At around 3:15 p.m., vinyl chloride monomer (VCM) was released from a basic-plastics manufacturing plant. At around 2:50 p.m., the emergency diesel generators started up to power the safety and I&C systems after the plant’s mains electricity supply shut off and the 20 kV emergency electrical supply failed to take over. Without electricity, the polymerisation reactors were no longer being stirred. As a result, substances known as ‘polymerisation stoppers’ or ‘reaction killers’ were automatically fed into them. At the time of the accident, the inhibition system of one of the reactors failed to operate. As a result, polymerisation inside the reactor was uncontrolled and its internal pressure rose. Only one of the two relief valves (safety devices) opened, allowing the pressure inside the reactor to continue to rise. Once the pressure rose to 20 bar, the safety valve opened and discharged 90 kg of VCM to the atmosphere via the cold flare stack. At 3:30 p.m., after the site was powered by the 20 kV backup, the reaction in the reactor was halted by the operator and its pressure went back down.

Causes :

• The significant rise in temperature in the electrical network, which occurred when the main network’s fans shut off, is what caused the mains electricity supply to trip. The electrical panel powering the network’s fans had a faulty outlet that caused the panel to trip when a device was plugged into it;
• A programming fault in managing the alarms of the 45 kV network prevented the switchover to the 20 kV secondary network.
• The inhibition system failed to operate because the nitrogen pressure was insufficient to allow the killer to be injected into the reactor.
• One of the relief valves failed to open due to a lack of pressure in the compressed air ring main. The compressor supplying the network is backed by a diesel generator but requires that a technician be physically present to restart it. This operation took some time while the reactor pressure was increasing, which caused the valve to open.

The operator took a number of actions after this event:

• It corrected the programming fault that prevented the 45 kV from automatically switching over to the 20 kV network if an alarm occurs on the 45 kV network.
• It added a diesel generator to nominally stir the reactors if the main power supply is lost and installed an associated supervision system.
• It replaced the compressor by a compressor with automatic restart.
• Electrical check reports and mains-to-backup power supply switchover tests are now monitored more closely.

The operator also analysed the electrical vulnerability. This analysis confirmed that the safety devices will operate properly in the event of a major accident (risk control measures). It is also considering whether to create a second reaction killer injection channel.

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