Pollution
Humain
Environnement
Economique

Vapour saturated with nitric acid droplets was released from the 60-m stack of an upper-tier Seveso fertiliser plant while a hot ammonium nitrate solution (mixture of nitric acid and ammonia) production shop was being restarted. The plant produces fertilisers and intermediates used in the manufacture of fertilisers. Denser than usual vapours were seen exiting the stack and a drift in the acid/ammonia ratio was observed in the control room. At 11:15 a.m., the operator implemented the internal emergency plan to require his employees to stay indoors and avoid exposure. The plant’s first responders monitored the cloud as it dispersed. As a precautionary measure, the workers a nearby refinery were ordered to stay indoors. The correct acid/ammonia ratio was restored after 30 minutes, allowing the vapours to be reneutralised. The internal emergency plan was lifted at 12:15 p.m.

The operator estimated the excess nitric acid at 10 m³ and, based on firefighters’ observations, stated that there were no off-site consequences.

The release occurred when a low-level sensor in the nitric acid feed tank caused the shop to shut down. Once the level was restored, the shop was restarted. During the start-up phases, the acid/ammonia ratio was adjusted manually but the system used to measure the pH of the vapours was not working. The safety instrumented system (SIS) monitoring the ratio activated 15 minutes later. When the shop was restarted, the acid/ammonia ratio was maintained until the evaporator at the reactor outlet was started up.

After inspecting the mist eliminator for damage and finding none, the operator explained the drift in the ratio as being due to an operating error. When the flows were being regulated, the technician overseeing the other start-up parameters did not notice the alarm triggered by the addition of excess acid. However, the start-up procedure requires that this ratio is monitored.

A mist eliminator is a piece of equipment built into an apparatus to remove droplets of liquid suspended in a gas stream.

For the start-up phase, the operator plans to:

  • add a second safety sequence to the ratio measurement;
  • raise his technicians’ awareness about the requirements in the procedure;
  • look into the feasibility of installing a pH measurement system that would also work during the start-up phases.

And, more generally:

  • installing a flashing alarm to warn of drifts in ratio;
  • optimising the alarm management system.

After visiting the plant, the inspection authorities for classified facilities required the operator to build a cloud dispersion model for this type of scenario. Scenarios resulting from an incorrect ratio were added to the safety report.