Release from the storage of a hydrogen production plant

The accidental hydrogen release occurred when testing the storage system connected to a PEM electrolyser.
The leak test consisted in filling the storage with hydrogen at step-wise increase of pressure. After the step at 3 bar (0.3 MPa), which did not reveal any leak, the storage was pressurised up to 5 bar (0.5 MPa). At that moment a hissing sound occurred from the storage at the outside of the building indicating a leak. The production of hydrogen was stopped and all shut-off valves were closed automatically.

Since the leak took place on the 'storage side of the shut-off valves, the leaking continued. The operator decided then to let the storage bleed out. Afterwards, the location of the leak was found and isolated by closing of nearby manual valves.
The leak was located on a flanged connection near the storage. Bolts on this flange were found loose after the incident.
The exact cause of the leak could not be identified.
A possible explanation was that the torqued connection might have become loose when installing the vent line due to the manoeuvring of the pipe. The vibration of the leak could have further loosen the bolts.
The gasket was replaced, the bolts were retightened and torqued and the storage was leak tested up to 33 bar (3.3 MPa) without any other leaks.

Approximately 224Nm³ were released in 60 minutes.

The leak was most likely caused by the manoeuvring of the piping during the installation of the vent line. The case showed that the storage system did not have a depressurisation system by its own.

Flange, vent, CGH2 storage vessel

The event took place during the commissioning stage of the hydrogen installation. The storage was being
filled with hydrogen up to a pressure of 5 bar.

DESCRIPTION of the FACILITY
2.5 MW (1.25 MW stacks x2) electrolyser connected to a wind farm

No damage or injuries were reported due to this incident

The alarm was given by the hissing sounds. The production of hydrogen was then stopped and all shut-off valves were closed automatically. The report does not says if the production stop was the action of an operator, or was triggered by an automatic system. it is highly probably that the 'hissing sound' did not triggered any of the preventing/mitigating automatic system in place.

The safety system in place worked as planned. However, since the leak was located on the 'storage side' of the shut-off valves, the leaking continued till emptying of the storage system.
After that, as immediate corrective measure, the gasket was replaced, the bolts were retightened and torqued and the storage testing restarted.
As long term measure correcting the initial design, it was decided to add an automated depressurisation system to the storage, to release pressure in case of a malfunctioning or failure.