Pressure increase in a LH2 tanker

A liquid hydrogen tanker was returning empty to its home terminal. When the drivers noted that the inner vessel was building pressure. The drivers advised their supervisor and were advised to find a safe location and to safely vent -off the pressure. The pressure was successfully reduced and the tank stabilised. The trailer could resume the travel.
The inspection found that the annular space vacuum had degenerated to 1000 microns, allowing an increased heat transfer which caused the pressure rise. A vacuum integrity check was completed and the vacuum restored before the next loading of the trailer.

The INITIAL was an increase of the pressure inside the LH2 trailer.
The ROOT CAUSE was a reduction of the thermal isolation capacity of the tank external shell.

vacuum thermal insulation

The LH2 trailer had a capacity of 3500 kg, but it was almost empty, containing only gaseous hydrogen

Only 1.0 kg went lost during the manual ' stabilisation' venting.

The safe systems and procedures worked as expected. The increase in pressure was 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.