Every pump datasheet contains a graph that looks like a few smooth lines crossing a grid. To a beginner it can be intimidating. To an engineer it is the most useful page in the whole document, because it shows exactly how the pump will behave in your system. Learning to read a pump curve lets you choose the right pump, predict performance, spot problems and save energy.

This guide explains each line on a typical centrifugal pump curve, shows how to find the duty point and describes how changes in speed, impeller size and system resistance move the operating point.

What a Pump Curve Shows

A pump curve is a graph of performance data measured by the manufacturer under test conditions, usually with clean water at a defined speed. The horizontal axis shows flow rate (Q), in units such as m³/h, L/s or GPM. The vertical axis shows head (H) in metres or feet. Several additional curves are usually plotted on the same chart or on stacked charts beneath it.

A complete centrifugal pump curve typically includes:

• Head versus flow (the H-Q curve)
• Efficiency versus flow
• Power versus flow
• NPSH required versus flow
• Multiple impeller diameters or speeds shown as a family of curves

Understanding each of these gives a complete picture of the pump.

The Head-Flow (H-Q) Curve

This is the main curve. It shows the head the pump can produce at each flow rate.

For most centrifugal pumps, the curve is highest at zero flow, called the shut-off head, and falls steadily as flow increases. At very high flow, head drops rapidly. This is because, as the pump moves more water, less energy is available to lift it.

Key points on the H-Q curve:

• Shut-off head: the head at zero flow, when the discharge valve is closed. It is the maximum head the pump can generate at that speed and impeller size. Running a pump here for long causes overheating and damage.
• Rated or design point: the flow and head at which the pump is specified.
• Run-out: the far right of the curve, at maximum flow and minimum head, where the pump is overloaded and prone to cavitation.

Steep versus flat curves. A steep curve means head changes a lot with flow, which gives stable operation and good control when system pressure varies. A flat curve means flow changes a lot for small changes in head, which can suit systems with variable levels, but needs care to avoid instability.

The Efficiency Curve

Efficiency is the percentage of input power converted into useful hydraulic power. The efficiency curve typically rises to a peak, then falls away on either side, forming a hill shape.

The peak is the best efficiency point (BEP). At this point, the flow pattern inside the impeller is smooth, hydraulic forces on the shaft are balanced and the pump runs with the least wear and the lowest energy cost per unit of water.

Many manufacturers also display iso-efficiency lines on multi-curve charts, linking points of equal efficiency across different impeller sizes. These show at a glance where the pump performs best.

A good selection practice is to choose a pump so that the duty point is within about 70 to 110 percent of BEP flow, and ideally close to 100 percent. This preferred operating region (POR) gives the best compromise of efficiency, reliability and flexibility.

Operating to the left of BEP (low flow):

• Internal recirculation increases
• Radial thrust on the shaft rises, loading bearings
• Heat builds up in the liquid
• Vibration and noise often rise
• Seals and bearings wear faster

Operating to the right of BEP (high flow):

• NPSH required rises and cavitation risk increases
• Motor may overload
• Hydraulic noise and vibration grow
• Impeller wear accelerates

The Power Curve

The power curve shows the shaft power the pump absorbs at each flow. For many centrifugal pumps with radial or mixed-flow impellers, power rises as flow increases. This matters for motor selection.

Non-overloading pumps have a power curve that peaks and then falls off at high flow, so the motor will not overload even at run-out. Overloading designs keep rising, so the motor must be sized for the highest flow the system might demand.

When selecting a motor, read the power at the duty point and at the maximum flow the pump could reach, then apply an appropriate margin. Power curves on datasheets are usually for water. For liquids of different density, power scales in proportion to specific gravity.

The NPSH Required Curve

This curve shows the net positive suction head the pump needs at each flow to avoid cavitation. It usually rises as flow increases, with a steep climb at high flows.

To use it, compare NPSH required at your duty flow with the NPSH available in your system, and ensure a safety margin, usually at least 0.5 to 1 metre. Always read NPSH required at the highest flow the pump might operate at, not only at the design point.

Impeller Diameter and Speed Curves

Most catalogue charts show several H-Q curves for the same pump casing fitted with impellers of different diameters. Each curve corresponds to a trimmed or full-size impeller. Smaller impellers produce less head and flow, while the largest produces the most. This lets a manufacturer cover a range of duties with the same casing.

Variable speed pumps may show curves for different speeds, such as 100, 90, 80 and 70 percent of nominal speed.

The System Curve and the Duty Point

A pump curve alone does not tell you what the pump will do. The pump delivers whatever flow results from its curve meeting the system curve, which describes the head your pipework requires at each flow.

The system curve has:

• A static component (elevation and any fixed pressure), which does not change with flow
• A friction component, which rises roughly with the square of flow

To find the duty point, plot the system curve on the same axes as the pump curve. The point where they cross is where the pump will operate. Read down to get the flow, across to get the head, then read the efficiency, power and NPSH required at that flow from the other curves.

If the duty point is not where you expected, either the pump or the system calculation needs revisiting. Common reasons include underestimated friction, incorrect static head or a pump whose curve is too high or too low for the job.

A Step-by-Step Example

Suppose you need 30 m³/h at 40 m of head. You check a pump curve with the following characteristics at the chosen impeller diameter:

• At 30 m³/h, head is 41 m
• Efficiency at 30 m³/h is 71 percent, and BEP is 32 m³/h at 73 percent
• Power at 30 m³/h is about 5.0 kW
• NPSH required at 30 m³/h is 2.8 m

You then read each curve:

1. Head: 41 m is slightly above your 40 m requirement, so the actual duty point will be a little to the right, with slightly more flow and slightly less head. That is acceptable.
2. Efficiency: 71 percent at the duty point is close to BEP efficiency. The duty flow is roughly 94 percent of BEP flow, within the preferred region.
3. Power: about 5.0 kW at the duty point. You would select a motor of, say, 5.5 or 7.5 kW depending on margin policy and the maximum flow expected.
4. NPSH: 2.8 m is required, so your system must offer an NPSH available of at least 3.3 to 3.8 m for a safe margin.

This pump is well matched. If the same duty fell at 15 m³/h on this curve, it would operate far to the left of BEP and be a poor choice.

How System Changes Move the Duty Point

Closing a discharge valve. The system curve becomes steeper, moving the duty point left along the pump curve. Flow falls, head rises, and efficiency often drops. Throttling wastes energy.

Opening valves or adding parallel branches. The system curve flattens, moving the duty point right. Flow rises, head falls and power may increase.

Pipe fouling and ageing. Friction increases over time, steepening the system curve and reducing flow.

Changes in tank level. Static head varies with level, moving the system curve up and down. A pump must perform acceptably across the whole range.

Changing speed. Reducing speed moves the pump curve down and left, increasing or decreasing flow according to the system curve. This is the efficient way to match flow to demand.

Trimming the impeller. Permanently lowers the pump curve to better match the duty.

The Affinity Laws

The affinity laws predict how performance changes with speed (N) for the same impeller:

• Flow is proportional to speed: Q₂ = Q₁ × (N₂ / N₁)
• Head is proportional to the square of speed: H₂ = H₁ × (N₂ / N₁)²
• Power is proportional to the cube of speed: P₂ = P₁ × (N₂ / N₁)³

A small speed reduction can therefore bring a large power saving. Reducing speed by 20 percent, for example, cuts flow by about 20 percent, head by about 36 percent and power by roughly 49 percent. This is why variable speed drives are so effective in systems with variable demand.

Similar relationships apply to impeller trimming, although they are approximate and become less accurate with larger trims. Always confirm with the manufacturer.

Pumps in Parallel and in Series

Parallel operation. Two identical pumps side by side add their flows at the same head. The combined curve is found by adding flows at each head value. In a real system, the duty point shifts along the system curve, so two pumps do not deliver exactly double the flow of one, because friction rises with flow. Pumps in parallel should have compatible curves to share load well.

Series operation. Pumps in series add their heads at the same flow. Multistage pumps work this way internally. This is useful for high head duties, provided casings and seals are rated for the higher pressure.

Common Mistakes When Reading Pump Curves

• Reading only the head-flow curve and ignoring efficiency, power and NPSH.
• Selecting a pump at the far left or right of the curve instead of near BEP.
• Forgetting the system curve and assuming the pump will deliver its rated duty.
• Mixing impeller sizes or speeds when reading the chart.
• Ignoring units, such as confusing feet with metres or GPM with m³/h.
• Assuming catalogue curves apply to dirty, viscous or hot liquids without correction.
• Using NPSH required at the design flow only, not at maximum flow.
• Overlooking manufacturing tolerances. Real pumps can deviate slightly from the published curve, and test standards allow small tolerances.
• Forgetting that pumps wear, with clearances opening and performance gradually declining.

Using Pump Curves for Troubleshooting

Pump curves are also diagnostic tools. If you measure flow, discharge pressure, and suction pressure, you can compute the actual head and compare it with the curve. Common patterns include:

• Low flow and higher than expected head: a partly closed valve or blockage in the discharge line.
• Low head at a given flow: impeller wear, wrong rotation, air entrainment, or a wrongly trimmed impeller.
• Low flow and low head together: possible cavitation, a blocked suction, or worn internals.
• High motor current: operation at high flow, a wrong impeller, a mechanical fault, or voltage problems.

Recording baseline measurements at commissioning lets you compare current performance with the original curve and spot deterioration early.

Conclusion

A pump curve is a map of everything a pump can do. The head-flow curve shows what it delivers, the efficiency curve shows where it performs best, the power curve guides motor selection and the NPSH curve warns of cavitation. Combine it with your system curve and the duty point becomes clear, along with the efficiency, power and suction requirements at that point. Aim to operate near the best efficiency point, use speed control rather than throttling, and check the curves whenever performance changes. With these habits, you will choose better pumps, run them more efficiently, and catch problems long before they become failures.