US Patent 8136546 Pressure relief valve

A pressure relief valve for blown fiber closures is provided having constituent components fabricated from plastic. An adaptor is provided for connection to a blown fiber closure, and a body welded to this adaptor, is provided to enclose a piston and a spring within, such that they are protected from the ingress of silt and water. The piston is arranged to provide a double seal such that a pressure relief valve plug and an opening in the enclosing body are closed or opened simultaneously to prevent the ingress of silt and/or water on the one hand and the unhindered escape of pressurized fluid on the other.