Cavitation

What is CAVITATION

Fig-1 (pic of impeller having cavitation effect). If the pressure at any point inside the pump drops below VP of the liquid, then under such condition the liquid can 'Flash'(boiling). i.e. changing from liquid to gaseous phase. The vapor bubbles or cavities are carried along with the stream through the impeller eye to regions where the pressure within the impeller shroud rises due to centrifugal force. In the higher pressure zone the bubbles collapse or implode with tremendous shock and noise. This phenomenon is called CAVITATION. Ref Fig-2

Why it happens

The absolute pressure at the pump suction nozzle and cavitation problems are closely linked. If the lift is high and pump operates beyond BEP, then suction velocity increases to such an extent that it causes further drop in suction pressure. when the pumps runs under such conditions then the risk of liquid getting vaporised cannot be ruled out. However, for cavitation to occur, the pump need not necessarily work beyond BEP towards the right hand side of curve. It will also happen if the pump is working far beyond left of BEP. Also, cavitation can happen at any point on the curve if NPSHA is less then NPSHR.

There is limit to which abs pressure in the suction pipe can be reduced. Centrifugal pump with ordinary design does not function properly when the liquid contains vapors.

How to prevent Cavitation

Pump must be selected to operate at BEP. This is most important factor. If there is variation at site, necessary arrangement, if possible is to be done to make pump operate at BEP.

To provide enough head on the pump suction side (say by reducing the suction lift by taking pump near the suction source) so that the suction pressure at the impeller eye will remain above the liquid VP.

We can also reduct the discharge to bare minimum and thus reducing the NPSHR. This can be done by installing pulley drive system.

Where and why it forms

Impeller eye forms the zone of lowest suction pressure because:-

Ref Fig-3

1) Rotating impeller causes forced vortex formation just in front of impeller eye causing pressure to drop.

2) Further pressure drop takes place because of abrupt change in flow direction and sudden increase in velocity due to centrifugal force.

3) Rotating vanes causes shock to flow and increases friction resulting in further drop.

This is why impeller eye zone after the inlet vane tip portion is prone to severe action of cavitation. Proper filing of vane inlet tip and finishing of the eye area may reduce gravitation to some degree.

Cavitation and liquid properties

1) Specific Gravity

If the sp.gr. of liquid increases, the danger of cavitation also increases as to that extent the available NPSH is reduced.

2) Liquid Temperature

Liquids have the ability to dissolve gases or air in varying quantities. The amount of gas/air which goes in to the solution depends on liquid temperature and pressure on its free surface. The volume of gas/air absorbed by the liquid decreases as temperature rises. However, dissolved gases are released from the liquid when the pressure decreases.