Cavitation occurs when the liquid in a pump turns to a vapor at low pressure. There are 2 kinds of cavitation – suction and discharge.
Suction cavitation occurs when there is not enough flow to the pump resulting in bubbles (cavities) at the eye of the impeller. When the bubbles pass from the middle of the impeller to the outer edges, the change in pressure inside the pumps causes the bubbles to collapse or implode with great force.
Discharge cavitation occurs when the discharge pressure is extremely high, resulting for some fluid to circulate inside the pump instead of discharging. Fluid trapped between the impeller and housing at high velocity causes a drop in pressure and the formation of bubbles and their eventual implosion.
A telltale sign of cavitation is excessive noise and vibration. Vibration causes problems for many pump components, including the shaft, bearings, and seals. Left unchecked, cavitation will eventually erode components beyond repair and lead to pump failure.
Causes of Cavitation
Suction and discharge cavitation share possible causes: clogged filters, pipe blockage, poor design, or the incorrect pump for the application.
The pump curve is also crucial – If the pump is running too far right on the curve, it can cause suction cavitation. If it is running too far left on the curve, it can cause discharge cavitation.
Poor suction conditions can also occur when a pump is selected, e.g., for high efficiency, but does not meet Net Positive Suction Head Required (NPSHr) –the absolute pressure that must be present in a liquid for a pump to avoid cavitation while pumping.
The second part to this is the NPSHr must be compatible with the NPSHa – the NPSH made available by the system in which the pump is installed. A pump that requires more NPSH than the system makes available will not operated as designed and internal damage can be expected in a brief amount of time. A rule of thumb is to require a 10% margin or 5 feet of difference between the NPSHa and NPSHr, whichever is greater.
How can cavitation be prevented?
The best way is to have a good system design, which include appropriate NPSH margins, correct piping, and selecting the right pump for the application. When determining the pump to use for your situation, the experts at JMI are happy to assist and provide a customized solution. Contact us at 262-253-1353 or sales@jmipumps.com.
