an Engineering & Service Bulletin

Technical Topics

J.H. WRIGHT & ASSOCIATES

CAVITATION

Most pump operators have seen centrifugal pump impellers that look like they were a smorgasbord for termites. This swiss cheese appearance is usually worse just inside the suction vanes. Usually, but not always, a pump impeller operating in this condition will sound like there is gravel in the pump casing.The cause of this attack is cavitation. Cavitation is a word that covers the entire phenomena of an impeller operating in a liquid very near its vapor pressure.

To understand, let's take an imaginary ride as a particle of water through an impeller where cavitation is occurring. The first part of our ride is uneventful. We're just coasting along through the suction pipe, our velocity is fairly low and although we're hot and would like to boil, the pressure of the other water particles is high and we just don't have room.

Now, we're entering the suction vane of the pump. The passageway becomes narrow, and we've speeded up ten or fifteen times. As a result, the pressure of the surrounding water particles is much less, and we have room to boil. Remember "Bernoulli's Theorem*".

We are now a bubble of vapor. Our size is vastly increased, and since we take up so much room, we've cut the pump's capacity way down. However, we move a little farther into the impeller and we find the pressure increasing. After all, an impeller is supposed to increase pressure.

Finally, the pressure is too great - we collapse - "we implode". As a final gesture, we take with us a particle of metal out of the impeller and make a loud pop. This process is cavitation.

One often asked question is "Why do some pumps operate for years in cavitation with no apparent damage or noise, and others break shafts and ruin impellers?" There are several theories as to the reason for this.

One that seems reasonable to me concerns the "energy level" of the cavitation. While this is rather difficult to explain, it means that a pump which has a net positive suction head requirement of 15 feet, would have five times the destructive cavitation of a pump with an NPSH requirement of 3 feet. In other words, when the bubbles collapse or implode in the l5 foot pump, the action is a lot more violent and does more damage.

Two other things must be remembered in connection with cavitation damage. One is that certain metals resist the pitting particularly well, and the other is that many liquids in a vapor form are much more corrosive than in liquid form.

*Bernoulli's Theorem (Paraphrased)

Total system energy remains constant in a flowing system (neglecting friction) therefore if velocity energy goes up - pressure energy must go down.

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