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Cavitation needn’t cause aggravation

18 May, 2020

Cavitation is usually bad news for any pumping system and,  if left unchecked, it can wreak havoc with impellers and other components. However, modern VSDs offer an easy, low-cost method to detect and prevent it, as ABB’s Andrew Bould explains.

Cavitation is caused by a pump’s impeller creating a vacuum on the suction side/inlet of the pump. When this vacuum gets too large, or if the pressure is too low, the liquid starts to vaporise and transforms from liquid into steam. When the steam bubbles reach areas of high pressure, they collapse back into a liquid state again, producing significant shockwaves.

While each individual shockwave is relatively small, when they occur in large numbers, they can have a negative effect on pump performance, and over time they can eventually destroy components, shortening pump lives by as much as half.

Cavitation can be caused by air getting into pipelines, or by backflow where a tank is filled to capacity, yet the pump is still pumping. The phenomenon has many symptoms, but common ones include a pump operating with unusually decreased flow or pressure, or unexplained vibrations. Noise is another a tell-tale sign; a cavitating pump will sound as if gravel is passing through it.

The key to avoiding cavitation lies in good pump system design, ensuring that the net positive suction head available (NPSHa) exceeds the net positive suction head required (NPSHr). This can be achieved by reducing the temperature of the fluid or pump, decreasing the number of elbows and valves in the pipework, shortening the length of the pipework, or increasing its diameter.

Of course, it isn’t always possible to achieve this, particularly for systems where the equipment is already in place, or where space or layout issues make it difficult to alter the system design significantly.

VSDs (variable-speed drives) are already used on many pumping applications and can now let you detect and prevent cavitation without having to redesign your system or spend money on extra sensors and PLCs. Some VSDs now incorporate dedicated anti-cavitation software. In the case of ABB drives, these use patented anti-cavitation algorithms to look for specific patterns in the motor and drive operating data. Because the VSD is measuring directly from the shaft, this means no latency and an instantaneous response should cavitation conditions be detected. When identified, the VSD will reduce the pump motor speed automatically, and resume normal operation once the pump has stopped cavitating.

Cavitation is a nuisance for any pump system, but knowing that your VSD already has the tools to combat it can save you substantial amounts of money on extra equipment and redesigns,  as well as improving the health and lifetime of your pumping equipment.

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