What is the positive displacement pump?

Positive Displacement Pumps has an expanding cavity on the suction side and a decreasing cavity on the discharge side.
Liquid flows into the pumps as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses.
The volume is constant given each cycle of operation.
The positive displacement pumps can be divided in two main classes reciprocating rotary

Positive-displacement pumps are another category of pumps.It operate by forcing a fixed volume of fluid from the inlet pressure section of the pump into the discharge zone of the pump.
Types of positive-displacement pumps are reciprocating, metering, and rotary pumps.The positive displacement principle applies applies whether the pump is arotary lobe pump, progressing cavity pump, rotary gear pump, regenerative (peripheral) pump, screw pump, peristaltic ans soon on.
When you use a Positive Displacement pump (“PD Pump”) is not always a clear choice.
To make a good choice between these pump types it is important to understand the at these two types of pumps behave very differently.