Inside a pectin extraction unit at a food processing plant, 102 m³ of nitrous vapours were discharged into the atmosphere during 1 or 2 min, following bursting of the rupture disc on a reactor. Fire-fighters were called to the scene, backed up by the chemical emergency squad and a helicopter dispatched by the police, to conduct toxicity measurements and monitor the advancing toxic cloud that had formed. This pink-coloured cloud had drifted towards a neighbouring high school; 200 people were evacuated; 13 inconvenienced by the rejection are hospitalized including 4 students who will be kept under observation less than 48 h.

This incident was caused by an exothermic reaction triggered by spillage of surplus nitric acid (due to accidental opening of a manual valve) on an organic mix of alcohol and pectin. This technician’s error was preceded by replacement of the unit’s nitric acid pump, whose new flow rate exceeded that of the former model. This modification was the source of frequent installation safety shutdowns, requiring restart in manual mode. Machine design flaws favored human error:

• manual mode can be used even if it is not necessary;
• there is no interlock between the stages of the system allowing the operator to intervene on one phase while another is in progress;
• the cycle can start automatically without all the steps being in automatic mode;
• the display of the valve open state (change from red to green) and of the acid flow rate on the control screens went unnoticed by the operator for 12 min, as he did not therefore not aware of the filling of the tank with acid;
• the safety of the acid valve limiting the quantity that can be delivered no longer exists in manual mode.

Several measures were adopted to reduce the probability of accident recurrence, including:automatic closing of supply valves once the reaction mix temperature reached 25°C, introduction of safety test alarms and procedures, backup safety redundancy on the reagent supply system, ban on reagent valve activation in manual mode.

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