Hydrogen cylinders explosion at a power plant

[NOTE of HIAD Validator: the sources are not completely clear in describing the system involved in this incident ; the following is the most plausible option ]

The explosion happened in two hydrogen tanks ('receivers') which were receiving and storing hydrogen produced by a water electrolyser via two compressors. The hydrogen was then supplied to a further non-specified generator.
The plant had been shut down on August 24 to 26 and resumed the hydrogen production and storage on August 27. On August 28, after that the tanks had supplied hydrogen to the generator for 20 hours, the hydrogen purity in the generator dropped suddenly to 85%. The two tanks were disconnected for hydrogen purity sampling and the generator was further supplied with standard hydrogen bottles. The quality check indicated a purity of 95%. One of the two tanks was then re-connected to the electrolyser after approximately 7 hours.
Again, the purity in the generator indicated 85% hydrogen purity. 20 minutes later, both tanks exploded, resulting in two fatalities, 18 injured by fragments and extensive blast damage in a 100 m radius.
An investigation from the incident reported that all the gas supplied to the receiver over a 20-hours period had been air. The explosion has been assessed at TNT equivalent 275 kg.

The INITIATING cause was the erroneous and undetected supply of air to hydrogen tanks. The investigation report is confidential and not available, however one of the sources reported preliminary findings. The most probable mechanism was a pipe between the electrolysers, gas holder and water ring compressors, was full of water. This pipe was 60 cm below ground level in a trench, and probably because filled with water during the re-commissioning of the plant. The water lock, combined with at least two vents being left open at the inlet to the water ring compressor, allowed air to be sucked into this section by the compressor.

Although the exact ROOT CAUSES were not reported, a source mentioned cumbersome and counter-intuitive operative schemes and deactivated existing safeguards. Moreover, personnel had no knowledge of the hydrogen physical and combustion properties and there was a lack of HAZOP study.

hydrogen cylinders

Two fatalities; 18 injured by fragments. Extensive blast damage IN A RADIUS OF 100 m (Roof, storeroom and concrete blast walls destroyed).