At approximately 6 p.m., the on-duty personnel heard an explosion near the boilers of a coking facility. Several other detonations were heard on site in the VOC capture network, all the way up to the desulphurisation unit. No outbreak of fire was reported. The site’s internal firefighters noticed that the boiler had stopped working after its PLC had shut down. They also noted some physical damage on the boiler’s VOC processing line:

• tearing of the rubber compensator on the primary air fan;
• breakage of the fan and sleeves on the desulphurisation VOC fan;
• broken windows, which were already in poor condition, in the boiler room.

These events occurred during a repetitive start-up phase of the air/VOC injection system following maintenance operations that caused the injection system to shut down and be secured. Blame for the incident was placed on the PLC controlling the boiler’s start-up sequence. It may have been inadvertently tripped during a test performed with another PLC having the same IP address. An operator had thus restarted the PLC in order to start the boiler, then sent the VOCs to the boiler for processing. The VOCs could not be sent as the wrong boiler had been selected in the PLC. The technician therefore manually changed the boiler number, and VOCs were sent about 20 minutes after the boiler was started.

Furthermore, the operator must conduct a risk analysis concerning the treatment of gases and the associated risk control measures must be updated, where necessary.