Positive Displacement Pumps

<b>Positive Displacement Pumps</b> Positive displacement pumps are pumps in which a reciprocating (oscillating) piston (displacer) sucks in the liquid and pushes it away. For this purpose, the medium is conveyed through self-contained volumes, and backflow is prevented by non-return valves or flaps. Apart from leaks due to the design, the fluid cannot flow through the pump in the opposite direction to the conveying direction, even when it is at a standstill. Positive displacement pumps are usually self-priming, whereby the maximum suction height (geodetic suction height) is limited by the achievable vacuum, the local air pressure, the density of the medium and the flow resistances to be overcome. Displacement pumps should not be shut off on the pressure side unless, for example, overpressure or bypass valves prevent an impermissibly high pressure from building up in the system. On board, for example, they are used as bilge and feed water pumps.