What is Cavitation?
Cavitation is when there is insufficient water available for a pump to meet the flow requirements of the system. This can occur when the pump cannot make sufficient discharge pressure to meet its manufacturer’s specification. It often occurs in process pumps where there is varying demand on the pumps, and typically in a chilled water system or a heating system.
On the graph below we are showing the volume pumped(Q) plotted against the pressure produced (H).
The blue line shows the capability of the pump. In the example below we show the pump producing 120 m3/h at 30m pressure. If the demand increases 150 m3/h then the pump will run off this curve which will result in cavitation. See the lower graph.
How can I tell if I have suction cavitation?
If you have a pump which fails more frequently than every 12 months, where either the mechanical seal fails, or the front motor bearing fails, it is highly likely that you have a measure of cavitation. If you were
to look inside the pump, the impeller would look as if it had been
attacked by chemical; it would have suffered from erosion.
Cavitation is more noticeable on larger pumps; a cavitating pump
is normally noisier than you would expect.
If you are experiencing cavitation within your pumps, the first sign will be leaking from the mechanical seal, or that the front bearing will seize and stop the pump operating. Ultimately, if cavitation is occurring, the pump will fail.
If you want to test to see if your pump is cavitating, you can slowly close your discharge valve.If you hear the noise reduce, this is a
good indication that the pump may be cavitating.
Typical causes of cavitation are:
1. Blocked filters on the suction side of the pump
2. Restrictions in the suction pipework
3. Pump is drawing water from below the pump (especially where the water temperature is over 50ºc.) This temperature might be lower than you’d expect,but as you reduce the pressure on heated water (by suction), the boiling point is reducedallowing it to vaporise at a lower temperature .
4.Discharge pressure drops below the minimum pressure the pump is designed to produce, which is normally a result of excessive demand for water.
5. Designed set flow rate is exceeded by the user.
6. Low water level in the suction tank.
Common applications where cavitation is found:
•Swimming pool pumps
•Chilled water supply pumps
The solution to pump cavitation is:
1. Analyse where the pump is performing
2. Assess what the demands of the system actually are
3. Adjust the valves and filters to suit the system
4. Alternatively, install the correct specification of pump. In many
cases the pump specification is either too small or the duty that has been selected doesn’t cover the full range of requirements under which the pump has to operate.Typically a larger pump is required.
If you suspect that you have cavitation, Dura Pump will carry out a full system analysis and assessment of your pumps to help you identify your problem. With our extensive knowledge and expertise, backed by specialist engineering software, we are able to provide you with the right and most cost effective solution. It is important to note that cavitation not only breaks pumps, but also wastes energy and money.