A positive displacement pump causes a liquid to move by trapping a fixed amount of fluid and then forcing (displacing) that trapped volume into the discharge pipe.

Positive displacement pumps can be further classified as either rotary-type (for example the rotary vane) or lobe pumps similar to oil pumps used in car engines. Another common type is the Wendelkolben pump or the helical twisted Roots pump. The low pulsation rate and gentle performance of this Roots-type positive displacement pump is achieved due to a combination of its two 90° helical twisted rotors, and a triangular shaped sealing line configuration, both at the point of suction and at the point of discharge. This design produces a continuous and non-vorticuless flow with equal volume. High capacity industrial "air compressors" have been designed to employ this principle as well as most "superchargers" used on internal combustion engines.

Reciprocating-type pumps use a piston and cylinder arrangement with suction and discharge valves integrated into the pump. Pumps in this category generally find some problem arrangements are from "simplex" one cylinder to in some cases four (quad) or more but usually are "duplex" two or "triplex" 3 cylinder. Furthermore they are either "single acting" independent suction and discharge strokes or "double acting" suction and discharge in both directions.

The pumps can be powered by air, steam or through a belt drive from an engine or motor. This type of pump was used extensively in the early days of steam propulsion (19th century) as boiler feed water pumps.

Though still used today reciprocating pumps are typically used for pumping highly viscous fluids including concrete and heavy oils. Another modern application of positive displacement pumps are diaphragm pumps which are used extensively for dewatering and generally use compressed air as a motive source of energy.

1. Periodic energy addition
2. Added energy forces displacement of fluid in an enclosed volume
3. Fluid displacement results in direct increase in pressure