Explosion in an electrolyser plant

[Note of HIAD event validator: The event description relates to a real accident, the details of which however are not publicly available. The descriptors are based on the analysis provided by the references source]
A small explosion occurred in the water-deionisation system of an electrolyser, specifically in the buffer tank receiving three water stream: (i) the make-up water from the demineraliser, (ii) the water coming from the cathode, and (iii) the water from the anode. The two latter water streams are required be the need to minimise water consumption and therefore to recycle the electrolyser water outputs. After the respective water treatments, these two work streams contain still respectively hydrogen and oxygen in solution.
A tentative cause of the explosion was the fact that various water streams carry into the buffer tank air, hydrogen and oxygen in solution, which can mix and generate an explosive mixture. Simulations show that an explosive mixture can be formed as early as 8 minutes from start-up. However, for an explosion an ignition source would be required, which seems not present in metal tanks which are always fully humidified and act as a virtual Faraday cages.

The INITIATING CAUSE was the production of an explosive air-oxygen-hydrogen mixture.
The ROOT CAUSE is related to an error in design

buffer tank, demineralised water system

With increasing electrolysers capacities, new hazards may emerging which were not considered or assessed as not relevant before, when dealing with smaller electrolysers . In this particular case, the increased water flows and the necessity to recycle the process water streams could bring into the water buffer tank gases in quantities enough to detonate.
It remains so far unanswered the question on the ignition: it does not seem to be present in metal tanks which are always fully humidified and act as a virtual Faraday cages. It is nevertheless important to be aware of the hazard, to assess the related risks and to develop preventing measures.

The electrolysers company put the electrolysers offline and retrofitted with a system able to prevent the occurrence. Nothing is known on the details, it was probably related with a water streams treatment step able to reduce/eliminate gases in solution.