Safety venting from a LH2 tanker

A full liquid hydrogen tanker left the home terminal with a internal pressure of 6 psig. Throughout the trip, drivers experienced nothing other than normal conditions. 12 hours later, at a scale, the tank pressure reading was 11 psig. When the driver checked the unit again approximately one hour later, the pressure had climbed at 28 psig causing hydrogen to be vented.
They notified the home terminal and were advised to proceed to a nearby plant of the company to stabilize the pressure in the tank. They accomplished this and proceeded on to the delivery site with no further problems.
On subsequent trips, this tanker did not experience no abnormal pressure conditions. Therefore, the abnormal pressure rise could only be attributed to drivers' failure to properly stabilise the load before departing.

The INITIATING cause of the hydrogen venting was an increase of the tank internal pressure.

The ROOT CAUSE was attributed to a shortcoming in following the stabilisation procedure of the liquid load before starting the journey.

Vent

PHMSA reports a loss of hydrogen comparable to the total quantity transported, not coherent with the narrative which speaks only of a stabilisation of the pressure. This could hint at an empty tank after delivery, but this journey started at the company plant for hydrogen production and liquefaction, so it is to be assumed that it was full load when it started the journey.