Venting from a LH2 storage vessel.

During venting from a from a 13,000-gallon liquid hydrogen storage vessel (approximately 50 m3), the vent line was damaged by unusually high winds.
The vent line toppled due to the breaking of the mounting brackets, releasing cold hydrogen near the ground. It did not shear or tear but experienced a bending that restricted the hydrogen flow and created a back pressure on the vessel relief system.

Repair efforts were hampered by the possibility of cold hydrogen gas releases. It was not possible to shut off or redirect the venting, and variable breezes made set up of safe zones uncertain.

The INITIATING CAUSE was the failure of the vent line mounting brackets. They were made of polymeric materials which had aged and disintegrated under the effect of abnormally strong wind. On top of the ageing, they were also all oriented with a degree of freedom in the same direction, so that drag forces from strong wind coming from just the right direction were able to dislodge the vent line and blow it down.
Despite periodic inspections and maintenance operations, the degradation of the bracket was not detected.
The ROOT CAUSES were a combination of inadequate design (wrong material, wrong installation design) and inspection shortcoming. An additional factor was that
plant operation did not have an emergency procedure covering this accidental scenario.

vent,mounting brackets

The source does not report the quantity of hydrogen contained in the vessel during the event. From the vessel volume a max capacity of approximately 3500 kg can be calculated.

Three lessons could be drawn:
(1) Hardware design must be adequate for weather conditions and materials selection must be compatible with temperature excursions and solar-UV exposure conditions. In this case, polymeric braces were used, which were not suitable for long-term exposure to sunlight and temperature extremes.

(2) Periodic inspection of mounting hardware should be in place and performed in a way to be able to monitor incremental damage.
(3) Periodic risk assessment review should catch all possible accidental scenarios, to enable (a) procedures covering the emergency created by these scenarios and/or (b) improving the original design. In this event, (a) the plant did not know what to do in case of hydrogen leaks hindering personnel to perform repairs, and (b) the facility did not have the possibility to zero the potential of hydrogen vent through the damaged vent line, for example, by switching to a second vent line.

An additional hazardous condition was created by the creation of a back-pressure of the pressure relief device, due to the vent line bending. Back-pressure has the effect to increase the value at which the relief device starts releasing hydrogen, reducing the safety design margin.

H2TOOLS reported that the plant developed a procedure by:
• Determining the approximate temperature and release rate of the hydrogen leaving the from the damaged vent,
• Finding computed hydrogen dispersion information based on diffusion and wind
• Using the dispersion information to establish a safe working area for repair operations and an exclusion zone around the hydrogen release point.