Thursday, November 27, 2008

Cavitation

What is cavitation?
Cavitation begins as the formation of vapor bubbles at the impeller eye due to low pressure. The bubbles form at the position of lowest pressure at the pump inlet (see Figure 1) which is just prior to the fluid being acted upon by the impeller vanes and then rapidly compressed. The compression of the vapor bubbles produces a small shock wave that impacts the impeller surface and pits away at the metal creating over time large eroded areas and subsequent failure. The sound of cavitation is very characteristic and resembles the sound of gravel in a concrete mixer.


Figure 1 Pressure profile at the pump entrance.


As you can see from Figure 1 the pressure available at the pump inlet which is the
pressure that we would measure if we put a gauge at that point, can be reasonably high but still drop considerably as it makes it way into the pump. The pressure may
be lowered enough that the fluid will vaporize and will then produce cavitation.
The same effect can sometimes be seen in control valves because they have a
similar pressure drop profile so that if the pressure is insufficient at the control
valve inlet cavitation will also occur.

Vapor pressure and cavitation
There is two ways to boil a liquid. One way is to increase the temperature while keeping the pressure constant until the temperature is high enough to produce vapor bubbles. In Figure 2 this is what happens when you move horizontally in the graph (that is at constant pressure) and increase the temperature. Eventually you hit the vaporization line of the particular fluid and the fluid starts to boil or produce
vapor bubbles.

Figure 2 Vapor pressure vs. temperature.

Figure 3 Making a liquid boil under low pressure.

We do the same thing every day when we boil water in a pot by using a stove top element for example.
The other way to boil a liquid is to lower the pressure. If you keep the temperature constant and lower the pressure you can make a liquid boil also. In Figure 2 this is what happens when you move vertically in the graph (that is at constant temperature) and decrease the pressure. Eventually you hit the vaporization line of the particular fluid and the fluid starts to boil or produce vapor bubbles.

If the pot were covered and you had a source of vacuum (see Figure 3) by lowering the pressure in the pot you would be able to make the water boil at a lower temperature. When the pressure is 7.5 psia or (14.7 – 7.5 = 7.2) or 7.2 psi less than the atmospheric pressure the water will boil at a temperature of 180 ?F and when the pressure is 1.5 psia the water will boil at 120 ?F. This is what happens at the pump suction when the pressure is low enough to make the fluid boil.

The pressure at which the liquid boil is known as the vapor pressure and is always specified for a given temperature because at different temperature the liquid will boil at different pressures. For example, the vapor pressure of water at 212 ?F is 14.7 psia the same pressure as the local atmosphere at sea level. The vapor pressure for water at 180 ?F is 7.5 psia and the vapor pressure of water at 120 ?F is 1.5 psia.
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