In a plastics factory, a rupture disc opened on the medium pressure return (MPR) of line 42 in the polyethylene (PE) workshop. The products released (1.4 tonnes) ignited at the grease bottle outlet 20 m high, generating a fire that lasted 90 minutes (with a 10-m flame height). The internal emergency plan was triggered. The circuit ‘dam’ valves were actuated in order to limit the quantity of product contributing to combustion; a monitor nozzle was used to protect adjoining installations from thermal radiation, and the MPR was inerted. All 3 PE workshop production lines were shut down. Of the 3 employees sustaining slight injuries, 2 resumed their functions following an onsite medical exam, while the 3rd was taken to hospital for X-rays (he had fallen in the staircase).

Shortly before the incident, production lines 42 and 43 had been in service and line 41 was down for maintenance. A pressure rise in the MPR circuit followed activation of the general emergency shutdown on all 3 workshop lines. This emergency shutdown measure was implemented in a completely unexpected manner during works performed on the electrical panel. Activation of this measure also triggered an automated schedule of steps placing the installation in safe mode in order to shut off the secondary compressor (reactor bypass and routing to the MPR circuit), deflate the reactor, and close the circuit’s shutoff valves. Closing the valves in turn caused a pressure rise and the opening of a rupture disc on the 1st grease bottle. Fire broke out in the duct very shortly after this protective disc opened (less than 1 sec later). The presence of static electricity in the duct caused the ethylene cloud to ignite. Following this incident, the operator launched several actions: revision of the functional analysis of backup positions for MPR circuit protection valves, verification and scheduling of positions selected in the automatic relays, improved signs in the electrical cabinets containing emergency shutdown cables, installation of continuous electrical braids at the base of the duct, and submission of a study on the effects of disc release at the level of MPR circuits. This incident came on the heels of 2 others occurring on 21st July and 21st September of the same year involving the MPR circuit (ARIA 30920 and 31232).