Over-pressurisation of a LH2 tanker

An empty liquid Hydrogen tanker was returning to the base terminal from a delivery at a space centre. At the departure, the internal vessel pressure was 8 psig. At the first safety check, the pressure had raised to 20 psig. At the second safety check (after approximately 240 miles), the pressure had risen to 42 psig. At this moment, the drivers received instruction to move the vehicle to a safe location and perform a controlled venting to reduce the pressure to 30 psig. The pressure remained at an acceptable level for the remainder of the trip.
An inspection of the unit revealed that the rise in pressure was due to moisture being released into the annular space from internal getters. The trailer was put out of service and corrective maintenance was performed to remove all excess moisture from the annular space.

The INITIAL CAUSE was the unexpected increase of the internal pressure of the liquid hydrogen vessel after the start of the travel.
This event could be classified as a near miss, because the hydrogen released to the environment was part of mitigating measures foreseen by the procedure.
Nevertheless, it is classified as an incident because the ROOT CAUSE was identified in the loss of thermal isolation capacity of the vacuum annular layer, due to condensation of moisture present in the vacuum system itself (getters).

LH2 tank overpressure

The tanker was empty, having just delivered hydrogen to a customer site. Only gaseous hydrogen, no LH2 was left.

5.5 kg of hydrogen were released.

A manual hydrogen vent was executed to reduce (in jargon 'to stabilise") the vapour pressure inside the LH2 tank. The drivers could handle the emergency alone.

Presence of condensed phase in the annular vacuum space of a cryogenic tank is something that was not considered possible, and that has been found out by the practice. It is not known if and witch remedial measures were taken.

The trailer was put out of service and corrective maintenance was performed to remove all excess moisture from the annular space.