Pump Curve

Definition

For a centrifugal pump, head falls as flow rises along a characteristic curve published by the manufacturer for a given impeller diameter and speed. The complete curve set includes: head vs. flow, efficiency vs. flow (peaking at the Best Efficiency Point, BEP), power vs. flow, and NPSHr vs. flow.

The duty point is the single point on the pump curve where the pump will actually operate in your system — determined by where the pump curve and the system curve intersect. Specifying a pump means identifying the head and flow at this intersection, then confirming the pump curve crosses it near BEP with adequate NPSH margin.

Engineering context

The pump curve only predicts behaviour when combined with the resistance of the connected system. Pumps are not “Do All” devices — they must always be matched to the pipe system. A pump selected at a nominal duty of 60 m³/h at 8 m head will not deliver that duty unless the system curve passes through that point.

Operating near BEP matters. In a worked liquid transfer system, a well-matched pump ran at 1870 RPM with a duty efficiency of 62.1% near BEP, drawing 2232 W with an NPSH margin of 3.02 — a reliable, efficient result. BEP is not just an efficiency target; operating far from it creates hydraulic instability, vibration, accelerated wear, and seal failure. The Hydraulic Institute recommends operating between roughly 70–115% of BEP flow for reliable service.

The over-sizing danger. When the same system was re-run with 200 mm pipe instead of the economically-sized 150 mm, the system curve flattened and the operating point moved well beyond the pump’s rated maximum flow. The NPSH margin fell from 3.02 to 1.09, the pump operated outside its reliable range, and power draw increased by over 6%. The larger pipe made the system less safe. The pump curve sets the outer boundary of safe operation; the system must be designed so the operating point stays within it — design safety factors belong in the head calculation, not the pipe size.

For variable-speed drives, the pump curve shifts with speed following the affinity laws: flow scales with speed, head with speed squared, power with speed cubed. The operating point tracks the system curve at different speeds, allowing flow control without throttling losses.

Manufacturer curves are published for water. For other fluids, the supplier corrects for temperature and fluid properties — particularly vapour pressure and viscosity — to confirm no cavitation will occur. Viscosity corrections follow ANSI/HI 9.6.7 (confirmed in FluidFlow’s supported standards). Leave final pump selection to the supplier: they confirm BEP alignment, correct for fluid properties, match the motor, and account for the working environment.

Related definitions

System curve · NPSH · Cavitation · Affinity laws · Darcy-Weisbach equation · Static head · Positive-displacement pump

See it in FluidFlow

In FluidFlow, manufacturer curve data is entered once and the solver finds the true operating point against the modelled network, including series and parallel pump combinations. The composite chart shows where the pump runs on its curve, the duty efficiency, power draw, and NPSH margin — so you can confirm BEP proximity and safe operating range before the pump is ordered.