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Date: 12th December 2009

Published in World Bunkering Magazine: Winter Edition 2009

H2S! Take Care!

Guardian Marine Testing, warns of the risk of hydrogen sulphide and looks at the debate on acceptable limits.

The presence of Hydrogen Sulphide (or H2S as it is commonly referred to) in marine fuels poses a dual danger through the storage and handling of fuels. Not only must we be aware of the associated health & safety risks but also the damage that this corrosive gas can cause to ships tanks and fuel lines.

If we are aware of danger we can act to mitigate the risks. Historically there has been no routine test to identify this gas under ISO 8217. Instead, the policing of H2S was left with the refinery. But does the testing and indeed specification limit adopted downstream help to fully assist in quantifying the potential risk?

This article aims to highlight the issues surrounding H2S and the developments with the testing of H2S in bunker fuel oils.

Just what is H2S?
Hydrogen Sulphide is a colourless chemical compound both toxic and flammable. It is often responsible for the foul odours of rotten eggs in volcanic gas, stagnant water or flatulence!

Although very pungent at first, it quickly deadens the sense of smell so that potential victims may be unaware of its presence until it is too late. This poisonous gas can attack several different systems in the body, although the nervous system is most affected. It forms a complex bond with iron blocking oxygen from binding and stopping cellular respiration. Exposure to lower concentrations will result in nausea, shortness of breath, eye irritation, a sore throat and fluid in the lungs. Long-term, low-level exposure may result in fatigue, loss of appetite, headaches, irritability, poor memory, and dizziness. H2S is heavier than air it will settle and accumulate at the bottom of tanks and poorly ventilated spaces. To demonstrate its lethal ability we can list the following affects of exposure:

  • 1 ppm is the level that most people will detect the characteristic odour
  • 10 ppm is Occupational Health exposure limit for 8 hours per day working.
  • 20 ppm experience eye irritation.
  • 200 ppm the olfactory nerve is paralyzed and the sense of smell disappears.
  • 350 - 550 ppm leads to pulmonary edema with the possibility of death.
  • 550 - 1000 ppm causes strong stimulation of the central nervous system and rapid breathing, leading to loss of breathing.
  • 800 ppm is the lethal concentration humans with 5 minutes exposure.
  • Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.


How is it created?
Crude oil is a complex mixture of hundreds of hydrocarbons, including many which contain sulphur. Refining the crude oil includes converting most of that sulphur into gaseous hydrogen sulphide. The hydro-desulfurisation process liberates sulfur from petroleum by the action of hydrogen and the resulting H2S is converted to elemental sulfur by partial combustion.

Raw natural gas also contains gaseous hydrogen sulphide and sulphur-containing mercaptans, which are removed in natural gas processing. The hydrogen sulphide removed in the refining and processing of crude oil and natural gas is subsequently converted into byproduct elemental sulfur.

H2S in Bunker Fuels
H2S is undesirable and should not be present in marine fuels. However, as there are no current limits set for the accepted levels of H2S at the point of delivery of bunkers, the adoption of an agreed level is being considered.

Storage tanks & fuel systems which use bunkers containing H2S have been known to encounter high levels of corrosion. Sulphate Reducing Bacteria (SRB) in the oil have been said to cause Microbial Induced Corrosion to the tank bottom plating. In addition the H2S gas released from the bulk liquid during storage will build up in the tank head space to create an ideal environment for pyrophor (pyrophoric iron sulphide). In basic terms this means that Sulphide will react with Iron Oxide (rust) without Oxygen to create Iron Sulphide in an exothermic reaction resulting in substantial heat, which for obvious reason is not desirable!

Wet cracking is the phenomenon of blistering to exposed steel in an aqueous environment which contains hydrogen sulphide. This hydrogen related corrosion results in blisters or blister cracks to the plate surfaces. Fuel containing entrained H2S gas will release some of that gas throughout its storage. The rate at which the gas will evolve will depend on many varying factors including:

  • The quantity of H2S in the liquid phase.
  • The length of time the fuel is stored.
  • The temperature of the oil.
  • The way in which fuel is agitated, pumped or transferred.
  • The bio-chemical decomposition.

Setting Limits
ISGOTT (International Safety Guide for Oil Tankers and Terminals) state in its guidelines for crew entering pump rooms and enclosed spaces, a limit of 10ppm TVL (Threshold Limiting value) and OSHA (Occupational Health & Safety Administration) has guidelines for H2S exposure also set at 10ppm. The Crux of the matter is understanding the potential risk that this gas poses as being over exposed to this gas can result in at worst a fatality so what limit should we apply? Should the limit be applied to H2S in the vapour phase or the liquid phase?

From a health and safety prospective, it is H2S in the gas phase which is of highest importance as significant concentrations of the gas are known to accumulate in the headspace of storage tanks. From an operational prospective, it is the H2S in the liquid phase which is considered more important. However, the concentration of gas in the vapour phase can only be measured in any given moment of time whereas the measurement of H2S in the liquid phase gives a total concentration and therefore the potential of a given fuel to emit the gas in to the vapour phase.

Unfortunately, the correlation between H2S in the liquid phase and that in the vapour phase is far from simple. The amount of gas emitted by a liquid to the atmosphere depends largely on partition coefficients. In simple terms, this is the amount of gas that is entrained in a liquid phase that would be expected to be released to the atmosphere (the equilibrium point). This can be measured in a laboratory, commonly using the ‘Shake-Flask’ method and can be predicted by calculation. The problem that we have with marine bunker fuel is that it is not a ‘pure distillate’ or ‘pure chemical’ and as such each one could have a different chemical make-up and therefore a different coefficient.

Various studies have been carried out in an effort to determine the partition coefficient of residual oil. Depending upon which study is referred to, so do the findings. However, in general, it is estimated that for every 1 ppm H2S in liquid phase, it represents anything between 80 and 400 ppm in the vapour phase.

Test Methods
Historically the industry has relied of two established test methods for the detection of H2S, but both need to be put into context with the appropriate application

To measure the H2S in the LIQUID PHASE of the oil (BY IP 399). This is the measurement of the trapped (entrained) gas in the liquid which over time can be released. This analytical method is a more traditional wet chemistry test which involves lengthy preparation and is considered a “complex” test. The test once complete will quote a results in milligrams per kilogramme (mg/kg or ppm). Refinery (downstream) specifications may vary but generally a limit of between 2 to 3 mg/kg is applied prior to the sales of the stock.

To measure the H2S in the VAPOUR PHASE, this is the measurement of evolved gas into the headspace of a sample container, this test is carried out using Gas detection tubes, the laboratory test (ASTM D5705) involves heating a known quantity of oil in an enclosed vessel for a set period of time, after which a gas detection tube in the inert headspace measure the H2S in the gas phase
Additionally these gas detection tubes can be used to check H2S levels in tank tops or worn continuously by barge or vessel crew to calculate the exposure over time.

This onboard VAPOUR Phase test is best suited for use to quantify and assess occupational health risks. In reality the only semi accurate way of measuring the evolved gas is to carry out onboard measurements in the tank tops or the affected confined working areas. These onboard tests will give a “snap shot” of the level of H2S at that given time. However the onboard test is not a standard test method, it is simply a means to measure the levels of exposure, if you remember ISGOTT recommend exposure be limits to 10ppm TVL (Threshold Limiting value)

Reports of increased cases of H2S in bunker fuels has lead to the ISO working group to add a new test and specification limit to the proposed (draft) of the ISO 8217, expected to be finalised and published in 2010.

The test new method (IP 570) has been adopted by ISO in an attempt to create a faster and more accessible way of routine analysis. But problems are a foot, due to poor historical data on the occurrences of H2S in bunkers fuels the inclusion of the parameter has been delayed until 2012 to allow time for data to be collated and an appropriate specification to be concluded.

The Way Forward
It is clear that the presence of H2S in marine fuel is undesirable. Defining an acceptable limit within the specification is something that we believe will require considerably more debate. The limits of 2 mg/kg put forward by the ISO working group has been question because an acceptable level of 2 mg/kg in the liquid phase could give rise to as much as 800 ppm in the vapour phase, a lethal concentration with five minutes exposure and well in excess of the occupation health recommended 10ppm value.

The fact that the ISO working group have added H2S to the latest draft of ISO8217 is in our opinion a very positive step as it serves to heighten the profile of what is a very important debate. Currently there is little research on H2S in marine fuels and it seems that there is a long way to go before an acceptable level will be agreed.

 

 

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