A hydrogen fire broke out in the acid etching tank of the ferric chloride manufacturing facility of a Seveso-classified site. The internal emergency plan was implemented at 11:40 p.m. The fire began dying down at 11:53 p.m. At around 12:50 a.m., a slight leak was present but the fire was out. The ferric chloride unit was shut down. The internal emergency plan was lifted at around 12:45 a.m. The leak stopped at 1:25 a.m.
The tank contains a hydrochloric acid solution that is used to etch scrap metal. The reaction produces ferric chloride and hydrogen. The hydrogen is swept with air supplied by a fan large enough to maintain the tank’s headspace below the Lower Explosive Limit (LEL).
A few days after the accident, the operator opened the tank to conduct an analysis to determine the cause of the fire. The fire was started by slow combustion of the hydrogen, which began while the scrap metal was being loaded. Despite the air sweeping, hydrogen built up at the bottom of the tank due to overfilling. The fire then spread outside the tank when the tank roof opened due to damage to the hydrogen analyser and melting of the explosion vents. Plastic materials on the surrounding equipment and pipes and the oil in the cylinders used to open the etching tank’s roof fuelled the fire.
The operator updated the risk analysis for this section of the unit and increased checks of the level of scrap metal in the tank and the hydrochloric acid analyses. It added safety devices to the air sweeping system’s flow rate and the tank’s high temperature limit and lowered the hydrogen analyser’s safety threshold.
The unit resumed service six months after the accident.