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