In describing a fuel oil several factors are measured. The more important of these are gravity, heat of combustion, viscosity, water and sediment, carbon, ash, flash point, pour point, sulfur, color, and distillation point. These terms are defined and described below:

Units of Heat Measurement

Both internal energy and heat is measured using the the calorie. The calorie is the amount of heat required to raise the temperature of 1 gram of pure water 1°C. One Btu equals 252 calories.

Another unit in which thermal energy maybe measured is British thermal unit (Btu). For most practical engineering purposes, 1 Btu is the thermal energy required to raise the temperature of 1 pound of pure water to 1°F.

Burning a wooden kitchen match completely will produce about 1 Btu.

When large amounts of thermal energy are involved, it is usually more convenient to use multiples of the Btu. For example, 1 kBtu is equal to 1000 Btu, and 1 MBtu is equal to 1 million Btu.

API Gravity:

A convenience term describing the grade and weight of oil.

With decreasing API gravity, viscosity, carbon content, carbon residue and weight increase. Heating value and hydrogen content decrease with increasing API gravity.

Heat of Combustion:

The heat of combustion of a fuel oil is the quantity of BTU's which could be released per gallon of oil.

A typical No.6 fuel oil nay have a heat of combustion of 150,000 BTU/gallon. No.2 oil will be slightly less than this.

Viscosity:

The viscosity of an oil is the measure of its resistance to flow. The viscosity of fuel oil decreases as temperature increases. Viscosity ranges from 34 SSU (Seconds Saybolt Universal) for No.2 oil, to 300 SSU for No.6 oil (at 100°F).

Water and Sediment:

Water, gum, resins, ashphaltenes, carbon, sand, wood, rust, gasketing and similar impurities are found in fuel oil. BSW (Bottom Sediment and Water) ranges from 0 to 255. This topic is discussed more fully in the section covering problems.

Carbon Residue:

The amount of carbon remaining after oil has been burned in the absence of air. Carbon residue range from 0% in lighter oils up to 15% in No.6 oil. If carbon residue is very high, burner tips may plug.

 

Ash:

The non-combustible impurities present in fuel oil. Some typical ash constituents are sodium, vanadium, potassium, silica, iron, etc. Typical No.6 oils show an ash content between 0.005 and 0.3%, Ligher oils are lower in ash.

Flash Point:

As fuel oils are heated, vapors are produced which at a certain temperature flash when ignited by an external flame. This temperature is called the flash point. Any oil not to be preheated should have a flash point of at least 100°F closed cup. If preheated, the oil should have a flash point of at least 150°F closed cup.

Ignition Temperature 

Ignition Temperature is the lowest temperature at which heat is generated by combustion faster than heat is lost to the surroundings and combustion becomes self-sustaining.

Pour Point:

The temperature at which an oil will just flow under standard conditions. The pour point of No.6 oil varies from 20 to 75°F. No.2 oil ranges from below zero to 75°F.

Sulfur:

The sulfur content of fuel oil may be due to hydrogen sulfide, alkyl sulfide, cyclic sulfides, or elemental sulfur. No.2 oil will generally show less than 0.5% sulfur. Sulfur in No.6 oil is typically between 0.5 and 4.0%. Sulfur will be discussed in detail in the following section.

Color:

The color of a fuel oil may range from clear to black. Color is important because it can give an indication as to the type and quality of the oil. A cracked heavy oil is almost always black.

Straight run residuals are usually dark green. A brown haze indicates water contamination.

Distillation Point:

The distillation test applies only to No.2 oil and tests its volatility.