Hydrogen release from a LH2 tanker

A liquid hydrogen tanker was en route on the highway when the driver noticed a vapour plume in the rear-view mirror. The driver exited the highway and stopped at the first available safe location. Upon inspection of the cargo, the driver noticed that the pressure in the inner vessel of the double-walled tank had risen above 3.45 barg (50 psig), causing the opening of the safety relief valve. Later, a thorough inspection revealed a leak in the annular space vacuum relief plate, leading to a loss of vacuum. That failure increased the heat transfer to the cryogenic hydrogen, consequently increasing vaporisation.

The INITIATING CAUSE was a loss of vacuum of he insulation shell of the liquid hydrogen tank. 

The safety measured worked as expected by releasing hydrogen at the set pressure through the  Pressure Relief Valve .

LH2 road tanker

This is an example of a correct mitigation of a mechanical failure. On a liquid hydrogen tank, the function of the Pressure Relief Valve (PRV) is to release hydrogen vapour, when the tank internal pressure increase due to the evaporation of liquid hydrogen. The reduction of the insulation capacity of the tank accelerated the evaporation rate and the PRD released more vapour. In this way, the internal pressure of the tank remained below the maximal allowed value.

Unfortunately, the source does not report the actions taken by the drivers before restarting the travel. In absence  of any action, at constant inward heat flow, the PRV will continue opening and closing till the LH2 tank empties. If the flow increases, it may happen that the PRV is not able anymore to release the pressure, which will increase up to the value at which the burst disc will open, accelerating the emptying.