Pump & Fan Selection Calculator — H-Q Curve, NPSH & Affinity Laws

None as a governing standard, and it is worth being precise about why. ISO 9906 — Rotodynamic pumps, hydraulic performance acceptance tests — is the standard people expect to see named here, but it is an acceptance-TEST standard: it tells a manufacturer how to measure an as-built pump against its guarantee, to tolerance grades 1B/1E/1U, 2B/2U or 3B. This calculator implements none of those grades and tests nothing against a guarantee. What it implements are physical laws: the Darcy-Weisbach equation with the Swamee-Jain explicit friction factor for the system curve, an energy balance for NPSH available, and the affinity laws for speed and impeller-trim scaling. Those are exact. The acceptance thresholds it applies — the NPSH margin, the ±20 % off-BEP band, the suction-specific-speed caution — are common design practice rather than clauses, and every panel that uses one carries a badge saying so. The full method is printed in the PDF report.

Cavitation happens when local pressure at the pump inlet drops below the fluid's vapour pressure, causing vapour bubbles to form and then violently collapse on the impeller — eroding metal and causing noise, vibration and loss of head. The headline NPSH margin on this page is computed from the suction-flange pressure and the vapour pressure only: it carries no suction lift and no suction-line loss, because the main panel has no fields for them, so it is the most optimistic case. The Cavitation Analysis panel adds both terms — NPSHa = (P_atm − P_v)/(ρg) − H_s − h_f — and is the one to use for a real installation; expect it to read lower. The tool flags a cavitation risk below a 1.1× ratio and calls the duty marginal below a 1 m margin or a 1.3× ratio. To fix a marginal result, raise suction pressure, lower the suction lift, reduce suction-line losses, or select a pump with a lower NPSH_r.

Pumps in series add heads at the same flow, so the combined H-Q curve is shifted upwards — useful for high static-head systems. Pumps in parallel add flows at the same head, shifting the combined curve to the right — useful for high-flow, low-head systems. The calculator finds the operating point of the combined curve against the system curve via bisection, so the result is correct even when the two pumps have different characteristic curves or the system has significant static head.

The affinity laws state that flow scales linearly with speed, head scales with speed squared, and power scales with speed cubed. Halving the speed therefore reduces shaft power to one eighth — an 87.5 % saving, but only in an ideal friction-only system with no static head. The calculator computes the real part-speed operating point by intersecting the speed-scaled H-Q curve with the actual system curve rather than affinity-scaling the duty point, which gives a saving that accounts for static head. There is a limit worth knowing: at 75 % speed a pump's shutoff head falls to 0.5625 of its full-speed value, and once that drops below the static head the pump delivers NO flow at all. The tool detects that case and reports no saving rather than a very large one — turning down a pump on a static-head-dominated system does not save energy, it stops the flow.

It says so, rather than returning a plausible-looking number. Three cases are reported explicitly. If the pump's shutoff head is at or below the static head the two curves never cross, so there is NO operating point and the tool says that instead of quoting a duty point at the edge of its search range. If the duty point lands beyond about twice the BEP flow, the parabolic efficiency model has run out of range and shaft power is reported as a floor, not a result. And if the required motor rating exceeds the largest standard IEC size the tool holds, it tells you the recommendation is the catalogue maximum rather than a valid selection. The same three flags are returned by the public API, so an integration sees them too. Non-physical inputs — a zero or negative diameter, density, efficiency or flow — are refused outright rather than turned into a finite but meaningless answer.

Because there is nothing honest to put there. At 75 % speed a pump's shutoff head falls to 0.5625 of its full-speed value. Once that drops below the static head the pump delivers no flow at all, so there is no part-speed duty point and no saving to quote — turning the pump down has not saved energy, it has stopped the flow. The tool detects that case and shows a dash. It also shows a dash when the duty point is outside the efficiency model's range, because a saving computed as a ratio of two floored shaft powers is an artefact of the floors rather than a property of the pump.

You can use the full calculator during a free 30-minute preview with no sign-up required, and a free 14-day account trial unlocks every tool with no credit card. The branded PDF engineering report and saved calculations are included in the free 14-day trial and in every paid plan.