Classification of Dusts Relative to Electrical Equipment in Class II Hazardous Locations (1982) / Chapter Skim
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Appendix A: Ignition and Explosion Hazard of Dusts
Appendix A: Ignition and Explosion Hazard of Dusts
Pages 23-28
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From page 23... ...
Among other parameters, the ease of ignition may be considered a function of the ignition temperature, minim energy for ignition, and minimum explosion concentration; the severity of an explosion is related to the maximum pressure and the rate of pressure rise. To facilitate evaluation of the explosibility of dusts and to give a numerical rating to the relative hazard, empirical indexes were developed that compare values obtained for these parameters with similar values for a standard Pi ttsburgh coal dust.
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From page 24... ...
19 61. The data for Pittsburgh coal dust used in quantifying the ignition sensitivity and explosion severity of dust are as follows: Cloud Ignition Temperature Minimum Ignition Energy Minimum Explosion Concentration Maximum Explosion Pre ssure Maximum Rate of Pressure Rise LABORATORY EQUIPMENT AND PROCEDURES *
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From page 25... ...
The top of the tube is connected by a glass adapter to a small brass chamber with a hinged lid for inserting the dust test sample. A full-port solenoid valve betweeen the dust chamber and a 500-ml air reservoir controls the dispersion of the dust.
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From page 26... ...
In limited trials with direct capacitor discharge at high voltages, comparable minimum ignition energies were obtained for several dusts. Minimum Explosion Concentration The minimum explosion concentration or the lower explosive limit of a dust is determined in the Hartmann apparatus except that an induction spark ignition source is employed rather than a timed capacitor discharge spark.
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From page 27... ...
The maximum pressure that can develop in the explosion tube from the dispersing air is 6.5 psig; however, because of rapid development of the dust explosion, the pressure from the dispersing air at the time of ignition is generally 2 to 3 psig. A full-port solenoid valve controls admission of the dispersing air, and a check valve prevents the combustion gases from escaping back into the dispersion reservoir.
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From page 28... ...
Application of the numerical values obtained in the laboratory to large-scale industrial situations must be done with caution. The factors involved are the generally incomplete and nonuniform dispersion in a large volume, the insufficient or excess dust present, the heat losses to the walls and enclosed equipment, the varying degrees of turbulence, and the intensity of the igniting source.
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