Pollution
Humain
Environnement
Economique

A natural gas-fired boiler at a lower-tier Seveso pharmaceutical plant overheated at around 11:50 a.m. after being restarted. A subcontractor’s technician in charge of operating the boiler plant restarted thermal oxidation of VOCs via the human-machine interface and sent the VOCs to the combustion chamber at around 11:30 a.m. Several minutes later, the LEL alarm sounded. The boiler went into bypass mode and then automatically into safe mode. The technician noticed the beginnings of a fire on the VOC supply pipe located in the area of the main fan on the first floor. The temperature rose to 600 °C and the discharge pipes glowed red. Informed of the situation, the shift supervisor implemented the internal emergency plan in order to secure the shops and turn off the heating of the fractionating tower. Fire-fighters monitored the cooling of the boiler, then left at 3:30 p.m. after using a thermographic camera to check the temperature of the VOC supply pipes near the treatment facility. The gendarmerie, the municipal police and the gas and electric utilities arrived at the scene. The operator monitored the temperature of the boiler until the next day. The special liquid waste (contaminated aqueous phases, solvents, etc.) linked to the shutdown of the fractionating column were sent to an offsite treatment facility.  The fire was caused by excess fuel laden with VOCs due to a design error in the VOC facility’s software program which made it possible to switch to incineration regardless of the LEL of the mixture. The VOC facility had been restarted following preventive maintenance performed by a subcontractor. However, the existing safety system, which was controlled by explosivity thresholds and deviated VOC streams if the thresholds were reached, only operated in incineration mode. The operator modified the VOC facility’s software program. A valve position error (withdrawal and control) due to different controls between automatic and manual mode caused all the heavily VOC-laden condensates to be reinjected. The operator supplemented the procedure for controlling the fractionating column and the PLC change management process (automatic/manual) with additional information. It also checked the condition of the boiler and replaced its flame arrester before restarting the facilities.