Analysis Data

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Comments on Analysis Data for Fuel Oils

Carbon Residue

The carbon residue of a fuel oil indicates its cokeforming tendency and can be used to determine the tendency of forming deposits in the combustion chamber and gas ways. The higher the carbon residue value, the higher the fouling tendency.

Some changes in the combustion process, requiring adjustment of the maximum pressure, may also be attributed to a high carbon residue content. The value is measured by standardized tests, such as the Conradson or Ramsbottom tests which give similar results.

The non-vaporized residue from the carbonizing test consists of carbonaceous material and inorganic impurities and is expressed in weight percentage of the fuel sample tested. Carbon residue and asphalteness content generally move in parallel, both related to the carbon-to-hydrogen ratio with increasing values for higher ratio.

The carbon-to-hydrogen ratio and thus the carbon residue too, is dependent on the source of the crude oil and the type of refinery processing used.

Theeffectofcarbonresidueisimpossibletocounteract by pre-treatment of the fuel oil, as centrifuging only influences solid inorganic contaminants and hard asphalts, which are only small amounts of the weight percentage called carbon residue.

Asphalteness

Asphalteness is defined as the part of a fuel oil sample being insoluble in heptane. The content of asphalteness is expressed in weight percentage of the fuel oil sample tested.

Asphalteness, which is aromatic, slow-turning, semisolid hydrocarbon compounds dispersed in the fuel oil, has a similar effect on the combustion process as the carbon residue, the main impact being fouling of gas ways. The stability of the fuel oil is related to the content of asphalteness.

Asphaltenessalsoinfluencesthelubricatingproperties

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of the fuel oil and, in extreme cases, high asphalteness content may lead to fuel pump sticking.

Fuel oils with a high content of asphalteness will have a tendency to form sludge, especially if the water content is also high. The asphalteness content of a fuel oil is influenced by pre-treatment, the heaviest semi-solid asphalteness, and asphalteness bound to water as sludge, can be separated by centrifuging.

Diesel Index

Diesel index is a calculated value for determining the ignition quality of a fuel oil. The ignition quality is related to the hydrocarbon composition, paraffin being of high quality, n-heptanes of moderate and aromatics of low quality.

Properties of the aniline point and the specific gravity reflect, with certain exceptions, the hydrocarbon composition of a fuel oil, and are therefore used in the following simple formula as an expression of ignition quality:

Diesel index = (aniline point °F x API gravity) x 0,01.

The aniline point is the lowest temperature at which equal volumes of the fuel and aniline become just miscible. The test depends on the fact that aromatic hydrocarbons mix completely with aniline at comparatively low temperatures, whereas paraffins require considerably higher temperatures before they are completely miscible.

Thus, a high aniline point indicates a highly paraffinic fuel, and consequently a fuel oil of good ignition quality. Similarly, a high API gravity number denotes a low specific gravity and high paraffinicity, and again a good ignition quality.

The diesel index provides a reasonable idea of the ignition quality, but generally gives figures slightly above the cetane number.

Fuel oils with poor ignition quality and low diesel index might especially cause problems in the starting of the diesel engines and running at low load.

The addition to starting difficulties, a prolonged ignition delay, may give rise to alternations in the maximum

pressure, leading to increased mechanical or thermal load.

Furthermore, fuel oils with poor ignition quality may cause retarded combustion and subsequent fouling gas ways.

Sulphur

Sulphur is present in fuel oil mainly in organic compounds, the amount present being expressed as weight percentage of an oil sample tested. If free sulphur is present, it may cause corrosion in the fuel system. The main problem caused by sulphur is low temperature corrosion. During combustion, sulphur oxides are produced in the form of gases. Since humidity also is present, sulphur, sulphuric acid may be formed on components in the combustion chamber and in the gas ways, where the temperature is below that of the dew point for sulphuric acid.

The detrimental effect of sulphur in fuel oil is counteracted by maintaining adequate temperature of the combustion chamber components and by using alkaline lubricating oil to neutralize the sulphuric acid produced during the combustion.

Vanadium and Sodium

Vanadium and sodium are constituents of the ash content. The amount of these are measured by analyzing the residue from the combustion test used for determination of the ash content. The amount of vanadium and sodium presence is expressed in ppm, parts per million, by weight related to the fuel oil samplebeingtestedforashcontent.Vanadiumderives from the crude oil itself and, being oil soluble, it cannot be removed from the fuel oil by conventional pretreatment. Sodium derives from the crude oil, and also from contamination with salt water during storage and transport of the fuel oil. Sodium is watersoluble and, regardless of derivation, tends to combine with the water present in the fuel oil.

Owing to its water-solubility, it is possible to remove or reduce the amount of sodium present in the fuel oil. During combustion, corrosive ash is formed from vanadium and sodium.

Especially if the weight ratio of sodium to vanadium exceeds 1:3, ash with very low melting point and

stiction temperature is formed giving rise to high temperature corrosion of exhaust valves and deposit formation in turbochargers.

It is possible to reduce the tendency to formation of detrimental vanadium-sodium ash by effective centrifuging, which will remove sodium salts together with water. If a very low content of sodium is ensured, a relatively high vanadium content might be acceptable.

Water

The water content of fuel oil is measured by a standardized distillation test and is expressed as a volume percentage of the sample tested. Water in fuel oil may lead to several detrimental effects of the fuel oil system, corrosion and cavitation of fuel pumps and fuel valves, and cause fouling of exhaust system and turbochargers.

Salt water, due to its content of sodium, contributes in combination with vanadium to the formation of lowmelting corrosive ash, which attacks exhaust valves and turbochargers. Water, when it disturbs the fuel atomization, will lead to poor combustion resulting in higher heat loads of the combustion chamber components.

It is possible to reduce the water content of a fuel oil primarily by centrifuging, and this should be done to the widest possible extent in order to avoid the detrimental effects of water in the fuel oil.

Ash

Ashcontentisameasureofinorganicnon-combustible material present in the fuel oil. The ash content is determined by a combustion test, and it is expressed as a weight percentage residue from complete combustion of the oil sample tested.

Ash-forming materials are present in the fuel oil as natural components of crude oil and due to external contamination of the fuel oil.

Ash-forming materials exist both as solid contami-

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nants and in soluble compounds. The solid contaminants may lead to abrasive wear in the fuel injection system. The solid contaminants may lead to abrasive wear in the fuel injection system. Ash formed during combustion may lead to abrasive as well as corrosive wear of combustion chamber components and give rise to formations of detrimental deposits. It is therefore essential, to the greatest possible extent, to reduce the amount of ash-forming materials by centrifuging.

Solid contaminants such as sand, rust, certain metal oxides and catalyst fines can be removed by centrifuging, and the same goes for water-soluble salts such as sodium.

Some of the components included in the ash content have been found to be particularly harmful and are therefore stated individually in the analysis data.

Silicium and Aluminium Oxides

Residual fuels produced by refineries using fluid catalytic cracking may be contaminated by catalyst particles in form of silicium and aluminium oxides. Catalyst particles, if any, are comprised by the value for ash content. Separate values for the silicium oxide contentandthealuminiumoxidecontentaremeasured by analyzing the ash content.

The amount of silicium and aluminium oxides is expressed in ppm related to the weight of the original fuel oil sample being tested for ash content.

As catalyst particles are very hard and abrasive, they can cause extreme mechanical wear of the fuel injection system, cylinder liners and piston rings.

Catalyst particles, being solid and insoluble, can be removed from the fuel oil. The guide-lines for dimensioning the centrifuge size is based on the fact that approx. 1/3 of the catalyst particles with regard to weight is removed.

Viscosity

Viscosity is a measure of the internal friction or resistance of an oil to flow.

Traditionally, the viscosity of fuel oils has been

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expressed in seconds Redwood No. 1 (sR1).

In 1977, however, this designation was officially superseded by the metric unit of Kinematic viscosity, centistokes (cSt), and the temperature at which nominal viscosity is to be related, was also changed.

Commercial designations of residual fuels for marine use, still being related to nominal viscosity expressed in centistokes at 50°C. The relationship between the two methods of expressing the nominal viscosity is shown in table.

Furthermore, the designation heavy fuel oil (HFO), bunker C fuel oil and No 6 fuel oil for the heaviest and cheapest grade of residual fuel oil for marine use, have been superseded by the common designation marine diesel fuel oil (MDO) or intermediate fuel oil (IF) for all grades of residual fuels followed by a number, indicating the nominal viscosity.

The use of the S.I. system also influences the

Table 1. Destignation of fuel.

nominal viscosity designation for marine gas oil (MGO) and marine diesel oil (MDO), both now to be expressed in centistokes at 40°C.

Viscosity is an important parameter in connection with pumping, pre-treatment and injection of fuel oil, since the possibility and efficiency of these processes to a large extent is dependent on adequate viscosity.

Adjustment of the viscosity into adequate values is

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possible by taking advantage of the interdependence between temperature and viscosity index of the fuel oil.

The nominal viscosity of a fuel oil is the determining factorforpreheatingtemperaturesnecessarytoobtain adequate viscosity for pumping, centrifuging and injection of the fuel oil, and thus also the determining factor for the capacities of the preheating equipment in the fuel oil system.

Density

Density is defined as the mass of a unit volume and it is expressed in g/cm³ at a temperature of 15°C (59°F).

Analysis Data for Fuel Oils

Table 2. Analysis data for fuels.

Specific gravity is the ratio of the mass of a given volume of liquid at 60°F (15.6°C) and the mass of an equal volume of water at the same temperature. For a given liquid, the specific gravity will generally give the same numerical as the density.

API-gravity is an arbitrary scale calibrated in degrees and related to specific gravity by the following formula:

specific gravity/50°F

As the formula indicates the API-gravity is in inverse ratio to density and specific gravity.

Density is an important parameter in the centrifuging process, where separating water and water-dissolved impurities from the fuel oil is based upon the difference in densities. If the density of the fuel oil approaches that of water, centrifuging thus becomes less effective, necessitatingreducedflowrateandthereforeincreased centrifuge capacity.

The water separation ability of fuel oil is increased by preheating the fuel oil prior to centrifuging since the densities of fuel oil and water change with the temperature at different rates, thus giving possibilities of obtaining optimal difference in densities.

To some extent, the quality of a fuel oil can be judged by the density, since this is directly proportional to the carbon-to-hydrogen ratio, which again is in direct ratio toaromativity,carbonresidueandasphaltenecontent, but in reverse ratio to calorific value.

Pour Point

The pour point is the lowest temperature at which an oil will flow or can be poured. The pour point is measured under specified test conditions. Fuel oil must be stored, handled and pumped at temperatures above the pour point to avoid wax crystallization, which may result in precipitation in storage tanks, blocking of filters and pipe lines and prevention of pumpability. Normally, the pour point of residual fuel oil does not create any problems since the temperature needed to reduce the viscosity to pumpable levels will be adequately in excess of the pour point.

Flash Point

The flash point of an oil is defined as the temperature at which it gives off sufficient vapour to create an inflammable mixture with air. This mixture will ignite or flash under the influence of an open flame, but will not support the combustion itself. The flash point of fuel oil is normally tested by the Pensky-Martens closed-up method.

In order to provide a sufficient margin of safety from fire risk during storage, handling and transportation, fuel oils for shipboard use must meet the classification societies' requirements of flash point, limited to a minimum of 60°C (140°F).

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