X-Mozilla-Keys:                                                                                 
Fred wrote:

Cavitation is a boiling phenomenon caused by the local pressure dropping
below the boiling point of the liquid for the prevailing temperature.  It
makes a very distinctive sound and so is very bad news for submariners
attempting to skulk around the ocean.  With propellers, the more power you
put into the prop, the lower the pressure on the low pressure side of the
prop, and the greater the likelihood of cavitation.  So if you want to move
at a reasonable speed, and keep quiet, you need to work the problem
diligently.

Cavitation for pumps is the same phenomenon, but in this case we are talking
about the amount or degree of cavitation.  A little on a small area of the
impeller will not impact performance measurements significantly although
prolonged cavitation could damage the surface.  Huge amounts of cavitation
would result in big bubbles of vapor which would occupy a substantial portion
of the total fluid flow and cause the pump to burp and generally have
indigestion.

So you are correct in stating that cavitation can occur at low temperatures.
Becasue of greater pressures at depth, it is less of a problem for subs
operating down deep.

It is question of degree.  Pumps are noisy (especially if they are spinning
3000-6000 RPM) but we don't care about a trace of cavitation noise since the
Soviets are not tracking us.  Moderate cavitation can cause long term erosion
of surfaces because when the bubbles collapse, they make a local high
pressure for an instant.  Massive cavitation causes pump surge, and noise
clearly audible to the observer, even over the noise of the pump, and should
be avoided if you want good data.

So I say put the feed tank such that its water level is above the pump inlet
(to easy priming), use tap water, put the flow control valve at the outlet,
and let 'er rip.

Fred

 
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