Whereas the total dynamic head developed by centrifugal, mixed flow or axial flow pump is usually determined for any given flow by the speed at which it rotates, positive displacement pumps and those which approach positive displacement will ideally produce whatever head is impressed upon them by the system restriction to flow. Actually, with slippage neglected, the maximum head attainable is determined by the power available in the drive and the strength of the pump parts. An automatic relief valve set to open at safe pressure higher than normal or maximum discharge pressure is generally required on the discharge side of all positive displacement pumps.
In general overall efficiencies of positive displacement pumps are higher than those of centrifugal equipment because of internal losses are minimized. On the other hand, the flexibility of each piece of equipment in handling a wide range of capacities is somewhat limited.
Positive displacement pumps may be of either the reciprocating or the rotary type. In all positive displacement pumps, a cavity or cavities are alternately filled and emptied of the pumped fluid by the action of the pump.
