Measures of Flammability

MEASURES OF FLAMMABILITY

             Several terms are important when evaluating the flammability of a material.The flash point is the temperature at which a liquid gives off sufficient vapors for an external ignition source to cause a flame to flash across the surface of the liquid.

However, if the ignition source is removed, the flame will go out because self sustained combustion is not possible at this temperature. Several methods are available for testing flash point; Tag Closed Cup ASTM D-56, Tag Open Cup ASTM D-1310, Cleveland Open Cup ASTM D-92, and Pensky-Martens Closed Cup ASTM D-93.



The ignition temperature or fire point is the temperature at which the material will begin self-sustained combustion if an external ignition source is used to initiate the process. This temperature is usually only slightly higher than the flash point. The auto-ignition temperature is the point at which the material has been sufficiently heated for combustion to occur without an external ignition source.

The flammable or explosive range identifies the percentage mixture of flammable vapor or gas in air that can be ignited. The flammable range is the area between the upper (UFL) and lower (LFL) flammable limits, also referred to as explosive limits (UEL and LEL). Gasoline, for example, has a lower flammable limit of approximately 1.5 and an upper flammable limit of approximately 7.5. This means that if its vapors are mixed in the surrounding air between 1.5 and 7.5%, and an ignition source is introduced, it will burn or explode. If the percent of vapors in air were 1%, the mixture would be too lean to burn because sufficient fuel would not be present. If the percent of vapors in air were 10%, the mixture would be too rich to burn because there would be too much fuel relative to the oxygen.


Figure illustrates a comparison of flammable ranges for several common substances. A solid material’s contribution to a fire is most commonly measured by its ease of ignition, flame spread, and smoke production. For testing and evaluation, solid materials are usually grouped into two primary categories: flexible solids, which include upholstery, furniture cushions, and clothing, and structural solids, which include solid building materials whether they are used in the structure or the contents.


Ease of ignition is tested to provide information about how much and how long heat must be applied to ignite the material under consideration. Flame spread addresses the speed at which a fire, once ignited, will travel across the surface of the material. Flame spread testing is typically done in a Steiner Tunnel. One example of a test standard is NFPA 255 Standard Method of Test of Surface Burning Characteristics of Building Materials. Smoke production has traditionally been evaluated based on the amount of visible smoke and not on the chemical composition of the smoke. The current trend is toward more accurate measurement of the smoke’s toxic components that may produce detrimental effects on people in a fire situation.