An explosion of polyamide powder was followed by its combustion inside a plastics factory. The unit involved was producing the powders themselves. At the end of the polymerisation step, the powder was dried and transferred by means of nitrogen pneumatic conveyance to the water washing tank.

Factory personnel were notified around 5:30 am by an audible thud and vibrations. They could spot flames in the workshop building and immediately activated the fire-fighting systems in place, in addition to placing the facility in safe operating mode and organising the emergency response. The blaze, which was brought under control shortly thereafter, was not visible from the outside.

During the night preceding the fire, a leak had been detected on the nitrogen blanketing valve of the transfer jar between the dryer and the washing tank (breach in the closure seal). The connection valve between the transfer jar and washing tank was open in order to decompress the pneumatic conveyor. Once the next batch of powder had been completely dried, the control room validated the operation, thus authorising continued operations despite the normally closed connection valve remaining open. This exceptional configuration triggered the simultaneous launch of 2 operations, which were typically staggered in time (i.e. filling the washing tank with water while keeping the valve closed, then transferring powder into the tank after opening this valve). The nitrogen valve, open for the powder transfer, raised pressure in the transfer line to the nitrogen line pressure (3.2 bar) since the connection valve had been closed. The balancing hose connecting the transfer jar to the transfer line, attached by interlocking onto the smooth taps and held in place by a Serflex type clamping system, broke free under the pressure, releasing powder to the outside. The triboelectric charges generated by the friction of powder exiting via a small-diameter orifice caused this ignition. The sudden ejection of powder under these conditions initiated the explosion and subsequent combustion. The insulation lining of the electric cables around the powder ejection point then caught on fire, spreading to the workshop building’s upper story via the cable paths, in damaging the asbestos cement roof, part of which collapsed. Some instrumentation and a few valves were also damaged, but none of the process containers were degraded.

Before restarting the installation, the factory operator planned on removing the asbestos and replacing the asbestos cement roof by steel siding. The facility was decontaminated and atmospheric measurements taken in order to detect the potential presence of asbestos fibres. The combustible walls were dismantled and also replaced by steel siding. All hoses installed on the workshop’s pneumatic conveyor belts were catalogued and replaced by metal hoses connected using threaded couplings. The equipment exposed to fire was inspected and tested, in repairing deteriorated components. Debris was removed while awaiting appropriate treatment. Moreover, the control system was modified to prohibit pneumatic conveyance with a closed discharge valve. From 7 to 9 November 2012, a specialist subcontractor evaluated the unit’s fire detection and protection system. A working group, composed of the operator, the process team, safety department staff and the Group’s Process Safety Division, was assigned to review both the physical and organisational features so as to ensure that all process-related risks had been taken into account in workshop design and operations.