advantageous to use body member 16 threaded into opening 12 in the composite wall of the vessel as shown and to add a sealing means 22 however it should be understood that the invention contemplates mounting plug member 18 directly into passage 20 wherein the passage itself is the opening in the pressure vessel thereby obviating the necessity of a separate body member with the above understanding plug member 18 is shown to be generally hollow or cylindrical as defined by outside walls 24 of a given thickness the upper end as viewed in the drawings of plug member 18 includes an outwardly projecting radial flange 26 for abutting against a shoulder 28 of body 16 as described hereinafter the lower end of the plug is dome shaped as at 30 and includes a rupturable section 32 of a reduced wall thickness as described hereinafter lastly the plug includes a plurality of holes 34 through outside walls 24 again for purposes described hereinafter as stated above body 16 is threadably mounted within opening 12 in vessel 14 as shown shoulder 28 surrounds passage 20 in the body for abutment by flange 26 of the plug an annular groove 36 is formed in the body surrounding passage 20 and plug 18 the groove communicates with an outlet 38 to atmosphere seal 22 can be a ring seal disposed between a shoulder 40 formed by a flange 42 of the body and an annular.