ASTM Specification D396 (Appendix E) classifies fuel oils into grades
and places limitations on properties of the oils in each grade. These
grades are described as follows:
2.4.1 Grade No. 1. This fuel is termed a light distillate oil because
it is capable of being vaporized at relatively low temperatures at
atmospheric pressure. Its intended use is almost exclusively for domestic
heating where vaporizing-type burners convert the oil to a vapor by contact
with a heated surface or by radiation. High volatility of the oil is
necessary to ensure that vaporization occurs with a minimum of residue.
This oil is a little heavier than kerosene, but modern-day practice places
both No. 1 fuel oil and kerosene in the same class.
2.4.2 Grade No. 2. This is a heavier distillate than grade No. 1 and
comes from the refinery fractionating tower after the No. 1 oil. Its
intended use is in atomizing-type burners, which spray the oil into a
combustion chamber where the droplets burn while still in suspension. This
grade of oil finds use in both domestic and medium-capacity
commercial/industrial applications where its cleanliness and ease of
handling sometimes justify its higher cost over that of residual oils.
2.4.3 Grade No. 4. This oil is a variable and complex mixed
classification. It is usually a light residual, but it is sometimes a
heavy distillate. This oil is of lower viscosity than No. 5 or No. 6 fuel
oils and is intended for use in burners equipped to atomize oils of higher
viscosity than domestic burners can handle. Its limited service includes
use in small boilers for schools and apartment buildings, forging furnaces,
and low-heat installations. Its viscosity range allows it to be pumped and
atomized at relatively low temperatures requiring no preheating for
handling except in extremely cold weather. A refinery usually does not
attempt to produce this grade of oil because of its low sales volume.
2.4.4 Grade No. 5. This is a residual fuel of intermediate viscosity
for burners capable of handling fuel more viscous than grades 1, 2, or 4
without preheating . Preheating may be necessary for use in some types of
equipment for burning and for handling in cold climates.
2.4.5 Grade No. 6. This is a heavy, black residual of the refining
process and is sometimes referred to as "Bunker C." It is a high viscosity
oil used primarily in commercial and industrial heating. It requires
preheating in the storage tank to permit pumping and additional heating at
the burner to permit atomization. The equipment and maintenance required
to handle this fuel usually prevents its use in small installations.
Table 1 of ASTM D396 lists the various grades of fuel oils with their
respective specifications (see Appendix E).