A cap for closing an opening of a receptacle such as for a liquefied natural gas container. The cap includes a main body and a sealing surface for sealing the opening of the receptacle. A lock/release mechanism is provided to lock the cap onto the receptacle, or enable the cap to be removed from the receptacle. A relief valve of the cap is activatable to relieve pressure within the receptacle. A grip overlies the main body and a relief valve actuator, enabling a user to activate the relief valve via the grip. The cap is configured to relieve pressure in a first stage of removal, and then unlock from the receptacle in a second stage of removal. A catch and secondary pressure relief features are provided to control removal of the cap. Magnets may be provided for proximity sensors to indicate proper closure of the cap on the receptacle.
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