In a Seveso-classified fluorochemical plant, an operation was underway to dilute 40% anhydrous hydrofluoric acid (HF, toxic and corrosive) in a 5,000-litre tank, when the tank pressure began to increase. The tank’s dome connection ruptured at around 10:45 a.m.; the piping supplying the tank was damaged and began to leak. Spilt HF began flowing into the tank’s double wall and onto the floor. The operator initiated its internal emergency plan. Concentrations of 7 ppm were measured at ground level. The HF vapours released triggered the fire detection system. The staff was evacuated, and the workshop was blocked off. The site’s response teams stopped the transfer pumps and pumped the spilt acid into a retention basin for neutralisation. In the end, the emergency services did not intervene. Operators sprayed down the HF that had dripped onto the ground and pumping operations allowed 100 l of water to be recovered in the building’s retention pit.

An analysis of the accident showed that 48 hours prior, over the weekend, an on-call employee had closed the tank’s vent valve after HF had been detected in the building. On the day of the accident, while HF was being transferred into the tank, the gaseous release due to the mixing of the anhydrous HF with the water could not be discharged by the vents, increasing the tank’s pressure. During the works, this valve had been installed to isolate the old part of the vent lines from the part that had been built more recently. When the works were completed, its operating handle had been removed so that it would remain in the open position by default. This made it possible to connect the old and the new vent line, both being connected to the building’s scrub column (neutralisation of the HF). Subsequently, new work required that the handle be put back in place temporarily in order to isolate part of the vent lines. In the end, the valve was returned to the open position, but the handle remained in place. The lockout of this handle in the open position was not specified and the individual on-call had thus closed the valve with all the other valves in the zone, due to lack of information and as stipulated in the instructions in the event HF is detected.

Since fire detection has proven to be effective in leak detection, the operator plans to slave the automatic safety devices to fire detection in the event of an HF leak. The HF detection system was, in fact, placed too high in relation to the leaking piping to trigger on the day of the accident (the tripping threshold was at 3 ppm, max. measurement at 0.7 ppm). A more stringent post-work equipment lockout procedure is being considered.