Around 4 a.m., a major leak occurred at the outlet of the diesel hydrodesulfurization furnace in a refinery. The goal of this unit is to lower the sulfur content of diesel fuels produced in other units, by a catalytic reaction in the presence of hydrogen. At the moment of the leak the furnace was restarting. The alarms of the temperature sensors at the oven outlet were triggered. At 4:08 a.m., the operator shut down the hydrogen compressors and diesel charging pumps, closed the supply valves and called the emergency status. The first emergency operation was to decompress the unit. Preventive foam mats were then placed under the oven, as well as under the heat exchangers. It was decided to not blown out the flame, to avoid introduction of air into the circuit. The drop in pressure in the unit caused eventually the decrease of the flame and smoke intensity. At 9 a.m., the emergency plan was terminated and the fire was gradually extinguished around 10 a.m. The leak occurred at the joint of a flange of the piping at the furnace outlet. This connection was not provided by evidence of controlled tightening or formal verification. Moreover, many successive shutdowns had already occurred, the last 4 the day before the event. The day before at a small leak had already been observed, but no LEL measurement had been taken, and it was decided to continue the start-up procedure.