(104be) “Explosibility Index” – Extension to Dust-Explosion Protection

The U. S. Bureau of Mines [USBM] developed the "Explosibility Index" to rank the relative hazards of different varieties of coal dust.   To highlight differences in coal-dust properties, components of the Explosibility Index were separated into “Ignition Sensitivity” and “Explosion Severity”.  A purpose of this paper is to show a physico-chemical basis for these two indices.

These characteristics of dusts have been utilized in comparing dusts of a wide variety of materials, from metals to pharmaceuticals.  Also, the National Fire Protection Association [NFPA] has further utilized these indices by stating that “area [electrical] classification is not considered necessary” if the Ignition Sensitivity is less than 0.2 AND the Explosion Severity is less than 0.5 when compared to Pittsburgh Coal dust.  Further, NFPA states “Dusts with explosibility parameters that fall below these limits are generally not considered to be significant explosion hazards”.

The above statements infer that values of Ignition Sensitivity and Explosion Severity that exceed 0.2 and 0.5, respectively, would indicate a significant explosion hazard.  Thus, higher values of either or both of these indices would justify proportionate explosion-protection measures in equipment or areas that could contain dense dust clouds.  A second purpose of this paper is to suggest a “matrix” method for determining the extent of explosion protection that should be provided, as functions of these indices.  An additional “layer of protection” is suggested for each half-order-of-magnitude increase in the Explosibility Index, from 0.03 to 30.