Marine Fuel Specifications

Ash

Ash is the inorganic solid residue left after combustion, and is important because it gives an indication of the propensity of a fuel to form post-combustion deposits.

For petroleum fuels, the ash content quoted is normally that obtained by simple high temperature combustion, sometimes called 'oxide ash'.

Ash represents solid contaminants as well as metals present in the fuel in soluble compounds (vanadium), and part of the ash may even be comprised of catalyst particles from the refining process. Such particles are highly abrasive therefore solid ash should be removed as much as possible by centrifuging.

Calorific Value

This is the heat liberated by the combustion of a fuel. It is also known as 'specific energy', 'heat energy', or 'heat of combustion'. The gross (upper) value is normally quoted, but the net (lower) value is also used in many

calculations .

Carbon Residue

This is a laboratory measurement which can give an indication of the carbon deposit forming tendency of a fuel oil .

The significance of carbon residue is that fuels with high carbon residue content may cause increased fouling of gas ways in the engine necessitating more frequent cleaning, especially of the turbocharger and exhaust gas boilers.

Cetane Number

This is a measurement of the ignition quality of distillate fuel, expressed as a percentage of cetane that must be mixed with liquid methylnaphalene to produce the same ignition performance as the fuel being rated.

A high cetane number indicates shorter ignition lag and a cleaner burning fuel.

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Density

This is the mass (weight in vacuum) of a liquid per unit volume.

When quoting the density of a fuel or lubricant, it is normally quoted at 15 Degree C.

The preferred S.I. unit used to describe density is kg/m³, however it is also frequently quoted in kg/l or g/ml.

'Density' has largely replaced 'Relative Density' or 'Specific Gravity', which is the ratio of the mass of a given volume of liquid at one temperature, to the mass of an equal volume of pure water at the same or another temperature.

Water separation of the fuel using centrifuges / purifiers is achieved by limiting the density of the fuel to comply with the centrifuge manufacturers specifications A limit of 991 kg/m³ must be observed when traditional centrifuges are used (purifiers & clarifiers), however fuels with a density of up to 1010 kg/m³ are acceptable when centrifuges capable of handling fuel of this density are installed.

Diesel Index

This is an indicator of the ignition quality calculated using the relative density and aniline point. It is applicable to distillate grades only, and cannot be used for residual fuels. Although in the same order as cetane number / index, it may differ widely and should be accepted with reserve.

Flash Point

The flash point limit is set as a safeguard against fire and is the only parameter of a marine fuel which has a legal significance. National & International regulations absolutely prohibit the supply and use of a marine fuel having a measured flash point below 60 Deg C.

The 60 Deg C limit is an absolute requirement of both Classification Societies, and 'Safety Of Life At Sea' (SOLAS) mandatory maritime regulations.

Unlike other parameters of marine fuels, where from time to time exceptions may be granted from meeting certain specification limiting values, with flash point there can be no exceptions to meeting the 60 Deg C minimum specification under any circumstances whatsoever.

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If a marine fuel is found to have a flash point below 60 Deg C, the flash point cannot be raised by mixing the fuel with another fuel having a higher flash point. This is because the mixing relationship is not linear, and it only takes a small amount of light volatile vapor in the lower flash point fuel to significantly lower the flash point of the higher flash point fuel.

. Ignition Quality

This is the ability of a fuel to ignite, and is measured as the delay between the start of injection and the start of the combustion. (See also 'Cetane Number' and 'Diesel Index').

Ignition quality can to some extent be predicted by calculations based on viscosity and density, using formulas such as Shell CCAI. High density in combination with low viscosity may be an indication of poor ignition quality.

Sulphur

Sulphur in fuel is of great concern because of it's potentially harmful effect when converted to sulphuric acid during the combustion process.

In diesel engines, the corrosive effect of sulphuric acid during combustion is normally counteracted by the use of alkaline cylinder lubricants.

Vanadium

Vanadium is present in fuel in soluble compounds and cannot be removed.

High vanadium content, in combination with sodium, may lead to exhaust valve corrosion and turbocharger deposits, especially if the weight ratio of sodium to vanadium is approximately 1:3.

The weight ratio is considered of less importance when the vanadium content less than 150 mg/kg.

Viscosity

Viscosity is the resistance of a fluid to flow.

When viscosity is used to classify a fluid it is necessary to report the viscosity at a reference temperature e.g. 40 Deg C.

Viscosity is only one of many quality criterion for fuel oils, and is quoted mainly for reasons of storage, handling, and treatment.

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The viscosity of distillate fuels is quoted at 40 Deg C.The viscosity of residual fuels is quoted at 50 Deg C.

Different reference temperatures are used depending upon the viscosity classification used, the type of fluid being measured, and the national preference for viscosity measurement.

The higher the delivered viscosity of a residual fuel, the more heat is required to reduce the viscosity prior to injection. Some older vessels, designed and equipped to burn 180 cSt fuel, do not physically have the heating capacity to use 380 cSt fuel.

If a fuel oil heating system is incapable of lowering the viscosity at injection to an acceptable level, then the fuel will not atomize efficiently, and this means that the combustion chamber will contain relatively large droplets of fuel which will be difficult to ignite, and combust.

Difficulty in igniting fuel leads to a characteristic diesel "knock" which can lead to engine damage, and reduces the power available from the engine.

The viscosity of fuels on board ships cannot be altered by simply mixing different viscosity fuels together, this is because a homogeneous mixture cannot be achieved without the use of proper blending equipment. Without the proper blending equipment, pockets of fuel of varying viscosity form throughout the tank, and this can result in uneven combustion. fouling of injectors, and injection difficulties.

Water

Water in fuel should be removed by centrifuging the fuel before use. This applies especially to salt water, as the sodium content of seawater may result in deposits on valves and turbochargers

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