Preventing pump cavitation the easy way
Clayton Mead, water industry manager for ABB Motion in the UK, explains how to prevent damage and extend the lifespan of pumps by using in-built drive functions to detect and control cavitation.
If a pump system is suffering persistent failures, one of the most common culprits is a phenomenon known as cavitation. Its tell-tale signs include erratic pump behaviour, unexpected vibrations, premature seal and bearing failures, and erosion of the impeller. The noise it creates is also an obvious sign – if it sounds as if there are small rocks moving around in the pipes, then there’s a strong chance that cavitation is the cause.
Left unchecked, persistent cavitation will eventually cause a failure, so it’s important to prevent it. There are several ways of doing this. Vaporisation cavitation is caused when the pressure required at the inlet (suction) to fill the impeller for a given flow – known as Net Positive Suction Head, or NPSH – is too low.
Another type of cavitation is known as discharge cavitation. This occurs when the discharge pressure is too high, forcing fluid to flow at rapid velocity between the impeller and the housing, rather than out of the pump as it should.
There are several ways to reduce cavitation. Lowering the temperature of the fluid or pump can help, as can decreasing the number of elbows and valves in the pipework. However, the latter is not always practical, especially in existing systems where it is difficult to make significant alterations to the layout. Another method is to install pressure sensors to monitor the pressure at the inlet. The sensors feed into a PLC or other control device, which can then instruct the motor to reduce its speed and in turn reduce the pressure.
Rather than installing extra sensors and other equipment, a drive can be used instead. Some drives have built-in anti-cavitation software, which uses algorithms to detect and analyse torque ripples directly from the pump motor’s shaft, and reduces the pump speed automatically when cavitation is detected, and resumes normal operation once cavitation has stopped.
If a drive is already being used to control the pump, this can save money and simplify the system’s configuration by avoiding the need for any additional hardware. Because the drive is measuring directly from the shaft, it will generally result in a faster response than a PLC-and-sensor arrangement.
For more information on anti-cavitation, visit: https://new.abb.com/drives/anti-cavitation-solution-in-abb-drives
