What is a pump curve?
A pump performance curve (sometimes called a characteristic curve or H-Q curve) is a graph published by the pump manufacturer that shows how a specific pump model performs across its range of flow rates. It is measured on a test rig using clean water at a standard temperature (typically 20°C).
The primary curve — the H-Q curve — shows how much head (pressure energy) the pump can deliver at any given flow rate. As flow increases, head decreases. This is the fundamental characteristic of centrifugal pumps.
Pump curves are used to:
- Select the right pump size for an application
- Determine the actual operating point
- Predict pump behaviour after system modifications
- Diagnose performance problems
The six components of a pump curve
1. Head-flow (H-Q) curve
The primary curve. Plots differential head (m) vs flow rate (m³/h). At zero flow (shut-off head), the curve is at its highest point. As flow increases, head falls — steeply in a "steep" curve (preferred for control stability) or gradually in a "flat" curve.
2. Efficiency curve
A second curve (usually dashed or in a different colour) showing pump hydraulic efficiency (%) across the flow range. It peaks at BEP and falls off rapidly on both sides. Running far from BEP dramatically reduces efficiency — a pump running at 50% of BEP flow may only be 60–70% as efficient as at BEP.
3. Power curve
The shaft power (kW) required to drive the pump at each flow rate. For most centrifugal pumps, power increases with flow. Motor sizing should be based on the maximum power point — often the run-out flow (maximum flow on the curve), not the design point.
4. NPSHr curve
Net Positive Suction Head Required — the minimum suction head the pump needs to avoid cavitation, plotted against flow rate. NPSHr increases with flow. You must maintain NPSHa ≥ NPSHr + 0.5–1.0 m margin. See our full NPSH guide.
5. Preferred operating region (POR)
Many modern pump curves show a shaded POR — typically 70–120% of BEP flow. Operating within this region ensures acceptable vibration, seal life, and bearing loads. Operation outside the POR voids some manufacturer warranties.
6. Impeller trim lines
For pumps with trimable impellers, the curve shows performance at multiple impeller diameters (e.g., 285 mm, 265 mm, 245 mm). Each trim shifts the H-Q curve down and to the left, following the affinity laws.
The Best Efficiency Point (BEP)
BEP is the single most important point on the pump curve. It is the flow rate at which the pump operates at peak hydraulic efficiency. At BEP:
- Fluid enters and exits the impeller at the ideal angle (no shock losses)
- Radial and axial hydraulic forces on the impeller are balanced and minimal
- Vibration levels are lowest
- Seal and bearing loads are minimised, extending service life
Pumps operating below 70% of BEP flow risk internal recirculation — the fluid reverses direction inside the impeller, causing cavitation-like damage, high vibration, and rapid seal failure — even when NPSHa is adequate.
BEP specific speed (Ns)
Specific speed (Ns) classifies impeller type and characteristic curve shape. Low Ns (500–2,000) = radial impellers, high head, flat curves. High Ns (6,000–15,000) = axial impellers, high flow, steep power curves. Knowing Ns helps predict how far you can trim an impeller and whether a curve will be stable.
The system curve
The system curve is not published by the pump manufacturer — it is a property of your piping system. It describes how much head the system requires at every flow rate:
Hsystem = Hstatic + Hfriction = Hstatic + k × Q²
Where:
- Hstatic = static head (elevation difference + any discharge pressure requirement) — constant, does not change with flow
- k × Q² = friction losses in pipes, valves, and fittings — proportional to flow²
A system with high static head (e.g., lifting liquid 20 m) has a system curve that starts high on the Y-axis. A system with all friction and no static head (e.g., a circulation loop) passes through the origin.
Finding the operating point
Plot both the pump H-Q curve and the system curve on the same axes. The operating point is where they intersect. This is where the pump will actually run — delivering exactly the head and flow where supply meets demand.
If the operating point is far to the left of BEP, the pump is oversized for the application. Options: trim the impeller, add a variable frequency drive (VFD), or replace with a smaller pump. Running an oversized pump permanently wastes energy and shortens pump life.
Effect of system changes
When you partially close a control valve, you add resistance to the system — the system curve steepens, and the operating point moves up and to the left (higher head, lower flow). When you fully open valves or remove a filter restriction, the curve flattens and flow increases. The pump curve itself does not change — only the system curve moves.
Affinity laws and impeller trimming
The affinity laws predict how pump performance changes with speed or impeller diameter:
| Parameter | Speed ratio | Diameter ratio |
|---|---|---|
| Flow (Q) | Q₂/Q₁ = N₂/N₁ | Q₂/Q₁ = D₂/D₁ |
| Head (H) | H₂/H₁ = (N₂/N₁)² | H₂/H₁ = (D₂/D₁)² |
| Power (P) | P₂/P₁ = (N₂/N₁)³ | P₂/P₁ = (D₂/D₁)³ |
Trimming an impeller from 300 mm to 285 mm (5% reduction) reduces head by ~10% and flow by ~5%. Reducing speed with a VFD by 10% reduces head by ~19% and power by ~27% — much more energy-efficient than throttling.
What happens when you run off-BEP
High flow (right of BEP): Risk of cavitation as NPSHr increases, high bearing loads, motor may overload if run at run-out point.
Low flow (left of BEP): Internal recirculation causes vibration, noise, and cavitation-like erosion of the impeller even when NPSHa > NPSHr. Temperature rise in the pump casing. Mechanical seal distortion and premature failure. For most centrifugal pumps, minimum continuous stable flow (MCSF) is 50–70% of BEP flow.
If you regularly throttle a pump below MCSF, install a minimum flow bypass line or recirculation valve — or better, switch to a VFD to reduce speed instead of throttling.
FAQs
Shutoff head (or zero-flow head) is the maximum head the pump can produce with the discharge valve completely closed (zero flow). It is the leftmost point on the H-Q curve. The pump generates maximum pressure but does no useful work — all energy goes into heating the fluid. Never run a centrifugal pump against a closed valve for more than 30–60 seconds without a bypass or the fluid will overheat.
A steep curve has a large head change per unit flow change — it gives a stable operating point that does not shift much when system resistance varies. It is preferred for applications where precise pressure control matters. A flat curve has little head change across a wide flow range — it is suited to systems where flow must be maintained despite varying system resistance. Running two identical flat-curve pumps in parallel can cause instability (one pump "stealing" flow from the other) if not carefully matched.
Pump curves are always published for clean cold water. For viscous fluids above ~50 cSt, you must apply viscosity correction factors (per the Hydraulic Institute method or ISO 9906). Viscosity reduces head, flow, and efficiency — and increases power. Positive displacement pumps are usually more suitable than centrifugal pumps for fluids above 500 cSt.