Presence of hydrogen sulphide in crude oils
Hydrogen Sulphide (H2S) is a very toxic, corrosive and flammable gas. It has a very low odour threshold and a distinctive odour of rotten eggs. H2S is colourless and is heavier than air, having a relative vapour density of 1.189. It is soluble in water.
Sources of H2S
Many crude oils come out of the well with high levels of hydrogen sulphide (H2S), but a stabilisation process usually reduces this level before the crude oil is delivered to the vessel. However, the amount of stabilisation may be temporarily reduced at times. Thus, a tanker may receive a cargo with a hydrogen sulphide content higher than usual. In addition, some crude oils are never stabilised and always contain a high hydrogen sulphide level.
Hydrogen sulphide can also be encountered in refined products such as naphtha, fuel oil, bunker fuels, bitumens and gas oils. Cargo and bunker fuels should not be treated as free of H2S until after they have been loaded and the absence of H2S has been confirmed by both the results of monitoring and relevant MSDS information.
Expected Hydrogen Sulphide Concentrations
It is important to distinguish between concentrations of hydrogen sulphide in the atmosphere, expressed in ppm by volume, and concentrations in liquid, expressed in ppm by weight.
It is not possible to predict the likely vapour concentration from known liquid concentration but, as an example, a crude oil containing 70 ppm (by weight) hydrogen sulphide has been shown to produce a concentration of 7,000 ppm (by volume) in the gas stream leaving the tank vent.
The H2S concentration in the liquid phase does not alone determine the concentration in air. Precautions against high H2S concentrations are normally considered necessary if the H2S content in the vapour phase is 10 ppm by volume or above. The effects of the H2S at various increasing concentrations in air are shown below.
(ppm by volume in air & Physiological Effects)
Prolonged exposure to concentrations at 100 ppm induces a gradual increase in the severity of these symptoms and death may occur after 4 -48 hours exposure.
The H2S concentration in vapour will vary greatly and is dependent upon factors such as:
Amount of air circulation
Temperature of air and liquid
Liquid level in the tank
Amount of agitation
The TLV-TWA for H2S is given as 5 ppm over a period of eight hours (ACGIH 2004 data). The adverse effects of H2S can be reversed and the probability of saving the persons life improved if prompt action is taken.
Hydrogen Sulphide (H2S) Gas in Fuel Oil Prior to taking bunkers, the possibility of H2S content in the fuel supplied must be considered. Regardless of bunker type, testing for H2S must be carried out during and after every bunkering operation with the testing requirements must be detailed in the Bunker Planprepared by the Chief Engineer.
The vessel and those personnel involved with bunkering should be aware of this possibility. Precautions must be taken to prevent exposure of personnel to H2S gas. Hydrogen sulphide gas is colourless, transparent and is commonly recognized by its characteristic smell of rotten eggs. However, you are not to rely upon this method of detection as H2S deadens the sense of smell very quickly. Dangerous concentrations may be present that cannot be detected by smell alone. In the event of H2S detection during bunkering, the advice of the Ship Management Office is to be sought urgently and access to areas adjacent to and downwind of the loading area and tank vents, should be prohibited.
The Company provides equipment to monitor levels of H2S onboard in the form of a portable H2S meter together with a Draeger Multigas Detector and tubes for H2S.
b) Threshold Limit Value (T.L.V.) of Hydrogen Sulphide
The following guide, with the effects under various atmospheric concentrations on exposed persons, gives an indication of the dangers involved:
Measured In Air by Volume
50 – 100ppm : Eye and respiratory tract after exposure of 1 hour
200 – 300ppm : Marked eye and respiratory tract irritation after exposure of 1 hour
500 – 700ppm : Dizziness, headache, nausea etc within 15 minutes, loss of consciousness and possible death after 30 – 60 minute’s exposure
700 – 900ppm :Rapidly suffers loss of consciousness and death after a few minutes
1000 – 2000ppm Instantaneous death however prompt and efficient artificial respiration should be administered
The TLV-TWA for H2S is given as 5ppm over a period of eight hours, however, working procedures should aim at ensuring the lowest possible gas concentration is achieved in work locations.
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