• Catalyst Fines
• Cloud Point
• Density
• Flash Point
• Ignition Quality
• Pour Point, Cloud Point & CFPP
• Specific Energy
• Viscosity
• Viscosity Conversion Table

Calorific Value - See Specific Energy

Catalyst Fines
(Ref. ISO 8217:1996 - Annex D - Informative)

Catalyst Fines are the main source of potentially abrasive material in bunker fuels.

Measurement of aluminium plus silicon, with limiting values for all fuels in the Shell Specification and ISO 8217 : 1996 Fuel Tables, is intended to limit catalyst fines contamination to a level that will ensure minimum risk of abrasive wear, providing that adequate fuel pre-treatment is carried out.

The proportions of aluminium and silicon compounds that comprise catalyst fines, varies significantly from refinery to refinery, and the combined aluminium and silicon limit value of 80 mg/kg is intended to ensure that catalyst contamination will be no higher on average than has previously been implied by the limit of 30 mg/kg aluminium, that has been used in the Shell Marine Fuel Specifications for over 10 years. The aluminium plus silicon requirement of max. 80 mg/kg is therefore to be used in place of, not in combination with, the 30 mg/kg aluminium limit.

The lower aluminium plus silicon control applied to grade ISO 8217 : 1996 - Grade DMC (25 mg/kg) is based on the proportion of residual fuel that may be expected to be part of this product.

CCAI, Cetane No. and Cetane Index - See Ignition Quality

Cloud Point / Cold Filter Plugging Point (CFPP) - See Pour Point

Density

Knowledge of a fuels density is used to determine the optimum size of purifier gravity rings, to calculate a fuels calorific value, but most importantly to convert from volume to weight for invoicing purposes.

All densities listed in this publication are in terms of kg/m³ at 15°C. They should be divided by 1000 if the density in kg/l at 15°C is required.

When density is determined in accordance with ISO 3675, the hydrometer readings obtained at ambient temperature on distillate fuels, and at elevated temperatures of between 50 Deg C and 60 Deg C on fuels containing residual components, has to be converted to results at 15 Deg C using Table 53B of ISO 91-1.

When density is determined in accordance with ISO 12185, an appropriate correction for glass expansion coefficient has to be applied to readings obtained by digital density analyser at any temperature other than 15 Deg C, before conversion and application of Table 53B of ISO 91-1.

Flash Point - Residual Fuel Oils
(Ref. ISO 8217:1996 - Annex E - Informative)

Flash point is a valid indicator of the fire hazard posed by residual fuel oil, but information is available which shows that it is not a reliable indicator of the flammability conditions that can exist within the head spaces of tanks containing such fuels.

This means that residual fuel oil has the potential to produce a flammable atmosphere in the tank head space, even when stored at a temperature below the measured flash point.

Consequently residual fuel oils should be considered to be potentially hazardous and capable of producing light hydrocarbons which could result in tank head space atmospheres being near to, or entering, the flammable range. Appropriate precautions are necessary therefore to ensure the safety of people and property.

Further information and advice on precautionary measures are given in ' The Flammability Hazards Associated with the Handling, Storage and Carriage of Residual Fuel Oil - published by the Oil Companies International Marine Forum (OCIMF) December 1989. Additional information can also be found in 'International Safety Guide for Oil Tankers and Terminals (ISGOTT)', published by the International Chamber of Shipping.

Ignition Quality
(Ref. ISO 8217:1996 - Annex B - Informative)

Ignition quality of marine diesel fuels is a major factor which effects engine operation, particularly high speed units.

The Cetane Number or Cetane Index of distillate fuel indicates performance relative to a reference fuel.

The ignition quality of residual fuels is more difficult to predict because they consist of blends of many different components. However, residual fuel ignition quality may be ranked by determination of Calculated Carbon Aromaticity Index (CCAI) from density and viscosity measurements. A formula for CCAI determination is given below.

Ignition performance requirements of residual fuels in marine diesel engines are primarily determined by engine type and, more significantly, by engine operating conditions. Fuel factors influence ignition characteristics to a much lesser extent. For this reason no general limits for ignition quality can be applied, since a value which may be problematical to one engine under adverse conditions may perform quite satisfactorily in many other instances. If required, further guidance on acceptable ignition quality values should be obtained from the engine manufacturer.

Calculated Carbon Dromaticity Index (CCAI)

The viscosity and density of a fuel oil can be used to calculate its Calculated Carbon Aromaticity Index (CCAI) value, which allows ranking of its ignition performance. CCAI is calculated by using the following formula:

CCAI = D-81-141 Log₁₀Log₁₀ (Vk + 0.85) - 483 Log₁₀ ((T + 273)/323)

where
Vk = Kinematic Viscosity (mm²/s) at temperature T °C:
D = Density kg/m³ at 15 °C

Pour Point, Cloud Point & Cold Filter Plugging Point (CFPP)

These characterisitics are used to assess the performance of a fuel in cold operating conditions, and to determine the temperature at which fuel filters may begin to become blocked.

Shell Marine Fuels are manufactured so that they will be suitable for the environment in which they will be used, and their characterisitics may vary slightly at different locations to ensure that they are suitable for different climatic conditions.

For this reason, the specifications for MFO up to 80 cSt at 50°C give two maximum levels for Pour Point, and the specifications for GO and MDF give two maximum levels for Cloud Point or Cold Filter Plugging Point (CFPP) as appropriate.

Pour Point, Cloud Point & Cold Filter Plugging Point (CFPP) are controlled according to the International Load Line Zone in which any particular port is located. This is done on the basis that load line zones have a reasonable relationship to ambient temperature conditions. The acceptability of the higher levels in deliveries at ports in summer and tropical load line zones should be assessed if vessels are proceeding to colder zones, particularly during winter months.

Specific Energy / Calorific Value
(Ref. ISO 8217:1996 - Annex A - Informative)

Heat of combustion, specific energy or calorific value, is a measure of the energy content of the fuel. It decreases as density, sulphur, water and ash content increase.

Specific Energy is not controlled in the manufacture of fuel except in a secondary manner by the specification of other properties.

Specific energy can be calculated with a degree of accuracy acceptable for normal purposes from the equations given below :-

Specific Energy (Gross) MJ/kg
Qg = (52.190 - 8.802 p² 10^-6) [1 - 0.01 (x+y+s)] + 9.420 (0.01s)

Specific Energy (Net) MJ/kg
Qn = (46.704 - 8.802p²10^-6 + 3.167p10^-3) [1-0.01(x+y+s)] + 0.01 (9.420s - 2.449x)
p = the density at 15 °C, kg/m³
x = the water content, % (m/m)
y = the ash content, % (m/m)
s = the sulphur content, % m/m

Viscosity

Viscosity is an important fuel characteristic, and although in itself is not an indication of quality, knowledge of a fuels viscosity is essential to enable the ship operator to determine both the temperature to which the fuel should be heated in storage to remain pumpable, and the temperature required at injection to ensure efficient atomisation.

For sales purposes the kinematic viscosity of distillate fuels is quoted in centistokes (cSt) at 40 Deg C, and the kinematic viscosity of residual fuels is quoted in centistokes (cSt) at 50°C.

The actual viscosity measurement is more usually carried out at higher temperatures, e.g. 80°C or 100°C, particularly with the more viscous and/or higher pour point fuels. The equivalent viscosity at 50°C is then calculated using the Shell conversion method. This gives results that are the same as those given by the viscosity / temperature chart in the "Shell Book of Useful Tables", and Annex C of the ISO 8217 : 1996 Specification.

In the event of any query or complaint, viscosity measurements are carried out at the original control measurement temperature with any subsequent conversion to an equivalent at 50°C calculated using the method described above.

In many new fuel specifications tables, viscosity is being quoted with reference to the unit mm2/sec, but in practice, reference is constantly made to centistokes. 1 mm²/sec is equivalent to 1 cSt.

Viscosity Conversion Table
(Ref. ISO 8217:1996 - Annex C - Informative)

The ISO 8217 : 1996 Standard specifies limiting values of kinematic viscosity at 100 °C for the fuel categories contained in the Residual Fuel Table, but as described above, in some cases kinematic viscosity is measured or quoted at other temperatures.

The table below gives approximate relationships of fuel viscosity at different temperatures.

The data should be used with caution :-

• Firstly since measurements at temperatures other than 100 °C may have precision that is different
• Secondly because of variations in the 'viscosity - temperature' relationships due to the variability of residual fuel composition.

Viscosities estimated from those measured at 100 °C

(1) 1 mm²/sec = 1 cSt