Hydrogen cylinders explosion at a power plant

The explosion happened in two hydrogen tanks which were receiving and storing hydrogen produced by a water electrolyser via two water ring compressors. The hydrogen was then used as coolant of the power generator of the plant.
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 failure mechanism was a pipe full of water, located between the electrolysers, gas holder and water ring compressors. This pipe was 60 cm below ground level in a trench, and probably 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

WATER RING COMPRESSOR
A water ring compressor is a rotary positive displacement machine that uses a rotating ring of water, formed by an eccentric impeller, to trap and compress gas. As the impeller spins, centrifugal force pushes the water against the casing, forming a ring that creates individual compression chambers between the vanes. The eccentrically positioned impeller causes these chambers to increase in volume (sucking in gas) and then decrease in volume (compressing [as) during each revolution.
{from Wikipedia: https://en.wikipedia.org/wiki/Liquid-ring_pump]

ASSESSEMENT OF TANKS CONTENT
The tanks were 8.7 m high and had a 1.12 m diameter. The nominal pressure deliverd by the high-pressure compressor was 13.6 MPa., but the tank pressure before the explosion is reported to be 6.9 MPa.

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