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Pump curves explained — system curve × pump curve

Pumps don't deliver a fixed flow. They deliver where their own head-vs-flow curve crosses the system's resistance curve. This guide walks through the visual.

Published Updated by Pipe Flow Lab Editorial

The most common pump-sizing mistake is thinking of a pump as something that produces a fixed flow rate. It doesn't. A centrifugal pump produces a curve: at zero flow it can develop its maximum (shut-off) head, and as flow rises the head it can develop drops. The actual operating point is where this pump curve crosses your system curve.

The visual

Flow QHead HPump curveSystem curveH_staticOperating pointQ_opH_opBEP

The two curves

System curve

For a fixed pipe geometry, the head loss your system requires depends on flow:

Hsystem(Q) = Hstatic + k · Qn

H_static is the elevation rise from source to discharge (independent of flow). k · Q^n is friction (n ≈ 1.85-2 in turbulent flow). The system curve is a parabola that starts at H_static and curves upward.

Pump curve

A centrifugal pump's head-vs-flow curve is published by the manufacturer for every model. It typically slopes downward from the shut-off head at Q = 0 to zero head at the runout flow. Manufacturers also publish efficiency overlays showing the best efficiency point (BEP) — usually 70-90% of runout flow.

Operating point

Whatever flow makes the two curves intersect is the flow your pump will actually deliver. If you size the pump exactly at design flow on a perfectly drawn system curve, you'll land near the BEP and life is good. The hazards are at the edges:

  • Operating far left (low flow, high head): low efficiency, recirculation inside the pump, increased radial loads, vibration.
  • Operating far right (high flow, low head): NPSH problems (suction cavitation), motor overload as power rises with flow.

Why the system curve matters

People focus on the pump curve because it's vendor-published. The system curve is the one you control through pipe sizing, fitting count, and valve settings — and it determines where on the pump curve you actually live. A throttled balance valve adds k to the system curve and pushes the operating point left; replacing a globe valve with a ball valve drops k and pushes the operating point right.

Designing in 2026: variable-frequency drives shift the pump curve up and down (speed × Q is constant), letting you ride near the BEP across a range of demand. The pump sizing calculator gives you the design point; a VFD lets you hold near it under varying load.

Reading a manufacturer's curve sheet

  1. Find the model that brackets your design flow and head.
  2. Confirm BEP is within ±15% of your design flow.
  3. Check NPSH-required at design flow ≤ NPSH-available.
  4. Note shaft power at design — sets motor sizing.
  5. Note efficiency at design — sets electricity cost.

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