Leak from a cryogenic tank

A large hydrogen gas leak from a dewar resulted when bolts holding fitting the top of the tank was loosened. The fitting contained instruments. On loosening the third bolt, hydrogen gas excaped from the opening around seal. The Viton or neoprene 0-ring was blown out of its groove and was immediately frozen, making it impossible to reseal the fitting cover. The area was evacuated, dewar vented and gasket replaced. The ullage space was not purged with helium gas during the gasket replacement which may have been responsible for small leaks which developed during the transfer.

The INITIATING CAUSE was a release from a flange sealing.
The fitting which contained the instruments was mounted on a flange, which in turn, was secured to another flange. A set of long bolts and short bolts were used to retain the components. The bolt heads were identical and not labelled, therefore an error was made in loosening the wrong bolts. Leaks after gasket replacement probably due to condensation of moisture on related vent valve components.
The ROOT CAUSE was a human error facilitated by lack of clear guidelines/procedures.

cryogenic tank

H2TOOLS proposed improvement based on the need to clearly label bolts according to their function/role
Bolts could be identified by means of colour coding. Obviously, an explanation of the colour coding should be easily available at the working place.