High pressure boilers of the type used by electrical generating plants operate at a water pressure generally in the range of 2,000-4,000 psi. Before such a high pressure boiler can be fired up, it must be supplied with water under pressure, for example, on the order of 500 to 1,000 psi, depending upon the boiler design. Water under pressure is supplied by a series of feed pumps, one feeding the other. Initially, the boiler and the series of feed pumps are usually filled with condensate from a condenser using only the condensate pump. In normal operation, the condensate pump usually takes water from a condenser and increases the pressure to about 150 psi and supplies a condensate booster pump which boosts the pressure to approximately 300-600 psi. In turn, the condensate booster pump supplies water to a boiler feed pump which increases the pressure to 1,000 to 4,000 psi depending upon boiler design and the operating condition, such as start-up, part load, or full load.
A very conventional arrangement for boiler feed pumps is to have two boiler feed pumps, one being a start-up pump that is limited in size and driven by a constant speed motor, without a fluid drive, and a second separate main “full-size” pump that is used for normal operation and is driven by a variable speed power source, either (a) a mechanical drive steam turbine, (b) a variable speed fluid drive that is in turn driven by the main turbine-generator, (c) a variable speed fluid drive that is driven by a large constant speed electric motor, or (d) a motor driven by a variable frequency power source based on solid state electronics. When a pump is used for boiler feed pump service and it operates a constant speed, the water flow is controlled by a discharge flow control valve (sometimes called a pressure control valve).
For boiler feed pump service, it is common to use a two-pole motor, and for 60 hz systems, such motors rotate generally at 3600 rpm if it is a synchronous motor, or between 3575 to 3585 rpm if it is an induction motor (3000 rpm for 50 hz systems). Another motor design that is also commonly used is a four-pole motor, and for 60 hz systems, such motors rotate at or near 1800 rpm (1500 rpm for 50 hz systems), but these motors typically use a step-up gear to increase the pump speed to the 3600 rpm range, or higher, depending upon the pump design.
Another conventional arrangement is to have two main pumps “usually approximately 60% capacity each”, that are each driven by mechanical drive steam turbines, wherein for start-up, steam from another boiler, either a dedicated start-up boiler, or a boiler of another operating unit, is used to provide steam to drive one or both of these mechanical drive steam turbines during the start-up phase of this unit. In some of these plants where there are two main boiler feed pumps each driven by a mechanical drive steam turbine, a smaller boiler feed pump with discharge flow control valve is driven by a constant speed motor for start-up, for a total of three pumps. The advantage of this arrangement is that the boiler and turbine-generator can be started using electric power either from the grid or from a “black-start” generator, so that no steam source is needed. Clearly, there are advantages to being able to start using a motor driven by a “black-start” generator located at the plant.