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Re: Pump cavitation

Ok, I'm not Steve, but this is a topic I know something about.

Cavitation is what happens when the water pressure at the trailing edge of the 
impeller blade is too low to keep dissolved gasses in solution. This is 
dependent on water temperature, water pressure, impeller speed and geometry, and 
gas concentration.

If the pressure drops low enough, the water itself will actually vaporize, 
creating a pocket of cold 'steam'.  This is technically 'true' cavitation; gas 
coming out of solution doesn't really count, although it's essentially the same. 
True cavitation requires one HELL of a pump (like an OHIO-class submarine going 
full tilt).  I'd be a bit surprised if this happens in an aquarium pump, but who 
knows what you people are using! :^)

The noise you hear is from these pockets of gas imploding as they move out of 
the low pressure zone.  The implosion is also what damages your impeller, as it 
packs a lot of force in a very tiny space, leaving pock marks or worse on your 

There are only two ways that you can be getting bubbles out of your pump by 
restricting the intake.

1) A leak in your casing or hose connection (restrict intake => lower pressure 
inside pump => suck air through leak)

2) Very high gas concentration which prevents the released gas from 
re-dissolving fast enough.

In either of these cases, you will not hear cavitation noise.

Best - 

 - Anthony

> Steve<<<
> Gee, thanks for setting me straight on this Steve.  Maybe you could
> explain to me what's happening when I get a lot of bubbles in the return
> line of my Iwaki pumps when I restrict the flow at the input of the
> pump.  They're amazingly quiet while this is going on. You might also
> explain why the manufacturers of pumps warn against restricting the flow
> at the input of a pump because it causes cavitation which can damage the
> impeller.
> I was under the impression that the restricted flow caused the water to
> vaporize due to the low pressure created.
> Best regards,
> Don