A boiler feed pump is a special type of pump used to pump feed water into a steam boiler. The water may be supplied fresh, or it may be condensed water due to the condensation of steam produced by the boiler. These pumps are usually high-pressure units that draw from the condensate return system and can be of the centrifugal or positive displacement type.
Feed pumps can be several kilowatts in size, and the motor is usually separated from the pump body by some form of mechanical coupling. Large industrial condensate pumps can also be used as feed pumps. In either case, in order to force water into the boiler, the pump must generate enough pressure to overcome the steam pressure produced by the boiler. This is usually done by using a centrifugal pump. Another common form of feed pump runs continuously and is equipped with a minimum flow device to stop overpressure on the pump at low flow. The minimum flow usually returns to the tank or deaerator.
Stationary applications such as steam engines used on steam locomotives and ships, and power plants also require feed pumps. In this case, however, the pump is usually driven using a small steam engine that runs on steam produced by the boiler. Of course, there must be a way to inject initial water into the boiler (before steam power can be used to operate the steam-powered feed pumps). The pump is usually a positive displacement pump with a steam valve and cylinder on one end and a water supply cylinder on the other; no crankshaft is required.
Jet pumps use the Venturi effect and steam condensation to deliver water to the boiler.