For concrete, vinyl and fiberglass pools, Hayward SP1056 1.5"/2" MPT Hydrostatic Relief Valve is available in a wide range of plumbing configurations, plaster collar options and colors. It offers protection for in-ground pools by safely releasing built-up groundwater pressure beneath the pool shell. Constructed from durable ABS plastic, this spring-loaded valve fits standard 1-½” and 2" male pipe thread (MPT) connections and installs directly into the main drain. This valve is a must-have safeguard against structural damage from hydrostatic lift and its self-cleaning O-ring seal reduces the risk of sticking or leaking.