Centrifugal Pump Affinity Laws

The Affinity Laws of centrifugal pumps or fans indicates the influence on volume capacity, head (pressure) and/or power consumption of a pump or fan due to
change in speed of wheel - revolutions per minute (rpm)
geometrically similarity - change in impeller diameter

Note that the affinity laws for fans are not identical with pumps.
Pump Affinity Laws
Volume Capacity

The volume capacity of a centrifugal pump can be expressed like

q1 / q2 = (n1 / n2)(d1 / d2) (1)

where

q = volume flow capacity (m3/s, gpm, cfm, ..)

n = wheel velocity - revolution per minute - (rpm)

d = wheel diameter
Head or Pressure

The head or pressure of a centrifugal pump can be expressed like

dp1 / dp2 = (n1 / n2)2 (d1 / d2)2 (2)

where

dp = head or pressure (m, ft, Pa, psi, ..)
Power

The power consumption of a centrifugal pump can be expressed as

P1 / P2 = (n1 / n2)3 (d1 / d2)3 (3)

where

P = power (W, bhp, ..)