CAVITATION IN PUMPS. HOW TO PREVENT THE CAVITATION
EVIDENCES OF CAVITATION IN PUMPS
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Noise. Rattling noise which comes from the pump. It seems that the pump handles stones.
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Not enough pump head. The pump develops not enough head.
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Erosion of the impeller.
THE CONDITION FOR CAVITATION FREE PUMP OPERATION
NPSHA > NPSHR
NPSHA - Net Positive Suction Head Available - defined by the suction line
NPSHR – Net Positive Suction Head required – defined by the pump characteristics.
POSSIBLE REASONS FOR CAVITATION AND POSSIBLE REMEDIES
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The level of the liquid in the suction tank has lowered
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The strainer is clogged.
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The suction pipe is corroded, has sediments or clogged.
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The valve in the suction line is not fully open or stuck.
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The impeller has been trimmed.
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The pump operates with a high flow.
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The old pump was replaced by the new pump.
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High resistance of the suction pipeline.
1. The level of the liquid in the suction tank has lowered
Hin
NPSHA
NPSHA= Hatm – Hin - Hvapour- hlosses
NPSHA < NPSHR !!!
CONTROL THE LEVEL OF THE LIQUID IN THE TANK DO NOT LET DOWN LESS THE MINIMUM LEVEL
2. The strainer is clogged
Hlosses
NPSHA
NPSHA= Hatm – Hin - Hvapour - hlosses
Control the pressure drop on the strainer.
If the pressure drop is more than standard clean the strainer.
3. THE SUCTION PIPE CORRODED, HAS SEDIMENTS, OR CLOGGED
hlosses
NPSHA
NPSHA= Hatm – Hin - Hvapour - hlosses
CLEAN THE SUCTION PIPE REDUCE THE RESISTANCE
4. THE VALVE IN THE SUCTION LINE IS NOT FULLY OPEN OR BROKEN
Check all valves in the suction line including non-return and bottom valves
5. THE IMPELLER HAS BEEN TRIMMED
If the pump impeller was trimmed The NPSHR has increased
NPSHA > NPSHR
For the imp 390 mm NPSHR 2,3 m
For the imp 300 mm NPSHR 3,8 m
Less impeller higher the NPSHR
6. THE PUMP OPERATES WITH HIGH FLOW.
At the Q=420 m3/h (blue OP) pump has NPSH=3,75 m
At the Q=340 m3/h (red OP) pump has NPSH=2,25 m
If at the Q=420 m3/h NPSHR can be more than NPSHA
The pump can cavitate.
This is also evidence of inefficient operation
WHAT TO DO?
APPLY THROTTLING
The pump operating point will move toward the less flow and lower NPSHR
7. THE OLD PUMP WAS REPLACES BY A NEW PUMP.
BUT
The new pump cavitates.
WHY?
Because The new pump has a higher NPSHR.
The old pump had less speed and less NPSHR.
The old pump was another type for example double suction pump and was replaced with the end suction pump.
Reduce the speed of rotation
install the pump with less NPSHR.
8. HIGHT RESISTANCE OF THE SUCTION PIPELINE
1. The suction pipe is too long. Reduce the pipe length. Remove the pump closer to the tank.
2. Increase the diameter of the pipe
3. Remove local hydraulic resistances pipe bends, changes of diameters, etc.
4. If the flexible pipe is used. It can be shrieked because of the vacuum inside.