Protect Pumps from loss of flow with undercurrent sensing using Model CDP Motor Protection Relay.

A Problem with Pump Cavitation
Copyright © 2006 Francis J. Martino

Question: Presently my customer is controlling flow of his pump by closing a discharge valve.
He has complained that when the valve is closed to less than 50% then the pump makes a
vibration noise...he believes it is cavitating. If he uses a VFD to control the flow, will he gain
any advantage with his cavitation issue and how is the pump efficiency affected?

John L.


Answer:

The most common forms of cavitation are a condition in which a pump is either operating
with no water in the pump or is filled with water and operating but the system does not
allow water to flow. In either case, the pump is not cooled properly by the appropriate
amount of water flow.

Water within the pump may then vaporize from the heat build-up and therefore cause cavities
to form in the water that is within the pump. In both cases, heat will build up due to a lack
of cooling and damage to the pump will then occur.

Another cause of cavitation is flow turbulence. I suspect that flow turbulence is what your
customer has encountered.

If a short distance exists in the piping system between the pump and the valve, water will back
up and circulate between the valve and the pump.

If there is a long distance between the valve and the pump (over ten pipe diameters of
distance) then the closing of the valve will appear to the pump as an increasing head rather
than as presenting a turbulence. The operation of the pump will then shift on the pump system
curve and the pump will operate normally if the system does not present an overload to the pump.

Removal of the valve and the addition of a VFD to control the flow of water will allow the
water flow and velocity to be lowered. Turbulence within the system will be reduced.

Replacement of a valve with a VFD for flow control will always yield an energy savings.

Regards,
Frank

Protect Pumps from loss of flow with undercurrent sensing using Model CDP Motor Protection Relay.