SUBJECT: The affect of cavitation 9-10

Cavitation means different things to different people. It has been described as:

Just what then is this thing called cavitation? Actually it is all of the above. In another section of this site I described the several types of cavitation, so in this paper I want to talk about another side of cavitation and try to explain why the above happens.

Cavitation implies cavities or holes in the fluid we are pumping. These holes can also be described as bubbles, so cavitation is really about the formation of bubbles and their collapse. Bubbles form when ever liquid boils. Be careful not to associate boiling with hot to the touch. Liquid oxygen will boil and no one would ever call that hot.

Fluids boil when the temperature of the fluid gets too hot or the pressure on the fluid gets too low. At an ambient sea level pressure of 14.7 psia (one bar) water will boil at 212°F. (100°C) If you lower the pressure on the water it will boil at a much lower temperature and conversely if you raise the pressure the water will not boil until it gets to a higher temperature. There are charts available to give you the exact vapor pressure for any temperature of water. If you fall below this vapor pressure the water will boil. As an example:
Fahrenheit
Centigrade
Vapor pressure lb/in2 A
Vapor pressure (Bar) A

40

4.4

0.1217

0.00839

100

37.8

0.9492

0.06546

180

82.2

7.510

0.5179

212

100

14.696

1.0135

300

148.9

67.01

4.62

Please note that I am using absolute, not gauge pressure. It is common when discussing the suction side of a pump to keep everything in absolute numbers to avoid the use of minus signs. So instead of calling atmospheric pressure zero, we say one atmosphere is 14.7 psia at seal level and in the metric system the term commonly used is one bar, or 100 kPa if you are more comfortable with those units.

Now we'll go back to the first paragraph and see if we can clear up some of the confusion:

The capacity of the pump is reduced

The head is often reduced

The bubbles form in a lower pressure area because they cannot form in a high pressure area.

A noise is heard

Pump parts show damage

The higher the capacity of the pump the more likely cavitation will occur. Some plants inject air into the suction of the pump to reduce this capacity and lower the possibility of cavitation. They choose this solution because they fear that throttling the discharge of a high temperature application will heat the fluid in the pump and almost guarantee the cavitation. In this case air injection is the better choice of two evils.

High specific speed pumps have a different impeller shape that allows them to run at high capacity with less power and less chance of cavitating. You normally find this impeller in a pipe shaped casing rather than the volute type of casing that you commonly see.

For information about my CD with over 600 Seal & Pump Subjects explained, click here  

 Link to Mc Nally home page