Fire-safe valve structure

In an expanding gate valve having a valve body with a flow passage therethrough, a valve chamber in the body communicating with the flow passage, and an expanding gate assembly movably mounted in the valve chamber for opening and closing the flow passage, and wherein the valve chamber becomes sealed off from the flow passage in the open and closed conditions of the valve, means defining an annular recess in said valve body circumferentially surrounding the flow passage on the upstream side of said valve element and opening between said valve chamber and said flow passage, said recess having a rear wall extending from the flow passage generally radially thereof and a circumferential cylindrical wall extending substantially coaxially to the flow passage from the valve chamber to said rear wall, and sealing means adapted to sealingly engage the gate assembly in its opened and closed conditions whereby said valve chamber becomes sealed off from said flow passage;

• an improved valve structure comprising:
• an annular metallic seat disposed in said annular recess, said annular seat having an inner annular cylindrical surface substantially coaxial to and facing said flow passage and an outer peripheral annular cylindrical surface substantially coaxial to said inner annular cylindrical surface and facing said recess circumferential wall, said annular seat having a port therein extending in a generally radial manner from the flow passage between said inner annular cylindrical surface and said outer peripheral annular cylindrical surface such that the outer end of the port is exposed to the interior of the valve chamber and the inner end of the port is exposed to said flow passage;
• and
• a relief plug of a selected temperature degradable material positioned in a plugging relationship within said port, said relief plug being exposed to both the fluid environment within said valve chamber on its outer end and said flow passage on its inner end whereby the valve chamber is sealed off from the flow passage during the open and closed conditions of the gate valve and said relief plug is caused to evacuate the port into the upstream flow passage upon the fluid environment in the valve chamber reaching a preselected temperature condition or when a preselected pressure differential exists between the valve chamber and flow passage.