What is a positive displacement pump?

A positive displacement pump is a pump that engages two opposing cavities in order to move fluid throughout a system. With an expanding cavity on the suction side of the pump, and a collapsing cavity on the discharge side, the pump follows this cyclical pattern to mechanically move a fixed volume of fluid through the system. This action can be driven by pistons, screws, gears, rollers, diaphragms, or vanes. Pumps are identified according to class as either reciprocating or rotary positive displacement pumps.

Reciprocating positive displacement pumps use the repetitive stroke movements of a piston, plunger, or diaphragm mechanism to move fluid through reciprocation cycles. This backwards-and-forwards movement is predictable and ideal for metered applications such as chemical sprays, water treatments, paints, corrosive liquids, and high-pressure washing. Reciprocating positive displacement pumps have the added benefit of being self-sealing, which minimizes the chance of spillage or cross-contamination.

Rotary positive displacement pumps, on the other hand, depend on the transfer of fluids using the action of rotating gears or cogs. The most common type of rotary pump is the gear pump, which moves fluids using either external or internal gears. An external gear pump uses two interlocking gears to trap fluid and moves fluid between the teeth from the inlet to the discharge. An internal gear pump uses the action of two interlocking gears of different sizes. It depends on the smaller of the gears rotating inside of the other. At the discharge point, the action of the smaller gear allows for the expulsion of the fluid from the system. Rotary positive displacement pumps need to be lubricated and should not be run dry. It is best to use rotary positive displacement pumps for high viscosity liquids because low-viscosity liquids have a higher likelihood of leakage and wear.

What is the difference between centrifugal and positive displacement pumps?

Positive displacement pumps differ from centrifugal pumps in both their applications and how they are used. A positive displacement pump is a constant flow machine and produces the same flow at a given speed regardless of the pressure exerted upon discharge. Flow is measured in RPMs. Unlike a centrifugal pump, positive displacement pumps have no shut-off head. Operating a positive displacement pump against closed discharge valves can cause the line to burst or other severe damages. Centrifugal pumps are capable of processing higher flow volumes and are more tolerant of low viscosity fluids. These pumps are generally less complex and easier to maintain. However, positive displacement pumps are often preferred when working with high viscosity fluids, high-pressure / low-flow operations, and when accuracy is needed for metered applications.

How do positive displacement pumps create pressure?

Positive displacement pumps create pressure as they move fluid through the system. This pressure is driven by the mechanism of the two opposing cavities working against each other.