Positive-Displacement Pump

Definition

In a positive-displacement pump, each cycle of the mechanism displaces a defined volume of fluid from the inlet to the discharge. The delivered flow is therefore essentially constant regardless of the discharge pressure, up to the mechanical and pressure limits of the machine.

This produces a characteristic that is the opposite of a centrifugal pump:

• Centrifugal pump: flow and head are linked by a performance curve — the operating point falls where that curve intersects the system resistance curve. Flow decreases as discharge pressure increases.
• Positive-displacement pump: the Q–H characteristic is nearly vertical — flow is fixed and the pump develops whatever pressure the system demands. If the discharge is blocked, pressure rises without limit.

Common types include reciprocating (piston, plunger, diaphragm) and rotary (gear, lobe, screw, vane) machines.

Engineering context

PD pumps appear wherever precise, repeatable flow control is needed regardless of downstream pressure: chemical dosing, hydraulic power systems, high-viscosity fluid transfer, slurry injection, and high-pressure cleaning.

Two consequences of the fixed-flow characteristic are critical for system design:

1. Overpressure protection is mandatory. A blocked or closed discharge will cause system pressure to rise to the point of mechanical failure or pipe rupture. An independent relief valve or pressure safety valve must be in the circuit. PD systems must always have independent overpressure protection — this is not handled by the pump itself.
2. The system resistance determines the discharge pressure and power, not the flow. Correctly modelling the system resistance — pipe friction, fittings, elevation, and back-pressure — tells you the pressure the pump must develop and lets you check it against equipment and pipe pressure ratings.

For booster arrangements — where a second pump is added in series with an existing driver to raise the operating pressure or recover head lost along a long route — the combined behaviour is the series sum of what each device contributes at the shared flow. See Positive-Displacement Pumps: Modelling Fixed Flow and Discharge Pressure.

In relief-valve hydraulics, the relieving load for a PD system is the pump’s fixed delivered flow — not a point read from a centrifugal pump curve.

Related definitions

System curve · Pump curve · NPSH · Centrifugal pump · Duty point · Boosters

See it in FluidFlow

FluidFlow models positive-displacement pumps using the dedicated Rotating Positive Displacement Pump component — distinct from the centrifugal pump element. The component represents the fixed-flow behaviour: the network solver calculates the discharge pressure and power required to sustain the set flow through the defined route. A Flow boundary can also impose a known flow where appropriate. The system curve for a PD pump is the pressure–resistance relationship of the route, not the pump’s performance curve.