Page 44 - Know and Understand Centrifugal Pumps

Cavitation

- 14.7 psi x 2.31
-
1 .o

= 33.9 ft
You can only raise a column of cold water in a pipe a maximum of
33.9 ft with a pump in suction lift. Beyond 34 ft, the water will boil or
vaporize. This is the reason whp submersible pumps and vertical turbine
pumps exist. There is no limit to the distance you can push a liquid
from below, but you can only aspirate a liquid a maximum of 34 ft from
below the pump.

Question: If you put a straw into a glass of milk and suck on the straw, are you really
sucking on the milk?
Reply: If you could really suck on the milk, then you wouldn't need the straw. What
you're actually doing with your mouth on the straw is lowering the atmospheric
pressure inside the straw, so that the atmospheric pressure outside the straw pushes
the milk up into your mouth. This is why we say that a pump does not suck. The pump
actually generates a zone of low pressure in the eye of the impeller, thereby lowering
the atmospheric pressure inside the suction piping. Atmospheric pressure outside the
suction piping pushes the liquid up toward the impeller a maximum of 34 ft under
ideal circumstances.

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.

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

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