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Pump Affinity Laws

For a centrifugal pump or fan, flow scales with speed, head with speed squared, and power with speed cubed: Q∝N, H∝N², P∝N³. The same exponents apply to a small impeller trim at constant speed.

Change

From (rpm)

To (rpm)

Base flow Q₁

Base head H₁

Base power P₁

New flow Q₂

—

New head H₂

—

New power P₂

—

Impeller trims beyond ~10–15% drift from the ideal law (efficiency falls) — treat large trims as approximate.

Affinity curves

Ratio of new to base value vs speed (or diameter) ratio. ■ flow ∝ r ■ head ∝ r² ■ power ∝ r³. The dashed line is your ratio — power runs away fastest.

Ideal affinity relations. A helper, not a substitute for engineering judgment.