These are the most common parameters used to describe and measure fuel quality:
Fuel Density
Density is a very important factor in the assessment of fuel quality. Indirectly, density indicates the extent of refining to which the oil has been subjected.
Fuel Density is the major factor affecting fuel oil treatment efficiency because most fuel contaminants, such as water and sediments, are of greater densities than the oil itself. Density is the principal factor in the empirical calculations for both net energy content and ignition performance (CCAI). Fuel supplies with density in excess of that capable of being handled by onboard fuel systems, are not uncommon. This is a particularly serious failing where the potentially abrasive content is appreciable.
The ISO standards set a maximum limit on the density of 0.991 kg/l at 15 °C for residual fuels. This is for systems using normal water seal purification plant. There are other ISO grades where no density limit is stated, and these grades are intended for steamships or vessels equipped with purifier plant specially designed to handle densities up to 1.01 kg/l at 15 °C. The unit of measurement is kg/liter at 15 °C.
Water content
Water is a contaminant present to a greater or lesser degree in all residual fuel oils. There is however no reason that significant quantities of water should be present in bunker fuel deliveries.
Water in the fuel oil can result in a number of operational problems. In the fuel supply system the principal problems are foaming of unpressurized return tanks, cavitation at pump suction, and corrosive action. In the combustion space atomisation sprays can be disturbed, and the cylinder lubricant film diluted. Saline water has a particular affinity for heavy fuels, and heavy sludging can be experienced during purification of fuels with high emulsified water content. High water content may in extreme cases cause main engine malfunction and loss of propulsion.
The international ISO limit for water content is 1% by volume for residual fuels. The unit of measurement is % by volume.
Ash
The ash content of fuel oil is a measure of a fuel's incombustible constituents from compounds inherent in the crude stock or material introduced into the fuel.
The sources of ash are threefold.
- Organic metal compounds existing naturally in crude stock
- Metals introduced during the refining process
- Materials introduced inadvertently during storage, transfer, or delivery
The nature of constituents making up the ash content may be corrosive, erosive, or inert. Exceptionally high levels of ash in the fuel oil are experienced in instances where high levels of saline water are present, significant quantities of waste automotive lubricants have been blended into the fuel oil, or where the fuel is coal rather than petroleum based.
The international ISO limits for ash content in residual fuels vary from 0.10% up to 0.20%. The unit of measurement is % by mass.
Carbon residue
The carbon residue value is the proportion of fuel remaining after the destructive distillation during a fixed period of severe heating. Both crude type and the degree of refining have an effect on the carbon residue value.
A fuel's carbon residue is indicative of the degree of fouling that can be expected in both combustion chamber and exhaust system, particularly in adverse conditions such as low engine load. In trunk piston engines, the carbon residue will also reflect the level of lubricant fouling.
The carbon residue value can vary widely, from as low as 5% up to 20% for the same grade of fuel. The engine design is an important factor in deciding which levels of carbon residue that can be tolerated.
The international ISO limits for carbon residue vary between 15% and 22%. The unit of measurement is % by mass.
Sulphur
Virtually all marine fuels, distillate or residual, contain significant quantities of sulphur as a result of the crude stock used.
In diesel engines, high sulphur has two particularly undesirable actions, corrosion and energy loss.
As sulphur trioxide is formed during combustion, assisted by vanadium present in any residual fuel, sulphuric acid is formed leading to pitting action on any exposed steel surfaces i.e. cold corrosion. Although modern lubricants and attention to maintaining adequate temperatures have gone a long way towards limiting the effects of sulphur on engine components, the problem may still manifest itself in the later stages of the exhaust system.
High levels of sulphur will reduce the available energy content of any marine fuel. This may cause loss of speed or increased consumption compared to low- sulphur fuels. Thus the commercial value of any high-sulphur fuel is less than comparable low-sulphur fuels.
The international ISO limit for sulphur in residual fuels is 5%, The unit of measurement is % by mass.
Stability
Of all the various characteristics which are used to determine the quality of residual fuel oil, none is more important than the stability of the fuel.
An unstable fuel, for whatever reason, is an unusable fuel. Extreme difficulties can be encountered in fuel treatment, and severe engine damage can occur from using unstable fuel oils. Testing for stability consists of a range of tests made in order to determine this extremely important fuel property. Instability may be caused by incompatible components from which a fuel has been produced, excessive heating, or low stability reserve. Use of an unstable fuel may cause engine stoppage and subsequent long-term loss of propulsion.
The international ISO standard gives a limit of 0.10 for the TSA [Total Sediment Accelerated] value. The unit of measurement is % by mass.
Flash point
Minimum flash point is specified for reasons of safety alone.
The internationally accepted and enforced minimum flash point is 60 °C. This is a statutory requirement. Thus vessels carrying fuels with flash point less than 60 °C may run the risk of being considered unseaworthy.
The international ISO limit for flash point is 60 °C for all residual fuel grades. The unit of measurement is °C [degrees Centigrade].
Pour Point
A fuel will solidify when its temperature gets below the pour point. This is due to the fuel's wax content.
Once fuels are solidified due to waxes present in the fuel coming out of solution, they are not easily re-liquified. Hence all fuels must be maintained at a temperature in excess of the pour point for the fuel. A margin of approximately 7 °C is normally advised to avoid wax solidification.
The ISO limit for pour point is 30 °C for residual fuels. The unit of measurement is °C [degrees Centigrade].
Aluminium + Silicon
Presence of aluminium and silicon in bunker fuels can represent remaining catalyst fines from the refinery process.
Although trace levels of aluminium and silicon can be present from the crude stock, any significant quantities of these elements will be indicative of highly abrasive catalyst fines. Silicon alone would indicate sand or dirt in the fuel. High levels of catalyst fines may affect fuel treatment and main engine performance adversely. Excessive and accelerated wear of engine parts, especially fuel pumps and injectors, can very often be traced to this contaminant. When high levels of catalyst fines are combined with significant levels of waste automotive lube oil, the fuel treatment system may not be relied on to reduce the levels of catalyst fines to tolerable levels for the engine. Thus caution should be exercised in using fuels with high levels of aluminium + silicon and significant levels of waste lube oil.
The international ISO limit for aluminium + silicon [al+si] in residual fuels is 80. The unit of measurement is mg/kg [ppm].
Vanadium
Concentrated into the residual oil by refining, vanadium represents the most common of the trace elements present in crude oil.
Vanadium, when combined with sodium, can form highly corrosive deposits on high temperature engine components. Very high levels of vanadium are mostly found in fuels originating from South American crude stocks. The level of vanadium tolerable for use in marine diesel engines is determined to a large extent by the engine design and fuel grade.
The international ISO limits for vanadium in residual fuels vary from 200 to 600 for different IFO grades. The unit of measurement is mg/kg [ppm].
Sodium
Sodium can be present in limited quantities in any residual fuel oil. Significant levels can be indicative of saline water or salt contamination.
Sodium not removed prior to use will result in heavy post-combustion particularly in way of turbochargers.
Sodium will also significantly reduce the temperature at which vanadium compounds exist in a corrosive form, thus causing high-temperature corrosion problems.
The international ISO standard has no limit on sodium content in fuel oils. The unit of measurement is mg/kg [ppm].
Lead
While trace quantities may be present from the crude stock, significant levels are indicative of waste lubricants being blended into the fuel oil.
The practice of blending waste automotive lubricating oils by some suppliers, and in particular used waste automotive lubricating oils, is an unacceptable addition to the fuel oil resulting in rapid wear of cylinder liners and piston rings.
Automotive waste lubricating oil in the fuel can also cause damage to the purifiers from an imbalance of the bowl due to much less mobile sludge forming in the purifier.
The ISO standard has no limit on lead content in fuel oils but clearly states that non-petroleum components are not to be blended into marine fuel oils. The unit of measurement for lead is mg/kg [ppm].
Calcium
While minimal amounts will be present from the crude stock, significant levels are indicative of contamination from sea-water or waste lubricants having been blended into the fuel oil.
Blending waste lubricating oils into the fuel oil introduces additives which make it difficult for fuel treatment purifiers to remove the contaminants held in suspension in the fuel. This results in the retention of potentially harmful contaminants, leading to purification and filtration problems and the possibility of abrasive particles passing through to the engine.
The unit of measurement is mg/kg [ppm].
Phosphorous
While minimal amounts will be present from the crude stock, significant levels indicate contamination of the fuel oil.
Presence of phosphorous alone in residual fuels may be indicative of coal distillation fractions. Significant levels of phosphorous in association with other elements, i.e. calcium and zinc, indicates that waste lubricating oils have been blended into the fuel oil.
The international ISO standard has no limit on phosphorous content in fuel oils, but clearly states that non-petroleum components are not to be blended into marine fuels. The unit of measurement is mg/kg [ppm].
Zinc
The typical levels of zinc in most crude oils are very low and below 5 ppm.
The presence of significant amounts of zinc in association with specific other elements may be indicative of waste lubricants being blended into the fuel oil.
The international ISO standard gives no limit for zinc content in fuel oils, but clearly states that non-petroleum components are not to be blended into marine fuels. The unit of measurement is mg/kg [ppm].
CCAI [Calculated Carbon Aromaticity Index]
The CCAI indicator was developed by Shell and is recognized as the most reliable indicator available to describe the ignition properties of bunker fuel oils.
The CCAI value is calculated from a formula using the density and viscosity values for a fuel oil. Caution should be observed in using fuels with CCAI in the range 850-870. Use of fuels with high CCAI values may cause main engine stoppage and subsequent loss of propulsion.
The international ISO standard has no limit for CCAI. The CIMAC recommendations stipulate that for all fuel grades ignition problems can occur for fuels with CCAI in the range 850-890. CCAI is a non-dimensional parameter and consequently has no unit of measurement.
Waste automotive lube oil
While trace quantities of lead may be present from the crude stock, significant levels are indicative of waste lubricants being blended into the fuel oil.
Waste automotive lube oil contains chemical additive packages of detergents and dispersants specifically designed to retain moisture and particulate matter in suspension. When waste automotive lube oil is added to marine fuel, the additives continue to be effective and severely reduce the effectiveness of the onboard purification plant in separating water and abrasive elements from the fuel. Therefore, waste automotive lube oil in marine fuels may cause increased wear rates, and component failure has occurred.
Specifying fuel oils according to the international ISO8217 standard may not protect the buyer against contamination of marine fuels from waste automotive lubricating oils, even when referring to ISO8217 subsection 4.1. Buyers are therefore advised to specify explicitly that the bunker fuel should not contain any waste lube oils.
It is not possible to determine the amount of waste automotive lube oil present in any fuel. The simultaneous presence of high levels of calcium, phosphorous and zinc in fuel oils indicate that waste automotive lube oils have been mixed into the fuel. The unit of measurement for the elements calcium, phosphorous, and zinc is mg/kg [ppm].
