For a 20-minute period, a 0.7-ton leak of residual gas containing 1% HCN spread at a chemical facility. The gas alert and internal emergency plan were both activated, only to be lifted 1 hour 25 min and 2 hours later, respectively. Plant personnel were evacuated at 10:30 am once the 2 gas detectors had been activated. The cloud spread 350 m south of the site, with no environmental impact recorded (detectors showed 2 ppm at the site boundary and were not activated).

HCN was produced by 2 parallel conversion lines from an intake composed of NH3, natural gas and O2-enriched air; 135 tonnes/hr of residual gas were conveyed to a recovery boiler (with 0.8 tonnes/hr directed to the process flare), whose installation was operating slightly above atmospheric pressure. The unit was operating normally when the boiler started up (according to the operator, under normal safety conditions) around 10 am on an excessive residual gas intake, as the operator was seeking to push the boiler to a higher operating regime. The entire volume of residual gas was automatically routed to the process flare, and the conversion lines dropped to a lower regime (from 135 to 60 tonnes/hr). The flare system comprised 2 upstream flame arrestors assembled in parallel: one open during normal operations, the other opening when gas was sent to the flare. At 10:20 am, pressure was rising between a column and a separation chamber located upstream of the flame arrestors. According to the operator, these arrestors consisted of metal strips placed very close to one another in order to “shear” the flames inserted into a metal cube base that was quickly plugged. The pressure expelled the hydraulic shield and residual gases were discharged into the atmosphere. Around 10:40 am, insulation jacks could be successfully closed between the conversion lines and the residual gas treatment lines, while the hydraulic shield was being filled automatically given that the relative pressure had dropped below 0.1 bar.