A boiler should have at least two means of feeding water, because one might fail to work. The water inside a boiler is usually kept at a proper level by either pumps or injectors. Steam pumps (Fig. 167) are most commonly used on stationary and marine boilers, and may be classified as boiler-feeders, general surface pumps, tank pumps, or water-work pumps.

The steam pump is commonly used in power plants to supply feed water to the boiler. It is very important for the engineer in charge of the plant to see that the pumps supplying the steam boilers are in first-class order at all times, as any failure to maintain the water at a proper level in the boilers may result in serious injury to the boilers; an explosion may even occur.

One end of a boiler pump is called the engine or steam end and the other the pump or water end. A boiler-feeder is intended to feed water into steam boilers while they are under pressure. To illustrate: If the boiler is under a pressure of 100 lbs. to the square inch, and the steam piston in the pump receives 100 lbs. to the square inch, it is clear that there will be equilibrium between the steam pressure and water pressure of the pump. This is overcome by reducing the plunger diameter to perhaps one-half the size of the steam piston. In this way an unequal area in the steam piston and pump plunger is obtained. This difference enables the pump to force water against a pressure greater than that of the boiler. The necessary allowance for friction varies from 5 to 40%.

When a pump takes in water at only one end of the piston, it is called a single-action pump; when it takes water in at both ends, it is called a double-action pump.

All single, direct-acting pumps make use of an auxiliary plunger to carry a valve which gives steam to the main piston. By means of various devices, steam pressure is made to drive this auxiliary plunger backward and forward.