A black cloud was released on the polyethylene production line No. 1 at a basic plastic materials plant. The plant had resumed operations on March 31 following a ten-year inspection lasting 4 weeks, only to be shut down due to an incident after just 4 days of production. The unit restarted April 5th at around 2.20 pm. At 4.49 pm, overpressure on the unit’s medium-pressure separator led to the emergency shutdown and then the opening of a rupture disc. A cloud of gas and black particles located within the separator (roughly 310 kg) were released. Actuated simultaneously by the automatic sequence, the reactor’s safety valves opened in order to decompress it. The ethylene released by the reactor’s 2 stacks caught fire; the 2 torches formed were smothered by the water injection system and the incident was brought under control in less than 10 minutes by the operating crew on site. There was no property damage. Only a loud noise (caused by the opening of the rupture disc, then the ethylene catching fire) worried the local residents who alerted the external rescue services. The unit was shut down at 6.10 pm, decompressed and inerted with nitrogen. Several operations were then performed: a pressure-reducing valve was removed for inspection (a polymer of normal colour at the valve’s inlet and black at the outlet); the primary separator of the apparatus’ bottom trap was removed to purge the residual polymer; removal and replacement of the rupture disc of the medium-pressure separator. All the inspections conducted on the various elements (seals, safety devices…) showed no sign of anomaly. The findings relative to the pressure-reducing valve show that it had opened although it was slowed down slightly (~1 sec) due to the partial obstruction of the pneumatic exhaust by an insect next. As the temperature of the gas at the stacks’ outlets were less than the ethylene’s self-ignition temperature, it is probable that it was ignited by incandescent soot released through the chimney from the rupture disc or by the residual gases in the event of insufficient cooling. Several measures were taken: the valve was protected against insect intrusion, recording of valve parameters in the event of a replacement for at least one week, temperature recorded before and after the valve, and a valve repair log.