Affinity Laws for Pumps: Principles, Formulas & Calculator

Calculate the influence of speed (rpm) and impeller diameter changes on pump volume capacity, head (pressure), and power consumption using affinity laws for centrifugal pumps.

Affinity Laws for Pumps Calculator

The Affinity Laws indicate the influence on pump performance (volume capacity, head/pressure, and power consumption) due to changes in wheel speed (rpm) or impeller diameter.

Two Main Scenarios:

Changing Wheel Velocity (constant impeller diameter)
Changing Impeller Diameter (constant wheel velocity)

Calculator

Inputs

Initial Volume Capacity (q1) (gpm)

Initial Head/Pressure (dp1) (ft)

Initial Power (P1) (bhp)

Initial Speed (n1) (rpm)

Final Speed (n2) (rpm)

Results

Final Volume Capacity (q2)-- gpm

Final Head/Pressure (dp2)-- ft

Final Power (P2)-- bhp

Calculator

Inputs

Initial Volume Capacity (q1) (gpm)

Initial Head/Pressure (dp1) (ft)

Initial Power (P1) (bhp)

Initial Impeller Diameter (d1) (in)

Final Impeller Diameter (d2) (in)

Results

Final Volume Capacity (q2)-- gpm

Final Head/Pressure (dp2)-- ft

Final Power (P2)-- bhp

Key Formulas

Changing Wheel Velocity (Constant Diameter):

Volume Capacity: q₁/q₂ = n₁/n₂
Head/Pressure: dp₁/dp₂ = (n₁/n₂)²
Power: P₁/P₂ = (n₁/n₂)³

Changing Impeller Diameter (Constant Velocity):

Volume Capacity: q₁/q₂ = d₁/d₂
Head/Pressure: dp₁/dp₂ = (d₁/d₂)²
Power: P₁/P₂ = (d₁/d₂)³

Important Note:

If pump speed increases by 10%:

Volume flow increases by 10%
Head increases by 21%
Power increases by 33%