A positive displacement pump is one in which a definite volume of liquid is delivered for each
cycle of pump operation. This volume is constant regardless of the resistance to flow offered by
the system the pump is in, provided the capacity of the power unit driving the pump or pump
component strength limits are not exceeded. The positive displacement pump delivers liquid in
separate volumes with no delivery in between, although a pump having several chambers may
have an overlapping delivery among individual chambers, which minimizes this effect. The
positive displacement pump differs from centrifugal pumps, which deliver a continuous flow for
any given pump speed and discharge resistance.
Positive displacement pumps can be grouped into three basic categories based on their design
and operation. The three groups are reciprocating pumps, rotary pumps, and diaphragm pumps.