Pressure increase in a LH2 tanker

A liquid hydrogen tanker was returning to the base after having deliver data customer site. The tank was empty. Nevertheless, on the road the drivers noticed a continuing elevated pressure. To avoid the possibility of having the safety vents open while traveling, they decided to pull off the road and perform a controlled, manual venting .
Not having a customer "safe haven" within the immediate area, and after having seen an pressure increase from 47 to 51 psi within 10 minutes, they decided to make a controlled venting alongside an isolated section of the highway.
The drivers followed their standard venting safety protocol, and after approximately 3 minutes of venting, the tanker pressure had dropped to an acceptable 13 psi. After having waited for approximately 60 minutes, noting no significant pressure increases, they restarted the travel.
3 hours later, they had to perform again the same procedure, reducing the pressure from 26 psi to 13 psi.

The INITIAL was an increase of the pressure inside the LH2 trailer.
The ROOT CAUSE was probably a reduction of the thermal isolation capacity of the tank external shell, but the PHMSA does not explicity state so.

over-pressure

The LH2 tanker had a capacity of 3890 kg, but it was almost empty, containing only gaseous hydrogen

Only 2.5 kg went lost during the manual ' stabilisation' venting.

The safe systems and procedures worked as expected. The increase in pressure was probably due to a decrease of the thermal isolation capacity of the tank external shell.
For these cases, in this type of LH2 trailer-tractor the drivers could continuously monitor the pressure from inside the cabin, and notified the head quarter, receiving instructions. They were trained in performing a procedure called in jargon a ‘ stabilisation’ of the tank pressure, consisting in a manual venting aiming at reducing the pressure well below the nominal value.