EXPLOSION IN FLARE LINE

A series of severe electrical storms caused refinery wide unit shutdowns, including the fluidised catalytic cracking unit (FCCU). The crude distillation unit was shut down as a result of a fire, which had been started by a lightning strike. A process upset in the FCCU’s gas recovery section ultimately led to a high liquid level in the flare drum and several shutdowns of the wet gas compressor, together with other process anomalies. As a result of the wet gas compressor shutdown, there was a large vapor load on the FCC flare system, which led to a high liquid level in the flare drum. When the hydrocarbon liquid overflowed into the outlet line of this drum, the line ruptured due to mechanical shock and approximately 20 tonnes of flammable hydrocarbons escaped. This mixture found a source of ignition 110 m from the flare drum and subsequently exploded. This caused a major hydrocarbon fire at the flare drum outlet itself and a number of secondary fires.
Key learning points
The explosion was caused by flammable hydrocarbon liquid being continuously pumped into a process vessel that, due to a valve malfunction, had its outlet closed. The only means of escape for this hydrocarbon once the vessel was full was through the pressure relief system and then to the flare line. The flare system was not designed to cope with this excursion from normal operation and due to liquid breakthrough at the FCCU flare knock out drum, a failure occurred in the outlet pipe. The investigation revealed, that internal acidic corrosion also contributed to the pipeline rupture. Apparently, the company did not inspect the weakest points of the pipeline which were exposed to corrosion

Source:IChemE