A valve ( 61 ) for enabling release of pressurized steam from a pressure vessel has a displaceable closure member ( 67 ) which is maintained in its closed disposition by exposure to the pressure of the steam within the pressure vessel. The closure member ( 67 ) is displaced between its closed disposition and an open disposition by a double-acting actuator ( 69 ), suitably an air-driven piston/cylinder device, the closure member ( 67 ) being preferably mounted at one axial end of a spindle ( 68 ) extending between the closure member ( 67 ) and the actuator ( 69 ). The closure member ( 67 ) is suitably mounted for substantially metal-to-metal contact with a valve seat portion ( 63 ), without interposition of any sealing element, preferably for substantially vertical displacement between its closed and dispositions.
