If your pump is operating and it sounds like a “bag of spanners”* or “marbles rattling around”* then it is highly likely your pump is cavitating. Pump cavitation is a result of the pump not receiving enough flow of liquid through the inlet and will mean an increase in energy and eventually catastrophic damage to the pump.
Cavitation is the process whereby bubbles of vapour formed when a fluid is under low pressure, spontaneously collapse as they are transported back into a region of higher pressure, also known as flashing. When running at 3000rpm, there are thousands of these mini implosions happening every second, and the damage will literally tear through the pump as you can see in the title images. Typical causes of cavitation happen when the pump is running too far to the right on its performance curve. These poor suction conditions mean the Nett Positive Suction Head available, or NPSHa, is LESS than the NPSHr, the Nett Positive Suction Head REQUIRED from the pump. NPSH is the measure of the pressure experienced by a fluid on the suction side of a centrifugal pump. Its purpose is to identify and avoid the operating conditions which lead to vaporisation of the fluid as it enters the pump.
Put simply, to prevent cavitation the Net Positive Suction Head available in the system must be at least half a metre higher than the required NPSH of the pump. If you are experiencing cavitation, there are a few things you can try to relieve the issue: After checking for blockages, look to increase the suction pipe diameter and reducing the flow velocity. Other options become a little harder to implement, but include lowering the temperature of the liquid, reducing the motor speed, using an impeller inducer, or installing two pumps running in parallel so they are operating at a lower capacity. Otherwise you could look to use a booster pump to feed the principal pump, or try installing a plastic pump instead of metallic as they tend to have a better resistance to cavitation damage as the plastic material of construction reacts in more of a shock absorbing effect. All these methods will help lower the velocity of the liquid entering the eye of the impeller, and so reducing the pumps NPSH requirement thus reducing the risk of cavitating.
*Both of these are real life descriptions from customers when they’ve heard a pump cavitate.
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