At around 3.20 pm, there was a loss of external power supply in a refinery. The site was supplied by two distinct lines on two separate sets of tower members without interconnection and supported by distinct towers operated by a power supply company independent of the refinery. There was an auto-reclosing device onsite with a 3-second time lag. Beyond this time frame, the circuit-breakers had to be reset manually by the technician. This required only a few seconds. On the day of the incident, the disturbance lasted 22 s: 2/3rds of the site’s units automatically shut down (distillation, high pressure hydrogen units, gas-plants, H2S conversion units, distillation residue conversion units). The catalytic cracker shut down. It was supplied by a gas turbine (gas supplied by the unit) and the buffer load bearing capacity ensured supply for 10 minutes. Subsequently, a pump directly connected to the power network supplied the catalytic cracker. At 3.25 pm, the power supply was restored in the entire site and the various units resumed functioning. At 4.04 pm, there was external another power failure that lasted 22 s. The units stopped along with the catalytic cracker supplied by the electrical pump. The gas turbine that was still functioning due to the depressurisation gas allowed the boilers to function until the power supply was restored (16h06) but was stopped by the lack of gas at 4.22 pm. Meanwhile, the supply to all units reverted to the main power grid.

The incident resulted in the burning of 12.8 tonnes of hydrocarbons and the emission of 3.2 tonnes of SO2 from the flare during the day. Following the abrupt stopping of the catalytic cracker, various checks were performed. During one of them, the safety pressure of oil caused the cracked gas compressor to stop for 1 hour and 27 minutes: 11.9 tonnes of hydrocarbons were burnt and 2 tonnes SO2 released for a short duration. This resulted in black smoke that drifted to a nearby village due to the north-east winds blowing at 4 to 6 m/s. Residents complained or panicked because of the situation. SO2 concentrations recorded on the town’s analyser revealed peaks as high as 285 µg/m³. The operator and the electricity company published a press release and informed the mayors of the neighbouring towns.