Fuel Specifications

1. Density

Density is the absolute relation between mass and volume at a stated temperature. The unit for density is kg/m3 and for fuel a reference temperature is always stated, which is usually 15°C.

A reference temperature has to be given because the density of fuel varies with temperature. The terms “density in vacuum” and “density in air” are sometimes found on bunker receipts. Density is the relationship of the mass of a substance to its volume, not its weight to volume ratio, and therefore density by definition is in vacuum.
For bunker fuels with a density range of 800 to 1010 kg/m3 at 15C, the conversion calculation approximates to a difference of 1.1 kg/m3. Hence to convert density at 15C to the weight factor at 15C, 1.1kg/m3 should be deducted.
Specific gravity is the ratio of the mass of a given volume of a substance to the mass of an equal volume of water at the same temperature.
As it is a ratio there are no units but as temperature is stated, eg. 15 / 15 Sometimes specific gravity is quoted at 20/4C, but this is not specific gravity as the temperatures are not identical. It is however relative density, which is the ratio of the mass of a given volume of substance at a temperature t1, to the mass of an equal volume of pure water at temperature t1.
Since 1m3 of pure water at 4C has a mass of 1000kg, the density of a substance at t1C is equivalent to the relative density at t1 / 4

2. Kinematic Viscosity

Among the various characteristics of fuel oils, the most important thing is viscosity. Kinematic viscosity is the ratio of absolute viscosity to density – a quantity at which no external force is involved.

As these units are large, it is usual to divide them by 100, to give a smaller unit called centistokes (cSt), and these are the units used for marine fuel. numerical value for the kinematic viscosity of the residual fuel in centistokes (cSt) at 50C
For example, IFO 180, IFO 380. Knowledge of viscosity is necessary for the determination of heat temperatures required for a fuel for transfer purposes. It is also necessary for the estimation of the required temperatures for storage, pumping and injection.
The minimum viscosity for fuel transfer is determined by the maximum viscosity acceptable by the transfer pump. If the actual viscosity at transfer is lower than the pump design, the rate of transfer will be faster than the rated capacity of the pump. The viscosity of all marine fuels varies with temperature, and reduces as the temperature increases.
Knowledge of the viscosity/temperature characteristics enables the temperature for a required viscosity to be estimated. The viscosity/temperature relationship for any particular fuel depends on the crude oil source and the refinery processes employed during manufacture. If the measured viscosity is marginally above the ordered viscosity, various practical aspects have to be considered to determine the suitability of the fuel.
These are the increased heating for correct atomization and the operation of the fuel treatment plant. In practical terms the required increase in temperature will only be a few degrees, which the existing heater should be able to accommodate. Should the actual viscosity be significantly in excess of that ordered the fuel may be unsuitable because of the constraints of the fuel treatment system.
For example a fuel of 35 cSt at 100C (ISO grade RM35) would be unsuitable for a system designed to handle a maximum of RM25 grades. Also in the case of smaller tonnage where no fuel heating exists, fuel in excess of 10 cSt at 100C would also be unsuitable, because of the requirement for heat at a higher viscosity.

3. CCAI

The calculated carbon aromaticity index (CCAI) is an index of the ignition quality of residual fuel oil.

The running of all internal combustion engines is dependent on the ignition quality of the fuel. For spark-ignition engines the fuel has an octane rating.
For residual fuel oil two other empirical indexes are used: CCAI and Calculated Ignition Index (CII). Both CCAI and CII are calculated from the density and kinematic viscosity of the fuel.
Conventional type separators of purifier type have a maximum density limit of 991 kg/m3; with modern high density type separators it’s possible to clean fuel oils with a maximum density of 1010 kg/m3/150 C.

4. Flash Points

The flash point of a fuel is the temperature at which vapour given off will ignite when an external flame is applied under specified test conditions.

A flash point is defined to minimize 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.
According to the ISO Standard requirements the flash point for all distillate and residual grades except DMX shall be a minimum of 60’C .
Ship classification society rules specifically stipulate that fuels with the flash point of less than 60’C are not permitted but with some exception.
Flash point is considered as a useful indicator of the fire hazard with regard to the storage of marine residual fuels.
Flammable vapours may still develop in the tank headspace even if fuels are stored at temperatures below the determined flash point.
Some basic precautions are:
1. Flame screens on tank vents shall be maintained in good order and condition.
2. Temperatures in the fuel system shall conform to recognized codes of practice.
3. All electrical fittings in tank headspaces must be designed for hazardous conditions and meet appropriate safety standards.
4. Any sources of ignition in the vicinity of the vents shall not exist.
5. In case that levels of fuel in storage tanks is low, heating coils should be shut down.
6. Gas detectors shall be calibrated correctly before they are used to check the flammability of headspace gas.
7. Headspace flammability readings shall be considered hazardous if they exceed 50% lower flammable limit (LFL). Low pressure air purging of the headspace will assist to reduce the hazard.
8. The risks of electrical charges should be taken into account when using metallic sounding or sampling devices. Such devices should be earthed or bonded to the tank structure.

5. Pour Point

The pour point is the lowest temperature at which a marine fuel can be handled without excessive amounts of wax crystals forming so preventing flow.

If the temperature of a fuel is below the pour point, wax will begin to separate out which will block the filters.
The wax will also build up on tank bottoms and on heating coils. However, when heat is adjusted properly it can be difficult to get the wax to be dissolved because of its insulating nature.
In extreme cases hand cleaning of the tanks becomes necessary. The actual pour point of a fuel depends on various factors, which include the source of crude oil and the refining processes used in manufacture.
Although for the majority of residual grades in ISO 8217 (those with a viscosity higher than 15 cSt at 100oC – RM15 and above) the limit on pour point is 30oC, in practice the great majority have a pour point of less than 0oC. If the pour point is known an informed operational decision can be taken as to a safe storage temperature of the fuel.
A fuel is stored at a temperature at least 5oC to 7oC above the pour point in order to avoid the waxing problems described. Knowledge of the pour point is not usually available at the time of delivery, but it is one of the parameters routinely determined by fuel testing services.

6. Carbon Residue (Micro Method) OR Carbon residue on 10% Volume distillation residue

The carbon residue of a fuel is the tendency to form carbon deposits under high temperature conditions in an inert atmosphere, and may be expressed commonly as Micro Carbon Residue (MCR) or alternatively Conradson Carbon Residue (CCR).

It should be noted that numerically MCR is effectively the same as CCR. The overall relationship between actual diesel engine performance and carbon residue is poor, however, the carbon residue value is considered by some to give an indication of the combustibility and carbonaceous deposit forming tendencies of a fuel.
The carbon residue provides information on the carbonaceous deposits which will result from combustion of the fuel. For fuels with a high carbon- high carbon/hydrogen ratio, it is proved more difficult to burn them fully, which results in increased deposits in the combustion and exhaust spaces.
Fuels with a high carbon residue value may cause problems in older engines when they are operating under part load conditions. The carbon residue value of a fuel depends on the refinery processes employed in its manufacture.

7. Ash

It should be noted that numerically MCR is effectively the same as CCR. The overall relationship between actual diesel engine performance and carbon residue is poor, however, the carbon residue value is considered by some to give an indication of the combustibility and carbonaceous deposit forming tendencies of a fuel.
The carbon residue provides information on the carbonaceous deposits which will result from combustion of the fuel. For fuels with a high carbon- high carbon/hydrogen ratio, it is proved more difficult to burn them fully, which results in increased deposits in the combustion and exhaust spaces.
Fuels with a high carbon residue value may cause problems in older engines when they are operating under part load conditions. The carbon residue value of a fuel depends on the refinery processes employed in its manufacture.

8. Water

Water is the most common fuel contaminant and the level of water present is very low. The standards allow water up to a maximum of 1% in residual fuels, however, the majority of fuel deliveries have water contents below 0.5%. The ingress of water can come from a number of sources, which include tank condensation and tank leakage, and can generally be avoided by good management. Where steam is used for tank heating purposes the pressure in the coils is usually greater than the head pressure in the tank, hence any leakage will result in an increased water content in the fuel.

A further potential source is the purifier if an incorrect gravity disc is used for the fuel being treated, or if the water control solenoid valves are leaking on the centrifuge. Sounding pipes should be securely capped at all times when not in use. Air pipes must remain open and consequently be arranged to terminate as high as possible, or be fitted with approved self closing devices to avoid breaking water. They should be in protected locations, safe from mechanical damage during cargo handling or from flooding when washing down decks.
Gross water contamination may be reduced by gravity separation in the settling tank. The practical effect of this depends on various parameters, which include the density and viscosity of the fuel and the height of the settling tank.
- Both the density and viscosity are affected by the fuel temperature and the rate of settling is governed by Stokes’ law. Further reduction in the water content should take place in the purifier. Water can become intimately mixed with the fuel to form a stable emulsion, through churning in pumps or throttling through valves. This stable emulsion is very difficult to remove by conventional means. Various problems caused by water are as follows ;
To cause some retardation in the speed of combustion, resulting in still burning particles striking the cylinder wall and crown To cause to corrode tanks and pipelines with a consequent rise in the iron content Steam formation, resulting in vapour locking in heaters and foaming in the mixing tanks To erode injectors with steam, caused by cavitation To disrupt the atomizing spray pattern To incur sludge formation To dilute the cylinder liner oil film To foul turbo chargers To corrode exhaust valves

9. Sulphur

Sulphur is a naturally occurring element in crude oil, concentrated in the residual components of the crude oil distillation process. The amount of sulphur in the fuel oil depends mainly on the source of crude oil, and to some extent on the refining process.

Crude oils have a natural sulphur level and this is the primary feature which determines the Sulphur level in any particular blend of fuel oil. During the combustion process in a diesel engine the presence of sulphur in the fuel can give rise to corrosive wear.
This can be minimized by suitable operating conditions and lubrication with an alkaline lubricant for the cylinder liner. Considerable work has been done by the various engine manufacturers to ensure cylinder liner surfaces do not approach the dew point, which is the temperature at which gases condense to liquid. In a diesel engine the sulphur in the fuel first burns to SO2, then combines with excess oxygen to form SO3.
In the presence of water vapor the SO3 is converted to sulfuric acid. If the temperature of the cylinder wall is below that of the dew point the acid will become deposited on the wall. This dew point is a function of the sulphur content of the fuel and also the cylinder pressure. Only a relatively small proportion of sulphur is converted to acid and the remaining oxides pass out of the cylinder with the exhaust gases.
A broad spectrum of measures intended to control air pollution from shipping has been discussed within the International Maritime Organization (IMO) for over a decade. One of these pollutants is sulphur dioxide (SO2) which one causes of acid rain.
This can result in deforestation and damage to man-made structures. Some restrictions now exist with respect to sulphur in some marine fuels and these are often on a voluntary basis. Examples are the use of low sulphur fuels on ferries in Scandinavian and Baltic waters, and in tanker movements on the Prince William Sound in Alaska.
In 1997 a new treaty was signed with members of IMO adopting Annex VI to 1973 MARPOL. Upon notification of this Annex by sufficient member states (or review by 2002) a maximum sulphur level of 4.5% m/m will be applied to all marine fuels.
If regional areas are legislated through IMO, it would seem that vessels trading to those areas would have to carry two types of fuel. While such an approach could be incorporated into new construction, it might not be straightforward to convert existing ships.
Any new requirements which are imposed on merchant shipping to reduce air pollution could have a major impact on ship-owners.
- The SO2 emissions are a direct function of the sulphur content of the fuel and essentially there are two ways of reducing them:

Formation of SO2 can be limited by restricting the sulfur content of the fuel.

10. Sodium

Sodium occurs naturally in crude oils and is concentrated in residual streams during refining. It can be introduced into fuel streams as a scavenger, used to control the hydrogen sulphide content of fuel oil via salt water contamination or through sodium ingress into a marine diesel engine due to salt water-saturated air.

11. Vanadium

Vanadium is a metal present in all crude oils. The levels found in residual fuels depend mainly on the crude oil source, with those from Venezuela and Mexico having the highest levels.

Problems associated with high vanadium are largely overcome by good engine design, correct fuel treatment (water removal) and correct engine operating conditions.
The actual level is also related to the concentrating effect of the refinery processes used in the production of residual fuel. Most residual fuels have vanadium levels of less than 150 mg/kg. In general, residual fuel contains a small amount of sodium when delivered, typically below 50 mg/kg. The presence of sea water increases this value by approximately 100 mg/kg for each per cent sea water.
Normally sea water can be removed from the fuel by gravitational separation in the settling tank and centrifugal purification. Very occasionally, sodium hydroxide used in the refining process may be a source of contamination. Some of the sodium may be present in an oil soluble form that cannot be removed on board ship. Particular attention is paid to the amounts of sodium and vanadium in the fuel as these elements have a low “striction” temperature.
This is the temperature at which the ash becomes semi-liquid and can adhere to components in the combustion system if the component temperatures are high enough. The melting point of the ashes depends on the constituents of the ash. There are no practical methods by which vanadium can be removed from fuel on board ship.
Therefore the only practical way to restrict vanadium is by limiting its content in the fuel purchasing arrangement. One possible operational solution for older engines which are sensitive to vanadium and sodium is the use of a fuel additive which acts as a combustion modifier.

12. Total sediment aged

Fuel stability testing There are three sediment tests covered within ISO 8217. They all use the hot filtration test method [described in ISO 10307 Petroleum Products] and all aim to define the total amount of sediments contained in a fuel sample.

Total Sediment Existent (TSE): ISO 10307-1 A fuel sample is heated to 100ºC and passed through a filter paper. The amount of dry sludge retained on the filter paper correlates with the amount of sludge that is likely to be separated by an on-board centrifuge.
Total Sediment Potential (TSP): ISO 10307-2 (Thermal Aging) A heated sample of the fuel is placed in a flask, which is then placed in an ageing bath at 100ºC ± 0.5ºC for 24 hours ± 15 minutes. After 24 hours the flask is removed from the bath and shaken vigorously prior to passing through a filter paper. The result of the test is reported to the nearest 0.01% m/m and is expressed as Total Sediment Potential (TSP).
Total Sediment Accelerated (TSA): ISO 10307-2 (Chemical Aging) A sample of the fuel is heated to achieve a viscosity of approximately 50mm2 /s. After 10 minutes, a measured amount (10% of sample size) of hexadecane is added and the sample is placed in an ageing bath at 100ºC ± 0.5ºC for 60 minutes ± 2 minutes.
The sample is shaken vigorously prior to passing through a filter paper. The result of the test is reported to the nearest 0.01% m/m and is expressed as Total Sediment Accelerated (TSA). The agreed limit for both TSP and TSA is 0.10% m/m – a fuel that falls below this limit should be viewed as thermally stable and able to homogenously maintain asphaltenic phase suspension.
As far as ISO 8217 is concerned, the ISO 10307-1 method is used to determine the level of sediment in distillate fuels whereas the ISO 10307-2 method is used to ascertain the level of sediment in residual products (ISO 8217 2010/2012 allow for the TSA method to be used for sediment determination but TSP is still classed as the official reference method).

13. Aluminium + Silicon

Total Sediment Existent (TSE): ISO 10307-1 A fuel sample is heated to 100ºC and passed through a filter paper. The amount of dry sludge retained on the filter paper correlates with the amount of sludge that is likely to be separated by an on-board centrifuge.
Total Sediment Potential (TSP): ISO 10307-2 (Thermal Aging) A heated sample of the fuel is placed in a flask, which is then placed in an ageing bath at 100ºC ± 0.5ºC for 24 hours ± 15 minutes. After 24 hours the flask is removed from the bath and shaken vigorously prior to passing through a filter paper. The result of the test is reported to the nearest 0.01% m/m and is expressed as Total Sediment Potential (TSP).
Total Sediment Accelerated (TSA): ISO 10307-2 (Chemical Aging) A sample of the fuel is heated to achieve a viscosity of approximately 50mm2 /s. After 10 minutes, a measured amount (10% of sample size) of hexadecane is added and the sample is placed in an ageing bath at 100ºC ± 0.5ºC for 60 minutes ± 2 minutes.
The sample is shaken vigorously prior to passing through a filter paper. The result of the test is reported to the nearest 0.01% m/m and is expressed as Total Sediment Accelerated (TSA). The agreed limit for both TSP and TSA is 0.10% m/m – a fuel that falls below this limit should be viewed as thermally stable and able to homogenously maintain asphaltenic phase suspension.
As far as ISO 8217 is concerned, the ISO 10307-1 method is used to determine the level of sediment in distillate fuels whereas the ISO 10307-2 method is used to ascertain the level of sediment in residual products (ISO 8217 2010/2012 allow for the TSA method to be used for sediment determination but TSP is still classed as the official reference method).

14. Used lubricating oil (ULO)

Some used lube oil may contain components harmful to an engine, but not all used lube oils are necessarily unfit for purpose. Some additives used to identify used lube oil, such as calcium, are naturally occurring in crude oil and hence residual fuel. Test methods are designed to eliminate false positives.

15. Calcium and Zinc, or Calcium and Phosphorous

Calcium: A soft, grey alkaline earth metal, the fifth most abundant element in the earth’s crust. It is essential for living organisms, particularly in cell physiology, and is the most common metal in many animals.

Calcium occurs naturally in crude oils. It is introduced into the combustion space via cylinder lubrication oil.The alkaline Total Base Number (TBN) additives of cylinder lube oil contain calcium.
Calcium is concentrated in the residual part of the refinery process as lighter products are removed.
A fuel shall be considered to contain ULO when either one of the following conditions is met: Calcium > 30 and zinc > 15 or calcium > 30 and phosphorus > 15

16. Hydrogen sulphide

Hydrogen sulphide (H2S) is naturally present in crude oils and can be formed during the refining process and in storage tanks downstream of the refinery. Residual marine fuels and marine distillate fuels can contain H2S at various concentrations in the liquid phase.

The H2S concentration in the liquid phase will depend on the crude oil, the specific refining processes (such as Vis breaking or thermal cracking) used to produce the residual marine fuel, and the conditions under which the fuel is stored after production.
H2S in the liquid phase of a petroleum product can evolve into the available vapour space of the product’s storage tank. The relationship between liquid and vapour phase H2S concentrations is difficult to predict, however, because the vapour phase concentration depends on a number of fuel- and storage-related factors.
Although marine distillate fuels do not typically contain enough H2S in the liquid phase to cause safety concerns, higher liquid-phase levels of H2S can be found which can result in higher than expected vapour phase concentrations of H2S under some circumstances.

17. Cloud point

In liquids, the cloud point is the temperature below which a transparent solution undergoes either a liquid-liquid phase separation to form an emulsion or a liquid-solid phase transition to form either a stable sol or a suspension that settles a precipitate.

In the petroleum industry, cloud point refers to the temperature below which paraffin wax in diesel or biowax in biodiesels forms a cloudy appearance. The presence of solidified waxes thickens the oil and clogs fuel filters and injectors in engines.
The wax also accumulates on cold surfaces (producing, for example, pipeline or heat exchanger fouling) and forms an emulsion or sol with water. Therefore, cloud point indicates the tendency of the oil to plug filters or small orifices at cold operating temperatures.

18. Cetane Index

Cetane index is used as a substitute for the cetane number of diesel fuel. The cetane index is calculated based on the fuel’s density and distillation range. Cetane index calculations can not account for cetane improver additives and therefore do not measure total cetane number for additized diesel fuels.

Diesel engine operation is primarily related to the actual cetane number, and the cetane index is simply an estimation of the base (unadditized) cetane number. Cetane number should equal or exceed cetane index, depending on the amount of additive used.

19. Acid Number

The Acid Number is stable until the Base Number is reduced to a sufficient level that will allow the acid number to increase. The Acid Number is usually deemed out of tolerance once it reaches 3.5 to 4.0 mg KOH/g with the exception of no zinc EMD/GE engine oils or marine applications using heavy fuel oils.

20. Lubricity, corrected wear scar diameter (wsd 1,4) at 60 OC h

The term lubricity is often defined as the ability of a lubricant—in this case diesel fuel—to minimize friction between and damage to surfaces in relative motion under load. Generally the tests used to evaluate diesel fuel lubricity try to create conditions of boundary lubrication.