At around 3:30 a.m. in a plant manufacturing liquid inks, a technician noticed an increase in temperature on the piping at the outlet of a ball mill containing solvents, nitrocellulose in solution, liquid additives and pigments. As the pipes had turned red, he stopped the installation by pressing an emergency stop button and sounded the alarm. The internal contingency plan was initiated at 3:50 a.m. The 12 employees present in the workshop were evacuated, and the internal emergency services sprayed down the installation with a hose. The city’s fire brigade was alerted and set up a 50-m safety perimeter and sprayed down the mill with water for 3 h in an attempt to cool it down. The 14 m³ of water used during the operation was recovered and transferred to a waste treatment centre. The internal contingency plan was lifted at about 6:30 a.m.
The Inspection authorities of classified facilities were informed of the accident that same day and then visited the site on 12 March.
After analysis, the operator settled on 2 hypotheses to explain the incident:
- a lack of product feed resulted in the mill running dry, which may have been related to either clogging of the screen (by the balls or by the pigment) or to clogging of the non-return valve located after the pump near the product in the mill,
- rupture of the mechanical seal due to overpressure in the mill, impact, wear or a faulty part.
However, a rupture would have caused the loss of barrier fluid, which was not the case. It was likely to have been caused by the sudden rise in temperature at the mill. In addition, when the sieve was dismantled, the operator noticed that 95% of the lower part of the sieve was clogged and that 50% of the mill balls were oval, and therefore worn. The lack of product feed to the mill, associated with the clogging of the sieve, is the hypothesis that was finally accepted by the facility operator.
The following improvements were foreseen:
- installation of means to measure product flow at the mill inlet, which was slaved to the shutdown on the 8 lines, costing 80 k€,
- review of the control table with a decrease in the maximum pressure parameter from 2.8 to 2.3 bar and an integration of the flow parameter after installation of the flow meters,
- monitoring the quality of the mill balls.