At a chemical plant, an ignited leak of synthesis gas was observed around an ammonia (NH3) unit reactor during its restart routine following a service shutdown. The plant’s internal emergency plan was activated; the steam-based extinction system was also switched on, resulting in a near immediate extinction of the flame. The unit was turned off, verified and started back up 4 days later. The shutdown prior to the accident stemmed from an incident on the turbo air compressor lubrication circuit of the NH3 unit that stopped the machine and nearly the entire unit. Once the lubrication circuit had been restored, the workshop restarted production and 14 hours later the synthesis process was reinitiated. Upon restarting the synthesis compressor, the gas leak was noticed at the reactor’s upper joint, spontaneously igniting. A thermal dilatation difference between the joint (cooled by the gas upon starting up the synthesis) and the reactor body (still hot) caused this leak, with the high hydrogen concentration of the synthesis gas explaining its spontaneous ignition. Moreover, the procedure of quickly and completely exposing synthesis loop components to air by systematically opening all sectional valves, even in the event of short-term shutdowns, increased the unit’s vulnerability to this type of incident. The corrective actions undertaken entailed: keeping the reactor pressurised by injecting nitrogen (80-100 bar) during short down periods; and automatically closing the synthesis compressor connection in order to reduce the pressure decrease rate.