H2S Threshold Limit Change
USCG has issued marine safety alert No 02-14 to remind to Coast Guard personnel and the maritime community of potential dangers during sampling of cargo tanks. During a recent Port State Control (PSC) tankvessel examination, a CG member was exposed to a dangerous concentration of Hydrogen Sulfide(H2S) gas and suffered a serious injury.
The exposure occurred during a tank vessel exam onboard atank vessel carrying Grade E Sour Crude. A PSC teamrequested a ship’s crew member check cargo tank oxygen levelsusing the ship’s portable gas meter. The crew member accessedthe cargo tank via a deck sounding valve similar to the adjacentphoto. When the valve was opened the pressurized cargo tankatmosphere escaped releasing inert gas and H2S vapors.
During the evolution the personal gas meters of two individualsinvolved (one CG and one crew) alarmed for H2S. The CG officerwhose alarm sounded was standing 1-2 feet downwind from thesounding valve. Within days the CG officer developed severeexposure symptoms consistent with H2S exposure. The othertwo CG officers involved were standing upwind. Afterward, onereported experiencing a minor headache
OSHA notes that H2S is a colorless, flammable gas with a “rotten egg” smell that occurs naturally incrude petroleum. Even at low concentrations this heavier-than-air gas can irritate the eyes, nose,throat and respiratory system with effects delayed for hours or days. At higher concentrations,nausea, vomiting, headaches, dizziness, unconsciousness or death may occur. While the initial”rotten egg” odor is present, an individual may lose the ability to smell that gas after becomingexposed. Personal monitoring equipment is, therefore, vital to protect against exposure. An alarm onH2S constitutes an acute exposure and should trigger immediate evacuation and initiation of acuteexposure procedures including medical attention.
Prior to using portable gas monitoring equipment, personnel should familiarize themselves withISGOTT Section 11.8 and safe work practices for conducting or witnessing these tests. ISGOTTrecommends when sampling tanks personnel should stand perpendicular to the wind to avoid beingdownwind or upwind and creating eddies. When monitoring cargo tank atmospheres, all personnelshould exercise diligence and great care. In all cases, personnel should completely assess the risks,to include the cargo type, tank pressure, venting arrangements, wind direction/speed and condition ofthe testing equipment. When H2S is suspected to be present, ISGOTT Section 2.3.6.5 recommendsthat a self contained breathing apparatus (SCBA) be worn if it is necessary to breach the integrity ofthe cargo system and if a vapor free atmosphere cannot be guaranteed.
The American Conference of Governmental Industrial Hygienists recently reduced the H2S dangerousThreshold Limit Value from 2 parts per million (ppm) to 1 ppm.
As a result the Coast Guard strongly recommends that those involved in cargo tank sampling oratmosphere testing:
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