Air-Blowing of Hydrocarbon Lines

When air is used to clear a line that had contained hydrocarbons, the potential for forming a flammable mixture of air with hydrocarbon vapors and/or mist is created. The elevated pressure which can be developed in a line or system broadens the flammable range, lowers the autoignition temperature, and can convert a previously fuel-rich mixture into a flammable mixture by lowering the vapor mole fraction of the fuel. Even though the temperature of a hydrocarbon liquid in a line may be below its flash point, mist formation can occur due to the sparging action of the air and can result in a flammable quantity of material being forced into the vapor space. 

Only a very thin film of oil is needed to produce an explosion in the pipework of compressed-air systems, even if the oil is not dispersed into the air prior to ignition. Many industrial fires and explosions have occurred in systems utilizing oil-lubricated air compressors. Therefore, adding air to an empty hydrocarbon line, or to purge a hydrocarbon line, is potentially hazardous because trace amounts of flammable materials may be clinging to the pipe walls. On one occasion, an explosion occurred in a cmde-oil pipeline to which air had been added, even though the line had been first emptied by displacement with water between two scraper plugs. 

Although autoignition is the probable cause of ignition in most compressed-air-line fires, other sources are possible. They include heat generated by the oxidation of iron sulfide, other sulfur compounds or carbonaceous deposits which may unexpectedly be present in the line, and friction or static-electricity-generated sparks. 

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