Page 45 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

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:

- (4,800 FT/SEC)2
-
2 x (32.16 FT)

- 23,040,000
-
64.32

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

Head x Sp. gr.
Pressure in psi =
2.31

- 358,209 x 1.0
-
2.31

= 155,069 psi
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.

L

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

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