Matching pump to system: the system curve H = Hstatic + KQ², the duty point, affinity laws (Q∝N, H∝N², P∝N³), best-efficiency-point selection and VFD vs throttling, with a worked duty-point example.
A pump does not have a flow rate — it has a curve. The flow it actually delivers is set by where that curve crosses the system curve of the pipework it feeds. Get the intersection wrong and the pump runs off its best-efficiency point: wasting energy, overheating, or cavitating. Pump selection is the discipline of matching curve to system so the duty point sits in the sweet spot across the whole operating envelope.
Total dynamic head the system demands rises with the square of flow (friction is ∝ v²) on top of a fixed static lift:
Hstatic = static lift + pressure difference (m)K = lumped friction + fittings coefficient (s²/m&sup5;)Q = volumetric flowThe duty point is where Hsys(Q) equals the pump head curve Hpump(Q). Speed changes move the pump curve by the affinity laws:
Because power scales with the cube of speed, trimming a variable-speed pump back even modestly yields large energy savings — the core reason VFD control beats throttling a valve.
Select so the duty point falls at 70–110% of best-efficiency flow. Far left = recirculation and vibration; far right = NPSH and overload.
Variable speed follows the load down the cube law; a throttle valve just burns the surplus head as heat.
Steep system curves suit centrifugal trimming; flat curves and high static lift may need PD or multistage pumps.
Verify duty at minimum, average and peak flow — not just the design point — so turndown stays stable.
Static lift 12 m; at 40 L/s the friction + fittings loss is 8 m, so K = 8 / 0.040² = 5000 s²/m&sup5;. Pump curve gives H = 25 m at 40 L/s.
Build the system curve and head loss to overlay against the pump characteristic.
Reynolds & Bauhm sizes pipework, pumps and valves and closes the head budget as part of a gated, fully documented bespoke design — every figure traceable from flow to fabrication.
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