Explosion of a storage vessel in a hydrogen production facility

This incident occurred at the hydrogen production and storage unit of (probably) a power plant using hydrogen as a coolant of the turbine-generator system. Hydrogen was produced probably by water electrolysis (the source does not say it) and stored in 6 steel tanks.
One of the six hydrogen storage tanks exploded killing a staff member. The investigation determined that a mixing of hydrogen and oxygen occurred in the tank. The needle valve of the hydrogen and oxygen pressure regulator jammed, and the oxygen leaked into the hydrogen main supply line and entered the hydrogen tank.
The explosion was evaluated to be equivalent to a TNT explosion in the range of 64-125 kg of TNT. The tank which exploded was pulled off from its position ripped open and completely destroyed, two other vessels fell due to the pressure wave caused by the explosion, and the other 3 were damaged in different degrees.

The INITIATING CAUSE was a defective valve which jammed and allowed oxygen to access the hydrogen gas stream.
The cause of the ignition was very possibly a spark generated by the friction of the high-speed gas flow, which can reach 100m/s when opening the valve of the vessel, driven by a pressure difference of about 392kPa.
Since the needle valve had already jammed twice before, one component of the ROOT CAUSE could be related to lack of timely repair process. Also, the required diagnostic system was not in place. Apparently, there was only a hydrogen purity measurement at the generator, and not on the line to the tanks. This system, foreseen by the "Chemical Operation Management System", would avoid escalation.

hydrogen tank, hydrogen supply line, pressure regulator

Operation practice proved that the needle valve with the function and structure adopted in this installation is very prone to jamming. The investigation revealed that it had already jammed twice before, on August 26, and again on August 31.

The investigation mentions also that a 'repair' had occurred before the incident, but it does not further explain.

DESCRIPTION OF THE COMPONENT
The stainless steel vessels had a volume of 10m3, (inner diameter: 1500mm, height 6400mm, wall thickness 12mm), Design pressure= 981kPa (9.8 bar), working pressure is 588kPa (5.9 bar). At the moment of the incident, the tanks were not completely filled: the exploded tank No. 1 tank was at 490kPa (4.9 bar).

The investigation formulated the following recommendations:
1. When rebuilding the hydrogen production unit, a new type of pressure regulator shall be used, with a liquid level limit interlock protection device.
2. A hydrogen purity meter or an oxygen meter shall be installed on the hydrogen side to continuously monitor the hydrogen purity.
3. The management of equipment (monitoring, defect repair, replacement) shall be improved/strengthen. An immediately stop the operations shall be triggered to eliminate defects affecting hydrogen purity.
4. The equipment belonging to the hydrogen production unit shall be maintained once a year and overhauled every three years.
5. According to the requirements of "Fire Inspection Manual" and "Code for Design of Power Installations in Explosion and Fire Hazardous Environments", anti-static measures shall be in place.
6. Technical training and awareness shall be strengthen.