Explosion of several hydrogen bottles

Hydrogen gas cylinders exploded at the same time in three industrial locations, when the opening valve was opened. The overpressure generated by the explosion caused the walls of the hydrogen cylinder storage room to collapse, six workers were killed.
The hydrogen cylinders used by the three companies were filled by the same company.

The investigation found that a large amount of air was mixed with hydrogen in many of the delivered cylinders, the air accounting for 50% of the total volume. When the operator opened the cylinder valve, the friction between the air flow and the bottle valve generated static electricity, which directly detonated the cylinder.

The company delivering the cylinders fills more than 1,000 cylinders in the same batch. These 20 MPa compressed hydrogen cylinders are filled from a hydrogen storage tank using two buffers tanks and two hydrogen compressors in parallel. According to the scientific article quoted in the references, the night before the accident, the company has experienced a reduction of the power supply. Answering to this, the operator ordered to close both the two valves delivering hydrogen to the compressors, after confirming that the compressors had stopped working.
When the hydrogen compressor restarted, one of the valves was open, while the other was still close. This caused the development of vacuum in front of one of the hydrogen compressor, which continuously increased with time. After the pressure difference reached a certain level, the explosion-proof film in the buffer tank cracked and a large amount of air was sucked by the compressor and pumped into the hydrogen cylinders, mixed with hydrogen. A contributing cause was the rain of the night before, which dampened the explosion-proof membrane in the buffer tank.

The INITIATING CAUSE was the explosion of the tanks due to presence of a hydrogen-air mixture inside them.
Instead of hydrogen, the tanks had been filed and delivered by the gas producer containing the the wrong (explosive) composition.

The ROOT CAUSE is related in first instance to an operative mistake, related to an erroneous manual control of the valves controlling the gas flow. However, several factors enabled the occurrence of this event: a bad design, a lack of automatic process system, a lack of gas composition control during the process and before delivery. Additional environmental and technical causes contributed to it. Finally, it is evident that that specific accidental scenario was not considered in during the designing phase.

standard 20 MPa hydrogen bottles, refillable

DESCRIPTION OF THE FACILITY
The three companies where the incidents occurred were producing quartz rods. To reach the melting point of quartz, tungsten crucibles are used for melting. Since tungsten is easily oxidized at high temperatures, a nitrogen protective atmosphere is used, containing small amount of hydrogen as reducing gas.

All three companies were using the same contractorsfor the refilling ofthe hydrogen gas bottles.

The overpressure generated by the explosion caused the walls of the hydrogen cylinder storage room to collapse, causing injuries to many people (10 is an approximated value) .
The instantaneous pressure of the explosion should have been be several times higher than 39.65MPa, which is the maximum design pressure of the cylinder.

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The investigation findings provide the following recommendations:
(a) Strictly control the inspection before filling the hydrogen cylinder. The gas in the cylinder should randomly sample gas for detection to determine the type of gas.
(b) Companies should not combine two lines into one pipeline. An additional hydrogen filling line should be installed and each line should be filled separately.
(c) Management change is required
(d) Improve the maintenance and repair of equipment, to guarantee at every time the integrity of the equipment.
(e) Unqualified products shall not be used. Specialised personnel should be in charge of repairing and maintenance of the equipment.
(f) Operators should keep records of shifts and strictly implement the process rules.
(g) Units using pressure vessels should be equipped with reinforced concrete explosion-proof walls and lightweight ceilings. In this accident, the use of a reinforced concrete ceiling in some locations caused the development of such an overpressure that the explosion-proof wall collapsed and caused secondary injuries.