Bill Freeman wrote:

Paul and all,

The cavitation issue apparently needs some more basic explanation.
I'll give it a shot, since it seems like we need to understand the water
pump issue better than we (as a group) are doing.  I'll apologize in
advance to those that have already "gotten it".  Hope this helps those
that haven't.

Any liquid will work the same way but let's stick to water for this
discussion.  At 70F and 14.7 psi (normal sea level conditions) water
is a liquid, but evaporates into the air at a specific rate.  If we keep
the pressure constant and heat the water it begins to increase the
rate at which it vaporizes until at 212F (100C) it rapidly converts to
a gas - gaseous water (steam) - not air.

If we keep the temperature constant (70F) and reduce the pressure we
do a very similar thing, the water evaporates more rapidly with
lower pressure and eventually just flashes into vapor (steam).
So, we see that water can exist as vapor at room temp if the pressure
is low enough or at room pressure if the temperature is high enough.

What happens with a cavitating pump is that the pressure is
LOCALLY (not everywhere in the pump or on the impeller or in
the inlet pipe) reduced to a level low enough that the water MUST
(due to the laws of physics) turn into vapor (steam, gas - NOT AIR)
as a bubble in the liquid flow.  Since it is likely that this critically low
pressure only exists in a small area (LOCALLY, probably at the tips
of the impeller or near by) the bubble (of steam, or water vapor, NOT
AIR) is carried with the flow.  Inside a pump the pressure is increasing
(this is the definition of a pump) so as the bubble of water vapor moves, it
will move into an area of higher pressure.  At some point the water
vapor bubble will reach a location where the pressure is higher than
the vapor pressure at that temperature and will HAVE TO turn back into
liquid water.  Liquid water has MUCH less volume than water vapor
so the bubble collapses, usually very violently.  This violent collapse can
cause very high local pressures and velcities and can literally dig chunks
of steel out of an impeller.

I the mean time the pump has been pumping a certain fixed volume of
flow with some portion of the volume being water vapor, which suddenly
decreases in volume at some point further downstream in the pump system.
This causes a loss of flow in the pump as it is "choked" with these water
vapor bubbles which suddenly disappear,  false pumped volume -- cavitation.

Anything that causes the pressure at the low pressure (inlet) side of the pump
to drop too low can cause cavitation.  Increasing the pressure at the inlet
of the pump will help to prevent cavitation.

Hope this helps.  Please correct any errors, thanks.

Bill

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