Marine Products Fuel Specification Guide
Acid Number
Strong Acid Number and Acid Number are tests used to measure the acidity of a fuel. The presence of acid in fuel, if not controlled, can give rise to acidic corrosion, both in the engine and in the fuel treatment and handling systems. The strong acid test and total acid test eliminate the possibility of the fuel being contaminated by chemical wastes from other refinery streams. Strong Acid Number is the weight in milligrams of base required to titrate a one-gram sample of fuel up to a pH of 4. Acid Number is the weight in milligrams of base required to neutralise all acidic components in the fuel.
Aluminium and Silicon
Measurement of Aluminium plus Silicon in marine residual fuel provides an indication of the quantity of catalyst fines in the fuel. Catalyst fines are important because in significant amounts, they can cause abrasive wear of engine components such as fuel pumps, injectors, cylinders and pistons. Catalyst fines are normally only present in a fuel if it contains components derived from a refinery process which uses a catalyst. Catalyst fines in the fuel occur due to the physical deterioration of the catalyst whilst in use, and they manifest themselves in the fuel in the form of aluminium silicate. Centrifuging of the marine residual fuel reduces catalyst fines to a safe level for engines provided that when the fuel is delivered, it meets the ISO 8217:2005 Marine Fuel Specifications for residual fuel oil.
API Gravity
In the USA an arbitrary scale known as the API Degree is used for reporting the gravity of a petroleum product. The API degree is related to the relative density scale (60°F/60°F) by the formula:
141.5
API gravity at 60°F = -131.5
relative density 60/60°F
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 their fining process. Such particles are highly abrasive therefore solid ash should be removed as much as possible by centrifuging.
Asphaltenes
These are high molecular weight components of fuel oil. The quantity of asphaltenes in fuel oil is dependent on the source of crude oil used to produce the fuel, and the process used to manufacture the fuel. Marine Residual fuel normally contains between 4% and 12% of asphaltenes, but as long as the fuel has sufficient reserve of stability, it is unlikely to cause any sludging problems, Fuel stability is assessed by the ISO 10307-2 test method, and cannot be assessed by the measurement of asphaltene content alone.
Blended Fuel Oil
This is a term used to describe a mixture of residual and distillate fuel.
Bottoms
In refining, this term is used to describe the high boiling point residual liquid that collects at the bottom or a distillation column including such components as heavy fuels and asphaltic substances.
Bunker C Fuel
This term is used by some bunker suppliers to describe their equivalent grade lo RMG/H380. The ASTM Designation D396-92 - ‘Standard Specification for Fuel Oils’, gives a complete summary of the characteristics of this product.Bunker C Fuel is also sometimes called ASTM Grade No.6. The specifications for Bunker C Fuel are fairly loose and minimal, with limited restrictions applying only to Flash Point (60°C min.), Water (2% max.), Sediment (2% max.) and Viscosity at 100°C (15 cSt min. and 50 cSt max.). Bunker C Fuel can be manufactured from 100% straight fuel oil, or mixed with gas oil depending upon viscosity requirement. If the fuel contains any ‘cracked components’, or any components manufactured by using a cracking process, it is vital to know the results of the ISO 10307-1 and ISO 10307-2 tests, to guarantee its stability and suitability for use.
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. Three test methods are used to determine carbon residue and these are known as Conradson, Ramsbottom, and Micro Carbon Residue Tests. The significance of carbon residue is that fuels with high carbon residue content may cause increased fouling of gasways in the engine necessitating more frequent cleaning, especially of the turbocharger and exhaust gas boilers.
Catalytic Cracking
This is the process of breaking down the larger molecules of heavy oils into smaller ones by the action of heat, with the aid of a catalyst. By using this process, heavy oils can be converted into lighter and more valuable products. This process is also commonly referred to as ‘cat-cracking’.
Catalyst Fines
These are fine particles of aluminium silicate which are deposited in residual fuel as a result of the physical deterioration of the catalyst during catalytic cracking processes.
Centistoke (cSt)
A unit of kinematic viscosity, numerically equivalent to the SI unit mm2/sec.
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.
Cetane Index
This is an approximation of cetane number based on the Specific Gravity and Mid-Boiling Point of the fuel.
Compatibility
This term refers to the ‘sediment forming tendency’ when different fuels are mixed together. As a general rule, wherever possible, all supplies of bunker fuel should be kept in separate storage tanks, and any mixing of fuels should be kept to a minimum to minimise the risk of incompatibility.
Cracking
The most common refinery process is called ‘thermal cracking’ in which pressure and heat are used to produce high quality products, such as gasoline, kerosene and gas oil. Other cracking processes are called hydrocracking and catalytic cracking.
During the cracking process, in addition to the high-quality products, some residual products may be formed which can be used for residual fuel oil supplies.
Cracked Distillates
This is a distillate fuel component which is manufactured by using a catalytic or thermal cracker. Cracked distillate produced from a thermal cracker aresimilar in characteristic to gas oil. Residues resulting from catalytic cracking processes are called‘cycle oils’. Cycle oils are sometimes used to improve the stability reserve of cracked residues; however, the resultant product may have a high density and be highly aromatic.
Cracked Residual Fuel Oil
This is a term that is used to describe residual fuel oil that contains cracked residues.
Cutback
This is a term used to describe the process of reducing the viscosity of a ‘heavier’ fuel by blending it with a ‘lighter’ fuel. The lighter fuel used is often called ‘Cutter Stock’ or ‘Diluent’.
Cutter Stock – see Cutback
Cycle Oils – see Cracked Distillates
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°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 alone 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 and 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. Density is normally physically measured at higher temperatures, and is then corrected to the standard temperature of 15°C using ASTM tables.
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. This is the term used to describe any of the wide range of products produced by distillation, as distinct from bottoms, cracked stock, and natural gas liquids. Distillate products have a ‘mid-boiling’ range and include gas oil and kerosene. Distillate fuels are also sometimes called Middle Distillates or Distillate Fuels.
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 and International regulations absolutely prohibit the supply and use of a marine fuel having a measured flash point below 60°C. The 60°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°C minimum specification under any circumstances whatsoever. If a marine fuel is found to have a flash point below 60°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 vapour in the lower flash point fuel to significantly lower the flash point of the mixture.
Heat Energy – see Calorific Value
Heat of Combustion – see Calorific Value
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 CCAI. High density in combination with low viscosity may be an indication of poor ignition quality.
Long Residue
This is the name given to the residue obtained from an atmospheric distillation unit.
Marine Diesel Fuel
In the marine industry this term usually defines a heavy distillate fuel that may contain small amounts of residual fuel. It is commonly called Marine Diesel Fuel (MDF) or Marine Diesel Oil (MDO).
Marine Fuel Oil
MFO is a Brand Name. Marine Fuel Oil is a term used to describe a broad range of residual fuels used in the marine market. Marine Fuel Oils are classified by a number of different standard bodies including the International Organisation forStandardisation (ISO), and the British Standards Institution (BSI). Marine Fuel Oils are usually thehighest viscosity, and lowest price marine fuel available at any given port. Under the ASTM system, fuels are graded from No.1 through to No.6, but only Grade No’s 2 and 6 are commonly used in the marine market. Grade 1 is a light, distillate, kerosene type fuel, that has the lowest boiling range. Grade 2 Fuel Oil is popularly called heating oil, has a higher boiling range, and is commonly used in home heating. It is comparable in boiling range to diesel fuel. Grades 4, 5 and 6 are called heavy fuel oils (HFO), or residual fuel oils; they are composed largely of heavy pipe still bottoms. Because of their high viscosity, No.5 and No.6 fuel oils require preheating to facilitate pumping and burning. No.6 fuel oil is also called Bunker C fuel oil. (See ASTM 396-92)
Marine Gas Oil
This is a lighter fraction and better quality fuel than diesel fuel, controlled to an ignition quality more appropriate to high speed marine diesel engines.
Metal Content
This is a term used to describe any metallic component of a residual fuel.
Redwood No.1
This is an obsolete unit of viscosity measurement. It is still quoted occasionally, but is calculated from measured kinematic viscosity.
Residual Fuel Oil
This is a term for fuel oil mainly comprising of the residues remaining after refining crude oil.
HFT (Hot Filtration Test)
The test for cleanliness and stability has now been replaced by ISO 10307-1 and ISO 10307-2.
Short Residue’
This is a residual fuel component that is produced in a ‘vacuum distiller’. It normally has to be cut back with gas oil. It is good because it has excellent stability and a low density.
Silicon - see Aluminium plus Silicon
Sodium
Sodium can be present in the fuel as a result of salt water contamination or may occasionally be present as a result of refinery processing. Sodium in fuel can be reduced by centrifuging.
Specific Energy - see Calorific Value
Stability
Fuel oils are produced from a wide variety of crude oils and refinery processes. When marine fuel oils are produced they are normally tested for stability, using ISO 10307-1 and ISO 10307-2. If it becomes necessary to mix different fuels together, great care must be taken because even the mixing of stable components can result in an unstable final blend. The use of incompatible fuels results in large amounts of sludge being taken out by fuel oil centrifuges and purifiers, and frequently leads to centrifuge blocking. To produce a stable mixture, it is essential to use proper blending equipment. Under no circumstances should blending be attempted onboard a vessel after delivery, as it is almost certain that the mixture will not be homogeneous, and is highly likely to be unstable.
Straight Run Fuel
This type of fuel is produced by the distillation of crude oil, with no secondary conversion process being used. The fuel can be either composed of short, intermediate or long residues, or a blend of any of these types.
Strong Acid Number - see Acid Number
Sulphur
Sulphur in fuel is of great concern because of its 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.
Thermal Cracking – See ‘Cracking’
Used Lubricating Oils
Used lubricating oils have been added to bunker fuels in some parts of the world for a long time, originating from both inland waste disposal and from the vessel itself. The third edition of ISO:8217 now states that ‘The fuel shall be free of ULO’ and includes limits on Zinc, Phosphorus and Calcium to determine if the fuel contains any ULO, only if ALL of these limits are exceeded can the fuel be deemed to contain ULO.
Vanadium
Vanadium is present in fuel in soluble compounds and there is no economic removal process. 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 is 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°C. Viscosity is only one of many quality criteria for fuel oils, and is quoted mainly for reasons of storage, handling and treatment. The viscosity of distillate fuels is quoted at 40°C. The viscosity of marine residual fuels is generally quoted at 50°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 atomise efficiently, with the result 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 onboard 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. Pockets of fuel of varying viscosity form throughout the tank, and this can result in uneven combustion, fouling of injectors and injection difficulties.
Viscosity Breaking
This is a thermal cracking process used to reduce the viscosity of long or short residues (Visbreaker), and produces some distillate product.
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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