In the ammonia manufacturing unit of a petrochemical plant, a 24″ pipeline carrying synthesis gases (43% hydrogen, 14% nitrogen, 12% carbon dioxide and 29% water) ruptured causing the resulting hydrogen cloud (60 kg) to explode. Even though the explosion was heard as far as 2.5 km from the site, the damage caused outside was limited (shattered glass panes, damaged roof). Under the effect of low pressure, an 8 m pipe that was further down rotated by 380° while another 12 m pipe that was further up rotated by 90° damaging a nearby rack and destroying the vent header of a desulphurisation unit of the refinery from where 600 kg of methane leaked and ignited. Hydrogen from the pipe further up ignited as well forming a 70 m long fiery stream. The alert was sounded and the units secured one after the other. The blaze was put out 5 min later by cutting off gas supply to the plant. Due to this emergency shutdown, ammonia was released into the atmosphere without any reported impact on the population or environment. The material damage on account of the accident was estimated at 0.65 M€ to the fatigue induced by thermal cycles. The rupture zone of the pipe connecting a converter to the CO2 absorber of the ammonia unit was near the water injection point that cooled the synthesis gas from 230°C to 179°C under 28 bar. The examination of the zone revealed several fatigue cracks on the internal wall of the pipe over a distance of 3 m from the water injection point. Moreover, the water injection nozzle that was probably distorted before the accident modified the direction of the stream but there was no proof to ascertain it as one of the causes of the accident. Lastly, several leaks on account of fatigue were identified on the pipeline over the past two years but were incorrectly diagnosed. The operator believed corrosion due to stress to be the cause. As part of the accident feedback, the facility was modified. The water injection system was replaced with a reboiler system. A general inspection was carried out in the facility with a view to identify similar phenomena. A preventive maintenance plan was implemented.