Explosion at an electrolyser at a refuelling station

This incident occurred at the electrolyser of a refuelling station. An employee of the electrolysers manufacturer was on-site to conduct a reset of the plant power breaker system and to restart gas generation. Whilst putting the system into generation, a loud ‘bang’ was heard and a cloud of water vapour was seen being emitted from the oxygen vent line of the electrolyser container.

The gas generation system was safely shut down, and an investigation was started to understand the cause and potential impacts.

The on-site investigation identified damage to the electrolyser stack, while no visible damage could be noted on other equipment, the water purification system or the oxygen exhaust pipe. The electrolyser stack showed tears on catalyst-coated membranes from cell 67 and cell 40. Moreover, the foil between cell 1 and the lower dielectric end-plate was torn The cause of this damage is unknown, it is nevertheless plausible to assume that this foil initially cracked, causing a leak and an increase in the pressure drop. However, this drop was not big enough to be detected during the pressure drop test. The stack could still generate at pressure as the rate of generation was greater than the leak rate. Therefore, the plant did not automatically alarm at the appropriate time to stop all operations.

Was the cloud noted at the vent line water vapour or smoke? From the absence of signs of a flame, heat damage to pipework and smoke residue, it was concluded that the cloud was water vapour. The absence of damage to the Balance of Plant suggests that no explosion happened, and that the loud bang heard was due to a resonance phenomenon in the large diameter vessel and its non-silenced, open-ended vent.
[Note of HIAD validator: the inspection and its conclusions were carried out by company-internal experts, without independent assessors]

According to the investigation, the INITIATING cause of the bang was probably a recombination of hydrogen and oxygen in the oxygen exhaust pipe. The report calls it a recombination, because no heat damage could be detected. But in general, the rapid recombination of oxygen and hydrogen occurring with a bang, is called an explosion. The present of hydrogen and hydrogen out of the stack in was caused by a damage of the sealing in the electrolyser's stack.
The alarm on high pressure-drop did not send an alarm becasue its alarm was set at avalue higher than the pressure-drop experienced during the incident, The report states also that "set-points designed to detect this type of failure mode where shown to have been adjusted from those initially set." This could suggest some operative mishaps.

stack

The electrolyser was being restarted after maintenance.

No consequences, no damage.

The electrolyser manufacturer have set up a dedicated team to review and report on the stacks optimal operating conditions to avoid stack damage leading to cross over situations. It also reviewed all active plants in respect to the management of the alarms, their set-points, escalations routes and improved data analysis function within the control room. An important improvement has been an earlier escalations of the alarms to the technology experts.