Positive Displacement is a general name for a pump type that does not have impellers but rather relies on rotating or reciprocating parts to directly push the liquid in an enclosed volume until enough pressure is built up to move the liquid into the discharge system. There are many types for specific applications.
Pumps that work on the Positive Displacement principle are either of a rotary or reciprocating type. They can be used in preference to centrifugal pumps if the liquids to be pumped have the following characteristics: high viscosity, fragile solids are present or shear sensitive liquids or low flow and high pressure is required.
Positive Displacement pumps are an excellent choice for higher viscosity services and when liquids need to be moved gently as they maintain a constant speed and flow, are self-priming and their capacity is not affected by operation pressure.