(47d) Upper Explosible Limits for Combustible Dusts

Upper Explosible Limits for Combustible Dusts

by Richard W. Prugh

Principal Process Safety Engineer

Chilworth Technology, Inc., a DEKRA Company

113 Campus Drive, Princeton, New Jersey 08540

609-799-4449; richard.prugh@dekra.com

Abstract

Although Upper Flammable/Explosible Limits are sharply defined for flammable liquids and gases, combustible dusts do not appear to have well-defined Upper Explosible Limits.  The causes for this behavior include the typically-wide particle-size distribution of dusts and difficulty in obtaining a uniform dispersion of dust in the test equipment.  During an explosibility test, the larger particles may act as inert materials, and the smaller particles represent a low concentration in the surrounding mixture of air and inert particles, and they burn readily.  If the composition and heat of combustion of the combustible dust are known, thermodynamic calculations can yield the concentration at which the combustion temperature is insufficient for propagation of flame through a cloud of small particles.  Examples are given for a typical carbohydrate dust and a metal dust.