At around 2:30 a.m., chlorine gas began leaking from the chlorine cabinet of a wire-drawing machine in the casting workshop of a metallurgical plant. The chlorine gas is mixed with argon in the wire-drawing machine’s treatment bag to purify molten aluminium oxide. The site’s chlorine gas detection system sounded. The 23 employees present were evacuated. The internal emergency plan was implemented at 3:00 a.m. and lifted at 4:25 a.m. The fedding of chlorine gas was cut off and then purged. Two technicians who went through the cloud of chlorine to fetch their masks near the cabinet were sickened. They were taken to hospital. The site was not shut down.

The sensor of the arm of the bag’s rotor, which initiates chlorine/argon treatment when the arm is correctly positioned in the bag, was either incorrectly positioned or not working. It had been alternating between ‘detection’ and ‘non-detection’ every second since 3:00 p.m., causing the motorised valve to continuously open and close that ultimately resulted in the leak. The failure of the sensor to detect the arm was registered as an event in the supervision software but was not directly reported as a failure or alarm on the control screen. The event had to be searched in the software to see what exactly it was. The chlorine flow rate was not maintained, but this was not visible on the supervision system because the curve was smoothed. The successive opening and closing of the chlorine feed valve caused it to loosen. It ultimately broke apart and started leaking. The cabinet containing the valve was not airtight. The machine’s beacon was not working due to maintenance that had been performed two days prior. No sign indicating that the beacon was no longer working had been posted.

The operator replaced the motorised valve and adjusted the rotor sensor. Shift managers are now instructed to monitor for rotor sensor events on the supervision system. An action plan has been put in place to see alarm messages on the supervision system in the event of failure of the motorised valve, to improve preventive inspections of the sensor, seal the chlorine cabinets, and improve chlorine training for the site’s technicians. Two other fires related to wire-drawing machines occurred at this site the same year (see ARIA 50301 and ARIA 49718).