Liquid hydrogen release from a truck

The leak occurred on a 44 m³ vehicle of liquid hydrogen during a delivery to the unloading station of a steel plant. The truck drivers had connected the tank container to the fixed storage managed by a subcontractor, carried out a nitrogen sweep of the hose before cooling it, and were pressurizing the hydrogen before unloading when they saw a white cloud.

The drivers closed the bottom valve of the container and the upstream and downstream valves of the heater before evacuating the area and sounding the alarm.

Two technicians of the subcontractor company arrived 30 minutes later, located the leak on the flange of one of the four protective valves of the tank container. Two of the four fixing bolts were missing from the leaking flange.
The technicians closed the three ways valve which allowed to isolate the valves 2 by 2. They could then re-seale the flange
correctly and performe a leak test.

The INITIATING cause was the lack of tightness of a cryogenic tank flange, which started releasing when raising the transfer pressure.

The ROOT CAUSE was inadequate reassembling of some of the flange bolts during the previous maintenance, performed 17 days before.

liquid hydrogen cryogenic tank, truck, liquid hydrogen heater

The tank drivers had started the procedure for unloading and evaporating the liquid hydrogen.

DESCRIPTION OF THE PROCESS
This process consisted in:
1) connecting the tank container to the fixed storage
2) execute a nitrogen sweep of the hose and cooling it,
3) pressurizing the H2 before unloading.

The tank had a volume of 44 m3 corresponding to approximately 3200 kg of liquid hydrogen at full load.
The customer was a steel manufacturer.

F

The public rescue services established a 150 m safety perimeter and evacuate 50 employees. The road traffic was interrupted for less than one hour. After the end of the emergency status, the operation of unloading the hydrogen was restarted and successfully brought to an end.

The ARIA report (see references) does not mention any corrective action, and also the real cause of the lack of tightness of the leaking flange is not identified.

There could be three non-exclusive root causes:
(1) Rad vibration which could loosen the bold.
(2) a human cause, because the maintenance procedure or the checks before tank delivery were not followed.
(3) an inadequate procedure, which (for example) does not foresees even a leak test under pressure after the maintenance, and/or a confinement integrity check before starting the hydrogen transfer .

In the first case, personnel training is required, in the second, a review of the procedure is required, probably assessed by a new risk assessment.