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Tuesday, January 16, 2018

REPORTS OF DIVERSE CARGOES OF FUEL OIL CONTAINING H2S

In recent weeks there have been reports of diverse cargoes of fuel oil, emanating from St Petersburg, that are reported as being destined for the bunker market and that apparently contain significant quantities of H2S. It is, therefore, felt worthwhile to highlight the topic of H2S with regard to bunker quality, its consequences for shipboard safety, its potential source when found in bunkers and its impact for corrosion.

H2S as a substance can be found in bunkers in either a dissolved state (that is in the liquid phase) or as an evolved gas originating from the bunker fuel. The example supplied within the ISGOTT Publication suggests a correlation between the dissolved state and the equivalent quantity of evolved gas is a factor of 100, e.g. 1 ppm dissolved in the liquid phase will potentially create 100 ppm concentration of gas in the vapour or head space.

Due to the relatively low concentrations of this substance in the liquid phase, there will be negligible impact upon the total Sulphur content of the fuel oil as delivered. Thus vessel’s personnel will not necessarily be aware of the presence of this type of substance in a bunker fuel on or prior to delivery to a vessel but, subject to its concentration, may detect it by smell (it smells like that of rotten eggs).

Given the TLV (Threshold Limit Value) of H2S gas at 10 ppm it can be readily seen that the extent of H2S in the dissolved state needs only to be at levels in excess of 0.1 ppm before health/safety problems can occur. The ISGOTT Publication supplies a series of expected symptoms for those suspected of inhaling this gas, which are as follows:

50 – 100 ppm          Eye and respiratory tract irritation after exposure of one hour.

200 – 300 ppm    Marked eye and respiratory tract irritation after exposure of 1 hour

500 – 700 ppm          Dizziness, headache, nausea, etc. within 15 minutes, loss of consciousness and possible death after 30-60 minutes exposure.

700 – 900 ppm    Rapid unconsciousness, death occurring a few minutes later

1000-2000 ppm          Instantaneous collapse and cessation of breathing.

Given the refining and production process involved to generate the oils used for blending to create an acceptable quality bunker fuel, it is not to be expected that substances such as H2S would be present. Given that H2S has a boiling point of roughly –60 degrees Centigrade, this substance would have been removed during the primary distillation of the crude oil, if present in the original crude oil feed stock. Thus in order to find a possible alternative source for this substance in a fuel oil it is perhaps necessary to look to sulphur degradation caused by micro-organisms – typically Sulphate Reducing Bacteria (SRBs). If this is the source of the production of H2S in the bunker fuel, which is a possibility, then fuel tanks onboard the vessel will become infested creating potential longer term problems for the vessel particularly with regard to efficient purification of the fuel and the potential for blockage of filter systems within the fuel supply system to the main engine. These later problems are caused by the generation of sludges and emulsions created by these organisms.

Hydrogen Sulphide (H2S) is a flammable gas having explosive limits created by a mixture of between 4.3% to 46% with air. Further H2S is soluble in water creating a very acidic (sour) water or water droplet. This sour water droplet will be a very good electrolyte promoting corrosion activity in the head space or vapour phase of the storage tank. Some crude oil tankers have already experienced the intensive pitting created by the “by products” of SRBs in their cargo tanks.

Thus, in conclusion, it is recommended that great care be exercised on board when it is suspected (by smell) that a bunker fuel contains H2S  particularly if the evolution of this substance is due to the infestation of the bunker fuel by SRBs.