Release from LH2 hydrogen storage

Approximately 250 kg hydrogen gas was released from a vent stack following failure of a relief valve on a LH2 stationary hydrogen installation.
No ignition and no injuries.
The system had been recently installed. It was equipped with a number of pressure relief controls. This controls were designed to be able to handle in various ways hydrogen overpressure situations. At first, the main regulator was opening at a cylinder pressure exceeding 8.6 bar; it was closing again at 9.7 bar. Above this pressure, a relief valve was opening at 10.5 bar releasing the hydrogen via a feed pipe to the process. If this operation was failing to relieve the pressure, a second relief valve was opening at 12.5 bar and the hydrogen was vented to the atmosphere via a stack. Finally, a pressure relief valve (PRV) activating at 16.5 bar represented the ultimate system protection.
In the case of this incident, the second relief valve opened but did not re-closed correctly, probably because of the freezing of the valve seat. This caused an uncontrolled release of hydrogen to the atmosphere. The situation was quickly recognised and steps were taken to stop the release.
During the release, a 5 mm pipe connecting the second relief valve to the vent snapped due to the shock caused by the hydrogen pressure wave.

[NOTE of HIAD event validator: the HSE report does not metion it, but the event in HIAD assumed that this was a liquid hydrogen storage system. The indirect evidences for this assumption are the set values of he pressure relief devices, typicl of LH2 storage,, and the fact that the affected valve did no close because ice had formed].

INITIATING CAUSE
The uncontrolled hydrogen release to the atmosphere was caused by a relief valve failure to re-close, possibly due to freezing. The reason for its opening is not given. It is plausible to assume that its opening was caused by the need to relief pressure due to boil-off.

CONTRIBUTING CAUSE
The hydrogen released by this valve was supposed to be discharged to the environment via a vent duct. However, the connection between the valve and the vent failed.

ROOT CAUSE
The failure of the valve has been related to its possible freezing. This suggest an ill-design, related to the choice of a wrong component unable or to failing to consider all occurring environmental conditions during operation. Also the snapping of a pipe due to the shock wave of the release is due to ill-design.

[NOTE of HIAD event validator: the pressure control system is not well described by the sources, so that it is impossible to assess its design-soundness in term of safety.]

vent stack, pressure relief valve, pipe.

250 kg of hydrogen released to the atmosphere, but they did not ignite.
No injury, no damage.

The situation was quickly recognised and steps taken to stop the release. The reference does not specify which steps.

S
High and high/high pressure alarms were installed between the operating pressures of the two relief valves.
The originally straight 5mm pipe were replaced by pig-tails to better absorb shock loadings.