Fixed Volume per Cycle

A positive displacement pump traps a fixed volume of liquid and forces it into the discharge. Flow depends on speed and displacement, and is almost independent of discharge pressure. This single fact drives every difference from centrifugal pumps.

The consequences

• It will generate whatever pressure the system resists with, up to the point where something breaks or the driver stalls. There is no shut-off head.
• A closed discharge valve is catastrophic. Within seconds the pump will burst a casing, a gasket, the pipework, or the driver. This is not a slow degradation as with a centrifugal pump.
• Flow is nearly constant with pressure, making these pumps suitable for metering and for high-viscosity fluids where a centrifugal pump would be hopeless.
• Flow is controlled by speed or stroke, never by throttling the discharge.
• They are self-priming to a degree, and can handle some entrained vapour.

Relief protection is mandatory

Every positive displacement pump needs a relief device on the discharge, upstream of any isolation valve, sized for full pump capacity at the relieving condition and set below the weakest component in the system. This is not optional and it is not a design nicety — it is the single control standing between a closed valve and a burst line. Relief valves on PD pumps are also routinely found isolated, plugged or wrongly set during audits.

If a positive displacement pump can be started against a closed valve, the question is not whether something will fail but when.