Pump Affinity Law — Flow vs Speed
Also known as pump laws · affinity laws · VFD flow change
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Learning zone
A centrifugal impeller is a volumetric scoop: every revolution flings out roughly the same slug of liquid, so turn it faster and the capacity climbs in lockstep. Slow a 1750 rpm pump to 1150 rpm and a 500 gpm duty becomes 500 × 1150/1750 ≈ 329 gpm. This is the whole economic case for variable-frequency drives — but the flow law is the gentlest of the three; head follows the square and power the cube, which is where the savings actually come from.
The trap is applying it across a duty the pump cannot reach. Affinity laws slide a point along a parabola through the origin, not along the system curve; if your system has 40 ft of static lift, dropping speed eventually drives the pump head below that lift and flow collapses to zero rather than scaling smoothly. Always plot the new curve against the real system curve before promising a customer a linear turndown.
- = Flow at speed 1
- = Speed 1
- = Flow at speed 2
- = Speed 2
- Flow at speed 1 — Pump Water Horsepower, Pump Brake Horsepower
- Speed 1 — Pump Affinity Law — Head vs Speed, Pump Affinity Law — Power vs Speed
- Flow at speed 2 — Pump Water Horsepower, Pump Brake Horsepower
- Speed 2 — Pump Affinity Law — Head vs Speed, Pump Affinity Law — Power vs Speed