Pool Hydrostatic Relief Valve - 50mm by Pentair Pool

For 50mm pipes but actual diameter = 70mm

Hydrostatic valves normal life is 5 to 6 years, after this you will find that pool chemicals have hardened or cracked the valve seal and "O" ring, the valve may seize and jam open due to calcium growth or spring failure.  

Your hydro-static valve plays a very important role in stabilising any imbalance between the water pressure underneath and inside your swimming pool.

Hydrostatic Valve are basically a safety valve for your pool to release excess water from underneath your pool.  

When water pressure builds up under the pool due to water run-off during a storm for example, the valve opens releasing the water harmlessly into your pool.

If this valve fails the walls can deform and split, floors can hump upward and crack open, or the entire pool could pop out of the ground.