<a href=showthread.php?s=&postid=8502266#post8502266 target=_blank>Originally posted</a> by samtheman
"strip the gas out of the water"
"building too much pressure in the volute, due to too high discharge head pressure."
Neither of these describe cavitation. Cavitation is where the pump does not have enough flow into the intake. The pump puts the water into a low pressure area where some of it vaporizes, causing bubbles.
I hate to get off topic, but I don't think you really understand cavitation. There are two kinds of cavitation, suction and discharge.
In suction cavitation, the suction, or vacuum, is so high that it causes the pressure around the eye of the impeller, which is where the water is sucked into the pump, to be low enough to "boil" the water, letting the water change to a gas. This bubble can cause damage to the impeller, usually around the inside of the impeller vanes close to the eye. It also sounds like rocks being chewed up in the pump. You probably won't see any bubbles with suction cavitation because as soon as the water moves to the higher pressure of the discharge side, the bubbles reform into water. That's where the noise comes from. Suction cavitation is fixed by reducing your suction head, most commonly by increasing the diameter of the suction piping. Throttling the discharge valve will increase the discharge pressure, and slow the pump down, decreasing the amount of suction. But you sacrifice flow.
Discharge cavitation occurs when the discharge head pressure is so high that very little water leaves the volute, so the water just spins in it. The very tip of the impeller vane creates a low pressure area behind it when it spins, much like a submarine propeller. When the impeller tip crosses the cutwater, which is right near the discharge tube, the pressure drops enough to cause cavitation. This can be fixed by increasing the diameter of the discharge piping, or some other method of reducing the discharge head.
Remember, there are two different sides to a pump. Suction and discharge, vacuum and pressure.
If you don't understand something, just try a quick google and learn.
http://en.wikipedia.org/wiki/Cavitation
http://www.pumpworld.com/troubleS2.htm
http://www.mcnallyinstitute.com/01-html/1-3.html
And my bread and butter, sewage pumps.
The important part:
All these articles will say the same thing: when the pressure increases again, like when the water moves to a higher pressure area of the pump, the cavities or bubbles will implode or collapse. That's what causes the noise and damage. This doesn't mean there has to be "high" pressure, just high-
er.
Like the difference between the inside of your pump and your aquarium.
What I'm getting at is that the chances of seeing bubbles in your aquarium that are caused by cavitation are almost impossible. It's like trying to boil water in a pot on the stove at room temp. Not gonna happen.
And throttling the suction valve can cause cavitation, too. It's kinda like a venturi.. the restriction causes low and high pressure areas after and before the restriction, respectively. This cavitation can eat away the valve pretty quickly.
