positivedisplacement

Positive displacement is a term used in engineering to describe devices that move a fixed amount of fluid with each operating cycle. In a positive-displacement mechanism, a sealed cavity changes size as a moving element enters or leaves contact with a boundary, trapping a precise volume of fluid and then discharging it. The output flow per cycle is defined by the geometry of the device and is largely independent of discharge pressure within the device's limits, making these mechanisms well suited to high-pressure pumping and precise dosing.

Positive-displacement devices come in two broad families: reciprocating and rotary. Reciprocating devices include piston and plunger pumps, which draw fluid into a cylinder during the suction phase and then push it out during the discharge phase. Rotary devices include gear pumps, lobe pumps, vane pumps, screw pumps, and diaphragm pumps, where meshing elements or flexible membranes create moving cavities.

Key characteristics include the ability to deliver high pressures, handle viscous or particulate-laden fluids, and provide relatively constant flow at a given speed. The main drawbacks are pulsating flow in some reciprocating designs, greater seal wear and leakage risk, and the potential for damage if run dry or operate without adequate relief in a system prone to overpressure. They are also sensitive to fluid properties and require proper priming and filtration.

Applications span hydraulic systems, fuel and lubricant delivery, chemical dosing, pharmaceutical syringes, and water treatment. Positive-displacement pumps are often chosen when precise flow, high pressure, or handling of viscous fluids is required, and are commonly compared with dynamic pumps such as centrifugal types.