A pump curve shows how much water a pump delivers at each head. Flow is the volume per unit of time, head is the height in metres the pump can push water, and efficiency is the share of input power that ends up as useful water power.
Reading the curve properly is the best defence against buying a pump that looks right on paper and underperforms in the field.
Flow
Flow, or discharge, is usually given in cubic metres per hour, litres per second or litres per minute. It sits on the horizontal axis. Check the unit carefully when you compare catalogues, because a quote in litres per minute can look larger than the same flow in cubic metres per hour. Some curves also mark a recommended operating range, shaded or bracketed, which is the band the maker stands behind.
Head
Head is the energy the pump adds to the water, expressed as a height of water in metres. It sits on the vertical axis. Total head is not just the vertical lift. It includes the vertical distance from the water level to the outlet, plus friction losses in pipes and fittings, plus any pressure needed at the outlet.
A centrifugal pump gives its highest head at zero flow, called shut-off head. As flow rises, head falls. That is why a single figure such as "maximum head 100 m" tells you very little.
Efficiency and power
Efficiency is the ratio of hydraulic power to input power. The curve usually shows pump efficiency, and sometimes overall efficiency of pump and motor together. Make sure you know which one you are reading.
The highest point of the efficiency curve is the best efficiency point (BEP). A pump runs smoothest, quietest and longest near its BEP. Running far to the left or right of it raises vibration, heat and wear.
Most curves also show input power and NPSH required. NPSH required matters for surface pumps, because if the suction conditions cannot supply it, the pump cavitates and erodes the impeller.
How to use the curve
- Work out the flow you need and the total head at that flow, including friction losses.
- Find that duty point on the curve and check it sits close to the BEP.
- Read the input power at that point and make sure the motor is not overloaded across the likely operating range.
- Ask which test standard the curve comes from and what tolerance applies to it.
A catalogue curve is a type curve, not proof of your batch. Our article on pump performance test reports explains how to check that production pumps match it, and choosing the right pump covers the selection steps.
Next step
We check curves against test bench results before shipment, so you know what you are selling. Book a free sourcing review.