Positive displacement pump

Positive displacement pump is a collective term for all pumps operating according to the positive displacement principle. These pumps are also referred to as volumetric pumps as they pump the fluid handled in an enclosed volume. Displacement bodies periodically increase and decrease the working volumes, thus transferring energy to the fluid handled. The working volumes are separated from the inlet and discharge by separating elements. In the case of reciprocating positive displacement pumps (see Fig.1 Positive displacement pump) the separating elements are lift check valves or swing check valves which are automatically controlled by pressure differences, or, for higher rotational speeds gate valves actuated by a crank mechanism. These valves prevent the fluid handled from flowing back into the suction line  or discharge line. In the case of rotary positive displacement pumps, controlled gap seals changing with the rotating movement of the displacement bodies serve as separating elements. The function of all separating elements is to control the inlet and outlet of the fluid to and from the working volume at the right time in order to create a flow (see Flow rate) by means of alternate intake and displacement via the displacement body according to the corresponding design principle. The head is independent of the pump speed.

A distinction is made between fixed and variable volume displacement (fixed and variable displacement pumps). The pumps are further distinguished by the motion of their displacement bodies, which can be reciprocating (moving to and fro) or rotary. The reciprocating motion can be either linear (e.g. piston or diaphragm pump) or crescent-shaped (e.g. reciprocating vane pump). Examples of rotary positive displacement pumps are gear pumps, screw pumps, progressive cavity pumps, rotary piston pumps, water ring pumps, rotary vane pumps, roller vane pumps and peristaltic pumps.

The most commonly known positive displacement pumps are piston pumps / plunger pumps. Their displacement body, e.g. a plunger, describes a linear motion. See Fig. 1 Positive displacement pump

In double-acting displacement pumps two working volumes are arranged opposite each other and use the same displacement bodies, which leads to a more even torque curve and flow rate.

Pulsations in the flow can further be absorbed by air vessels (accumulators) acting as short-term storage vessels in the suction line and discharge line or by several cylinders arranged in parallel with a common crankshaft (e.g. duplex or triplex pump). The cylinders can be arranged horizontally or vertically. The crankshaft is usually fitted in a horizontal position. See Fig.1 Positive displacement pump

Positive displacement pumps are primarily used in systems requiring pressurised water and oil (pressure pump) in the chemical industry (e.g. dosing pump) and frequently also on board vessels and in small and medium-sized industrial plants.

Large quantities of positive displacement pumps are employed by the automotive industry as injection pumps for diesel engines. In drive systems positive displacement pumps are widely used as radial and axial piston or plunger pumps for continuously variable energy transmission.