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 

EXPLOSION IN SPENT ACID TANK DURING HOT WORK

A massive explosion destroyed a large storage tank containing a mixture of sulphuric acid and flammable hydrocarbons at an oil refinery. One contract worker was killed, eight others were injured, and sulphuric acid from collapsed and damage tanks polluted the local river. The explosion occurred during welding operations to repair a catwalk above the sulphuric acid tank, when flammable hydrocarbon vapour was ignited by welding sparks. This resulted in a powerful explosion inside the tank.
Key learning points
On the day of the accident, flammable gas testing was performed only at the start of the hot work, but monitoring was not conducted for the duration of the repair activities. It is also important as to where the explosive atmosphere measurements were made. Through the five hours between the last gas test and the explosion, the ambient temperature raised significantly. This warming caused the hydrocarbons inside the tank to vapourise. The resulting flammable vapour leaked out from corrosion holes in the tank into the work area. The company had a hot work program that included written permits, but the program was inadequate. Hot work was allowed near tanks that contained flammable materials without continuous atmospheric monitoring and the control of welding sparks was not required.

Source:IChemE 

 

EXPLOSION IN TANK DURING MAINTENANCE

An explosion occurred in a wholesale and retail storage facility in 2001. Two contract workers were carrying out maintenance operations inside a 5,090 m³ atmospheric storage tank. The closed floating roof tank usually contained premium grade gasoline but it was emptied for the maintenance job. The event occurred during cleaning operations. The activity, performed by employees of an external company, consisted in removing residue from the tank bottom by means of scraping. The most plausible theory is that spark was created by one of the worker's tools (boot soles points, metallic scraper, steel snap hook, etc.). Within an explosive atmosphere, this could have caused an explosion. The two workers were seriously injured in the incident. The tank was not repairable and the operation of the storage facility was interrupted for approximately two months.
Key learning points
The distance of the floating roof from the tank bottom at the time of the accident was approximately 1.2 m. The working space in this situation was limited. The empty tank that exploded normally contained premium grade gasoline. The gasoline vapours which were still in the tank caused the explosion. The tank was equipped with only one manhole. Apparently, when the incident occurred not all the vents were open; it is likely that the atmosphere was not homogenous and that explosive pockets of vapour/air also existed. The ventilation system set up to evacuate gasoline vapours was shut down for the cleaning operations to take place. What is also not always recognised is that gasoline collects in welds, pontoons (of floating rooves), particularly if they are not well maintained, small pits and crevices on the surface of the steel, within the residues on the tank floor. This means that there is a continual production of vapours
even after the tank has been emptied. This means that explosive atmosphere measurements need to be regularly repeated as well as constant ventilation. Finally, the limited space of movement of the workers was also an adverse factor in their escape.

Source:IchemE