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Posted:  7/24/2008 9:56 AM #22379
Kevin MacDonald
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Total Posts:4
Last Post:7/28/2008
Member Since:7/24/2008
Subject: CAVITATION
I SEEM TO HAVE A PROBLEM IN MY PUMP. MY NEIGHBOUR (AN ENGINEER) SAYS IT'S SOMETHING CALLED CAVITATION. WE USED TO GET A HUGE "BACKWASH" FROM THE MAIN DRAIN AND SKIMMER WHEN THE PUMP WAS BEING SHUT OFF BUT THAT SEEMS TO HAVE STOPPED NOW THAT THE WATER LEVEL IS HIGHER. THERE ARE STILL BUBBLES IN THE PUMP. WE DON'T SEEM TO HAVE A LEAK IN THE PIPING BECAUSE WE'RE NOT LOSING WATER. NO ONE SEEMS TO KNOW WHAT'S GOING ON.



Posted:  7/24/2008 11:41 AM #22380
JohnT
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Total Posts:1533
Last Post:11/2/2009
Member Since:7/11/2005
You might have cavitation, but leaks on the suction side of the pump often don't leak water in significant amounts and are much more common. A loose pump basket lid, bad pump basket lid O-ring or a small plumbing crack can cause all of the problems you describe. Air is sucked into the pump while it runs, and accumulates in the filter where it is compressed by the water being pumped in. When the pump shuts off, the air expands and forces water out wherever it can go most easily.

Cavitation is less common, and is caused by inadequate flow into the pump. This can be caused by a sticky skimmer weir, oversized pump, undersized plumbing or excessive length or fittings on the inlet plumbing.




Posted:  7/25/2008 9:20 AM #22381
Kevin MacDonald
Member
Total Posts:4
Last Post:7/28/2008
Member Since:7/24/2008
Date: 7/24/2008 11:41:58 AM
Author: JohnT
You might have cavitation, but leaks on the suction side of the pump often don't leak water in significant amounts and are much more common. A loose pump basket lid, bad pump basket lid O-ring or a small plumbing crack can cause all of the problems you describe. Air is sucked into the pump while it runs, and accumulates in the filter where it is compressed by the water being pumped in. When the pump shuts off, the air expands and forces water out wherever it can go most easily.

Cavitation is less common, and is caused by inadequate flow into the pump. This can be caused by a sticky skimmer weir, oversized pump, undersized plumbing or excessive length or fittings on the inlet plumbing.



Thanks John. I checked the fitting of the lid this morning and it seems fine, and the o-ring looks brand new. I was afraid there might be a crack but not sure where to check. At the bottom (side) of my pump there's a small valve. It seems tight but could that be problem?




Posted:  7/25/2008 10:19 PM #22382
JohnT
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Total Posts:1533
Last Post:11/2/2009
Member Since:7/11/2005
Anyplace air can get in could be the source. I'd lube the O-ring with pool-lube just to be sure. Suction side leaks will often spit a little water when the pump shuts off. You can also locate them by covering areas with shaving cream. The air going in will create a small dimple in the foam.



Posted:  7/25/2008 10:26 PM #22383
SteveThePoolguy
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Total Posts:98
Last Post:9/14/2008
Member Since:6/26/2008
Cavitation is fairly rare. It is when the vacuum pressure is so strong that the water is actually converted into a vapor. Technically, the water “boils” at normal pool water temperature. This typically occurs just in front of the impeller. Once the water moves past the impeller, the higher pressure causes the water vapor to collapse. This continuous vaporization and implosion, or collapse, of the void causes a loud banging noise and a lot of vibration. Cavitation occurs when the pump is too strong for the supply. You may have some sort of blockage in the lines creating reduced water availability to the pump. When a pump is cavitating, it is usually straining very hard. You could do a vacuum test to determine if it is really cavitation or not.

Most likely it’s not cavitation, but rather a suction side air leak. That’s what you should investigate first. You can get a cup of water and pour it on a suspected air leak and watch for the problem to temporarily stop. Check all of your gaskets and drain plugs. Also check where the pipe goes into the front of the pump. Sometimes you can find an air leak just by turning off the pump and watching for water to squirt out.
Steve


Posted:  7/27/2008 8:37 AM #22384
Kevin MacDonald
Member
Total Posts:4
Last Post:7/28/2008
Member Since:7/24/2008
Here's the situation. We've had a lot of rain over the last few days/weeks. As a result my pool level has risen and after I've drained it a bit (I left the water level higher than usual - about 3/4 on an inch below top of skimmer plate) the pump seems to be working more normally. There are less bubbles within the pump and there is no "backwash'when the pump is shut off. I'm not sure if the overall problem is/was low water levels but the pump seems to be working as it should - the higher the level the less bubbles in the pump lid window. Before I drained the pool to it's present level (it was above the skimmer because of the rain) there were virtually no bubbles in the pump window.



Posted:  7/28/2008 7:28 AM #22385
JohnT
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Total Posts:1533
Last Post:11/2/2009
Member Since:7/11/2005
If water level makes a difference, I'd be looking at the skimmer weir. If it sticks in the up or closed position, even for short periods of time, you'll get air in the line.



Posted:  7/28/2008 1:59 PM #22386
Kevin MacDonald
Member
Total Posts:4
Last Post:7/28/2008
Member Since:7/24/2008
I'll keep an eye on that to see if it's the cause. Any other possibilities related to water level?



Posted:  7/28/2008 2:55 PM #22387
JohnT
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Total Posts:1533
Last Post:11/2/2009
Member Since:7/11/2005
Date: 7/28/2008 1:59:02 PM
Author: Kevin MacDonald
I'll keep an eye on that to see if it's the cause. Any other possibilities related to water level?


If your pump is well above the water level, more water in the pool would help, but I doubt just a few inches would make much difference.




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