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U.S. Coast Guard Marine Safety Alert - formation of dangerous levels of hydrogen sulfide within a waste oil or slop tank

The following is excerpted from a U.S. Coast Guard Marine Safety Alert issued June 6. The complete bulletin is available online.

This safety alert shares important information related to a unique, not very well known and potentially deadly hazard that recently developed on a vessel involving the formation of dangerous levels of Hydrogen Sulfide (H2S) gas within a waste oil tank. This was determined to be due to the use of biodegradable cleaners and the chemical reactions and resultant H2S byproduct involved in breaking down oils in an anaerobic (low oxygen) environment like that found in a full slop tank. In this case, vessel officers had detected significant H2S concentrations (>200 PPM) in the vessel's engine room bilge holding tank. This circumstance had existed for quite some time as crewmembers had previously reported smelling the distinctive odor of rotten eggs in the vicinity of the slop tank, which is an indicator of the gas.

Two hundred (200) PPM of H2S is considered Immediately Dangerous to Life and Health (IDLH). Furthermore, the Lower Explosive Limit (LEL) was found to be in the dangerous flammable range, at 100 percent LEL, in the same tank.


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