FAQs - Technical

Q. Why is it I have three samples taken at the time of delivery, by continuous drip and they are all different?

A. Although the three samples may have been taken from one bulk, continuous drip sample they could have different test results because the person who made up the three samples from the bulk failed to thoroughly stir/shake/blend the bulk sample before he poured into the three sample bottles. It can happen that if this is not done, heavier components of the fuel like Catalytic fines, water and other solids will not be proportional in all three samples. TIP:- Thoroughly shake and stir (heat if in cold places) the bulk sample and fill three bottles a little at a time, making up to ten passes across the bottles before they are full.

Q. What are the summaries and definitions for the following fuel parameters?

Density
Viscosity
Flash Point
Pour Point
Ash
Water
Sulphur
Vanadium & Sulphur
Aluminium & Silicon
Sediment & Stability
Specific Energy
Compatibility
Ignition Quality

Density

Essential for quantity calculations, setting purifier, indicates specific energy and ignition quality.

Density is the absolute relationship between mass and volume at a stated temperature and the SI unit is kg/m3. The standard reference temperature used in international trade for density calculation of petroleum and its products is 15°C.

Knowledge of density is required for quantity calculations. Its value also needs to be known in order to select the optimum size of gravity disc for the centrifuge. In addition the density gives an indication of other fuel characteristics, including specific energy and ignition quality.

Specific gravity of a substance is the ratio of the mass of a given volume to the mass of an equal volume of water at the same temperature. As it is a ratio there are no units.

Relative density of a substance is the ratio of the mass of a given volume at a temperature t1 to the mass of a given volume of pure water at temperature t2. Like specific gravity, relative densities are ratios and hence no units. For example, relative density at 20/4°C. Since 1m3 of pure water at 4°C has a mass of 1000kg, the density of a substance at t1 °C is equivalent to the relative density at t1/4°C.

In the United States and some other countries, the density of petroleum products is defined in terms of API gravity. This is an arbitrary scale adopted by the American Petroleum Institute for expressing the relative density of oils. Its relation to relative density is:

API gravity (degrees) = (141.5 / Relative density @ 60 / 60 °F) - 131.5

The terms "density in vacuo" or "density in air" are sometimes used on bunker receipt notes. As density is the absolute relationship between mass and volume and not its weight to volume, by definition density is in vacuo. Although often used the term "density in air" is incorrect and should be referred to as a "weight factor". The reason for this is because, to a small extent, a substance weighed in air is supported by the buoyancy of air acting on it. Thus the weight of a liquid in air is slightly less than the weight in vacuo. There is no simple relationship between density and "weight factor" but for bunker fuels the difference approximates to 1.1 kg/m3. To convert density at 15°C to the "weight factor" at 15°C, 1.1 kg/m3 should be deducted.

Densities are measured over a range of temperatures, usually for convenience, at the temperature at which the fuel is stored. The value is then corrected back by the use of standard tables to the reference temperature. (Technical - back to faq's)

Viscosity

Determines injection and transfer temperature

Dynamic viscosity is a property of the internal resistance of a fluid that opposes the motion of adjacent layers. The unit of measure of this resistance in SI units is a Pascal.s. Frequently the unit of a Poise is used, where 1 Pascal.s = 10 Poise. It should be noted that dynamic viscosity is also referred to as absolute viscosity.

Marine fuel viscosity is usually expressed as kinematic viscosity, which is measured in Stokes. Kinematic viscosity is the quotient of the dynamic or absolute viscosity divided by the density, with both expressed at the same temperature. As one Stoke is a large unit, kinematic viscosity is usually measured in - centiStoke (cSt) - (one cSt = 1mm2/s). Sometimes viscosity is quoted in Engler, Saybolt or even Redwood and Appendix 1 shows the conversion.

The storage and handling temperature is determined by the viscosity if the pour point of the fuel is low. Typical maximum fuel viscosity for transfer is 800 - 1000 cSt . The temperature for atomisation of the fuel also depends on viscosity.

For distillate fuels the reference temperature used is 40°C. However for residual fuels 50°C is still commonly used, even though the international marine fuel specification has a reference temperature of 100°C.

Oil suppliers publish temperature/viscosity charts, however it should be appreciated that these charts are based on average data of a large number of representative fuels. As the relationship depends on its crude oil source and the refinery processes employed, estimations made from the charts cannot be regarded as precise. In general for the lower viscosity fuels the difference is small but it becomes wider as the viscosity of the fuel increases.

The majority of motor ships are fitted with fuel viscosity controllers so it is not normally necessary to estimate the injection temperature. For a diesel engine, the injection viscosity may be in the range 8 to 27 cSt and 13-17 cSt is typical. For boilers the atomisation viscosity depends on the burner and may be in the range 15 to 65 cSt.

It should be noted that because of the viscosity/temperature relationship a few degrees change could make a big difference to the injection viscosity. In practical terms this means that if the actual fuel viscosity is greater than that ordered it is likely that this can be accommodated by the fuel oil heater. The table below shows the temperatures required for a range of viscosities for injection at 13cSt and 17cSt

Injection temperatures for a range of viscosities

Injection Viscosity Injection Viscosity

Fuel 13 cSt 17 cSt
IF 180 119°C 109°C
IF 200 121°C 111°C
IF 220 123°C 113°C
IF 240 125°C 115°C
IF 380 134°C 124°C
IF 400 135°C 125°C
IF 420 136°C 126°C
IF 460 138°C 127°C

Whilst a satisfactory injection temperature may be attained it must be appreciated that the performance of the centrifuge may fall below design conditions. Typically a fuel with a viscosity >15% above that ordered can still be successfully used in the fuel treatment plant and engines.

Flash Point

Flash point - Legal requirement

The flash point of a fuel is the temperature at which vapour given off will ignite when an external flame is applied under standardised conditions. A flash point is defined to minimise fire risk during normal storage and handling. The minimum flash point for fuel in the machinery space of a merchant ship is governed by international legislation and the value is 60°C. For fuels used for emergency purposes, external to the machinery space the flash point must be greater than 43°C. Even when residual fuels are at a temperature below their measured flash point they are capable of producing light hydrocarbons, and flammability is discussed in section 10.2. The normal maximum storage temperature of a fuel is 10°C below the flash point, unless special arrangements are made.

Pour Point

Pour Point - Fuel must be maintained above pour point

The pour point is the lowest temperature at which a marine fuel can be handled without excessive amounts of wax crystals forming out of solution. If a fuel is below the pour point wax will begin to separate out, which will block filters. Also the wax will build up on tank bottoms and on heating coils. When heat is reapplied difficulties may be experienced in getting the wax to re-dissolve because of its insulating nature. In extreme cases manual cleaning of tanks may be necessary. In order to avoid the operational difficulties just described, it is necessary to store the fuel at least 5°C above the pour point. The transfer pumps in the fuel system are usually designed to operate at a maximum fuel viscosity of 800-1000 cSt. Hence for efficient transfer, the fuel should be heated to give such a viscosity. However for less viscous fuels, say below IF100, the important parameter is pour point. Although some marine diesel oil is sometimes delivered as a clean product for the purpose of wax indication it is considered as "black". This is because it may be delivered through the same transfer lines as residual fuels.

Ash

Ash - If excessive can give fouling deposits

The ash value is related to the inorganic material in the fuel oil. The actual value depends upon three factors, firstly the inorganic material naturally present in the crude oil, secondly the refinery processes employed, and thirdly, upon possible subsequent contamination due to sand, dirt and rust scale. The ash level of distillate fuels is negligible. Residual fuels have more of the ash forming constituents as they are concentrated from the residue from the crude oil refining processes. Vanadium and other materials such as silicon, aluminum, nickel, sodium and iron are the main contributing components. Typically the ash value is in the range 0.03-0.07% m/m.

Water

Water - Can cause sludge and combustion problems

Usually the level of water in the fuel is very low and 0.1-0.2% by volume is typical. The introduction of water can come from a number of sources, which include tank condensation, tank leakage or deliberate adulteration. Where steam is used for tank heating purposes, heating coil leakage is another potential source of water. A further potential source is the purifier itself if the gravity disc is incorrect for the density of fuel being treated. In practice the nature of the actual water present may be fresh, brackish or salt depending on the level of sodium as determined by elemental analysis. On a worldwide basis the salt content of seawater varies, but usually in first order terms 100mg/kg of sodium is associated with 1% of water. Gross water contamination will be removed in the settling tanks with the final water being removed by the centrifuge.

Sulphur

Sulphur is a naturally occurring element in crude oil that is concentrated in the residual component of the crude oil distillation process. Hence the amount of sulphur in the fuel oil depends mainly on the source of the crude oil and to a lesser extent on the refining process. Typically, for residual fuel on a worldwide basis, the value is in the order of 2-4% m/m.

The level of sulphur has a marginal effect on specific energy as discussed in section 3.14. In the combustion process in a diesel engine the presence of sulphur in the fuel can potentially give rise to corrosive wear. This can be minimised by suitable operating conditions and lubrication with alkaline lubricant for the cylinder liner. Considerable work has been done by the various engine manufacturers to ensure that the cylinder liner surfaces do not approach the dew point. The dew point is that temperature at which gases condense to a liquid. In a diesel engine the sulphur in the fuel having first burnt to SO2, then combines with excess oxygen to form SO3. In the presence of water vapour the SO3 is converted to sulphuric acid, which forms on the cylinder walls if the temperature is below the dew point for condensation of the acid. This dew point is a function of the sulphur content of the fuel and the pressure in the cylinder.

Only a relatively small proportion of the sulphur is normally converted in this way, the remainder of the sulphur oxides passing out of the cylinder with the exhaust gases. Crosshead diesel engine lubricants are readily available in a range of high alkalinities and are capable of neutralising the sulphur levels found in present day fuels. In trunk piston engines an alkalinity reserve must be maintained at an adequate level in the oil charge.

Vanadium and Sodium

Vanadium and Sodium - Potential high temperature corrosion can be minimised by temperature control and materials selection

Vanadium is a metal that is present in all crude oils in an oil soluble form. The levels found in residual fuels depend mainly on the crude oil source, with those from Venezuela and Mexico having the highest levels. The actual level is also related to the concentrating effect of the refinery processes used in the production of the residual fuel. The majority of residual fuels have vanadium levels of less than 150mg/kg. However some fuels have a vanadium level greater than 400mg/kg. There is no economic process for removing vanadium from either the crude oil or residue.

In general, fuel as delivered contains a small amount of sodium, and typically this is below 50mg/kg. The presence of seawater increases this value by approximately 100mg/kg for each per cent sea water. If not removed in the fuel treatment process a high level of sodium will give rise to post combustion deposits in the turbo charger, which can normally be removed by water washing. High temperature corrosion and fouling can be mainly attributed to the vanadium and sodium in the fuel. During combustion, these elements oxidise and form semi-liquid and low melting salts that may adhere to exhaust valves and turbochargers. In practice the extent of hot corrosion and fouling is generally maintained at an acceptable level by the correct design and operation of the diesel engine. Temperature control and material selection are the principal means by which corrosion is minimised. It is essential to ensure that exhaust valve temperatures are maintained below the levels at which liquid sodium and vanadium complexes are formed, and for this reason valve face and seat temperatures are usually limited to below 450°C. Materials such as nimonic steels or stellite facing have been used by engine designers as these materials are resistant to the effect of fuel oil ash components. In older engines that do not include such materials, and may not have exhaust value cooling, the vanadium and sodium elements of the fuel are important. The actual temperature of the formation of the sticky low melting salts depends on the vanadium/sodium ratio, and a ratio of 3:1 results in the lowest temperature.

Aluminium and Silicon

Aluminium and Silicon - Usually present as catalyst fines which are abrasive, can normally be reduced to an acceptable level by a centrifuge

It is generally accepted that an indication of aluminium represents the potential presence of catalyst fines. These fines are particles of spent catalyst arising from the catalytic cracking process in the refinery. The fines are in the form of complex alumino-silicates. Depending on which catalyst is used the fines vary both in size and hardness. If not reduced by suitable fuel treatment the abrasive nature of the fines does damage to the engine, particularly fuel pumps, injectors, piston rings and liners. Historically the generally accepted parameter for limiting the amount of catalyst fines was by specifying a limit of 30mg/kg of aluminium. The percentage removed by fuel treatment depends upon the size and density of the particulate matter. Operational experience has shown that if there are greater than 30mg/kg of aluminium in the fuel before treatment it is likely that the fuel treatment plant will not be able to reduce the level sufficiently and abrasive wear will take place. The extent of this wear depends on the level of contamination and also the capacity of the fuel treatment plant for removal. Today (1996), the accepted way of limiting the amount of catalyst fines is by considering the combination of aluminium and silicon in elemental form to 80mg/kg. The reason for this change is because it is considered more realistic in limiting the presence of catalyst fines, which on a worldwide basis are of variable composition. It should be appreciated that the catalyst is an expensive material for the oil refiner and stringent measures are taken to retain it.

Sediment and Stability

Sediment and Stability - Fuel is stable if it does not break down giving heavy sediment

Sediment by extraction defines the insoluble residues remaining after extraction by toluene. These insoluble residues are contaminants such as sand, dirt and rust scale, and are not derived from the fuel. Such a definition and test method are suitable for clear distillate fuels, but are not applicable to residual fuels. What is of greater importance is the total sediment content of the fuel, including hydrocarbon material, which relates to stability. Stability of residual fuel may be defined as the ability of a fuel to remain in an unchanged condition despite circumstances which may tend to cause change; or more simply, as the resistance of an oil to breakdown. Conversely, instability would be the tendency of a residual fuel to produce a deposit of asphaltenic sludge as a function of time and/or temperature. In the 1980s three sediment test methods were developed which each relate to various aspects of sediment. They are the Total Existent Sediment (TSE), Total Sediment Accelerated (TSA) and Total Sediment Potential (TSP) methods. Each method is that of filtration through a double filter paper and what differs between each method is the treatment of the fuel sample prior to filtration. In the case of the TSE procedure there is no specific sample preparation, and the sediment result relates to the dirt in the sample. For the TSA procedure the sample is mixed with 10% cetane and heated for 1h at 100°C before filtration. In the case of the TSP test the sample is heated for 24h at 100°C to simulate thermal ageing of the fuel. In the event of lack of stability of a fuel it is likely that filter blockage will be experienced. Should there be any difficulty in identifying the nature of this material a small portion should be placed in an open container and allowed to float in a vessel containing water at a temperature of 60-70°C. A waxy material will melt but an asphaltenic sludge will not.

Compatibility

Compatibility - The ability of two fuels when mixed to remain stable

Whilst every fuel is manufactured to be stable within itself, in that it does not have the tendency to produce asphaltenic sludge, it does not necessarily follow that two stable fuels are compatible when blended or mixed together.

Problems of incompatibility between fuels are rare but when they happen the results are severe. Typical problems are sludging and blockage of bunker and service tanks, pipe runs, filters and centrifuge bowls. In extreme circumstances the only remedy is manual removal of the sludge build-up. It is impossible to give precise advice on the probability of compatibility problems between two fuels but the risk of incompatibility can be ranked

Fuel A Fuel B

High Risk Low Density High Density

Moderate Risk High Density High Density

Lowest Risk Low Density Low Density

Incompatibility is the tendency of a residual fuel to produce a deposit on dilution or on blending with other fuel oils. A blend is regarded as being stable if it is homogeneous immediately after preparation, remains so in normal storage and at no time produces or tends to produce sludge on a significant scale. Under these circumstances the fuels forming the blend can be considered as compatible with each other. By definition, residual fuels are the remainder of the crude oil after the more valuable components have been extracted for the manufacture of petroleum products. The chemical composition of residual fuels is difficult to define as it depends upon the source of the crude oil and the manufacturing processes used in the extraction of the petroleum products. However, by considering the chief constituents of residual fuels an appreciation can be made of the sludge forming mechanism. These constituents of a residual fuel include asphaltenes, resins and liquid hydrocarbons. The generic term "asphaltenes" covers a wide range of heavier hydrocarbon structures. Besides being of high molecular weight and high carbon/hydrogen ratios, they may also contain small amounts of other elements, depending on the source of the crude oil.

Asphaltenes are believed to exist in residual fuel as "micelles". The resins can be considered as low molecular weight asphaltenes and these resins produce true solutions (i.e. molecular dispersions). The resin molecules in an oily medium of a residual fuel are known as "maltenes" whilst the liquid hydrocarbons are an oily medium of still lower carbon/hydrogen ratio and molecular weight than the resins, which acts as a solvent for the other constituents. Thus a residual fuel oil is generally considered to contain a disperse phase of asphaltenes complexed with high molecular weight components of the maltenes (resins) and liquid hydrocarbons in the form of a micelle. The continuous or intermiceller phase consists of low molecular weight constituents of the maltenes. In this way one can visualise a general decrease in carbon/hydrogen ratio and molecular weight from the centre of the micelle through to the continuous phase. Although the hypothetical zones are shown separately, in fact they merge with each other, so there is no distinct interface between the micelle and intermicellar phases.

A state of equilibrium exists and the "micelles" are considered to be "peptized" (i.e. colloidally dispersed). If, however, the carbon/hydrogen ratio of the maltenes is lowered, say by the introduction of a paraffinic diluent, the resins which are absorbed in the asphaltenes are to a certain extent desorbed. This results in the asphaltene particles not being completely surrounded by resins and they are mutually attracted. This leads to a precipitation that appears as sludge.

Specific Energy

Specific energy - Net value for diesel engines and gross value for boilers, usually calculated from empirical equations

The specific energy of a fuel expressed in MJ/kg depends on the composition. For marine fuel the main constituents are carbon and hydrogen both of which release energy on combustion. Sulphur also releases energy on combustion but to a lesser extent than carbon and hydrogen. The fuel density is mainly proportional to the ratio of carbon and hydrogen atoms in the fuel.

Figure 15 shows the relationship of the net specific energy of fuel taking account of the density and sulphur content. On a worldwide basis the density of residual fuel is typically in the range 975-990kg/m3 and the sulphur level 2-4% m/mFor practical purposes specific energy can be calculated from empirical equations. Of these there are two, one for the net specific energy <<...>> which is applicable for a diesel engine, and the other the gross specific energy <<...>> for boilers when all the water vapour is condensed out.

<<...>>

<<...>> <<...>>

where <<...>> is the density at 15°C, in kilograms per cubic metre; <<...>> is the water content, expressed as a percentage by mass; <<...>> is the ash content, expressed as percentage by mass; <<...>> is the sulphur content, expressed as a percentage by mass.

Figure 16 shows the net specific energy taking account of variations in density, sulphur and water. Ash has a slight effect and may be accounted for by subtracting 0.02 MJ/kg for each 0.05% m/m of ash. Typically the ash value is in the range 0.03-0.07% m/m.

Ignition Quality

Ignition quality - Relates to part of the combustion process. For residual fuels empirical equations for CCAI and CII

The ignition quality of a fuel is a measure of the relative ease by which it will ignite. For distillate fuels this is measured by the cetane number, which is determined by testing in a special engine with a variable compression ratio. The higher the number the more easily will the fuel ignite in the engine. For residual fuel there are two accepted empirical equations both based on the density and viscosity of the fuel. These are the Calculated Carbon Aromaticity Index (CCAI) and Calculated Ignition Index (CII). The CCAI gives numbers in the range 800-870 which the CII gives values in the same order as the cetane index for distillate fuels. Of the two equations CCAI values are more frequently quoted.

CCAI = d - 81- 141 log log (VK + 0.85)
d = density in kg/m3 at 15oC
VK = viscosity in mm2/sec @ 50oC

Figure 17 is a nomogram that incorporates both CII and CCAI. If the viscosity is fixed and the density is raised the CII value falls and the CCAI value is increased. Similarly if the density is fixed and the viscosity lowered the CII value falls and the CCAI value is increased. In general values less than 30 for CII and greater than 870 for CCAI are considered problematical. If required, further guidance on acceptable ignition quality values should be obtained from the engine manufacturer. CCAI and CII predict performance under full load conditions, however damage tends to occur at part loads and on faster running engines.

BEWARE Fuel takes a finite time from the start of the injection to the start of combustion. During this period fuel is intimately mixed with the hot compressed air in the cylinder where it begins to vaporise. After a short delay known as the ignition delay, the heat of compression causes spontaneous ignition to occur. Rapid uncontrolled combustion follows as the accumulated vapour formed during the initial injection phase is vigorously burned. The longer the ignition delay, the more fuel will have been injected and vaporised during this "pre-mixed" phase and the more explosive will be the initial combustion.

The second phase or "diffusion burning" phase of combustion is controlled by how rapidly the oxygen and remaining vaporised fuel can be mixed as the initial supply of oxygen near the fuel droplets has been used during the pre-mixed combustion.

Rapid pre-mixed combustion causes very rapid rates of pressure rise in the cylinder resulting in shock waves, broken piston rings and overheating of metal surfaces. Large diesel engines are designed to withstand a certain rate of pressure rise within the cylinder although the figure will vary between different designs.

Rate of Pressure Rise (bar/degree crank)

Comments

Below 10 No problems
10 - 12 Acceptable
12 - 16 May cause problems
Over 16 Probably damaging

CCAI and CII are empirical attempts to estimate how long the fuel will take from injection to ignition and by implication the likelihood of engine damage. After calculating the CCAI or CII of a fuel, the operator must then judge the acceptability of that fuel for effective operation in the engine. Variations of engine load, rated speed and design affect the likelihood of poor combustion hence it is impossible to give precise figures that apply to all engines.