Positive displacement

A positive displacement pump causes a fluid to move by trapping a fixed amount of it then forcing (displacing) that trapped volume into the discharge pipe. The volume is constant given each cycle of operation. A positive displacement pump can be further classified according to the mechanism used to move the fluid: Rotary-type, for example, the lobe, external gear, internal gear, screw, shuttle block, flexible vane or sliding vane, helical twisted roots (e.g. the Wendelkolben pump) or liquid ring vacuum pumps. Reciprocating-type, for example, piston or diaphragm pumps.