Damage to a rocket LH2 tank during pressurisation (with He)

[Note HIAD validator: this an incident where a hydrogen system was affected, as effect of the initiating event, overpressure of LH2 tank with helium, but it is not a LH2 incident].

The second stage liquid hydrogen tank of the Saturn V rocket ruptured while undergoing tank pressurization tests with helium. No H2 was on board.
The tank pressure was between 22.6 and 23.5 psig at time of rupture.
Extensive damage was done to the test stand and the stage was demolished above the lower liquid oxygen tank bulk­ head. Considerable fragmentation of the LH2 tankage and forward skirt took place.

[Ordin, NASA (1974)]

The INITIATING CAUSE was a abnormal stress-induced material failure.

The ROOT CAUSE was caused by a wrong repair/maintenance intervention.
The pressure at which the rupture occurred was well below the design allowed pressure of 38 psig. The failure was considered to be caused by the stress induced by the poor fit of the LH2 fill-and-drain line in conjunction with the pressure applied during the test. Before the incident, a 5-inch gap had existed between the fill-and-drain line and the stage mating attachment. Hydraulic jacks were used to close the gap. Such a preload condition could have created about 28 000 to 35 000 psi hoop stress in the tank wall.
Considering the 12 000 psi stress present from the 23 psig differential pressure in the tank, the total stress in the hoop direction would be from 40 000 to 47 000 psi, well above the design value.

LH2 rocket stage tank

Before the incident, a 5-inch gap had existed between the fill-and-drain line and the stage mating attachment. Hydraulic jacks were used to close the gap. Such a preload condition could have created about 28 000 to 35 000 psi hoop stress in the tank wall.

Extensive damage was done to the test stand and the stage was demolished above the lower liquid oxygen tank bulk­ head. Considerable fragmentation of the LH2 tankage and forward skirt took place.

For a summary of the operation of purging liquid hydrogen systems, and the preferred purging approaches, see H2TOOLS platform at https://h2tools.org/bestpractices/operating-procedures/purging#:~:text=Helium%20is%20used%20because%20at,another%20part%20of%20the%20system.