In a plant manufacturing PVC, the atmospheric release of a mix containing nitrogen, chlorine (estimated at 2 kg) and HCl occurred following dislocation of the coupling sleeve on the degassing pipe connecting the PVC workshop to the bleach column. This leak was limited to the workshop periphery. The alarm went off. The station manager checked the circuits and performed a purge at the bottom of the cyclone separator on the collector pipe. Liquid flowed out via the purge. Upon reclosing the valve, a noise was heard by the station manager, who acknowledged the sleeve dislocation and the release of vapours into the atmosphere. All plant valves were isolated, the various installations placed in safe operating mode, and the circuits reconfigured. An assessment of the degassing pipe led to observing the presence of liquid inside the pipe; this liquid might have been sulphuric acid stemming from the chlorine drying equipment. It seemed that excess filling of the sulphuric acid receiving tank, which had been used for the pumice dryer regeneration operations, had driven the liquid into the degassing collector pipe, thus forming a hydraulic guard and disturbing degassing operations. Since the collector pipe had not been depressurised, chlorine triggered the workshop safety system. In attempting to purge the cyclone separator, this hydraulic guard had been partially suppressed. Upon reclosing the purge valve, the hydraulic guard was suddenly driven towards the bleach column due to a lack of liquid height. Liquid intake in the bleach production tank had been accompanied by a sudden pressure surge causing the dislocation. This incident created no noteworthy consequences for either the personnel or the environment.