Explosion in a laser cutting plant

The incident occurred on a laser cutting system, and affected the top of a water storage tank. The water was used to cool the cutting part. The plant is mainly dedicated to steel cutting, but the specific system affected by the explosion was cutting aluminium. The process produces hydrogen which recirculate with the water.
On the day of the incident, workers decided to weld a new throughput in the tank, but as soon as the melting arc was switched on, the hydrogen ignited and exploded injuring three workers and propelling the small welding set through the roof of the unit.

The INITIATING cause was the accumulation of hydrogen accidentally produced from water when laser-cutting aluminium.
The existing national guidelines for laser cutters mention the possibility of hydrogen formation by cutting aluminium, but apparently this had not been taken in consideration in the case of the affected plant. This could be seen as lack of risk assessment or of effective instructions & procedures.

Water tank "bell" section, of a CNC plasma cutter work area.

3 injuries. The welding set was propelled through the roof of the unit.

It would appear that this was caused by hydrogen gas produced by the 'dross' from the cutting of aluminium becoming trapped in the bell and being ignited by the arc. Samples were taken of the dross, the liquor, dross and gas bubbling from the second tank for analysis. This showed hydrogen to be present in the gas bubbling from tank 1.

Thanks to an interlock, the triggering of the alarm sent automatically a all to the fire brigade and to lab responsible.
In absence of a fire, the intevention strategy consisted in leaving the hydrogen to disperse and to wait for the conrentrtion to go down below the safety threshold.

The health & safety guidelines in place for laser cutters mention the risk of hydrogen production when aluminium is cut. The incident shows the importance of considering the available literature when performing a risk assessment of industrial processes. It shows as well the importance of considering them when designing preventive and mitigating measures.
The fact that aluminium cutting was a very minor part of the plant activities may hint at the possibility that the specific knowledge regarding aluminium was not present in house.