The purpose of these tests is to determine the minimum amount of concentration and the minimum amount of energy necessary to have an explosion when working with dry, powdery materials. Minimum Concentration is determined by testing various sample weights (dry powders) with a constant energy source (23 milliamperes). This test will determine how much of the material must be present in a dust cloud to be an explosive hazard. Minimum Energy is determined by using a constant sample weight and varying the energy. From this, data can be obtained to determine how much energy is needed for that material before an explosive hazard exists.

Explosibility Index

Hartman Minimum Concentration and Energy

Hartman Apparatus

The Hartman apparatus is used to determine the minimum explosive concentration and the minimum ignition energy of a dust cloud. This apparatus consists of a vertically mounted, 2-3/4-inch diameter combustion tube 12 inches long and auxiliary equipment for producing the dust dispersion. The clear tube is attached to a cylindrical metal base or dispersion cup by hinged bolts. The top surface of the base is machined somewhat hemispherical in shape.

The total volume of the chamber is 75 cubic inches (1.23 liters). Dispersion is accomplished by a single blast of air from a reservoir. The airflow is controlled by a solenoid valve and passes into the chamber to a mushroom-like deflector in the bottom of the dispersion cup. The quantity of dust dispersed ranges from 0.1 to 2.0 gm/L. The top of the tube is covered with a filter-paper diaphragm held by a locking ring. In the standard test, fixed electrode gaps are used. For more precise evaluations, the electrode gap is also varied.

Minimum Explosive Concentration

The minimum explosive concentration or the lower explosive limit of a dust is determined in the Hartman apparatus. An induction spark ignition is used in the minimum explosive concentration test. A weighed amount of dust is spread in a thin layer in the dispersion cup. A hole is made in the center of the filter paper to prevent pressure build-up in the tube from the dispersing air and the tungsten electrodes are adjusted to a gap length of 3/16-inch. The electric spark is struck and the current adjusted to 23.5 milliamperes.

The dust cloud is formed in the PMMA tube by releasing air from the 80 ml reservoir through the 1/2 inch diameter full-port solenoid valve; optimum air pressure ranges from 5 to 15 inches of mercury and is determined in preliminary trials. Following ignition of the dust, sufficient pressure must develop to burst the filter-paper diaphragm; appearance of flame in the tube is not considered propagation. The pressure required to burst the 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 a 5-milligram increment and another trial made until a quantity is obtained which fails to propagate flame in any of four successive trails. The lowest weight at which flame propagates is used in calculating the minimum concentration. Tests are made with the electrodes 2 and 4 inches from the bottom of the tube. The average of the two weights is divided by the volume of the tube (75 cubic inches) for the value of the minimum concentration.

Minimum Ignition Energy

The minimum electrical energy required to ignite a dust cloud is determined in the Hartman apparatus. The quantity of dust dispersed is eight times the minimum explosive concentration, as previously determined. This concentration approximates the most easily ignitable concentration, as determined in tests conducted by the Bureau of Mines.

The igniting spark passes between pointed tungsten electrodes normally 1/4-inch apart. The electrodes are a set distance above the base of the tube. The ignition spark is obtained from the discharge from a bank of capacitors charged to 5000 VDC. The spark energy level is controlled by the size of the capacitor used for discharge to the electrodes. An electronic timer, with adjustable delay, controls the spark discharge during dust dispersal; the optimum time is determined in preliminary trials.

The minimum energy for ignition of the dust cloud is the least required to produce flame propagation 4 inches or longer in the tube. Four trials are made at each capacitor value; however, if the dust ignites in initial trials, lower energy is tried until a minimum is obtained. The value of the minimum ignition energy obtained is approximate as a minor portion of the electrical energy is dissipated in the circuit and left stored in the capacitors.

Hartman Pressure Rate

K_st Value of Combustible Dusts

The purpose of this test is to determine the dust deflagration index or K_st value of combustible dusts. The description follows the test method specified in ASTM Designation E 1226 - 88. The K_st value is calculated by determining the ratio of the maximum pressure and the maximum rate of pressure rise. K_st is useful in evaluating the deflagration parameters of combustible dusts and in sizing explosion vents when the data is used in conjunction with the nomography published in NFPA 68.

SMS uses a 20 L spherical test chamber for obtaining the pressure and pressure rate data as a function of concentration as discussed above.

Twenty-Liter Dust Bomb

The twenty-liter dust bomb is a spherical test vessel used to obtain the maximum pressure and rate of pressure rise of dust samples as a function of concentration. The bomb is equipped with two injection ports, a piezoelectric pressure transducer, a pressure relief port, and an exhaust port. Samples are normally dispersed through a perforated dispersion ring fitted to the bottom port of the bomb. The sample is placed in the injection line and dispersed by a blast of air. After an injection delay, the ignition source is activated to ignite the dust. Two 5-kJ igniters are the standard ignition source. Other ignition sources may be used for special tests as desired. The pre-dispersion vacuum level in the bomb, the dispersion volume and pressure, and the ignition delay have been optimized to provide results comparable to the ASTM E-1226-88 standard, at an initial pressure of one ATM. The bomb is interfaced with a computer, which controls the dispersion/firing sequence and data collection.

Standard protocol is to test at increments of 0.25 gm/L, starting at 0.25 gm/L, to establish the concentration(s) at which the maximum pressure and the maximum rate of pressure rise occur. Three replicates are then completed at the concentration(s) at which the maximums occur, and at the concentration increment below and above it.

Twenty-Liter Pressure Rate