Petroleum refineries frequently need to clean storage tanks to check for corrosion, or to allow the tank to be used for a different purpose. Part of the process of cleaning a tank include gas freeing – removing flammable vapors from the tank -- to provide a nonflammable atmosphere in the tank for the cleaning crew. They will often hire companies that specialize in gas freeing to complete this operation.
ESI was retained to investigate a deflagration that occurred shortly after a large storage tank had been gas-freed. ESi found that the gas-freeing plan did not consider the density of the gas in the tank (methane) and so the air inlet and outlet were not properly located. The gas freeing operation was suspended when the readings in the air discharge from the tank showed a low methane concentration. However, the duration of the gas freeing operation was significantly too short to completely remove the methane from the tank even if the air inlet and outlet were properly located. ESi estimated that approximately 20% of the tank was gas freed and the remainder contained methane. This subsequently ignited resulting in a deflagration and failure of the tank.
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