The Affinity Laws – Pump Laws
The centrifugal pump is a very capable and flexible machine. It is not necessary to design specific pump for each specific system. The performance of centrifugal pump can be changed by the change in impeller diameter or its rotational speed. The affinity laws, or the pump laws, state how such changes influence the performance of the pump. These laws are summarized in the following points.
• The flow rate or capacity is directly proportional to the pump speed: double the speed / double the flow.
Q ∝ n
• The pump head is directly proportional to the square of the pump speed: double the speed/multiply the pressure by four.
Hp ∝ n2
• The power required by the pump motor is directly proportional to the cube of the pump speed: double the speed/multiply the power by eight.
P ∝ n3
These principles apply regardless of the direction of the change in speed or in impeller diameter. It must be noted the Affinity laws give results that are approximate. There is a discrepancy between the real hydraulic values and calculated. This discrepancy is due to hydraulic efficiency changes.