System Curve & Duty Point Finder

A pump does not run at one fixed flow — it runs where its curve meets the system curve. The system curve is the head your piping demands at each flow, H = H_static + K·Q²: a fixed static lift plus friction that grows with the square of flow. The duty point is the single flow and head where the pump curve and system curve cross, and it is the number that actually matters for selection.

How to use it

Enter the static head, one known system operating point (a flow and the head your system needs at that flow — this sets K), and 3–8 points from the manufacturer’s pump curve. You can paste the pump points straight from a spreadsheet: one “flow head” pair per line. The tool fits a least-squares curve through the pump points, draws both curves, and marks the intersection.

Worked example

A pump with the curve 150, 145, 130, 100, 55 ft at 0, 200, 400, 600, 800 gpm serves a system with 40 ft static head that needs 120 ft at 600 gpm (so K ≈ 2.22 × 10⁻⁴). The curves cross at about 552 gpm and 108 ft — that is where this pump will actually run in this system.

Read the duty point against BEP

Landing near the best efficiency point matters for reliability, not just energy. The Hydraulic Institute defines a preferred operating region of roughly 70–120% of BEP flow; run well below it and you risk suction/discharge recirculation, and well above it you risk high NPSH demand and cavitation, both of which shorten seal and bearing life. Enter the pump’s BEP flow and the tool flags where your duty point sits. If the duty point is in the wrong place, change the system (pipe size, control valve) or the pump (trim or speed — see the affinity laws calculator), not just the motor.