A release of ammonia (NH3, odorous and toxic gas) occurred between 8 p.m. and 10 p.m. in a synthetic fertilizer plant. Local residents complained of a strong smell of ammonia at 10:45 p.m. The sensors located on the periphery of the site and at the air quality monitory station in the neighbouring hamlet measured peak concentrations of 12.5 and 1.5 ppm of NH3, respectively.

After securing the refrigeration turbo compressor of the ammonia production unit’s synthesis loop (triggered by low oil pressure safety), the “Purgal” device for recovering the purge gas from the synthesis operated for about 20 minutes before tripping due to low pressure. The shutdown of the turbo compressor led to an increase in the amount of ammonia in the purge gases. The Purgal device was secured by the closure of two valves, the purge gases from the synthesis being directed to the fume line from the ammonia unit (processing and release at 90 m) via automatic opening of a third valve.

The analysis revealed that the by-pass of the Purgal device’s shutoff valve was blocked in the open position and allowed a residual flow of gas to flow toward this installation. The system upstream from the turbine and the unit’s stack was clogged with ammonia. At 7:50 a.m., a technician intervened, pending the restart of the turbo compressor of the ammonia unit’s synthesis loop, by opening a manual valve because he thought that the turbine was isolated from the Purgal diffuser following the trip (shutoff valve closed). As the latter remained open, the decompression of the turbine caused the decompression of the system towards the Purgal’s stack (45 m). This action resulted in the accidental release of an ammonia-enriched purge gas into the environment. The technicians in the control room were alerted by the site’s peripheral sensors. Patrols around the site were able to locate the outlet. The manual valve involved was closed at 10:05 p.m.

The facility operator introduced the following corrective actions:

• function and leak testing, lockout of the shut-off valve by-pass in closed position;
• modification of the quick reference card regarding the triggering of the turbo compressor and the operating guidelines relative to the start-up and shut-down of the Purgal device;
• addition of a low-temperature alarm on the temperature sensor located at the inlet of the Purgal stack.