During restart of an ammonia production plant, syngas (50% hydrogen mixed with methane, ammonia, and nitrogen) leaking from a flange directly downstream of the synthesis reactor ignited. The plant had been shut down for about 90 minutes due to a technical problem. Alerted by the plant fire alarm, the operator activated the emergency shutdown, which isolated and depressurized the synthesis loop. Steam was sprayed onto the leak site to dampen the fire, which was brought under control 55 minutes later. Property damages included pipe insulation, the reactor's protective shutters, concrete fireproofing of the reactor structure, and instrumentation cables within 3 meters of the leak site. The flames did not affect the synthesis reactor itself, which was protected by a deflector. The incident was caused by inappropriate tightening torque applied to the bolts of the leaking flange, which was not adapted to the exceptional operating conditions when the incident occurred (i.e., a large temperature difference between the bolts and the flange due to the relatively short shutdown period).
The incident was the result of a combination of factors leading to exceptional temperature conditions that were not taken into account in the mechanical design of the reactor. Corrective actions that were implemented by the plant management included:
- redefinition of the appropriate tightening torque on flanges
- improved design of the leak collector on the flange (which failed during the accident)
- creation of a nitrogen injection system in the leak collector
- installation of a steam-injection system to protect the bottom part of the synthesis reactor.