In a chlorochemical plant, a chlorine leak occurred at 8:26 a.m. on the DN80 gasket of a flange on a chlorine line (Cl2, toxic gas) feeding a chlorination reactor. The fail-safe control system switched to safe mode and triggered the alarms; the leak was stopped at 8:35 a.m. A hundred or so kilos of chlorine were released into the atmosphere, but the high winds were able to disperse the cloud rapidly. The workshop was shut down long enough to analyse the accident and take the necessary corrective measures.

After analysis, the operator identified the causes of the leak. System purging operations were performed following a sudden shutdown of the workshop the day before. Despite these purges, an abnormally higher pressure was noted in the upstream circuits of the reactor during the restart of the workshop, particularly in the liquid chlorine evaporator. The cause was attributed to a rise in reagent via the manifold bypass of a differential pressure transmitter between chlorine and the other reagent. The reagent had accumulated in the evaporator of the chlorine system. It then reacted with the chlorine just before the first mixer. This mixture resulted in a wave of overpressure (4 bar), causing the weakest seal in the chlorine supply line to rupture. This joint was, however, of good quality (appropriate material and class).

The operator took the following corrective measures:

• the pressure differential transmitter between chlorine and reagent was removed: the pressure was measured by calculation using measurements from other sensors on the Cl2 line,
• leak tests on the Cl2 lines were added to the unit’s restart procedure,
• modification of the programming on the PLC controlling the safety chains in order to prevent the workshop from restarting in case of an abnormal pressure differential between the reagent and chlorine in the Cl2 line.