The affinity laws lay down the relationship between hydraulic characteristics and rotational speed of centrifugal pumps.

Specific cases can use the general rule as follows:

1st Model Law


2nd Model Law


3rd Model Law


Q = flow
H = head
P = power input
N = speed
The indices relate to the respective speed.

The affinity laws are applicable to frictionless, incompressible flows. They are to be viewed as empirically suitable for technical application.

Multi-speed functions of small and medium size pumps are realised by changing the motor windings configurations to achieve stepped speed variations.

Also see:
Pump H-Q Curve