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Simplex Control with Dry Pump Protection.

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


Simplex Control with Dry Pump Protection.

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





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