FUEL OIL SPECIFICATIONS

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In order to fully define the grade of fuel loaded by the vessel, a fuel standard or specification should always be used. The old method of using the criteria of density and viscosity to specify the fuel required does not provide enough detail of the limits that must be adhered to in order to prevent premature damage to the main engine.

There is only one standard which is ISO 8217 (2005).  The attached fuel standard indicates the full specification for residual fuels of varying grades.

To specify which fuel should be ordered the engine makers books should be consulted, and this will indicate their suggestion to the permissible grade of fuel.

For example:

MAN B&W MC                      RMH 55, with 1010 density if correct purifier units installed

B&W GF                                Reference in manual only to limits of 600cSt at 50^oC, and 0.990 density

SULZER RTA                       RMH 55, with additional limit on sodium at 100ppm

MITSUBISHI UET45/80      RMD 15, with limits of 0.2% water, 3.5% sulphur, 10% carbon residue, and max viscosity 650cSt at 50^oC

WÄRTSILÄ VASA 32          RMH 55, with additional limits on sodium of 100ppm, and asphaltenes at 14%. Allows 1010 density with correct purifiers.

MaK M35 & M551                RMH 55

Thus we can see that the engine builders have constructed recent engines to consume the average `worst-grade' fuels in order to remain competitive, although continuously operating the engine at the fuel maximum limits will increase engine fuel related problems and fuel preparation difficulties.

The oil refiners also attempt to produce oil that is close to these standards, depending ultimately on the crude oil base. However the pricing of fuels is still governed by the final viscosity, with about a $4 discount per tonne for 380cSt oil against 180cSt. Both of these fuels is produced by `cutting' a high viscosity fuel (over 600cSt) with a lighter fraction to produce the desired viscosity, and hence the higher viscosity fuel used will often dictate the impurities in both oils. Thus buying a lighter grade fuel does not imply a better grade fuel, i.e. one with fewer impurities; in fact it may even produce more problems than the original residual stock.

Publications such as Lloyds List indicate the average grade of fuel in different areas of the world, and Northern Europe and the US Gulf have been chosen to show that bunker grades differ slightly around the world.

                        Density          Water              MCR               Ash     Alum/Si          Sediment

Europe           992.2              5.0                   17.5                0.11    88                    0.09

US Gulf          994.5              0.4                   19.3                0.08    58                    0.14

Hence although you may specify the correct grade of fuel, the supplier may deliver (sometimes unintentionally) fuel that can severely damage the engine. Fuel could be classified as unsuitable if the damage occurs over a small period of time, or if the fuel was obviously totally unusable, such as too low a flash point. Poor quality fuel would indicate that the fuel could damage the engine, but this would occur over a much longer period.

The only way of monitoring the delivered fuel quality would be to test the fuel using a recognised fuel testing agency. The results obtained from this sampling could be used in

1.         Disputes over quality with the supplier

2.         Highlights areas where operational adjustments of the main engine or preparation equipment may reduce damage

3.         Reduce frequency of poor bunkers, (supplier more likely to supply `other' vessel)

In order to obtain effective results, the sampling of the fuel must follow accepted guidelines to ensure that a representative sample is obtained. This sample should be obtained at the point of transfer, usually the ship’s rail or the closest point, (i.e. the ship’s manifold). Sampling using either the “Continuous Drip System”, or a “Flow Proportional Automatic Continuous Sampler” would be acceptable. This ensures that the small sample and the subsequent fuel oil analysis results give a true representation of the fuel oil quality that has been bunkered.

Once the sample has been taken, the sample bottle is sealed and signed by the engineer, before being sent to a recognised fuel testing agency. The test results will normally be available within three days of the test sample arriving at the testing agency, and will specify some of the following results:

à        Viscosity at 100⁰ C. This will allow the engineer to find the required heating temperature required for transfer of the oil within the vessel, and the injection temperature. Temperature ~ viscosity charts held on-board will be used to find these temperatures.

à        Viscosity at 50⁰ C. This is an alternate temperature for measuring the viscosity. For residual fuel oils the 100⁰ C measurement is considered more accurate. The 50⁰ C temperature is used as its results are still widely used within the marine industry  i.e . when 180cSt fuel is referred to, it will be the viscosity of the fuel at 50⁰ C; hence the viscosity measurement should always be read in conjunction with the measuring temperature.

à        Density at 15⁰ C. This measurement is required by the engineer to calculate the quantity of fuel in tonnes within the vessel’s tanks from volume measurement calculated from the tank gauging. There is a density limit for the fuel treatment plant i.e. purifiers, with the modern purifiers capable of cleaning oils up to a density of 1010 kg/m³. Older purifiers using a water seal can only accept oils up to a density of 991 kg/m³, and still maintain effective operation. Ignition and combustion characteristics of higher density fuels may be inferior and be an indication of poor ignition quality.

à        Water. The water limit of 0.5% for residual and less for distillate is defined in all fuel specifications. Quantity shortfall, gassing up, poor lubrication and reduction of combustion energy being some of the problems. Salt water being the main problem resulting in deposits and corrosion. It is possible to homogenise after centrifuging to emulsify (NO_x reduction).

à        Ash. This is a measure of all the incombustible material free of carbonaceous matter contained within the fuel. High ash content fuels will lead to abrasion of the fuel injection equipment, and fouling of the exhaust gas path. Ash levels can contain -

·         natural organic compounds of the oil such as silicon, sodium, vanadium etc

·         Contamination from the refining process, such as aluminium and silicon which are present in the catalytic fines used in some refineries

·         Contamination from the storage of the fuel, such as sand, scale, rust, etc

à        Micro Carbon Residue. This test replaces the Conradson Carbon Residue test (USA) and Ramsbottom (Europe), but gives the same readings. The carbon residue indicates the difficult to burn carbon and fouling nature of the fuel, and residual fuels always have higher carbon residue levels than marine diesel oils. As the combustion of the residual fuel always produces heavier exhaust fouling, then more frequent in service cleaning of the T/C and EGB and out of service maintenance will be required for these fuels.

à        Sulphur. This measurement of the mass of sulphur in the fuel indicates the corrosive nature of the fuel’s reactants once the combustion process is complete. When the sulphur in the fuel has been converted to a sulphur di or trioxide, then condensation of these products will form a highly corrosive sulphuric acid. This produces corrosive wear on components at a temperature of 170⁰ C or lower, such as the cylinder liner wall, piston rings, exhaust valve stems, and the cooler regions of the waste heat boiler. As discussed in the Emission section, most residual fuel oils have a sulphur level between 1 and 4%.

à        Total Sediment – Accelerated. This test is primarily for the residual fuels, and gauges the stability of the asphaltene phase of the fuel. When a fuel becomes unstable in storage, the various components of the fuel break-down, and the resultant sludge formed at the base of the tank is high in asphaltenes, and produces heavy filter blockage and fouling/damage to the components in the fuel injection system. The accelerated nature of the test attempts to replicate the long term instability of a fuel using a test, which only lasts one hour.

à         Pour Point. This is a measurement of the temperature at which the fuel oil ceases to flow. The pour point indicates the minimum temperature that a fuel has to be held at in order to avoid filter plugging from cold oil. Once a fuel starts to solidify, due to the formation of waxes within the fuel structure, reheating will not totally convert the fuel back to its original state (re-dissolve). Hence it is imperative that sufficient heat is given to the fuel in storage to maintain the oil temperature at 5-7⁰ C above the quoted pour point (transfer pumps designed with a max 800-1000cst). Residual and diesel fuels can have pour points up to 30⁰ C and 15⁰ C respectively.

à        Nett Calorific Value. This measures the energy of the fuel as determined by actual combustion of the test sample. Although it is desirable to have a high energy fuel, the chemical structure of the hydrocarbon dictates the energy available, with the residual fuels having a lower calorific value than the distillate fuels.

à        Flash Point. The international accepted minimum temperature for the flash point of a marine fuel is 60⁰ C. This limit is stipulated for safe storage of the fuel, store 10⁰ C below, as most fuels are heated in the tanks. Generally fuels have flash points in excess of 100⁰ C, thus it is only when the fuels have been mixed with volatile substances such as kerosene, crude oil, etc that the flash point will be low when tested.

à        Silicon. This element can naturally occur in crude oils but usually at low levels. When testing indicates a high level of silicon, then the presence of highly abrasive catalytic fines should be suspected within the fuel.

à        Aluminium. This element can naturally occur in crude oils but usually at low levels. When testing indicates a high level of aluminium, then the presence of highly abrasive catalytic fines should be suspected within the fuel. Note the catalyst would normally increase both aluminium and silicon levels.

à        Iron. This can include both the naturally occurring irons within the oil, but also the abrasive rust and scale particles present from the transportation of the oil.

à        Vanadium. Another natural occurring element, but this will cause corrosive deposits to be formed on the high temperature components of the engine, such as the exhaust vale and turbocharger exhaust gas side.

à        Sodium. High levels of this element usually indicate salt water contamination, which should be reduced by an effective purifier stage. It is important that the sodium level is reduced before the fuel is injected into the cylinder, as sodium will combine with any vanadium present to form highly corrosive deposits, as well as heavy fouling of the turbocharger unit.

à        Phosphorus (P), Lead (Pb), Calcium (Ca), zinc (Zn), iron (Fe). All of these elements are measured to test for the presence of waste lubricating oils within the fuel oil. Some suppliers add waste automotive lube oil for easy disposal and cost gains. The presence of such waste oil is highly undesirable, as the additives present in the lubricating oil are soluble and will prevent the dirt and abrasive particles being removed by the purifier unit. Hence abrasive damage of the fuel injection equipment, rings/liner and formation of deposits on components such as turbochargers will take place. Environmental issues involve the increase of particulates in the exhaust gas.

Note that none of the specifications include any limits on mixing compatibility, or sodium content, and only the DO grades have a reference to ignition quality.

For compatibility, the aromaticity reserve in the fuel is the important factor. The ashphaltene phase is kept in suspension by the aromatic of the fuel, and extreme care should be taken when mixing residuals with fuels of low aromaticity levels such as distillates. If this were to occur then the ashphaltene phase would precipitate causing filter blockage, and purifier difficulties. The biggest culprits of poor compatibility fuels were residuals from vis-breaker refineries, but as these units are being used less frequently this problem may not increase. Straight run residuals are fuels, which are highly paraffinic oils with the correct viscosity without light fuel dilution, and these tend to be the most stable fuels.              

Stability is the resistance to chemical breakdown on its own due to heating /moving which can form sludges.