Is your dust collection system an explosion hazard?

Chemical Engineering Progress, Oct 2003 by Ebadat, Vahid

Minimum explosible concentration - The minimum explosible concentration (MEC) test determines the lowest concentration, in g/m^sup 3^, of the dust cloud in air that can give rise to flame propagation upon ignition. The test, performed in accordance with the ASTM E1515 and ISO method 618411, involves dispersing a sample of the dust in a 20-L sphere apparatus and attempting to ignite the resulting dust cloud with an energetic ignition source. Trials are repeated for decreasing sample sizes until the MEC is determined. The MEC of a given dust cloud is influenced by the size of the ignition source. For instance, an increase in the size of the ignition source will result in a lower MEC value.

Minimum ignition energy - The minimum ignition energy (MIE) test determines the lowest electrostatic spark energy that is capable of igniting a dust cloud at its optimum concentration for ignition. The test is used primarily to assess the potential susceptibility of dust clouds to electrostatic discharges.

The MIE lest is performed in accordance with ASTM E2019, British Standard 5958, and IEC 1241-2-3 using the modified Hartmann tube apparatus. Dust samples of various sizes are dispersed in a 1.2-L vertical tube, and attempts are made to ignite the resultant dust cloud with discrete capacitive sparks of known energy.

MIE is influenced by particle size and the moisture content of the dust, and by process conditions, such as temperature and oxidant content. For instance, a decrease in particle size and moisture content of the dust particles results in a lower MIE. An increase in the temperature of the atmosphere in which the dust cloud is suspended will result in a decrease in MIE.

Minimum ignition temperature - The minimum ignition temperature (MIT) test determines the lowest temperature at which dust that is dispersed in the form of a cloud can ignite. The MIT is an important factor in evaluating the ignition sensitivity of dusts to ignition sources, such as heated environments, hot surfaces, electrical apparatus and friction sparks.

The MIT test is performed in accordance with ASTM E-2021. Dust samples of various sizes are dispersed into the furnace, and the minimum furnace temperature capable of igniting the dust cloud at its optimum concentration for ignition is determined. Its value is influenced by the particle size and moisture content of the dust. For instance, a decrease in dust particle size and moisture content results in a lower MIT.

In addition to AIChE's educational services (box), Chilworth is offering courses related to controlling electrostatic hazards; dust explosion prevention and protection techniques; gas/vapor explosion hazards; and electrical-area classification and chemical reaction hazards on Oct. 20-24 in Las Vegas, NV, and in the spring of 2004, in NJ.

Literature Cited

1. Abbott, J. A., "British Materials Handling Board Survey of Dust Fires and Explosions in the United Kingdom 1979-1984," Warren Spring Laboratory, Hertfordshire, U.K. (1986).

2. Eckhoff, R. K., "Dust Explosion in the Process Industries," Second Ed., Butterworth-Heinemann Linacre House, Jordan Hill, Oxford, U.K. (1997).