Reading a Pump Curve: BEP, NPSHr, and Why Your Pump Is Cavitating

Four curves share one sheet of paper, and every pump failure conversation eventually comes back to one of them.

The four curves

A performance curve plots flow rate along the bottom and stacks four different things above it. They are usually printed on top of each other, which is why the sheet looks busier than it is.

• Head (H–Q) — how much head the pump develops at a given flow, for a given impeller diameter and speed. It slopes down to the right: more flow, less head. The left-hand end, at zero flow, is shutoff head.
• Efficiency — hydraulic efficiency across the flow range, drawn as arcs or as iso-efficiency islands. The peak of that arc is the best efficiency point.
• Power (BHP) — brake horsepower absorbed. On a radial-flow pump this generally rises with flow, which is why a pump run out to the right of its curve trips motors.
• NPSHr — net positive suction head required, in feet of the liquid being pumped. It rises steeply with flow.

Best efficiency point is a mechanical spec, not an energy one

BEP gets discussed as if it were only about power bills. It is not. BEP is the single flow at which the liquid enters the impeller vanes and leaves the volute at the angles the hydraulic designer intended. Move away from it in either direction and the flow separates, which loads the impeller unevenly and puts a rotating radial force on the shaft.

That radial load is what actually breaks pumps. It deflects the shaft at the seal faces, which opens the faces and kills the seal; it loads the bearings off-axis, which shortens L10 life; and at the extremes it cracks impellers and fatigues shafts. A pump run at 40% of BEP is not merely wasting energy — it is grinding through mechanical seals on a schedule.

Two consequences follow. First, oversizing “for safety” is not safe: a pump specified two sizes too large will spend its life throttled back down the left side of its own curve. Second, the fix for a chronically low-flow pump is often a trimmed impeller or a smaller pump, not a bigger one.

NPSH: the number that is not on your pump

NPSHr belongs to the pump and appears on the curve. NPSHa — available — belongs to your system and is never printed anywhere. You calculate it:

NPSHa = h_a − h_vp ± h_st − h_f

where h_a is absolute pressure on the liquid surface (in feet of that liquid), h_vp is the liquid's vapour pressure at pumping temperature, h_st is static suction head — positive on a flooded suction, negative on a lift — and h_f is friction and entrance loss in the suction line.

Cavitation happens when NPSHa falls to NPSHr. Vapour bubbles form at the impeller eye where pressure is lowest, travel a few millimetres into rising pressure, and collapse. The collapse is violent and local: it pits the impeller vanes, sounds like gravel, and takes head and efficiency with it.

Why cavitation usually starts with a change nobody logged

NPSHa is fragile in ways that are easy to miss:

• Temperature went up. Vapour pressure climbs steeply with temperature. Water at 60°F needs about 0.6 ft of NPSH for vapour pressure; at 180°F it needs roughly 17 ft. Same pump, same piping, a summer process change, and the margin is gone.
• The suction strainer is loading up. Every foot of added friction loss comes straight off NPSHa.
• The tank level dropped. Static head is a direct term.
• Flow went up. NPSHr rises with flow, and suction friction rises with the square of flow. Both move the wrong way at once, which is why cavitation so often appears when a line is opened up rather than throttled.

Design for margin, not for equality. A common rule is NPSHa at least 3 ft or 1.35× NPSHr above NPSHr, whichever is greater, evaluated at the highest flow the pump will ever see — not the design point.

Minimum flow, and the other failure at the other end

Two further limits are worth marking on any curve before it goes into service.

Minimum continuous stable flow sits at the left. Below it, recirculation at the impeller eye and discharge tips causes pressure pulsation and vibration — and, on any pump with meaningful power density, the liquid retained in the casing starts absorbing shaft power as heat. On a hot service that heat can reach vapour pressure and flash the pump dry.

Runout sits at the right. Past the end of the published curve, NPSHr has usually climbed beyond anything the system can supply, and BHP has climbed past the motor rating. A pump that starts against an empty discharge line will run out to this point until the system fills.

Reading it in practice

A workable sequence for checking a selection against a curve:

1. Mark the duty point — design flow and head — and find the smallest impeller whose curve passes above it.
2. Check where that point falls relative to BEP. Inside 70–120%, comfortable. Outside it, look at the next size.
3. Read NPSHr at the duty flow and at the maximum expected flow. Compare both against calculated NPSHa at the highest expected temperature.
4. Read BHP at runout, not at duty, and size the motor for it — unless the system genuinely cannot run out.
5. Confirm the duty point is above minimum continuous stable flow, including during turndown and start-up.
6. Correct for the fluid: head in feet is independent of specific gravity, but power and pressure are not. Multiply BHP by SG. Viscosity above roughly 4 cSt requires the Hydraulic Institute viscosity corrections, which reduce head, flow and efficiency and increase power.

The Pinnacle-Flo selection program does steps 1 through 5 against your entered duty point and will sort candidates by NPSHr, efficiency or absorbed power — and it holds specific gravity, viscosity and temperature data for more than 120 chemicals so step 6 is not left to a spreadsheet.

General engineering reference. Conditions vary by application; confirm any selection against your actual duty point, fluid analysis and operating range before purchase. Pinnacle-Flo application engineers will review your data on request.