Pump Head and Flow: Reading a Pump Curve Correctly

Flow and head are not independent. As a centrifugal pump delivers more flow, the head it can generate falls. The pump curve plots that relationship.

What head actually means

Head is the height of liquid column the pump can support, expressed in metres. It combines static lift — the vertical distance the water must rise — with friction losses through pipe, fittings and valves. Friction losses grow sharply with flow rate and with smaller pipe diameters, and they are frequently underestimated.

The duty point

Your system also has a curve: the head it demands at each flow rate. Where the system curve crosses the pump curve is the duty point, and that is where the pump will actually run — regardless of what the nameplate says.

Why oversizing causes problems

A pump selected far larger than needed runs to the right of its best efficiency point. That means higher power draw, more noise, and in many cases operation outside the range the impeller was designed for. Undersizing is equally unhelpful: the pump never delivers the required flow at the required pressure.

Cavitation and NPSH

If pressure at the suction falls too low, vapour bubbles form and collapse violently inside the pump, damaging the impeller. The available NPSH on the suction side must exceed the pump's required NPSH with margin. Long, narrow or restricted suction lines are the usual cause of trouble.

Practical selection sequence

1. Calculate the required flow rate
2. Calculate static lift plus friction losses at that flow
3. Plot the duty point and select a pump running near its best efficiency point
4. Check NPSH available against NPSH required
5. Confirm motor power, voltage and phase against the site supply

Where the datasheet marks flow, head or power as not specified, request the manufacturer's curve before committing.