This is a commercial pool, but it is small as far as commercial pools ago. Only 38k gallons.
We have 3 skimmers, one of them we attach the vacuum hose to and the manual pole vacuum. The other 2 skimmers, we put in plugs (2" pvc with caps) which we fill with water, turn upside down and hook into the skimmer holes.
After hooking all those up, we go to the pump area, then we close the main drain valve until we reach the desired vacuum pressure of 15 inches.
I don't know why, but sometimes I get good vacuum pressure and most of the time, I don't get much. When I first started working here, we used to get the pressure to 15 inches then we start vacuuming. Now I'm lucky to get 10 without the pump losing water... then we gotta open the main drain valve a bit until the water catches up and maintains.
I am thinking it's the main drain valve, which hasn't been replaced in about 10 years. The motor has been replaced a few times over the years, the sand filter is about 5 years old, I'm getting good circulation (around 130 GPM).
Also, what's the best tool to replace this main drain valve? It has 2 unions, but they're so large that our largest pipe wrench won't fit around it. I'm thinking it'd be too tight for a strap wrench.