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.