a unit composed of a steam engine and a piston pump; the pistons of both devices are connected to the opposite ends of a single rod. A steam pump is used to pump water, petroleum, and other fluids and to feed small boilers and drain bilges on ships. Steam pumps are usually built in horizontal pairs. As it moves back and forth, the rod of one machine controls a slide valve on the other. Both pairs of pistons move simultaneously but in opposite directions. When the piston of one of the pumps is at an extreme, the piston of the other is at midpoint; this allows the fluid to be pumped without interruption. Steam pumps composed of a steam turbine and a centrifugal pump are called turbopumps.