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Appendix A IGNITION AND EXPLOSION HAZARD OF DUSTS DEFINITION OF IGNITION SENSITIVITY AND EXPLOSION SEVERITY Def inition of the terms "ignition sensitivity" and "explosion severity" requires a somewhat detailed description of the equipment and procedures used to quantify the parameters involved. The hazard of a dust is related to its ease of ignition and to the severity of the ensuing explosion. 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. The ignition sensitivity and explosion severity of a dust are def ined as: Ignition Sensitivity ~ ~ To x E x C ~ 1/( TC x E x C ~ 2 and Explosion Severity · = (P x P)2/(P X P)1, where subscripts 1 and 2 refer to Pittsburgh coal dust and the test dust, respectively; To is the cloud ignition temperature; E is the minimum i gnition energy; C i s the minimum explosion concentration; P i s the maximum explosion pressure; and P is the maximum rate of pressure rise. lrhe indexes are dimensionless quantities and have a numerical value of 1 f or a dust equivalent to the standard Pittsburgh coal dust. They were not derived from theoretical considerations but provide ratings of explosibility that are consistent with research observations and practical experience. The relative ignition and explosion hazard of dusts may be further classif fed by ratings of weak, moderate, strong, or severe . These terms are corre lated with the empirical index as shown in Table A-1. 23
24 TABLE A-1 Correlation of Indexes with Relative Degree of Hazard Degree of Hazard Severi ty Ignition Sensitivity Explosion Weak Mo aerate Strong Severe 0.2 0.2-1.0 1.0-5.0 5.0 0.5 0.5-1.0 1.0-2.0 2.0 Source: Jacobson et al . 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 * Preliminary Examination of a Dust Sample 610°C 0.06 J 0.055 g/1 83 psig 230 0 ps i/s A sample is initially screened through a No. 20 sieve (840 ~m); the fraction not passing through the sieve is weighed and discarded. A repre sentative portion of the through-No . 2 0 sieve dust then is screened mechanically through No. 100 (149 ,um) and No. 200 (74 ~m) sieves to evaluate the part icle-s ize distribution. The through-No . 2 00 sieve dust of a homogeneous substance i s prepared by sieving . For a nonhomogenous mete ria 1, the through-No. 200 sieve dust is prepared by grinding all of a representative portion. In practice, if 95 percent or more of the as-received dust passes through a No. 200 sieve, no further size reduction is made. A few tests are performed using the through-No. 20 sieve dust; complete tests are made with the through-No. 2 00 sieve dust. The moisture content of the as-received material, except coal, is determined by drying at 7 5°C for 24 h. Coal is dried at 105°C for 2 h in accordance with ASTM Standard Method D3176 and D3180. Heat-sensitive material s are dried over a suitable dessicant at room temperature. Explosibility tests are conducted on dusts having 5 percent or less moisture; however, if moisture at this level is observed to affect dispersibility, the dust is dried further before testing . *Based on methods described in Dorsett et al. 1960.
25 It is recommended that each dust be microscopically examined at magnifications of lOOX and 400X to ascertain the shape, size, and other physical characteristics of the particles. When applicable, the apparent density should be determined. Chemical analyses and X-ray or spectroscopic examination then may be desirable. This physical and chemical data will aid in interpretation of differing test results for similar materials. Ignition Temperature of a Dust Cloud The ignition temperature of a dust cloud is determined in a Godbert-Greenwalt furnace that consists of a 1-7/16-in. vertical Alundum tube, 9 in. long, wound with 21 ft of 18-gauge (0.824-mm2) Nichrome V wire (Dorsett et al. 1960~. The windings are spaced closer together toward the two ends than in the middle to provide a relatively constant temperature throughout the tube. The tube is mounted between two 1/2 tn.-thick transite plates in a 6-in. diameter sheet-metal cylinder with kieselquhr packing between the Alundum tube and the sheet-metal cylinder. 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. The air reservoir is pressured to a selected level, indicated by a mercury manometer or any suitable gauge from a compressed air line. Opening of the solenoid valve disperses the dust in the chamber downward through the furnace. The pressure used for dispersion ranges from 4 to 20 in. of mercury, depending on the density and dispersibility of the dust. Normally, 0.1 g of dust is used in the test, but the weight of the sample may be varied between O.05 to 1.0 g if the quantity affects the determination. The temperature of the furnace is measured with a 22-gauge (O.326~mm2) chromel-alumel thermocouple 1/32 in. from the interior furnace wall at mid-height. The temperature is maintained at the desired value (within +5°C) by automatic control. Ignition is indicated by the appearance of flame projecting below the mouth of the furnace. The ignition temperature is the minimum furnace temperature at which flame is observed in one or more trials in a group of four. The nominal test increment is 10°C. Appearance of excessive smoke is an indication that the quantity of dust placed in suspension or the pressure used for dispersion may need to be adjusted. Minimum Ignition Energy The minimum electrical energy required for ignition of a dust cloud is determined in the Hartmann apparatus which consists of a vertically mounted, 2-3/4-in. tube, 12 in. long, and auxiliary equipment for producing the dust dispersion. The tube, made of lucite, is attached to a cylindrical metal base or dispersion cup by hinged bolts. The top surface of the cup is machined to an approximately hemispherical shape. The total volume of the chamber is 1.23 1. Dispersion is accomplished by a single blast of air from
26 a 1.31-1 reservoir. The airflow, controlled by a full-port solenoid valve, passes into the chamber through a mushroom-like deflector in the dispersion cup. The air pressure in the reservoir, determined by trial, ranges from to 15 psig. The quantity of dust dispersed ranges from 5 to 10 times the minimum explosion concentration, which must be determined by trial. The top of the tube is covered with a filter-paper diaphragm held by a retaining ring. The spark constituting the igniting source passes between pointed 20-gauge (O.518-mm2) tungsten-wire electrodes located 4 in. above the base of the tube. Preliminary trials are made by varying the electrode gap to determine whether this distance affects the minimum igniting value; the normal gap distance is 1/4 in. The spark is obtained from the discharge of capacitors at 100 ~ (to increase the energy range, the voltage is increased to 400 V). Oil-impregnated, paper-dielectric capacitors ranging from 2 to 100 OF are used. The capacitors discharge through the primary of a luminous-tube (neon) transformer. An electronic timer, with adjustable delay, controls the spark discharge in relation to the dust dispersion; the optimum time is determined during preliminary trials to determine minimum energy. The energy of the spark (in joules) is calculated as 0.5 CV2, where C is the capacitance in farads and V is the charging potential in volts. The reported minimum energy for ignition of the dust cloud is the least required to produce flame propagation 4 in. or longer in the tube. Four trials are made at each energy setting; however, if the dust ignites in initial trials, lower energy settings are tried until a minimum is obtained. The value of the minimum ignition energy is approximate since some electrical energy is dissipated in the transformer circuit and some remains in the capacitors. For this reason, nominal rather than absolute values of energy are obtained. 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. This test was developed to provide data corresponding to those obtained in large-scale experiments in galleries and in the Experimental Coal Mine using Pittsburgh coal dust. A weighed quantity of dust is distributed in the dispersion cup. The top of the lucite Hartmann tube is covered with a filter-paper diaphragm held in place with a retaining ring. A 1/16-in. hole is made in the center of the filter paper to prevent pressure buildup in the tube from the dispersing air. The electrodes are adjusted to a 3/16-in. gap, and when the electric spark is struck, the current is set to 23.5 mA. The dust cloud is formed in the lucite tube by dispersing the weighed dust sample with air released from the reservoir. Optimum dispersing air pressure ranges from 5 to 15 in. of mercury and is determined in preliminary trials.
27 Following ignition of the dust, sufficient pressure must be developed to rupture the diaphragm to indicate an explosion. The pressure required to burst the filter-paper diaphragm is about 2 to 3 psi, depending on the rate of pressure rise. If propagation occurs for a given weight of dust, the weight is reduced by 5 mg and another trial is made until a quantity is obtained that fails to propagate flame in any of four successive trials. The lowest weight at which flame propagates is used in calculating the minimum concentration. Tests are made with the electrodes at 2 and 4 in. from the bottom of the tube. The average of the two weights is divided by the volume of the tube (1.23 1) to arrive at the minimum concentration. For materials that tend to agglomerate, 3 to 5 percent of Fuller's earth may be admixed to facilitate dispersion. In this test, a momentary dust cloud is produced by a single blast of air. This cloud is of short duration and is relatively nonuniform. To achieve controlled dust dispersion of known concentration, an apparatus was developed to produce a continuous dust-air stream. By varying the airflow and dust feed rate, a dust cloud of desired concentration was produced for studying the lower explosive limit. The results obtained with the continuous-stream method are similar to those obtained with the single-air-blast method in the Hartmann apparatus. Explosion Pressure and Rates of Pressure Rise Pressure and rates of pressure rise developed by a dust explosion are determined in a closed steel Hartmann tube. Dust disperson is accomplished by releasing air from a 50-m1 reservoir at 100 psig, instead of from the 1.31-1 reservoir at 14 psig previously described. 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. Ignition of the dust cloud normally is produced by the 23.5 mA continous spark son m e. For dusts that ignited with difficulty, a heated coil or guncotton source is tried. The explosion pressure is measured by electronic transducers. The maximum pressure and the average and maximum rates of pressure rise developed in an explosion are determined from the pressure-t~me records. The dispersion pressure (initial pressure in the tube at time of ignition) is subtracted from the peak explosion pressure to give a corrected maximum pressure. The average rate is obtained by dividing the maximum pressure by the tome interval between ignition of the dust cloud and the occurrence of the maximum pressure. The maximum rate is the steepest slope of the pressure-time curve. Normally explosion tests are made at dust concentrations of 0.1, 0.2, 0.5, 1.0, and 2.0 oz/ft3. l
28 Reproducibility of Tests In laboratory tests, small quantities of dust (usually 1 g or less) are dispersed in a relatively small volume. 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. Variations in particle shape and size distribution and the pretreatment of a dust also are important factors. It is assumed that test samples are identical with regard to ignition and explosibility. The variation in the measurement of the parameters of ignition sensitivity is appreciable. For example, based on 10 repetitive tests, the mean ignition temperature of cornstarch dust clouds is 430°C. Assuming that systematic errors are not involved, the actual test temperature may be 430 + 11°C at a 95 percent confidence level. When data obtained in laboratory tests are reported, specific values are given even though they may not be statistically valid. For example, the minimum energy required for ignition of coal dust is reported at 0.06 J. A more complete study might show the probability of ignition at 0.06 J to be 0.25 at a 95 percent confidence level. REFERENCES Dorsett, H. G., Jr., Jacobson, M., Nagy, J., and Williams, R. P., Laboratory Equipment and Test Procedures for Evaluating Explosibility of Dusts, Report of Investigations 5624, U.S. Bureau of Mines, Pittsburgh, Pennsylvania, 1960. Jacobson, M., Nagy, J., Cooper, A. R., and Ball, F. J., Explosibility of Agricultural Dusts, Report of Investigations 5753, U.S. Bureau of Mines, Pittsburgh, Pennsylvania, 1961.
