How to Read a Pump Curve

When a series publishes flow from 2 to 1100 m³/h and head from 2 to 152 m, as the Flowsteer FA series does, no single pump delivers both extremes. The figures describe the range the series covers across all its sizes and speeds.

Getting from that envelope to a pump means fixing four things: the flow, the head, the speed and the liquid.

The Four Numbers You Need First

Flow, in m³/h. What the process actually needs, not the pipe size.

Total head, in metres. Static lift plus friction losses at the design flow, not just the height difference.

Supply frequency. The FA publishes 1450 to 2900 rpm at 50 Hz and 1750 to 3500 rpm at 60 Hz, and the curve moves with speed.

The liquid: temperature, solids content and chemistry. These decide materials and impeller type, and they can rule out a hydraulically perfect selection.

Reading the Curve Itself

The head-flow curve falls from left to right. Where it crosses your system curve is where the pump will actually run — not necessarily where you wanted it to.

Efficiency is plotted as a separate curve or as islands across the chart. The peak is the best efficiency point.

NPSH required rises with flow. Check it against the NPSH available in your suction system at the worst case, not the design case.

Power rises with flow on most water pumps. Size the motor for the whole operating range, not just the design point.

Why the Best Efficiency Point Matters

Efficiency is the obvious reason, and on a pump running continuously it is a real operating cost.

The less obvious reason is mechanical. Running far from the best efficiency point puts uneven radial load on the impeller, which deflects the shaft and shortens both seal and bearing life.

A pump running at 60 percent of its best efficiency flow is not merely wasteful; it is wearing out faster.

Common Selection Mistakes

Sizing on the pipe diameter rather than on the flow required.

Adding a safety margin to the head. The pump then runs further left on its curve, at lower efficiency and higher radial load, and it still delivers the flow the system allows.

Ignoring the supply frequency, which shifts the entire curve.

Selecting on the duty point alone and leaving the material and impeller decisions to the supplier.

What To Send With an Enquiry

Flow in m³/h and total head in metres.

Supply frequency and voltage.

Liquid description, temperature and solids percentage.

Existing pump details if this is a replacement — nameplate, size designation and flange rating.

Pipe class and flange standard already installed.