## Holes in the liquid cavitation

A cavitation bubble is a hole in the liquid. If I should have bubbles in the suction of my pump then I have problems. Pumps can move liquid, but they cannot move air or gas bubbles. Compressors exist for moving gases. A gas will not centrifuge. Bubbles occupy space inside the pump and affect the pump's pressure and flow. With vapor bubbles in the low-pressure zones of the pump, the motor's energy is wasted expanding the bubbles instead of bringing more liquid into the pump. As the bubbles pass into the pump's high-pressure zones, the motor's energy is wasted compressing the bubbles instead of expelling the liquid from the pump. The bubbles can collapse as they pass from low- to high-pressure zones in the pump. The water is rather hard.

AUTHOR'S NOTE

You'll know this if you've ever done a belly flop into a swimming pool.

When vapor bubbles collapse inside the pump the liquid strikes the metal parts at the speed of sound. This is the clicking and popping noise we hear from outside the pump when we say that cavitation sounds like pumping marbles and rocks. Sound travels at 4,800 ft per second in water. The velocity head formula gives a close approximation of the energy contained in an imploding cavitation bubble. Remember that implosion is an explosion in the opposite direction.

Using the velocity head formula: V2

23,040,000 64.32

In pump terminology, the approximate energy in an imploding cavitation bubble is 358,209 ft. To convert this energy into pressure:

You can see, based on the velocity head formula, a cavitation bubble impacts the impeller and other pump parts at about 155,069 psi. Other experiments in test laboratories using a more precise rHv, have calculated the impact pressure at 1 Gigapascal, or 147,000 psi. This is the reason that the damage from cavitation appears like someone was beating on your impeller with a large ball pein hammer.

AUTHOR'S NOTE

In medicine, doctors use this same energy contained in cavitation bubbles (Lithotripsy) to treat and destroy kidney stones and tumors. The bubbles act like microscopic jackhammers, disintegrating kidney stones.

If your pump is in cavitation, you'll have one or more of the following:

■ Problems with pump packings.

■ Problems with mechanical seals.

■ Problems with alignment.

■ Problems with the bearings.

■ Problems with impellers, casings, and wear bands.

■ Problems with pump efficiency.

■ Problems with leaks and fugitive emissions.

And these problems won't go away until you resolve cavitation at its source.

There are five recognized types of cavitation:

■ Vaporization cavitation, also called inadequate NPSHa cavitation

■ Internal re-circulation cavitation.

■ Vane passing syndrome cavitation.

■ Air aspiration cavitation.

■ Turbulence cavitation.

Let's investigate each of these, their causes and resolutions:

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