Fire on the hydrodesulphurisation unit of a refinery

A major leak occurred at the outlet of the diesel hydrodesulphurisation furnace in a refinery. The furnace was just restarting. The released gaseous product ignited, and the alarms of the temperature sensors at the oven outlet were triggered.
8 minutes later the operator shut down the hydrogen compressors and the diesel charging pumps, closed the supply valves and called the emergency status.
The internal emergency services were able to bring the situation under control, by depressurising the unit and letting the fire terminate due to lack of fuel.
The emergency plan was terminated 5 hours later and the fire gradually extinguished one hour later.

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.

Poor inspection processes and wrong assumption of the severity of the initial leak are CONTRIBUTING or ROOT CAUSES. The failure to act on clear signal of malfunctioning provided by the previous near misses (multiple shutdown, small leak detection) suggests a lack of proper risk assessment and mistaken management decisions.

furnace, outlet flange

Several 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.

The unit was shut down for more than 35 days to allow for repair work. The material damage on the site was located within a radius of ten meters around the leak point.
The quantity of ignited product was 18 t, with an emission of 173 kg of SO2.

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.

The operator set up an environmental detection system by means of portable sensors and detectors). On-site measurements of H2, volatile oxide compound and SO2 did not show any anomalous signal. The measurements of air quality by the fixed measurement stations in the area and on the mobile stations did not show any effect from the fire. The water used to extinguish the flame were safely stored.