Explosions in dust collectors
Dust explosions are possible whenever the process produces combustible dusts. Not all combustible dusts will produce explosions. For instance, even combustible dusts may not have the characteristics to produce an explosion. A coarse combustible dust such as coal may burn well but not explode depending on how fine the dust is. To produce a conflagration the dust must have a sufficient ratio of surface area to weight to sustain the rapid oxidation for creating and sustaining an explosion. When a dust can sustain an explosion, the dust concentration must be within the explosive limits. These are often defined as:
L.E.L. (Lower Explosive Limit): Below this level of concentration, an explosion will not occur and propagate itself. There is not enough concentration of fuel to allow the flame front to grow. A typical range of values would be 20-30 grains/ cubic foot.
U.E.L. (Upper Explosive Limit): Above this limit the concentration of dust is so high that there is insufficient oxygen to oxidize the fuel and the unburned fuel stops the spread of the flame front.
Ignition of the dust depends on several factors
(1) Chemical Composition
(2) Shape and fineness, briefly described above.
(3) Dust distribution in the gas stream or atmosphere
(4) Concentration of oxygen in the gas stream.
(5) Initial temperature and pressure of the gas.
(6) Energy level available to detonate the explosion
Intensity of the explosion is dependent on the rate of pressure rise and maximum pressure developed. Factory Mutual ran lab tests to determine these values and are contained in their publications. It must be pointed out these tests and values are run with a spherical test chamber with power ignition source in the center of the sphere. These numbers are relatively high when referring to explosions in a dust collector housing, because the bags usually obstruct the expansion of the explosive flame front.
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