Fuel Oil - Classes


The ASTM classification system was originally based on early refining and combustion engineering practices and nomenclature. Some specifications have changed over the years to reflect new refining practices and refinery by-products. Other organizations have published modified versions of the original six fuel specifications to assist operators of older equipment find appropriate fuels from current refinery products.

Although the following trends generally hold true, different organizations may have different numerical specifications for the six fuel grades. The boiling point and carbon chain length of the fuel increases with fuel oil number. Viscosity also increases with number, and the heaviest oil has to be heated to get it to flow. Price usually decreases as the fuel number increases.

Number 1 fuel oil is a volatile distillate oil intended for vaporizing pot-type burners. It is the kerosene refinery cut that boils off right after the heavy naphtha cut used for gasoline. Older names include coal oil, stove oil and range oil.

Number 2 fuel oil is a distillate home heating oil. Trucks and some cars use similar diesel fuel with a cetane number limit describing the ignition quality of the fuel. Both are typically obtained from the light gas oil cut. Gas oil refers to the original use of this fraction in the late 19th and early 20th centuries - the gas oil cut was used as an enriching agent for carburetted water gas manufacture.

Number 3 fuel oil was a distillate oil for burners requiring low-viscosity fuel. ASTM merged this grade into the number 2 specification, and the term has been rarely used since the mid-20th century.

Number 4 fuel oil is a commercial heating oil for burner installations not equipped with preheaters. It may be obtained from the heavy gas oil cut.

Number 5 fuel oil is a residual-type industrial heating oil requiring preheating to 170 – 220 °F (77 – 104 °C) for proper atomization at the burners. This fuel is sometimes known as Bunker B. It may be obtained from the heavy gas oil cut, or it may be a blend of residual oil with enough number 2 oil to adjust viscosity until it can be pumped without preheating.

Number 6 fuel oil is a high-viscosity residual oil requiring preheating to 220 – 260 °F (104 – 127 °C). Residual means the material remaining after the more valuable cuts of crude oil have boiled off. The residue may contain various undesirable impurities including 2 percent water and one-half percent mineral soil. This fuel may be known as residual fuel oil (RFO), by the Navy specification of Bunker C, or by the Pacific Specification of PS-400.

Mazut is a residual fuel oil often derived from Russian petroleum sources and is either blended with lighter petroleum fractions or burned directly in specialized boilers and furnaces. It is also used as a petrochemical feedstock.

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