A leak of methylglutaronitrile (MGN), which is a by-product during manufacturing of intermediate adiponitrile from nylon for use as a fuel, occurred at 10 am on the burner feed line of a chemical plant boiler. As in-house first responders were securing this leak using foam, the substance infiltrating into the boiler’s heat insulation ignited. The site’s internal emergency plan was activated and the fire was attacked at 11:14 am. This response team was able to control the blaze with 1,000 litres of emulsifier and then immersed the heat insulation in water to prevent the fire from rekindling when opening the insulation. The municipal fire department, alerted out of precaution, was not needed on-site and the incident ended at 3:32 pm; the internal emergency plan was lifted at 4:50 pm. The plant’s technical unit waited for the damaged boiler to cool entirely before initiating the necessary repairs. During this incident, the plant was starting back up after a strike-induced shutdown. According to the operator, the fire behind the heat insulation had 2 causes: ineffective spraying of MGN in the combustion chamber, leading to the presence of liquid MGN in the throats of boiler burners 1 and 2; followed by the flow of MGN beyond the tube sidewall owing to the unsealed GR casing / tube assembly. Several measures were implemented: visual inspection outside the tube and inside the source; reconstitution of the tube connection seal inside the casing by welding a stripping plate to cover permeable zones; bleeding of the plate welds; hydraulic test conducted at the proof pressure (48 bar); and visual verification of the adequate seal of GR casings by means of partial weld bleeding.