At a chemical plant, during an exceptional operation to drain a gaseous chlorine collector pipe, nearly 21 kg of chlorine (Cl2) were discharged into the atmosphere for 90 s through a 30-m high chimney. No human or environmental impacts were recorded, as the forced ventilation system (32,000 N-m³/hour) generated a high dilution flow rate. This incident could be traced to absorption station that had become saturated due to improper valve handling. The production workshop where the discharge occurred was using gaseous Cl2 as a raw input. This particular operation, which consisted of draining a portion of the collector pipe containing gaseous Cl2 via a degassing circuit into an absorption station, was to be conducted every 3 years. Its protocol had been described in a clearly written and diagrammed document familiar to the technician (who photocopied it before starting the task). Failure to close the Cl2 intake valve prior to opening the intermediate circuit valves triggered degassing of the Cl2 distribution pipe leading to the gas washer and eventually saturated its absorption capacity, followed by chlorine release into the chimney until a technician detected the problem. Several corrective actions were introduced, namely: modifying the operating procedure to specify closing an additional manual valve; installing a calibrated orifice on the degassing circuit to limit flow rate into the absorption station; defining instrumentation to measure redox potential in order to evaluate the chlorination rate of washer soda solutions.