Why you should be worried about a few bubbles
by Jim Elsey
May 13, 2019

Local pressure forces involved in the microjet burst can have resultant shockwaves higher than 10,000 pounds per square inch gauge (psig). The bubble collapse phenomena can occur with a high periodicity of 300 times per second and all of this action happens at the speed of sound. The resultant microburst jet almost always directs at the adjacent surface in lieu of the fluid stream. The vane material substrate is subjected to a localized surface fatigue failure. The average lifespan of a vapor bubble from creation to collapse is about 2 to 3 milliseconds. Not everyone agrees if it is the shockwave or the reentrant micro jet burst that creates the damage. Likely, it is the combination.

Hopefully, with this perspective, you begin to understand how cavitation can damage an impeller in short order.

On a scientific level, besides the enthalpy equation mentioned earlier, the energy of the bubble collapse is simply a kinetic energy calculation and is a function of the mass and velocity.

equation 1

Note that vapor bubbles formed in water at ambient temperature are of a much larger size (mass) than if the water temperature was close to and approaching 200 F. The larger the bubble, the more energy and damage. Therefore, cold water cavitation is much more dangerous than hot water cavitation.

The root cause for vapor bubble evolution is often overlooked. Pumps do not so much generate heat to make the water flash to vapor, but instead it is a result of the drop in pressure near the impeller eye. Remember you can boil water at 33 F if you reduce the pressure low enough.

There is some correlation of cavitation noise (intensity) to impeller damage. I am not presently aware of a conclusive formula or method for accurate determination. I am aware that several people are conducting studies in this subject area. Noise level for cavitation falls in the general range of 10 kilohertz (kHz) to 120 kHz. The general accepted range of hearing for humans is only 20 Hz to 20 kHz. Perhaps I will devote a future article to acoustic detection of cavitation. If you hear cavitation noise, the pump is likely cavitating, but just because you do not hear cavitation noise does not mean it is not cavitating. Some of the most damaging cavitation occurs at noise levels outside the audible range. I also witness many people confusing cavitation noise with turbulent or high velocity flow noises.

Sometimes you just cannot have a cavitation-free system, and you may wish to treat the symptom in lieu of the problem. With all of this energy being dissipated near the surface of the impeller vane, it is important to note that all impeller materials react differently to the exerted force.

For impellers, 300 series stainless is better than cast iron. Higher chrome content steels are better yet, while CD4MCu (duplex alloy) is better than high chrome stainless. There is good information and engineering studies completed in this area. Your empirical results may differ.

Finally, note that even with high NPSH margins, where the NPSH available far exceeds the NPSH required, the pump may still experience some cavitation. It is nearly an impossible task to reduce it to zero.