In a chemical plant, 18 kg of aluminium chloride (AlCl3) in gaseous form were discharged into the atmosphere for 3 min on a pipe undergoing maintenance (pipe replacement). The facility’s internal emergency plan and gas alert were both activated. Chlorination reactions on the affected circuit were stopped by shutting down the chlorine supply line. AlCl3 hydrolysis into hydrogen chloride (HCl) and alumina (Al2O3) caused the formation of a white opaque cloud that traversed 100 m on the site, crossing the facility’s internal railway line, yet without the HCl detectors of other workshops sounding the alarm. Five railroad employees working in the zone had to resort to their leak mask due to the foul smell; they were seen by the plant’s nurse before returning to their posts. The works foremen, equipped with gas masks as a preventive measure, continued their intervention despite the discharge. The incident occurred on the gaseous AlCl3 collection circuit upstream of condensation. This circuit was held in low pressure by fans that suctioned the product towards condensers; in all, 3 condensers were permanently hooked up to the circuit whereas the 4th was emptied of its solid product while remaining isolated. The switch between condensers was performed automatically every 20 to 25 min. This incident resulted from a combination of 3 events: opening of the collection circuit so as to proceed with partial pipe replacement, automatic condenser switching (order of fan opening on one condenser and closing order on another), and a deficient condenser opening valve that had remained locked in the closed position. These 3 events caused an insufficient pressure drop in the pipe and an AlCl3 discharge by the circuit opening into the atmosphere. The suspect valve was replaced and analysed.