The relationship between flash point and LFL with application to hybrid mixtures



There usually is a significant difference between the flash point and the equilibrium liquid/vapor (“saturation”) temperature that corresponds to the lower flammable limit (LFL). This occurs because (1) the LFL is frequently not measured at the flash point temperature and (2) the LFL concentrations are typically for upward propagation of flame, whereas the flash point is obtained for downward propagation of flame, and upward propagation occurs at lower (“conservative”) concentrations. In this paper, guidance is given for “correcting” this difference. Since many “hybrid” mixtures of flammable vapors and combustible dusts occur at elevated temperatures (as in driers), it may be important that the effect of temperature on the LFL be taken into account.

For several hybrid mixtures, there is a linear relationship between the dust concentration and the vapor concentration that is required to make the mixture explosible. This relationship is more readily visualized if vapor concentrations are expressed in terms of grams per cubic meter (g/m3) like dust concentrations. A linear relationship is observed when the deflagration indices (Kst and Kg) are about equal, with strong deviations if the indices differ significantly. Testing is advisable to establish the relationship with confidence. © 2007 American Institute of Chemical Engineers Process Saf Prog 2008