In a metallurgical plant, 2 explosions occurred at 9:30 p.m. followed by a fire in a melting furnace containing 5 t of titanium. The technician on duty was alerted by deviations in control parameters: fusion was stopped by the safety barriers 20 minutes before the explosion and the water level of a tank was lowered. The plant’s internal contingency plan was initiated, and the workshop was evacuated. A safety perimeter was established around the building, traffic was interrupted on the D1212 for 2.5 hours, and the gas supply was shut off. The electricity supply was maintained to ensure monitoring and control of the nearby furnaces. The fire brigade arrived and was able to extinguish the fire at around 11:15 p.m.

The property damage was extensive, and the furnace was destroyed. The workshop’s roof and cladding were damaged, a load-bearing wall was cracked, and windows were broken. The melting workshop was shut down for at least 3 to 4 weeks.

The accident analysis showed that the first explosion was linked to water entering the furnace through the cooling circuit of the “electrode holder” rod. This water vaporised upon contact with the metal, producing hydrogen. The resulting vaporisation caused a mechanical failure in the furnace and its cooling system — the massive influx of water in contact with the molten titanium in the crucible generated hydrogen. Upon contact with oxygen, it triggered a second, more massive explosion.

The initial influx of water came from a secondary priming on the rod that caused the perforation of the cooling system of this rod in 29 seconds. The cooling water then flowed into the furnace for 12 minutes (i.e. 300 litres) until the first explosion occurred. According to the operator, secondary priming would be due to a geometrical problem with the parts (rod, stub,tip), or an issue with the surface finish and/or assembly quality (“clamping).

The operator proposed the following corrective actions:

• control the arc voltage variation to halt the fusion as soon as a secondary priming was formed in order to protect the stub/rod assembly elements;
• shut off the rod’s cooling water in the event of an abnormal variation in water flow rate;
• improve the reception and inspection methods for parts associated with the stub/rod assembly, based on the establishment of compliance reports, parts tracking sheets and the reinforcement of clamping and tip changing procedures.

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