Pump Terminology Glossary

Every trade defends itself with vocabulary, and pumping is no exception. This page once explained how to navigate a website; it now explains how to navigate a conversation with a pump engineer. Terms are grouped by theme rather than alphabet, because that is how the concepts actually connect. For the authoritative definitions, the Hydraulic Institute's standards on pump nomenclature are the final word.

Head, Flow, and the Duty Point

Head — The energy a pump adds to a liquid, expressed as the height of a liquid column (feet or meters). Head is the pump world's preferred unit because, for a rotodynamic pump, it stays constant regardless of the liquid's density — a pump that generates 100 feet of head does so whether it is moving water or gasoline, though the pressure produced differs with density.

Total dynamic head (TDH) — The total head a pump must generate at a given flow: static lift plus friction losses plus any pressure difference between source and destination vessels.

Duty point — The flow and head a system actually requires. Where the system curve crosses the pump curve, the pump operates — whether you like the spot or not.

System curve — The plot of head required versus flow for a piping system: a static component that does not vary with flow, plus a friction component that rises roughly with the square of flow.

Efficiency and Operating Range

Best efficiency point (BEP) — The flow at which a given impeller and casing convert shaft power to hydraulic power with the least waste. Reliability and efficiency both degrade as operation moves away from BEP; hydraulic loads become unbalanced, vibration rises, and bearing and seal life fall.

Allowable / preferred operating region — Bands around BEP, defined by the manufacturer and by Hydraulic Institute guidance, within which the pump may run continuously (allowable) or should ideally run (preferred, commonly about 70% to 120% of BEP flow for many centrifugal designs).

Specific speed — A dimensionless index computed from speed, flow, and head that characterizes impeller geometry, from narrow radial-flow impellers (high head, low flow) to axial propellers (low head, high flow). It lets engineers compare hydraulic designs across sizes.

Affinity laws — The scaling rules for rotodynamic pumps: flow varies with speed, head with the square of speed, and power with the cube of speed. They are the reason variable-speed operation saves so much energy in friction-dominated systems.

The Suction Side

NPSH (net positive suction head) — The margin between the absolute pressure energy available at the pump suction and the liquid's vapor pressure. NPSHa is what the system provides; NPSHr is what the pump demands. When available drops toward required, the liquid flashes to vapor inside the pump. Full treatment in our NPSH and cavitation note.

Cavitation — The formation and violent collapse of vapor bubbles in a flowing liquid as it passes from low- to higher-pressure regions inside the pump. It sounds like pumping gravel and chews impellers into lacework.

Suction specific speed — An index of an impeller eye's suction performance; very high values buy low NPSHr at the cost of a narrower stable operating range.

Positive Displacement and Metering Vocabulary

Positive displacement (PD) pump — A pump that traps a fixed volume and forces it to the discharge: gear, lobe, screw, vane, peristaltic, piston, plunger, and diaphragm designs. Flow is nearly independent of pressure — which is why every PD pump needs relief protection.

Metering (controlled-volume) pump — A reciprocating PD pump engineered for accuracy and adjustability of flow, typically specified to API 675 steady-state accuracy of ±1% or better over its turndown range. See the metering pump page.

Turndown — The ratio between maximum and minimum controllable flow (a 10:1 turndown means stable dosing from 100% down to 10% of capacity).

Pulsation dampener — A gas-charged or bellows device that absorbs the pressure pulses of reciprocating pumps, protecting piping and smoothing dosing.

Sealing and Containment

Mechanical seal — The precision pair of flat faces — one rotating, one stationary — that closes the gap where a shaft exits a pressurized casing. The single most common pump maintenance item; API 682 governs its engineering in hydrocarbon service.

Sealless pump — A design that eliminates the shaft penetration entirely: canned motor pumps enclose motor and impeller in one hermetic pressure boundary, while magnetic drive pumps transmit torque through a containment shell via permanent magnets.

Secondary containment — The backup pressure boundary (containment shell, outer casing) that holds the fluid if the primary boundary fails — central to specifying pumps for toxic or flammable liquids.

Drives and Control

Variable frequency drive (VFD) — Power electronics that vary motor speed by varying supply frequency. Transformative for energy use in centrifugal service via the affinity laws; for reciprocating pumps, remember the constant-torque caveat explained in our Engineering Notes.

Constant torque load — A load whose torque demand stays roughly flat across speed, like a reciprocating pump. Motors and drives must be rated for it, especially regarding motor cooling at low speed.

Dead-heading — Running a pump against a closed discharge. A centrifugal pump churns its own casing contents hot; a PD pump builds pressure until something opens or breaks. Neither ends well unattended.