The next and probably the most important parameter of combustion of a flammable liquid is its flash point. Flash point is the most important information for emergency responders to know about a flammable liquid. Flash point, more than any other characteristic, helps to define the flammability hazard of a liquid. If a flammable liquid is not at its flash point temperature, it will not burn.
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Flash point is defined as "the minimum temperature to which a liquid must be heated to produce enough vapor to allow a vapor flash to occur (if an ignition source is present)". After all, it is the vapor that burns, not the liquid, so the amount of vapor present is critical in determining whether the vapor will burn. Flash point is a measurement of the temperature of the liquid. Therefore, even if the ambient temperature is not at the flash point temperature, the liquid may have been heated to its flash point by some external heat source. For example, the radiant heat of the sun, heat from a fire, or heat from a chemical process may heat the liquid to its flash point. If an ignition source is present, ignition can and probably will occur! In order for ignition to occur if a flammable liquid is at its flash point temperature, there has to be an ignition source present that has a temperature at or above the ignition temperature of the vapor.
Courtesy Robert Burke
There are three basic methods in which ignition temperatures can be reached, or to put it another way, there are three types of ignition sources. They are external, external-internal (auto-ignition), and internal (spontaneous combustion). External ignition sources produce heat that will enter the vapor or liquid itself and transfer their own heat energy directly to the flammable material. Examples of external ignition sources are open flames, sparks (electrical, static, or frictional), and heated objects. Sparks are capable of developing temperatures, which range from 2000