Manual venting from a LH2 tanker in transit

A pressure build-up in the tank of a liquid hyrogen tanker caused the drivers to vent 6000 cu ft of hydrogen to reduce the pressure.

[Zalosh and Short, 1978]

The INITIATING CAUSE was the over-pressurisation of the tank when returning empty of liquid hydrogen after delivery.

The ROOT CAUSE was probably a small shortcoming in the execution of the procedure for stabilisation of the tank pressure after delivery.

manual vent

The facility was shut down and sealed off the operational part of the plant. An alarm was sent to the plant and the local fire brigades, which extinguished the fire after approximately 2 and half hours.
TIMELINE
00:53 The shift crew on hand decided to shut down the system and turn off the cold box to thaw.
01:40 There was a dull double bang in quick succession. This was probably the heater rupturing and a subsequent deflagration with subsequent fire.
01:42 Report to control centre
01.50 Block valves around the cold box were closed.
03:30 The fire was extinguished.

This event could be considered almost a near miss. The safety procedure (manual venting) preventing the pressure to increase beyond the allowed value functioned as planned, avoiding uncontrolled hydrogen release.

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 to transport , transfer and store hydrogen outdoors away from occupied areas."