Over-pressurisation of a LH2 tanker

A liquid hydrogen tanker was travelling back to the base after having made an earlier delivery. At the end of the delivery, the pressure in the tanker had been reduced to 75 psig using the vent stack installed at the customer unit.
During a routine stop to conduct a check of the tires and of the tank pressure, the drivers noted that the pressure had risen to 138 psig (9.5 barg), in excess of the anticipated pressure rise for a hydrogen tanker. To prevent any unexpected venting of hydrogen gas, they exited the highway and stopped at an abandoned gas station. There the drivers ("blew down") reduced manually the tank pressure to 75 psig (5.2 barg) and restarted the travel. When the tanker had travelled for approximately 6 hr, the pressure had climbed back to 120 psig (8.3 barg). It is not mentioned in the report, but it is probable that the drivers decided to repeat the blow down procedure before arriving at the home terminal (probably at additional 3 hr driving).
The trailer was removed from service because of the deterioration of the vacuum of the thermal insulation shell.

The INITIAL CAUSE was over-pressurisation of the LH2 tank, which caused an intevention of the drivers to reduce pressure.
The ROOT CAUSE was the degradation of the thermal insulation capacity of the tank.

vacuum thermal insulation

The tanker was almost empty, after delivery.

A negligible amount of hydrogen was released.