At 3:40 p.m, refinery operators detect a leak of aerosol hydrocarbons (HC: diesel, hydrogen and hydrogen sulfide) in the diesel desulphurization unit causing a release of hydrogen and hydrogen sulfide at the top of the unit .
The operator in charge of conducting the unit triggers the automatic shutting of the unit and the water walls of the furnace. The leak is located at the outlet of the desulfurization reactor, in the cooling towers. The plant firefighters team was alerted at 15:45 , then a warning containment for the 1500 employees of the platform (petrochemical units and refinery) is issued at 15:47. The company triggers the emergency plan at 3:50 p.m. and the firefighters team installs a water curtain around the unit. Public emergency services dispatch a firefifhter officer at the plant PC. Containment is raised 50 minutes later for the platform employees and 2h later for the unit and neighboring units employees , the emergency plan is then cancelled. The unit is depresurized by sending the diesel content to the torch, then is degazeifed with nitrogen. No casualties are registered but residents’ complaints concerning odors are recorded in the late afternoon in the nearby town located downwind of the refinery, their claims are confirmed between 16 am and 17 pm by air quality sensors located in this town. The company suffered production losses worth several million Euros due to the slowdown of the units of the platform, the closure of the desulfurization unit for several days and the non-desulfurized diesel producted which is more difficult to sell. The cloud of HC was released at a 10m height due to the fan of the cooling tower at the origin of the leak and quickly dispersed into the air, fixed hydrocarbon gas detectors in the units or mobile ones belonging to the employees and emergency staff never triggered. The company issues a press release.
The investigation shows that the leak came from a boring on a beam of ordinary carbon steel pipes on 1 of the 4 air coolers units, following corrosion related to the presence of ammonium disulphide produced by the combination of H2S issued by the desulfurization porcesse and the nitrogen issued from the denitrification process in the reactor. An injection of water upstream is assured at all times to dissolve the substance and avoid deposits, but a retrofit of the unit 5 years ago had increased the rate of denitrification in the reactor. Corrosion was aggravated by the erosion derived from the excess of penetration of the weld beads on an elbow of the tube, thereby creating a turbulent flow erosion after the elbow. An examination of the radiographic controls made three months before by the plant inspection service (SIR) shows a residual thickness of 1.5 mm on the tube which remained undetected by the radiography operator and next by the employee of this service, given the difficulties to detect it with the radiiographic device. The normal procedure involves replacing the tube when reaching a residual thickness <2mm. The uniform nature of the loss of thickness in this part of the tube is more difficult to detect than a ponctual loss of thickness on the radiographic films. Incidents of boring related corrosion / erosion had already occurred on some of the 92 tubes and 368 elbows belonging to the 2 oldest cooling towers (30 years old), with change of 41 tubes after a similar incident 5 years ago , 5 elbows changed following a checkup 2 years ago and 75 elbows changed after incident 3 months ago, but without any consequences on the environment. The company restarts the unit on a reduced mode with the 2 most recent cooling towers (15 years old) to avoid the complete paralyzation of the atmospheric distillation unit, the 2 oldest are replaced the next year with a special attention given to the nature of the metal and the design of tubes and elbows.