There are several physical properties exhibited by petroleum products which have a significant impact on their fire and explosion potential. These include the following:

1- Autoignition Temperature: Minimum temperature to which a fuel in air must be heated to start self-sustained combustion without a separate ignition source. This means that, should a leak occur on a line containing a petroleum product above its ignition temperature, ignition can occur independent of an ignition source.

2- Boiling Point: The temperature at which the vapor pressure of a liquid equals the surrounding atmospheric pressure. For purposes of defining the boiling point.

3- Fire Point: The temperature at which a liquid produces enough vapors to sustain combustion.

4- Flammable Range: A range of vapor-to-air ratios within which ignition can occur. The lower flammable limit (LFL) is the minimum vapor-to-air concentration below which igni tion cannot occur. Atmospheres below the LFL are referred to as too lean to burn. The upper flammable limit (UFL) is the maximum vapor-to-air concentration above which ignition cannot occur. Atmospheres above the UFL are referred to as too rich to burn.

5- Flash Point: The lowest temperature at which a liquid gives off enough vapor to produce a flammable mixture with air immediately above the surface. A source of ignition is needed for flash to occur. The flash point temperature can be very low for volatile petroleum products.

6- Specific Gravity: ratio of the weight of a given substance to the weight of an equal volume of a standard substance (water for liquids and air for gases). This is frequently referred to as vapor density for gases. Since the specific gravity of the standard equals one, liquids with a specific gravity less than one will float on water (unless they are water soluble like most alcohols). Most liquid petroleum products have specific gravities less than one.

7-Vapor Pressure: The pressure exerted by the vapor of a substance when the substance and its vapor are in equilibrium. Equilibrium is established when the rate of evaporation of a substance is equal to the rate of condensation of its vapor. In general terms, the higher the measured laboratory vapor pressure the more likely that liquid is to give off vapors under "real ambient" conditions.

The importance of these basic vapor generation and combustion properties can be seen in the way flammable and combustible liquids are classified.