Properly called a “Direct Acting Steam Pump”, it is a reciprocating (steam) engine and a pump liquid end built together as a unit. Although steam is implied as the driving medium, compressed gases such as air or natural gas can be used.

Figure 8.2 below shows the cross section of such a pump. The steam or driving end is on the left and the liquid or pump end is on the right. Steam is admitted to the driving end through a sliding valve on its top which moves back and forth in response to the piston drive rod through the rocker arm thereby causing reciprocating action of the pump rod. The liquid end acts the same as any reciprocating pump. 

The pump shown is a double acting pump. That is, it pumps water in both directions of the stroke. It can also be built as a duplex unit which has two pistons driven by two steam cylinders. Duplex units have a more steady pressure and rate of flow and are often preferred. More detail on steam pump construction can be found in ANSI/HI 8.1-8.5 Direct Acting Steam Pumps available at www.pumps.org.

Direct Acting Steam Pumps were first designed in 1840 by Henry R. Worthington to act as boiler feed pumps aboard early steamboats.  Today, they are still used in hazardous applications where electrical power might ignite explosive gases. In addition, they should also be considered where pressurized steam or gas is readily available and might otherwise be wasted. Some manufacturers still produce steam pumps, but they have mostly been replaced by air operated pumps.