A positive displacement pump makes a fluid move 
by trapping a fixed amount and forcing (displacing) that trapped volume into the discharge pipe. … Liquid flows into the pumpas the cavity on the suction aspect expands and the liquid flows from the discharge since the cavity decreases.
The most typical type among centrifugal pump is the radial flow pump. … The primary difference between these types of pumps and centrifugal is that positive displacement pumps will move fluid at the same speed regardless of the strain on the inlet end and centrifugal pumps will not