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A Boiler Feed Pump

A boiler feed pump is critical equipment used in a boiler house which supplies deaerated and LP dosed feedwater to the boiler. This pump has to pump feedwater into the boiler drum against the boiler drum pressure. So, discharge pressure of this pump is higher than the boiler drum pressure. Normally, the feed pump is a multistage pump driven by an electrical motor or turbine.

In a large boiler, where steam generation capacity is more, more feedwater is required to be pumped. In this case, turbo feed pump is preferred. Also, turbo feed pump is having another merit. During non-availability of power, a turbo feed pump can pump feedwater into the boiler, by utilizing steam of the boiler. So, starvation can be avoided. 

The suction pipe of the boiler feed pump is connected to the deaerator storage tank through an isolation valve and a strainer. The suction strainer restricts entry of foreign material into the pump. To monitor the chocking condition of the strainer, differential pressure across the strainer is monitored. Two mechanical seals are provided at both ends of the pump to avoid leakage from the shaft end. The discharge line of the pump has a pneumatic or motor operated control valve. A non-return valve (NRV) is provided to restrict back flow of feedwater. A disc is fitted to the discharge end of the shaft of the pump, which is acted on by the discharge pressure of the water to counter the axial force of the impeller. This is called balance disc.

A multistage pump cannot be operated at zero discharge (when discharge valve is closed). Some minimum flow is required to avoid the damage to the pump. To ensure minimum flow of boiler feed pump at any operating condition, automatic recirculation valve (ARC) is provided. When feed pump flow becomes less than the required minimum flow, the ARC opens and the water discharged is recirculated to the deaerator through a minimum circulation pipeline.

In a boiler, feedwater is evaporated continuously, and the steam generated is taken out. So it is required to pump that much water into the boiler to keep the water level in the boiler within the safe limit. With the help of a boiler feed pump, this level is maintained in a steam drum. In this drum, both steam and water exist. 

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