confused on suction leaks

[cvaparo]

My Pentair VS pump keeps getting air bubbles(not enough to activate the auto prime).
I have the timer set to circulate two hours twice a day at 1500 rpm. This seems to be enough to keep the sides from getting slimy and keeps the water crystal clear and 92 degrees.
The bubbles seem to develop when the pump is off and stay there at 1500. If I raise it to 2300 the air disappears and stays that way until the next timed cycle.
I tried a new o-ring and tightening all the unions on suction. I cracked the vent on the filter and heater plate on the pressure side to check for air trapped in high spots. All joints are visible including the jets and skimmer.
Everything is above the water level in the pool, and I do here water dripping back in the heater plate, that is the highest point in the system, when the pump shuts off.
Could it be air trapped on the pressure side that creeps down the pump outlet? Any thoughts?

 

[Richard320]

You're not the first to report this. It seems common on variable speed pumps and is supposedly normal.

If you try the search function (the google one at the bottom might be better) using terms like bubbles variable speed you will get a bunch of hits.

 

[cvaparo]

Thank you Richard, that eases my mind somewhat, but its gotta be coming from somewhere. If I set the auto prime to its default which is the highest sensitivity, it will prime at every start up.
I reduced the sensitivity to prevent this from happening.

 

[duraleigh]

Thank you Richard, that eases my mind somewhat, but its gotta be coming from somewhere

I agree. It is a common problem with VS pumps and the only scenario I can think of is.....

When you lower the speed of a VS, you reduce the vacuum that is formed on the suction side of the pump.

This reduced vacuum then allows air to be pulled into the pump basket, probably from the large O-ring of the pump basket, because there is no longer sufficient vacuum to pull the O-ring down tight and make a perfect seal as it does on higher speeds.

 

[mas985]

This might an interesting read:

poll-for-users-of-variable-speed-pumps-please-read-t30272.html

 

[cvaparo]

I have just read posts of other people having the same types of problems. The next time I have to shut down to clean the filter, I am going to make a shim out of gasket paper or mylar paper to put between the threaded cover and the clear plastic cover that holds the o-ring. That seems easier than trying to find an 0-ring with a larger cross section. In case I go to a bearing house and find the perfect o-ring size, what material is best for the pool water? buna or epdm? I know Viton usually must be ordered and is expensive.

 

[mas985]

If it is an air leak, pool lube should be able to seal any pump lid unless the gasket has a lot of cracks in it.

But it may not be an air leak and could be just out gassing which there is really nothing you can do about.

 

[duraleigh]

mas985,

if outgassing is a source, does it only occur at low speeds?

 

[mas985]

Outgassing can occur at all speeds but at higher speeds, the flow rate is higher and there is more turbulence in the pump basket making easier for the pump to push the air out of the pump basket. This is also why it is easier to prime a pump on high speed than low speed, in addition to more suction lift.

 

[duraleigh]

It will occur at all speeds but at higher speeds, the flow rate is higher and there is more turbulence in the pump basket making easier for the pump to push the air out of the pump basket. This is also why it is easier to prime a pump on high speed than low speed, in addition to more suction lift.

That makes perfect sense...Thanks!

Does the out-gassing occur as a result of the reduced pressure in the basket?

 

[mas985]

Yes, it is the same principle as when you heat up a pan of water on the stove, you will see the outgassing bubbles form well before the water boils. Both higher temperatures and lower pressure can force dissolved air out of the water. Here is a more technical explanation:

http://www.engineeringtoolbox.com/air-s ... d_639.html

When the air pressure is lowered enough where it is below the vapor pressure of water, the water boils and we all know that as cavitation. But that can only occur at extremely low pressures > 28" hg.

 

[duraleigh]

Thanks, Mark. We all learn from you everyday. :lol: