Gate valve

A slab gate valve assembly comprising a housing comprising an inlet, an outlet, and a gated fluid passage between the inlet and the outlet, at least one valve seat disposed in a seat pocket of the housing, a gate-carrying shaft, and a slab gate plate arrangement in slidable engagement with the at least one valve seat and to be operated via the gate-carrying shaft between an open condition and a shut condition, wherein the at least one valve seat is threadedly disposed in the seat pocket, wherein the at least one valve seat comprises a seating surface and a radially extending peripheral seal integral with the at least one valve seat, the peripheral seal permitting splaying, and wherein the seat pocket comprises a radially extending abutment surface for the seating surface and a radially extending seal seat for the peripheral seal, axially spaced such that the seating surface comes into abutment with the abutment surface no sooner than the peripheral seal with the seal seat, whereby splaying of the peripheral seal is limited by abutment of the seating surface against the abutment surface and the degree of splaying of the peripheral seal is relative to the axial distance that the valve seat moves from where the peripheral seal abuts the seal seat until the abutment of the seating surface against the abutment surface.