The Hydrostatic valve is usually on the lowest part of the pools floor. The hydrostatic valve is there to protect the pool from being lifted by ground water in the event that it is empty. it allows the ground water under the pool to get into the pool thereby stopping it from floating out of the ground on top of the ground water.
A ball valve.
Perhaps it is not a drain but a hydrostatic valve. These are fitted at the bottoms of pools to let ground water that has built up under an empty pool into the pool in order to stop it from floating out of the ground.
I'm not sure if it can be installed after a pool has been installed, try contacting a company who makes these hydrostatic valves and see if it can be done. good luck.
A Hydrostatic valve is basically a plug that stop water coming down out of the pool, but when the water pressure under the pool gets higher then the pressure in the pool it will let water into the pool from underneath in order to stop the pool from floating in ground water.
exspanion tank or relive valve
No, not usually.
For as long as you like just make sure that the pool has a hydrostatic valve or some other means to stop it from floating out of the ground if it starts raining or the water table underneath it rises. In gunite and concrete pools, the hydrostatic relief valve is located in the bottom drain. Sump or ShopVac the remaining water covering the drain, take out the screws, and remove the cover. Again, using a ShopVac, remove the water just beneath the cover and look inside the drain. There are two possibilities: 1) a relief valve that can be unscrewed and removed, or 2) a second pipe that doesn't point toward your pump and filter. Inside this pipe is a hydrostatic relief valve that probably won't open because of sand or dirt clogging it. Using the handle of a short screwdriver, tap the valve inside the pipe (you may not see it, but it is there) several times until water comes into the pool. There may also be hydrostatic relief valves near the bottom of the steep slope that can be removed. Good luck. -Bill
A "relief valve" can only be accessed when the pool is empty, and no swimming pool should ever be left empty. The only exception is when absolutely necessary due to repair or resurfacing, and that should be completed within the shortest possible time. Any pool that is empty, and that has a relief valve, should have the valve removed for the entire time, regardless of the weather forecast. A "hydrostatic" relief valve opens and closes when needed as it attempts to equalize the pool water pressure with the ground water pressure. Human intervention is not involved because the pool is still filled. Hydrostatic valves are great in theory, but seldom work after a few years.
the hydrostatic valve is only to equalize pressure of ground water to that of the pool the only time it does anything is when the water table becomes higher in the ground water than in the swimming pool . it doesn't connect to anything. it is simply to keep the pool shell from floating out of the ground