At 8:35 a.m., flammable gases were released from the exit of a steam cracking furnace at a petrochemicals plant. An alarm warned the operating crew that the vent valve was open on the operating furnace. The technicians activated the furnace’s emergency shutdown, causing the gas fuel valves and the valves supplying the furnace with naphtha to close and the dilution steam valves to open. This stopped the flammable-gas leak in two minutes. At 8:50 a.m., a technician went to the furnace and closed its vent valve. Whilst the furnace was being shut down, the operator evacuated the area and warned the downwind workshop. The safety department conducted downwind measurements, which were negative.

The operator estimated that 1,200 kg of hydrocarbons consisting of the cracked gases were released into the atmosphere. A plume of greyish smoke (the hydrocarbons at the start of the event) and then steam (the opening of the steam valves) was visible outside. The operator had cloud dispersion models run. As the cloud was flammable, the thermal and overpressure effects were simulated. Since the cloud’s emission point was located 40 m away, only ground thermal effects would have been felt if the cloud had ignited. The cloud dispersed up to 2.6 km from its emission point. The operator reported production losses due to the shutdown of the furnace and costs of cleaning the coke particles that fell onto the furnace.

The release was caused by unintentional opening of the furnace’s vent valve. An impact on the local furnace decoking control box is believed to have set in motion the valve operating sequence. The operator reported that the local control button was not protected by a guard and local/remote control was not protected by a padlock.

The operator instituted inspections of the guards protecting the local control buttons and reinforced the protections that could lead to releases. He put a padlock on the control that led to the incident and checked that padlocks were fitted on other controls of the same type. Regular checks of their condition were added to the checks already in place.