An explosion occurred at 1:30 am inside an urban heating plant (500 MW, 6,000 m² floor area), with the energy dissipated into the ground estimated at the equivalent of a 50 kg charge of TNT. Operational since 1987, this heating unit comprised 5 boilers (2 coal-fired, 2 fuelled by a coal/gas mix, and 1 gas-powered). During the previous shift, several attempts to start up one of the mixed fuel boilers failed. Unable to restart the equipment and with the gas inlet pressure gauges indicating zero pressure, the foreman of the night shift ordered opening both valves a quarter turn towards shutting off the gas inlet on the main circuit. Since the indicated pressure remained at zero, the shift foreman requested the boiler technician to open a blowout preventer and then a butterfly control valve to feed the mixed fuel boiler with gas. This operation resulted in a major gas leak. A gas boiler underwent emergency shutdown, and 2 technicians exited the unit to cut the general gas supply at the regulator station, 110 m from the building, when the explosion happened.
One of the 5 employees was killed on the spot. A girl 10 years old living 40 m from the plant died 4 days later due to the injuries she sustained; 59 other neighbours were also hurt. The installation was devastated and nearby districts severely damaged. A total of 600 personnel from local businesses had to be laid off temporarily and 250 residents were displaced from their homes. While awaiting hook-ups to neighbouring utility lines, some 140,000 users and 2.2 million m² of office space had no heating or hot water service. Operations of large-scale computer networks that relied on the plant for cooling, were disturbed. Total damages were valued at 544 MF (or 83 million). Investigation results indicated that 3,750 normal m3 of gas would have been released until the gas utility company was able to cut the supply line 30 min after the explosion.
The defective pressure gauges might have been damaged by a pressure surge occurring sometime prior to the accident. The orders issued by the night shift foreman fell under the exclusive responsibility of the maintenance crew; in case of emergency, plant technicians should have requested intervention from the gas company. The blowout preventer had not been designed for handling within a pressurised environment, and moreover the butterfly valve upstream of the blowout preventer should have been adjusted by the boiler operator while the device had remained in the intermediate position, where it was no longer sealed since the flanges were slightly spaced. The gas cloud ignited upon contact with the coal-fired boiler, which was operating at the time of the accident. No scenario involving a leak and gas explosion had ever been assessed in any of the site’s previous safety reports. The risks related to coal dust had not been addressed either. Dust particle behaviour was also likely to have contributed to the strength of this explosion.