Fast opening valve

A fast opening valve having an opening movement driven by inlet pressure, comprising an inlet port, an outlet port, a valve passage linking said ports, a plug, a plug mounting which permits movement of the plug to and fro between a forward position thereof in which the plug closes the valve passage and a rearward position in which the plug lies clear of the valve passage on the outlet port side thereof, a wall member which is fixed in relation to the plug mounting, a chamber with the plg which defines in combination with the wall member a pressurizable compartment having an enclosed volume which increases as the plug moves from the rearward to the forward position, a discharge passage leading from the pressurizable compartment, a closure means operable to seal the discharge passage and thereby the pressurizable compartment, a pressurization means operable to introduce fluid into the pressurizable compartment under pressure and thereby to cause forward movement of the plug to its forward position in the absence of valve inlet pressure said closure means being opeably linked to movement of the plug to the extent that movement of the plug from the forward position opens the discharge passage to allow exit of said fluid from the pressurizable compartment, a receiving chamber at the outlet of the discharge passage rearward motion of the plug under the impetus of valve inlet pressure causing said fluid to exit from the pressurizable compartment through the discharge passage to the receiving chamber, and a choke means operable to restrict exit of fluid from the pressurizable compartment through the discharge passage as the plug nears the rearward position thereby to decelerate the plug.