Ask the Expert ...about Diesel Fuels
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How can I interpret my diesel fuel analysis?
PURPOSE OF ANALYSIS
To detect changes in diesel fuel which result from either long- term storage or contamination and which may render the fuel unfit for use in a diesel engine.
FUEL CHARACTERISTICS MEASURED
Appearance
Fuel may vary in color from white to dark amber. Fuel should be clear and bright. This indicates that the fuel contains only normal amounts of dissolved water. Fuels that appear dim or cloudy may contain entrained water or other forms of contamination.
ASTM Color
Some fuels appear darker than others as a result of the refining process. The higher the number the darker the fuel. A darker color fuel is not necessarily an indicator of poor fuel quality. As fuel ages and products of oxidation form fuels tend to darken due to the accumulation of varnish particulate. Fuels with an ASTM color of 3 or greater are suspect, as this degree of darkening may be a sign of advanced oxidation.
Density
Density is the weight of fuel in Kilograms per litre (measured at 15degrees C). Typical summer diesel fuels range from 0.845 to 0.870. The denser the fuel the higher the energy content.
Odor
The fuel should have a robust typical diesel fuel odor. Contaminants such as gasoline can often be detected by their smell. A flat varnish like odor can signify microbiological activity.
Flash Point
This test is useful for detecting contamination of the fuel with more volatile materials such as gasoline or solvent. Diesel fuel must have a flash point no lower than 40 degrees C.
Water Content
Most diesel fuels will have a dissolved water content which is normal and harmless. Typically at room temperature this dissolved water content could be 200 ppm (.02%). As water content increases it can no longer exist as dissolved water and becomes present as free water. The water content becomes critical at concentrations of .1% and above.
Viscosity
The minimum acceptable viscosity for fuel system protection is 1.4 cSt at 40 degrees C.
Existing Particulate
Particulate can result from either contamination (dirt, rust, etc.) or from the presence of oxidized fuel materials (varnish). The maximum allowable particulate level in fuel is 10mg/l. Short fuel filter life will be experienced at elevated contamination levels.
Fuel Stability
The fuel stability test measures the propensity of the fuel to deteriorate with the formation of gums and varnish. Typically as fuel ages it becomes increasingly unstable. By subjecting the fuel to extremes of heat and oxygen the fuels stability can be measured. This is a test to determine the future storage capacity of the fuel. The maximum limit for fuel stability is 10 mg/100ml.
What is microbiological contamination?
Fuel bugs can develop in any stored diesel fuel given the right conditions. In order for this microbiological contamination to occur, water must be present, as is the case with tank bottom water. Once started these fuel bugs can multiply rapidly with resulting short fuel filter life. Quite often the infested fuel will develop a flat varnish like odor. It is possible to test for fuel bug contamination in the lab. If bugs are present they can be neutralized with the use of a biocide.
What is the difference between fuel API Gravity and Density?
API Gravity is a special function of relative density (specific gravity) 60/60 F, represented by:
Degrees API = 141.5 / (sp gr 60/60 degrees F) - 131.5
API gravity is an old empirical expression of gravity, which is still in use in the US. Nevertheless, US fuel standards quote density, which is used in most countries around the world. Note that API gravity is the opposite of density. As API gravity increases, density decreases.
What is the relationship between density and energy value?
There is a direct relationship between density and the energy contained in a diesel fuel. The greater the density the more energy contained in the fuel. As a result a winter grade of diesel fuel which has a density less than a summer grade will contain less energy per unit volume.
How should I store diesel fuel?
Diesel fuel should be stored under clean, cool, dry conditions. Bulk fuel tanks should always be kept full to minimize "breathing" which results in condensation. Translucent plastic or fiberglass tanks should not be used above ground as light promotes the formation of gums and varnish. (If water is present algae growth may also occur). The storage tank should be tested frequently for water and any free water should be drawn off.
While most diesel fuel is quite stable "straight run" and hydrotreated fuel is more stable than fuel with catalytically cracked components.