Hydrostatic Valves 40mm by Pentair Pool 

For 40mm pipes but actual diameter = 55mm

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

Hydrostatic Valve are basicallly 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.

To remove the old valve your will require this tool the Hydrostatic Tool