Manual venting from a LH2 tanker in transit

Before loading it with liquid hydrogen, the cryogenic tank had been purged with nitrogen for maintenance work.
After the loading, the drivers failed to stabilise the internal pressure of the tank, which was still too warm. A release of 6000 cu ft (approximately 15 kg) of hydrogen was necessary to avoid over-pressure.

The INITIATING CAUSE was the risk of pressurisation which triggered a manual venting of hydrogen to reduce the internal pressure.

The ROOT CAUSE was that the usual procedure for the loading of the tank had been performed, without considering that the tank had been completely emptied of cold hydrogen and was at room temperature. An adaptation of the usual procedure should applied in these cases.

manual vent

manual venting

Zalosh et al (see references), concluded their analysis of hydrogen transportation accidents till 1978 with the following conclusions:
"Finally, an encouraging aspect of the transportation incident compilation is that 71% of the hydrogen releases did not lead to an ignition. ... The relatively few ignitions may be due to either to lack of ignition sources or to the rapid dispersal of hydrogen into the atmosphere. In any event, the accident data provide further incentive t o transport , transfer and store hydrogen outdoors away from occupied areas."