What is positive displacement pumps?

Positive displacement pumps move fluids by trapping a fixed amount and then forcing (displacing) that trapped volume into the discharge pipe. Some positive displacement pumps use an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pump as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. They are often used where high viscosity fluids need to be pumped or where a precise flow rate is required.