# PUMP GUY: Considering the  Variable-Speed Clock

Feb. 27, 2013

The Pump Guy, Larry Bachus, answers a reader’s questions: At what speed will cavitation begin in a pump?; and is it advisable to have a variable-speed motor on a pump?

Larry Bachus
(a.k.a. "The Pump Guy")

Dear Pump Guy,

I stayed awake all night reading your articles on the Internet. I really liked your article on treating the symptoms of pump failure while ignoring the real illness. Your explanations of cavitation reach through many layers of understanding.

I have two questions:

1. At what speed will cavitation begin in a pump?
2. Is it advisable to have a variable-speed motor on a pump?

I hope to attend your seminar before I retire.

Hello Dr. Bukari,

First, you asked: At what speed will the pump cavitate?
Each pump is different. It is impossible to look at a pump in a pipe system and say that cavitation will begin at some pre-determined speed.

Cavitation and pump speed are generally not related. Cavitation is a function of the design of the suction side of the pump and impeller eye. Pump speed is determined by the driver or electric motor and the motor’s internal design.

But strangely, there is a remote link between pump speed and cavitation—cavitation can start if and when the liquid’s energy (head and flow) leaves the pump on the discharge side faster than the liquid’s energy can enter the pump on the suction side.

The energy (head and flow of liquid) arriving into the pump is mostly a result of the design of the suction pipe, valves, elbows, fittings and process devices. This suction energy is mostly limited by the pipe diameter and pipe length between the suction tank or vessel and the pump. This energy is not tied to the pump’s speed.
As pump speed increases (with a variable speed motor, or pulleys, or a gearbox), the energy and liquid leaves the pump faster and faster as speed increases. There will be a point (a certain RPM) where the energy leaves the pump faster than the energy can enter into the pump. Cavitation will start.