The affinity laws

Any machine that imparts velocity and converts a velocity to pressure can be categorized by a set of relationships that apply to any dynamic conditions. These relationships are referred to as the affinity laws. They can be described as similarity processes, which follow the following rules:

  1. Capacity varies as the rotating speed — the peripheral velocity of the impeller.
  2. Head varies as the square of the rotating speed.
  3. Brake horsepower varies as the cube of the rotating speed.

Source: Handbook of water and wastewater treatment plant operations