Explosion in a hydrogen gas holder

A series of explosions occurred at a renewable hydrogen installation facility part of a Research & Development project. It was caused by the formation of a flammable hydrogen-oxygen mixture in the hydrogen section of the water electrolyser unit. The hydrogen was produced by water electrolysis, was first sent to a buffer storage, then compressed and finally stored at high pressure. The oxygen, as byproduct, was vented to the atmosphere.

On the day of the event, it was snowing, and the temperature was -5 C. The oxygen vent line was frozen, and the oxygen flow found another path through the alkaline electrolyte circulation reservoir, into the hydrogen buffer tank, which experienced an increasing concentration of oxygen.
Though a compressor, the hydrogen-oxygen mixture reached the oxygen removing catalyst (catalyst burner), installed after the compressor. There, two ignitions occurred, triggering an alarm.
The operators stopped the whole facility manually, and this action stopped the hydrogen compressor as well. This it created the possibility of a hydrogen backflow from the catalyst to the buffer tank. At this point, a third ignition occurred, and the ignition reached the buffer tank, causing a large explosion.
The lid of buffer tank was projected towards the sealing of the facility building, destroying part of the sealing.

The ROOT CAUSE is not provided by the source. Nevertheless, assuming as correct the explanation of the sequence of explosions, two root cause components could be identified: (a) the stopping of the compressor should go together with presence of non-return valves which exclude the possibility of backflow also in the case of an emergency shutdown of the compressor. It is not clear if this safety measure was not available due to a design shortcoming, or due to an erroneous manual execution of the facility shutdown. (b) Possible contamination of the oxygen and hydrogen flows mut be checked by means of detectors or gas analysers, which have an important safety function. Therefore, the number and choice of detectors locations must be selected according to a careful risk assessment. In this installation, they were apparently installed on gas holder line, and not after the compressor.

vent, compressor

DESCRIPTION OF THE PROJECT
The hydrogen production facility belonged to an R&D project studying storage and utilization of variable renewable electricity using hydrogen-based technologies. It run from Feb 2014 to March 2019
Electricity: from wind, 2.2 MW
Electrolyser: alkaline, 135 kW
Buffer hydrogen gas holder: low pressure, <1 MPa, ceiling floating on water sealing.
Hydrogen carrier: LOHC (toluene - MCH)
Hydrogen application: heat (mixed with LPG at a boiler at local hot spa)

The damage was probably limited to the destruction of the gas holder and the ceiling of the building hosting it.