Minimum dust concentration

The screening test is used to determine the rates of pressure rise during an explosion, the maximum pressure reached, minimum dust concentration needed to support an explosion and the minimum... 

There are several different devices for determination of dust explosion characteristics. All devices include a vessel which may be op>en or closed, an ignition source which may be an electrical spark or electrically heated wire coil and a supply of air for dispersion of the dust. The simplest apparatus is known as the vertical tube apparatus and is shown schematically in Figure 15.4. The sample dust is placed in the dispersion cup. Delivery of dispersion air to the cup is via a solenoid valve. Ignition may be either by electrical spark across electrodes or by heated coil. The vertical tube apparatus is used for the classification test and for determination of minimum dust concentration for explosion, minimum energy for ignition and in a modified form for minimum oxygen for combustion. 

The minimum dust concentration for explosion is measured in the vertical tube apparatus and is used to give an indication of the quantities of air to be used in extraction systems for combustible dusts. Since dust concentrations can vary widely with time and location in a plant it is not considered wise to use concentration control as the sole method of protection against dust explosion. 

Minimum Explosible Concentration (MFC) The lowest concentration of combustible dust necessary to produce an explosion. 

Dust Minimum ignition temperature (°C) Minimum explosible concentration (g/l)... 

Material Median particle size, pm Minimum explosive concentration g/m= p max bar ga (dp/dt)max> ba r/sec Ksi bar-m sec Dust Hazard Class... 

Material Median particle size, Mm Minimum explosive concentration g/m3 P oux bar ga (dP/dl)max, bar/sec KSt Dust bar-m Hazard sec Class ... 

Table 5. Representative ignition temperatures, minimum explosion concentration, and minimum ignition energy for selected dusts, from NFPA Fire Protection Handbook (National Fire Protection Association, 1986) and other sources.

Dust Median particle size (Mm) Minimum explosive dust concentration (9/m3) Pmax (bar g) st (bar-m/s) Minimum ignition energy (mJ)... 

Minimum explosive concentration for explosive and pyrotechnic dusts have been reported in the range of 40 to 1000 gm/mm, (40 to 1000 oz/ft ). With the exception of location 5 in Building 1619 at Louisiana AAP, all the dust concentrations determined for the various plants were below the maximum average concentrations. 

The lower explosive limit and minimum explosive concentrations of flax, wool, cotton, jute, hemp and sisal fibres are of the same order of magnitude as those of highly explosive dusts [15], The explosibility of pyrites dusts with sulfur contents above 20% was evaluated experimentally. Dusts of 30% sulfur content gave explosion pressures of 3 bar at pressure rise rates of 16 bar/sec. Mixtures of 60% pyrites and 40% powdered limestone still showed significant pressure effects, and the proportion of limestone actually needed to suppress explosions was considerably above the values currently accepted by mining industries [16], Effects of mixtures of particle sizes in combustible dusts upon minimum ignition temperature (T ") and upon presence or absence of explosion were studied. Presence of 30% of fines in a coarse dust lowers Tf significantly [17], Experimental explosions of polyethylene,... 

Cashdollar, K.L., and Chatrathi, K., Minimum explosible dust concentrations measured in 20 liter and 1 cubic meter chambers, Comb. Sci. and Tech not, 87, 157-171, 1992. 

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