Flash point (FP) temperature is the lowest temperature where a fuel will give off sufficient vapors for ignition under ambient conditions. It is an estimate of the lower flammability limit.
To better understand FP temperature we will start with a discussion of the classical "Fire Pyramid." Four elements, (1) heat or energy for ignition, (2) fuel vapors, (3) oxygen and (4) free radicals are necessary for a sustained hydrocarbon fire. The first three ["Fire Triangle": (1) ignition, (2) fuel, and (3) oxygen] must be in the proper range and state for ignition and (4) chain branching reactions must occur to maintain the reaction process. Without these four elements, a sustained fire is not possible. In addition, the first three must be available in the proper balance for a sufficient period of time for ignition to occur. This time increases near the flammability limits and the ignition energy limits.
The three elements of the fire triangle may be independently varied with ignition occurring only under a certain combination of conditions. A change in one element may affect the other two elements. A stronger ignition source (heat) may lower the amount of oxygen required (lower oxygen concentration) and/or reduce the amount of fuel vapors necessary at ignition. Increasing the amount of oxygen available may lower the energy necessary for ignition (heat) and/or decrease the amount of fuel vapors needed.
An ASTM test method is used as follows to determine the FP temperature. The test fuel is placed in a container, the liquid fuel is slowly and uniformly heated with its temperature monitored. Periodically as the temperature is increased, a large ignition source is inserted into the container and this is repeated until the lowest temperature where a flash occurs is identified. This temperature is defined as the flash point temperature. Since this ASTM test is conducted at ambient conditions, the amount of oxygen present is fixed and the ignition source is specified. It, therefore, may be stated that FP temperature has nothing to do with ignition. Excluding fuel sprays, the fuel must be near or above its FP temperature for ignition to occur at ambient conditions. However, ignition does not always occur when the fuel is above its FP temperature. If this was not true, we would have a major safety problem with gasoline that has a FP temperature of -36 oF.
The flash point temperature is very close to the lower flammability limit temperature at ambient conditions. The flash point temperature, however, is slightly higher (5-10 oF) than the lower flammability limit temperature since the test for flash point temperature involves downward flame propagation whereas the lower flammability limit test involves upward flame propagation. Upward flame propagation is easier to achieve than downward propagation, therefore, it occurs with fewer fuel vapors.
Again, flash point temperature is an estimate of the minimum amount of fuel vapors necessary for a combustion reaction at ambient conditions. There is no reaction even with a fuel above its FP if the necessary conditions for the independent variables, oxygen and heat, are not satisfied.
All fire prevention and protection technigues must negate at least one of the four elements of
the "Fire Pyramid" in some way.