Centrifugal pump Cavitation and it's causes and indication.

Centrifugal Pump Cavitation:

With on a Centrifugal pump, the flow area at the eye of the pump impeller is usually smaller than either the flow area of the pump suction piping on the flow area through the impeller vanes. When the liquid being pumped enters the eye of a pump, the decrease in flows area results in an increase in flow velocity accompanied by a decrease in pressure. The greater the pump flow rate, the greater the pressure drop between the pump suction and the eye of the impeller.

If the pressure drop is large enough ( below the vapour pressure) on if the temperature is high enough, the pressure drop may be sufficient to cause the liquid to flash to vapour when the local pressure falls below the saturation pressure for the fluid being pumped. Any vapour bubbles formed by the pressure drop at the eye swept along the impeller vanes by the flow of the liquid.

When the bubbles enter a region where the local pressure is greater than saturation pressure, the vapour bubbles abruptly collapse. This process of the formation and subsequent collapse of vapour bubbles in a pump is called cavitation.

Causes: 

1. Drop in pressure at the suction nozzle due to low NPSH a.
2. Increase in temperature of the pumped liquid.
3. Increase in the fluid velocity at the pump suction.
4. Reduction of the flow at pump suction (clogged filters).
5. Undesirable flow conditions caused by obstructions or sharp elbows in the suction piping.
6. The pump is not selected correctly.
7. Running the pump too fast.
8. Having too small dia of Suction pipe, too long of a distance of suction pipe, too many fitting on suction pipe.

Indication of cavitation :

1. Vibration.
2. Delivers less flow.
3. Noisy operation.