Cavitation

[cliffcc]

Since replacing the 5hp pump with a 3hp pentair challenger, I have been having problems with what was initially thought to be an air leak. Constant air bubbles being released into the pool, driving the ph up. Replacing the shut off valves and adding a manifold where the skimmer line and main drain come together at pump to lower the vac pressure side did not help, but hurt the situation. Upon opening the pool this year I noticed upon closing the main outlet valve a little bit to bring up pressure in the filtration system to check for leaks, the pump immediately went quiet and the bubbles stopped at about 8lbs of pressure. I assume that if you can have cavitation from having too much vac pressure, you could have it on having too little head pressure. This seems to be the only answer I can come up with. The vac side reads about 7lbs, the pressure side however when the filters are clean reads nothing until they get at least 50% blocked. The filtration system is an old harmsco 900 (150 filters to fill the drum). I have been keeping about 8lbs of pressure and it seems to have temporary fixed the problem. Would putting back in a 5hp help? Is there one that does not require head pressure to not cavitate? Any help would be great.
Cliff

[Denniswiseman]

Hi Cliff
A 5hp pump and a 3hp pump are massive as is the filter (500gpm capacity) do you have a very large pool or a lot of head
If you didn't have a problem with air before downgrading your 5hp to a 3hp then you have a suction side air leak. Check your pump basket "O" ring is lubricated with silicon grease and your pump inlet connection
Reducing the flow at the outlet side of the filter isn't a good thing as it puts extra pressure on all your equipment and you are just reducing the water flow
What is your water level at the skimmer, it should be 1/2 to 2/3rds up the slimmer opening