What happens to the horsepower requirement if a centrifugal pump motor is replaced with a motor that has a 25% higher speed?

When a centrifugal pump motor is replaced with one that operates at a higher speed, the horsepower requirement does not increase linearly but rather follows a specific relationship governed by the affinity laws for pumps.

According to the affinity laws, if the speed of the pump increases, the flow rate increases proportionally to the speed increase. However, the pump power requirement typically increases with the cube of the speed ratio. This means that if the motor speed increases by 25%, the new speed is 1.25 times the original speed.

Using the affinity laws, the relationship can be understood as follows:

• Power at new speed = Power at original speed × (New speed / Original speed)³.

• If new speed = 1.25 times the original speed, then Power increases by (1.25)³ = 1.953125.

This represents an increase of approximately 95% in the horsepower requirement when switching to a motor that is 25% faster. This cubic relationship demonstrates why the horsepower requirement would significantly increase, reflecting the demands put on the pump as a result of operating at a higher speed.