System Curve vs Pump Curve
How the system curve and the pump curve interact, why the operating point is where they intersect, and how static head, friction, valve throttling, and speed move each curve — with what low flow can be telling you.
Definition
A pump curve and a system curve are two head-versus-flow relationships plotted on the same axes. The pump curve is the head a given pump delivers at each flow, falling as flow rises — it comes from the vendor for a fixed speed and impeller. The system curve is the head the piping demands at each flow: a fixed static (elevation/pressure) head plus friction and minor losses that grow roughly with the square of flow. Where the two curves cross is the operating point — the single flow and head at which the pump's delivery equals the system's demand. Everything in pump behaviour at steady state is a story about how those two curves move and where they intersect.
Why it matters
You cannot set the flow of a centrifugal pump directly; you can only change one of the two curves, and the operating point follows. Raise the static head or add friction (a partly shut valve, fouling, a longer line) and the system curve steepens or lifts, so the intersection slides up and to the left — less flow at more head. Change pump speed or impeller diameter and the whole pump curve scales by the affinity laws, moving the intersection the other way. This is why a pump rarely delivers its catalogue 'maximum' flow, why throttling a valve reduces flow without touching the pump, and why a flow problem is often a system-curve problem rather than a pump fault. Reading the two curves together is what turns a vague 'the pump is weak' into a specific, fixable cause.
Formula
Units involved
- •Q — volumetric flow in m³/h, L/s, or gpm
- •H — head in metres (m) or feet (ft)
- •H_static — elevation/pressure head, independent of flow
- •K — lumped system resistance (head per flow², from friction + minor losses)
- •N — rotational speed in rpm (scales the pump curve)
Concept diagram
Worked example
A system has 12 m of static head and, at 50 m³/h, develops 7 m of friction + minor losses, so its resistance coefficient is K = 7 / 50² ≈ 0.0028 m per (m³/h)². The pump curve passes through (50 m³/h, 19 m). Check the operating point and see what throttling does.
- 01System head at 50 m³/h: H_sys = 12 + 0.0028 × 50² = 12 + 7 = 19 m
- 02Pump delivers 19 m at 50 m³/h → the curves intersect there: operating point ≈ 50 m³/h, 19 m
- 03Throttle a discharge valve (add ~5 m loss at 50 m³/h): the system curve steepens
- 04New intersection moves left — say to ~44 m³/h at a higher head — flow falls without changing the pump
- 05Slow the pump 10% instead (affinity): pump curve drops (H ∝ N²), intersection also moves to lower flow but at lower head, saving power
Operating point ≈ 50 m³/h at 19 m. Throttling cuts flow by raising system resistance; slowing the pump cuts flow by lowering the pump curve — both move the intersection, but only the speed change saves energy.
Common mistakes
- •Expecting the pump to deliver its catalogue maximum flow — that is the curve's no-head end, not the operating point set by your system.
- •Treating static head and friction head as interchangeable — static head is flat with flow, friction rises with Q², and they move the operating point differently.
- •Throttling the suction side to control flow — this attacks NPSH available and invites cavitation; control on the discharge side.
- •Assuming a low flow means a faulty pump, when a steeper-than-expected system curve (closed valve, fouling, wrong line size) is the more common cause.
- •Comparing a measured duty against the pump curve without correcting for density, viscosity, or speed — the published curve is for clean water at the rated speed.
When to use the calculator
Build the system curve with the Total Dynamic Head calculator for the static and pressure terms, and the Pipe Head Loss, Darcy-Weisbach, and Minor Loss calculators for the flow-dependent friction. Use the Pump Affinity Laws calculator to see how a speed or impeller change scales the pump curve and moves the operating point.