What is a Positive Displacement Pump?
Positive displacement pumps move entrapped volumes of fluid mechanically through the system. On the intake (suction) side, the volume expands, while on the outlet (discharge) side, the volume contracts. The volume per revolution is therefore fixed and theoretically constant regardless of outlet pressure, inlet vacuum or fluid properties. Positive displacement pumps are also self-priming, creating strong vacuums on the inlet. This can simplify the overall system design and allow maintenance without manual re-priming.
The behavior of positive displacement pumps is considerably different from centrifugal pumps, which rely on the momentum of the accelerated fluid to deliver flow at pressure and are very sensitive to pressure changes. The graph on the right compares an external gear pump (common positive displacement pump) and centrifugal pump using the same size motor. While centrifugal pumps can reach considerably higher flow rates, they are extremely sensitive to pressure.
DPP Centrifugal Versus Gear Pump Flow Rate Graph
Flow independence from pressure is only theoretical. Material flex, internal leakage (“blow-by”), wear, and other variables result in slight pressure dependence. The amount of pressure dependence is a function of the type of pump as well as the precision of the pump components. When choosing a pump type a tradeoff must often be made between accuracy, life and system response. The negative aspects of the tradeoff can be minimized by working closely with the pump designer and using high precision, quality components.
Positive displacement pumps can be broken into two subcategories that have very different pump characteristics and applications.
- Reciprocating
- Rotary
Reciprocating Positive Displacement Pumps
Reciprocating positive displacement pumps operate by repeated linear movement of a mechanism. The movement is often called a stroke, and the size of a pump is often specified as the volume per stroke. The flow profile is pulsed due to the once per revolution discharge of a reciprocating pump. If improperly implemented the pulsed flow may cause excessive vibration and/or damage to the hydraulic system, sometimes referred to as “water hammer”. The pulsed flow also causes peak flow rates higher than the average flow rate, requiring careful design of the hydraulic circuit. Reciprocating pumps are ideal for precise, repeatable fluid metering and dosing. The most common types of reciprocating pumps are:
- Diaphragm pump
- Piston pump
- Plunger pump
Diaphragm Pump
A diaphragm pump uses a flexible membrane (often called the diaphragm) that flexes inward and outward. The movement of the membrane changes the volume internal to the pump and, when coupled with valves, allows fluid to flow into and out of the pump. Diaphragm pumps are ideal for vacuum, air, and low pressure corrosive fluids.