Over pressurisation of a LH2 trailer

A LH2 trailer was traveling to delivery hydrogen to a customer, while the drivers noticed an increase in trailer pressure. 

To avoid the possibility of having the safety vent open while on the road, they pulled into a truck stop. There they performed a controlled, manual venting of the trailer in a remote section of the truck stop. 

The drivers then continued on to destination without further incident.

The INITIAL CAUSE was over-pressurisation of the LH2 tank, which caused an intervention of the drivers to reduce pressure. 

LH2 road tanker

The trailer was full loaded, on the road towards a customer

A negligible amount of hydrogen was released.

The emergency was quickly ended by the intervention of the drivers, who performed a manual pressure release operation.

From this and several other cases reported in PHMSA database, it appears that a corrective measure often adopted in case of a slight increase of LH2 internal tank pressure is to perform a manual, controlled release of the hydrogen vapour to reduce pressure. Although not stated in the reports, the value of the pressure at which the drivers take action is less than the pressure value at which the automatic safety pressure control system activate. Often the cause of the pressure increase is not stated, in other cases it is assumed having been a partial loss of the thermal insulation capacity (loss of vacuum in eh annular space between the internal and external shells of the cryogenic tank. Anyhow, the corrective measure appears to be successful to continue the delivery to the customer or to return to the base with an empty tanks, despite the fact that the cause of the pressure increase is not eliminated, and it is expected that it will occur again.