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November 2002 — Combustible Gas Detectors A 2000 gallon tank that had contained leaded gasoline was steam-cleaned, purged with nitrogen gas, and analyzed using a combustible gas detector. The detector readings were confirmed to be below the Lower Explosive Limit. When welding began, the tank exploded. Why didn't the combustible gas detector alert the operator to the presence of flammable vapors? The combination oxygen and combustible gas meter is used to test atmospheres for sufficient oxygen content for life support and/or the presence of combustible gases or vapors posing a potential flammability/explosion hazard. The particular combustible gas indicator being used was designed to measure combustible gas or vapor content in air. In other words, "air" was essential to the proper operation of the instrument as a background gas. In fact, the particular instrument involved would not indicate any combustible gas content in atmospheres containing less than 10% oxygen. When the tank was purged with nitrogen, the oxygen was displaced, causing the instrument to give a false reading. Although it seems reasonable to analyze the purge gas to ensure the removal of flammable vapors, an understanding of how the equipment was designed to operate would have shown otherwise. Always review the equipment manufacturer's literature and understand your equipment's operating parameters before relying on the results. Your life may depend upon it.
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© 2000 Prest-O-Sales & Service, Inc.
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