(65a) The Relationship between Flash Point and Lower Flammable Limit

There is a sound basis for the observation that the vapor concentration at the Flash Point of a flammable liquid is near the Lower Flammable Limit [LFL] concentration. One might expect that these concentrations would (or should) be identical. However, there are two reasons why they should not be expected to be identical:

1. In most cases, the LFL is measured at a temperature that is not the Flash Point temperature. Since the LFL's are typically measured at ?room temperature? (20oC to 25oC), only if the Flash Point was also near 20oC to 25oC would the temperatures be the same. In fact, for some flammable and particularly combustible liquids, the LFL is measured at temperatures that are well above the Flash Point. Somewhat contrarily, for flammable liquids having Flash Points below ?room temperature?, the LFL would be measured at temperatures well above the Flash Point.

2. The reported LFL concentrations are typically for upward propagation in cylindrical apparatus, whereas the Flash Point determinations are determined in an apparatus where downward propagation produces a ?flash?. LFL data clearly show that downward propagation of flame requires a higher concentration of flammable vapor, as compared to upward propagation.

Figure 1 shows the difference in (1) LFL concentration and (2) the equilibrium concentration of flammable vapor over a liquid at the Flash Point. For this material [Cyclohexanone], the difference in concentrations is about 0.6 volume-percent (12.8 mmHg minus 8.35 mmHg partial pressures, or 1.68 volume-percent minus 1.10 volume-percent). Also shown on this graph is the essentially straight-line relationship between the decrease in LFL concentration as the initial temperature increases toward the flame temperature of LFL mixtures (as based on the heat of combustion and the specific heats of the products of combustion [Van den Schoor, J. Haz. Materials, 128 (1): 3 (Jan. 2006)]). If this relationship is extended to the vapor pressure curve, the difference in LFL and Flash-Point concentrations is about 0.38 volume-percent.

Figure 2 shows that the difference between the LFL and Flash-Point concentrations at the Flash Point temperature averages about 0.35%, with some considerable ?scatter?. However, about 75% of the data show that the Flash-Point concentration is above the LFL concentration at the Flash Point temperature, representing the difference between downward and upward propagation.

Thus, as described above, there are two components to the difference in LFL and Flash Point concentrations. Although Flash Points and LFL concentrations are best determined by test, the flame-temperature extrapolation and the downward/upward propagation difference can be used to confirm one of these properties if the other is experimentally determined.