 

Fignre 18.6 shows the power plant subsystem water flow paths for a typical steam turbine with two separate water circulation loops. In the working fluid loop, steam is generated in the boiler and then expanded through a steam turbine to generate electricity. Condensed water is returned to the boiler to be recirculated through the cycle. A small fraction of the condensate is purged as blowdown to prevent the buildup of solids. Makeup boiler feedwater is added to maintain sufficient fluid levels. 

The boiler shown in Fig. 21.2 relies on natural circulation. The density difference between the water plus steam in the downflow pipes and the riser pipes causes the water to circulate through the pipes. A typical circulation rate results in 10 to 15 percent of the water flow being vaporized into steam. The produced steam flows from the steam drum into the steam header. The water level in the steam drum is maintained by the makeup boiler feedwater valve. 

The hot oil to the kettle boiler is a circulating pumparound stream from a fluid catalytic cracker fractionator, slurry-oil circuit. There is a fundamental difference between this sort of boiler and the utility plant boilers discussed previously. In the kettle boiler, the heating medium is inside, rather than outside, the tubes. To obtain the full capacity of the kettle boiler, the uppermost tubes should be submerged in boiler feedwater. This requirement creates a few problems. 

Figure 21.6 shows another common form of steam generation from waste heat. Boiler feedwater is circulated, via a pump, through the convective tubes of a fired heater. Most of the heat released by the fuel... 

This specific consumption figure is on an LHV (lower heating value) basis and is equivalent to approximately 31.5 MMBtu/t. This value is representative of the energy required not only to produce 2500 t/day of methanol plant but also to support all required off-sites, such as power generation, desalination of sea-water for boiler feedwater if required, and cooling water circulation. Battery limits energy consumption for a plant of this capacity would be about 2 MMBtu/1 less. 

Hot water or boiler feedwater is sometimes more effective than steam at removing heavy sludge deposits. The hot water tends to wash out waxy solids. If used for cleaning a distillation column or tower, the water can be circulated through the reflux piping. 

In order to limit thermal stresses and to reduce corrosion of the steel vessel, the reflector temperature was regulated near 350°F. About 50 kw of heat conducted from the fuel core to the reflector liquid was removed by circulating the heavy water with a 30-gpm canned-rotor pump through a reflector cooler which acted as a boiler feedwater preheater. A jet was located in this high-pressure circulating loop, the suction of which drew a continuous stream of gas from the vapor space above the reflector to a catalytic recombiner so that the concentration of deuterium and oxygen gases in this vapor space could be kept below explosive limits. 

In natural circulation type boilers, the water system and steam system are divided by a steam drum. Boiler feedwater is preheated at the economizer and fed into a steam drum, then evaporated at the water wall (Evaporator) connected to the steam drum, before coming back to the steam drum as water-steam mixture. Water and steam are separated at the steam drum, then steam is led into superheaters and water is led into the water wall (Evaporator) again. Therefore, impurities of silica, etc., contained in boiler feedwater concentrates during boiler operation. The drum has a blow-down line to avoid concentration with a continuous blow-down to the... 

On the other hand, in a once-through type boiler, boiler feedwater is fed once-through and preheated at the economizer, evaporated water wall and evaporator, superheated at superheater and led to the steam turbine. Therefore, impurities contained within boiler feedwater will deposit inside the evaporator or be carried into the steam turbine. Consequently, once-through type boilers require more severe water quality control than natural circulation type boilers. AVT has been applied as feedwater treatment for all once-through type boiler plants for many years, but Combined Water Treatment (CWT) Oxygen Treatment has been used with good results since about 10 years ago. Since then, water treatment in once-through type boilers has been switched from AVT to CWT in sequence. 

The second area will be feedwater pumps. It is normal to have a duplicate standby feedwater pump. Sometimes this may be two for each boiler or one duplicate pump to serve any selected boiler. These will usually require manual changeover in the even of failure of the duty pump. It is practical to automate this changeover by using pressure sensors and motorized valves. The same can apply to oil-circulating pumps, gas boosters, water-treatment plant and any other valves and motors. It is possible to do most things, but in the end there is the cost to be considered. An... 

Feedwater pumps will not normally constitute a noise problem unless the area is particularly sensitive. Two alternatives are then available. One is to install reduced-speed pumps and the other is to site the pumps in a separate acoustic enclosure within the boiler house. Oil-circulating pumps are usually low speed and, as such, do not cause a noise problem. 

Another common problem is that particulate and soluble iron returns to the boiler by way of the condensate and feedwater. When water circulation is discontinued, these solids are much more likely to settle out on internal surfaces. Frequent outages increase the opportunity for these deposits to grow to the point where problems can result, since cleaning of the surfaces rarely takes place under these circumstances. 

The steam and water circulation rate within the pressure vessel decides the effectiveness of the heat-transfer surface. When new feedwater is added to the system, precipitates fall out which might be removed as blowoff. Provisions are usually made in the lower portion of the convection section whereby a boiler operator may remove these particular precipitates by opening boiler blowoff valves. 

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