A heat treatment furnace was in the process of being restarted after its annual maintenance shutdown when a fire broke out at 1:06 p.m. on the salt quenching tank. A technician raised the alarm after seeing white smoke exiting the oven. The process gases were shut off, and the furnace was inerted with nitrogen. The fire alarm was triggered, and the personnel were evacuated. At 1:15 p.m., the fire was brought under control by site personnel using a powder type fire extinguisher. The fire brigade arrived at 1:20 p.m. and left at 2:00 p.m. after performing a few checks. The oven was shut down for two weeks and the operator disposed of 12 t of used quenching salts and 500 kg of metal materials. The cost of the repairs was 100 k€.
The quenching salt used consists of a mixture of 50% sodium nitrite and 50% potassium nitrate. Used for its thermal properties, it is recycled in situ with added salt. The quality of the bath is followed by semi-annual analyzes. The volume in the quench tank is 12 t.
Since 2015, the site has accepted to process very greasy parts that generate unburnt residues and which become attached to the muffle. The portion not eliminated during the soot burning process falls into the quenching tank and generates carbonates and insoluble materials. The absence of a methodology to trap these compounds and the lack of cleaning of the storage tank made it impossible for them to be eliminated. Furthermore, an inlet of air on the gasket between the muffle and the spray unit caused the atmosphere to deteriorate soot to form in the muffle. The insoluble elements present in the salt blocked the slits that generate the “curtain” of salt in the spray unit. Without a curtain of salt, the oven radiates at high temperature in the spray unit onto the film of pollutants on the surface of the bath in the quenching tank. The atmosphere of the muffle deteriorated due to rising vapours. This intense thermochemical reaction resulted in a clean cut of the spray unit and allowed a massive influx of air into the muffle, which activated the burning of the soot.
The analysis of the causes led to the implementation of the following actions:
- improvement of procedures to better manage how carbonates and insoluble materials are trapped and how the storage tank is cleaned;
- improved access to the storage bin to facilitate its cleaning;
- improvement of the procedure for burning soot in the furnace’s muffle;
- cooperation with the customers to eliminate or limit heat treatment on greasy parts;
- verification of the quality of the salt used with the supplier;
- improvement of the worksheets concerning the tightening checks of threaded fasteners on the spray unit and the condition of the salt when restarting equipment following maintenance.
The accident highlighted the effectiveness of the nitrogen inerting system in the furnace, which was quickly activated by the technicians and facilitated by the low volume (0.7 m³) of the furnaces’ muffle.