Pump Affinity Law — Head vs Speed
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Head is a velocity effect — the impeller tip speed sets how much energy the liquid leaves with, and energy goes as velocity squared. Trim a 1750 rpm pump making 120 ft of head down to 1450 rpm and you keep 120 × (1450/1750)² ≈ 82.4 ft. Note the head is expressed in feet or metres of the pumped liquid, not of water: the affinity laws are blind to specific gravity, which is exactly why manufacturers publish curves in feet rather than psi.
The classic field error is reading a psi gauge and scaling that by the square. Convert to head first. A pump on 1.2 SG brine showing 52 psi is making 52 × 2.31/1.2 ≈ 100 ft of head — and it is that 100 ft, not the 52 psi, that follows the speed-squared rule.
- = Head at speed 1
- = Speed 1
- = Head at speed 2
- = Speed 2
- Head at speed 1 — Pump Water Horsepower, Pump Brake Horsepower
- Speed 1 — Pump Affinity Law — Flow vs Speed, Pump Affinity Law — Power vs Speed
- Head at speed 2 — Pump Water Horsepower, Pump Brake Horsepower
- Speed 2 — Pump Affinity Law — Flow vs Speed, Pump Affinity Law — Power vs Speed