How to read a pump curve

Head runs up the vertical axis, flow along the horizontal, and the main line slopes down from left to right. As flow rises, the head the pump can make falls. The top-left end is shut-off: maximum head, no flow. The bottom-right end is run-out: maximum flow, almost no head. Read that line first, because every other line on the chart is taken at a flow you pick off it.

That slope is also why curve-reading is a centrifugal discipline. A centrifugal pump's flow moves significantly as the pressure it works against changes, and the line is what tells you by how much. A positive displacement pump does not behave that way. It holds its rate to within a few per cent and takes the pressure change on the motor instead, so on a rotary lobe or progressing cavity pump you are checking power and speed rather than chasing a duty point up and down a curve.

The other three lines answer three different questions at the same vertical line on the chart.

Efficiency is usually drawn as islands across the curve, peaking at the best efficiency point (BEP). That is where the pump turns most of the motor's power into flow, and where it runs with the least vibration and the lightest load on the bearings and the seal. Sit your duty near it. A duty out on either flank will still make head, and it will still wear the pump out early.

The power curve gives the shaft power the pump absorbs at each flow, in kW, and it is what sizes the motor. Read it at the duty, then read it again at the highest flow the installation could ever push the pump to. On most centrifugals power keeps climbing with flow, so a motor sized only for the duty can overload once the system runs the pump out to the right.

The NPSHr line is the odd one out. It describes what the pump demands of your installation rather than what it gives you: the net positive suction head it needs at that flow, in metres. It rises steeply as flow rises, so it has to be read at your duty and not somewhere convenient in the middle of the chart.

Run-out is not simply the end of the useful range. A centrifugal pump run off the end of its performance curve needs back pressure it is not getting, so it draws more than the suction can supply and starts pumping air. Size for 10 m³/h when the tank will only ever give you 5, and that is exactly what you have built.

That is cavitation, and it is not a quiet failure. It damages the wet end, it takes the mechanical seal out, and it puts vibration and noise into the pipework of a factory that may well be sensitive to both. On a food or pharmaceutical line, the noise alone becomes a complaint long before the pump actually fails.

How a duty point ends up that far to the right is the same story nearly every time it reaches us, and nobody in the chain is being careless. The customer states a flow. The consultant adds margin for future duty, reasonably. The contractor adds margin. The subcontractor doing the installation adds a bit more flow or pressure. Four sensible decisions, and the pump that comes out of the other end is far too big. You then pay for it twice, because power rises with flow and pressure: 3 kW duties end up carrying 11 kW motors, and they carry them for the life of the installation. That is a selection fault rather than a curve fault, and keeping it out of your own figures is a matter of working the duty out properly before you open a catalogue.

Read the NPSHr at your duty flow, compare it with the NPSHa your installation actually provides, and keep a margin between the two. If NPSHa falls below NPSHr, the pump cavitates. The curve tells you that before you buy the pump, which is most of the reason for reading it.

In practice this rarely goes wrong on the arithmetic. What we get called out to is the suction side of the installation: a suction tank that is too small, or a suction head that is too small. The pump was fine. The thing feeding it was not, and the pump is what fails.

Check the margin at the highest flow the pump could see as well as at the duty. NPSHr climbs steeply to the right, so a margin that already looks tight at the duty point has gone entirely by the time the pump runs out.

Reading a curve comes down to one vertical line and four readings taken off it. If any of the four fails, the pump is wrong for the duty, and you change the pump or the impeller rather than accept the reading.

A performance curve describes the pump against clean water. It says nothing about what you are actually pumping, and that is where the rest of the trouble sits.

Abrasive media and shear-sensitive media both want pumping slowly. Speed is what wears an abrasive duty out, and what breaks up a product you were meant to keep intact, and there is no line on a performance curve for either. Solids are the other one. Past roughly 3–4% solids as a rule of thumb, a centrifugal is the wrong machine whatever its curve says, and you are into positive displacement instead. Viscosity moves that boundary too, though solids content is the number most people actually have to hand.

So read the curve last rather than first. It proves or disproves a pump you have already selected on the medium, the temperature, the materials and the sealing arrangement. A curve that fits a duty you have described wrongly is only a tidy way of buying the wrong pump.