A feed pump supplies feed water to the boiler as and when required. An essential part of the Boilder feed water system, a boilder feed pump is selected according to the quantity and the amount of pressure required by the boiler. The feed pump raises the pressure of the feed water to a level high enough for the water to enter the boiler. The type of the boiler also plays an important role in selecting a feed water pump. For example, in case of auxiliary boilers, where the amount of feed water required is less, a steam driven reciprocating positive displacement pump is generally used. Read on to find more about boiler feed pump.
Reciprocating Boiler Feed Pump
The positive displacement reciprocating pumps are double acting pumps; which means that the liquid enters the boiler feed pump on either sides of the pump piston. Thus, when the piston moves up, suction is taken in the area below the piston and the liquid is drawn in. During the intake of the liquid, only the suction valve is open and the design is made in such a way that discharge valve remains shut. As the piston moves up, discharge valve on the top side of the piston opens, pushing the water out, while the suction valve remains closed. As the piston moves in the downward direction, the same operation is repeated in the upper part of the piston. In this way, the suction and discharge occurs on the opposite sides of the piston of the boiler feed pump.
All the positive displacement reciprocating pumps are self priming pumps, which produce the pressure required by the system. The pump is not affected by the intrusion of vapours or gases inside the pump system. Yet the pump is always provided with an air vessel at the discharge pipe to reduce the effects created by the pressure variations at the time of discharge. Due to high occurrences of pressure fluctuation inside the pump, the pump system is fitted with a relief valve in between the suction and discharge chambers in order to protect the pump.
Turbo-Feed Boiler Pump
The systems having steam turbine driven feed pumps usually have a high pressure water tube installations. The turbo-feed pump is a two stage horizontal centrifugal pump, which is driven by an impulse turbine. The whole arrangement includes a diffuser and a volute casing along with the impulse turbine. The pump runs with the help of direct steam from the high pressure boiler. The exhaust steam from the turbine is send to the pre-heater for feed heating. The bearings of the pump are lubricated with the help of filtered water supplied from an inlet opening provided in the first stage of the pump.
In most of the systems, the feed water supply is controlled with the help of a feed water controlled system, also known as the cascade system. The pressure of the feed discharged is controlled with the help of a governor. For the safety of the system, overspeed protection trips are also provided.
Another type of feed pump known as "electro feeder" is generally used in packaged boiler installations. An electro feeder type of a pump is a multistage centrifugal pump, driven by a constant speed electric motor. The number of stages required by the boilder feed pump depends on the feed quantity and discharge pressure.
References and Image Credits
Vandagriff, R. (2001) Practical Guide to Industrial Boiler Systems. New York: Marcel Dekker