Abstract: A tubular insert is adapted to be secured into an opening in the vessel and is formed with an internally threaded section adapted to receive a closure. The closure is a subassembly which includes a core hosuing having a threaded section threadedly engaging the insert and an internal threaded section and formed with a valve seat at its lower end. A core body is threadedly disposed in the housing and a valve element engages the seat with a compression spring inbetween the body and valve element. A stop is provided on the insert and retaining means are provided on the housing so that when the core housing is unscrewed, it is loosely retained in the insert indicating to everyone that the vessel is depressurized and the metal of the vessel can be heated and melted without any fear of explosion.