Release from a hydrogen storage

Malfunctioning of the non-return valve of the hydrogen compressor has caused the pressure between the hydrogen bottle and the compressor to rise to the maximum allowed pressure of 275 barg. As a consequence, as foreseen by the safety system, the rupture disc of the safety valve broke and the hydrogen content of the gas bottle and the pipe section involved has been released through the vent above the roof of the building.
The released hydrogen ingited and a flame could be seen for some instants. Ignition could have been caused by the occurrence of the following events:
- Expansion of hydrogen at the end of the exhaust pipe,
- Consequent mixing of hydrogen and air up to a near-stoichiometry and increase of gas temperature
- Mixture ignition due to sparks from static electricity potential generated by gas molecules friction against suspended dust particles.

The INITIATING CAUSE was the internal leaking of the return valve of the compressor.
The fact that the valve, after dismantling and cleaning, functioned as designed, suggested a ROOT CAUSE in the lack of regular maintenance. The fact that a flame could develop at only 2.5 m above ground made the operator realise a shortcoming of the risk assessment and the related safety design.

non return valve; burst disc

DESCRIPTION OF THE FACILITY
The laboratory’s main components were:
(1) a compressed hydrogen storage: one standard 50 l cylinder at 20 MPa
(2) A diaphragm compressor with the role to maintain the hydrogen supply pressure to the laboratory always at the same pressure, by raising hydrogen pressure above 20 MPa and then delivering it to the lab via a pressure reducer.
Both components were in a technical building with limited personnel access. The safety relief device (a burst disk) on the connection between compressor and cylinder was venting above the roof of the building.

An Occupational Incident Report was issued as near-miss, with corrective actions (see Corrective Measures)

The event was noticed by a guard who was making the standard tour of the site early in the morning. It was still dark; therefore, the flame was visible. The guard reported the event to the site safety officer. No action was necessary, the flame quickly extinguished due to the limited amount of hydrogen in the storage.

This small incident could be also considered a near-miss. Despite the malfunctioning of one component, the safety measures installed were able to mitigate its consequence.

Nevertheless, the event acted as a ‘wake-up call’ and was the occasion for a review of the risk assessment of the whole laboratory set-up. On top of the direct corrective actions aiming at improving the preventing maintenance and the venting process, this review brought to the placement of the hydrogen cylinder outside the compressor building and the installation of a hydrogen safety sensor in compressor building, interlocked with an automatic shutdown and depressurisation. Eventually, the compressor itself was replaced with a small booster directly installed on cylinders manifold.

1) Dismantling, cleaning and testing of the non-return valve. Following positive testing, the system has been restarted and pressurised without any further faulty operation, thus restoring its normal safety function.
2) Extension of the hydrogen discharge pipe from the roof of the building to the roof of the next building. In this way a hydrogen ignition can only take place above approximately 6 meters from ground, farther away from personnel than the 2.5 meters in the previous situation.
3) The compressor has been sent for preventive maintenance and review to the producer, aiming at reducing the frequency of malfunctioning of components.
4) A periodic maintenance of the non-return valve was put in place and documented.
5) A flame arrestor was mounted at the end of the exhaust pipe on top of the building.