Explosion and fire in a metal hydride storage production plant

An explosion and fire occurred at around at a company specialised in hydrogen storage in metals. The accident occurred while a press used to compact magnesium hydride and natural graphite into pellets was being reassembled following maintenance.

The manufacturing of the pellets created dust inside the vacuum chamber. This dust built up on various areas of the press (bellows, front and rear of the press, valve, stripper, inside of the airlock and dispenser). To prevent excess powder build-up, the press was cleaned three times a day at the end of each shift. The explosion occurred after the press had been cleaned and when the employees had placed the pressing cavity under vacuum before the injection of argon.

Three employees were injured. Damage affected only the press and its surrounding.

The INITIATING cause was suspected to be the desorption of hydrogen from the magnesium hydride (MgH2).
The post-incident investigation postulated an explosion of the magnesium hydride and/or hydrogen dust inside the pelleting chamber containment. Hydrogen may have been desorbed at ambient temperature from the magnesium hydride powder. Moisture in the air can produce this phenomenon (according to MgH2 + H2O → 3H + MgOH). A failure of the vacuum system after the doors of the press were closed may have caused the powder mixture to become suspended and form a confined cloud that spontaneously ignited in contact with air. This theoretical assumption had however not been confirmed, so that certainty on the real accidental sequence and the substance involved remained guess-work.

The ROOT CAUSE was a lack of sound safety procedures, and the failing to learn from the experience of past accidents.

solid-state hydrogen storage , pellets press, mechanical workshop

Two incidents with serious consequences had already occurred in the past.
The first, in 2010, involved an experimental powder mixture that caught fire, burning the arm of a technician.
The second occurred in 2012, when the press was cleaned with a vacuum cleaner although this was prohibited.

Three employees were injured: one suffered slight burns and the two others has ruptured eardrums.

Property damage was primarily limited to objects immediately adjacent to the press. The front and rear doors of the press were blown off. The fasteners on the front door were ripped off and the rear door was blown off. The catwalk of the press was damaged. The manway of the proportioned used to mix powder in the crushing chamber was blown off when the filter housing of the vacuum pump exploded. The wall of the shop located behind the press as well as the lighting, valves and pipes were damaged. However, the structure of the building was left intact.

The 16 employees were temporarily laid off pending the identification of the cause and circumstances of the accident. The pellet production line was shut down for several months. Industrial safety bodies and companies specialised in explosive materials and hydrogen were contacted. The decision to build an extension to the shop was pushed back.

The employees extinguished the flames in the casing of the vacuum pump and the press airlock before the fire-fighters arrived.

This incident is an example of how plant operators fail to learn from past accidents and from near-misses. Two similar incident with severe consequences had already happened in the past.
After this last accident, the workshops were cleaned and rearranged (separate testing and storage areas, etc.) and employees training improved.

The operator also planned to reorganise production by:
1. adding more maintenance staff, setting up hydride teams (furnaces and pelleting) and assembly teams (cartridges and tanks), creating testing units (NDT, etc.) and adding a compliance and monitoring team to the HSE department.
2. overhauling the documentation and quality procedures.
3. creating a simpler and safer magnesium hydride pelletizing line (and ultimately automate it) and revise the production conditions for the various machines.