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Hydrogen sulphide is a colourless, toxic, flammable gas, also known as sulphuretted hydrogen or sewer gas. It has a foul odour frequently referred to as ‘rotten eggs’. In nature, bacteria under anaerobic conditions can reduce sulphate materials to release hydrogen sulphide. Natural gas and crude petroleums contain varying quantities of hydrogen sulphide. The characteristic odour of hydrogen sulphide can be detected around petroleum refineries, coke ovens in steel works and coke oven plants.
The U K Health and Safety Executive Document EH 40 /2005, ‘Workplace Exposure Limits’, in force from October 2007, sets the following workplace exposure limits for hydrogen sulphide
Workplace Exposure Limit
|Hydrogen Sulphide||Long term exposure limit (8 hour TWA reference period)||Short term exposure limit (15 minute TWA reference period)|
|(Figures quoted in vapour phase)|
Concentrations above these limits lead to headaches, dizziness and eventually, in higher concentrations, severe respiratory effects leading to death. Standard gas detecting instruments in use in the industrial and marine environments have warning alarms fitted at 5ppm of hydrogen sulphide in air. It is essential that persons who are at risk of exposure to sources of hydrogen sulphide are provided with gas monitoring equipment which is calibrated at regular intervals by makers or appointed agents. Furthermore such persons should be fully trained in the use of the equipment.
To rely upon the sense of smell to detect levels of hydrogen sulphide is a dangerous practice. Following exposure to very low levels of hydrogen sulphide the olfactory senses are no longer responsive, thus exposure to higher concentrations will not be detected by smell and the result can be fatal.
Anaerobic conditions favouring the formation of hydrogen sulphide by bacterial action may exist in tanks containing stagnant water with dissolved or suspended solids. Crusts may form over stagnant water which, if broken, can allow trapped hydrogen sulphide to escape suddenly in lethal concentrations. Wetted lentils, wool and milk powder, carried in containers, have also been known to produce hydrogen sulphide. For these reasons tests of the atmosphere alone in the tank may not be sufficient to determine whether a hydrogen sulphide hazard exists. Proper tank entry precautions should always be followed.
As previously stated, crude oils may contain hydrogen sulphide and the concentration in the liquid covers a wide range. For instance Al Rayyan crude may contain 1500ppm in the liquid phase whilst Gulf of Suez crude may contain less than 1ppm in the liquid phase.There is no correlation between the hydrogen sulphide in the oil phase and the concentration in the vapour phase. Generally this hydrogen sulphide is removed by sweetening before shipment. However there is always the risk that some may remain in the vapour phase during shipment and this is confirmed by tests of the two crudes referred to above.
The ISGOTT guide (International Safety Guide for OilTankers and Terminals) gives extensive advice on hydrogen sulphide and refers to it in the vapour phase being 100 times greater than the liquid phase.
Thus the recommendation is that ships’ crew, cargo inspectors, jetty staff etc, should all wear hydrogen sulphide monitors during gauging and sampling operations related to crude oil and be guided by the information in ISGOTT.