In a chemical fertilizer plant, a plume of orange smoke was seen at around 6 a.m. above the nitric acid unit, after a 3-week maintenance shutdown period. Upon restarting the unit’s nitrous effluent absorption column at 5:35 a.m., the technicians noticed that the pressure in the column was excessively low and attempted to close the valves supplying it from the turbine. Although the closing command was recorded in the operating system, the pressure remained low in the column. During rounds conducted in the unit 6:30 a.m., it was discovered that the valves in question were not supplied with instrument air as their supply line had been shut off while works were under way. An emergency intervention made it possible to connect the pipe to the valves making them manoeuvrable again. The pressure in the column and its absorption rate returned to normal at 7:20 a.m.: the flow of NOx to the stack gradually decreased and the nitrous plume disappeared from the atmosphere.

Analysis of the accident shows that the location of the instrument air supply was located a certain distance from the valves connected to the column. An external technician in charge of dismantling an electrical panel consisting of valve control had disconnected and shut off the air line supplying the panel. He also mistakenly shut off the air supply to the valves which were located in the same place, thinking that their supply could not possibly be located so far away. Furthermore, the operating mode for this procedure did not offer a level of precision that would allow someone from outside to understand the actual function of the air supply at this location.

Two factors delayed the operating crew’s identification of the problem on the day of the accident.

• the mimic diagram in the control room did not display the actual position of the valves, but simply that the closing order had been taken into consideration. The only way to realise this was to go into the unit, once they understood that the adjustment attempts from the control room had no effect;
• the distance between the valves and their supply point delayed the identification of the problem by those performing the rounds and looking into the problem.

The operator shall improve the identification of critical parameters, any deviation from which may increase the NOx content of stack emissions. A ‘reflex sheet’ has been formalised with regard to the measurements to be taken by technicians in the event of a deviation from these parameters. The position of the column supply valves was added to the mimic panel in the control room and the crews were reminded of the need for high accuracy in operating procedures when performing scheduled work.