Ignition energy minimum

The minimum ignition energy (MIE) is the smallest quantity of energy stored in a capacitor which is sufficient to ignite the most flammable mixture of dust and air. The test is carried out under standardized conditions (vid. [33]). Table 2.26 gives values for selected materials. 

Materials are categorized according to their minimum ignition energy MIE 10 mJ normally sensitive to ignition 3 ml MIE 10 mJ especially sensitive to ignition MIE 3 mJ extremely sensitive to ignition 

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The minimum ignition energy of Hquid acetylene under its vapor, when subjected to electrostatic sparks, has been found to depend on the temperature as indicated in Table 3 (86). Ignition appears to start in gas bubbles within the Hquid. 

Table 3. Minimum Ignition Energy of Liquid Acetylene...

Turbulent Diffusion FDmes. Laminar diffusion flames become turbulent with increasing Reynolds number (1,2). Some of the parameters that are affected by turbulence include flame speed, minimum ignition energy, flame stabilization, and rates of pollutant formation. Changes in flame stmcture are beHeved to be controlled entirely by fluid mechanics and physical transport processes (1,2,9). 

From a safety standpoint, priority must be given to the measures in item 1. Group 2 cannot be used as a sole protective measure for flammable gas or solvent vapors in industrial practice with sufficient reliability, but can be applied as the sole protective measure when only combustible dusts are present if the minimum ignition energy of the dusts is high (>10 mj) and the operating areas concerned can easily be monitored. 

Minimum Ignition Energy (MIE) Initiation of flame propagation in a combustible mixture requires an ignition source of adequate energy and duration to overcome heat losses to the cooler surrounding material. Dust and vapor... 

Intrinsic Safety. Static electrical concepts such as minimum ignition energy do not directly apply when assessing the safety of electrical circuits such as radios, flashlights and instmmentation. Intrinsically safe electrical equipment is usually available which has been subjected to fault analysis and testing. The equipment must be certified for the flammable atmosphere in which it will be used (NFPA 497). Refer to texts on Intrinsic Safety such as [63]. 

If it is assumed that explosible dust will be present above the MEC, and equipment design data are not required, explosibility testing forP g, and Kg usually has no direct application. However, minimum ignition energy (MIE) testing should be considered to help determine the likelihood of ignition. Since MIE is extremely sensitive to particle size it is especially important to test a sample that is sufficiently fine to represent the worst credible case. 

TABLE A-4-1-3a. Minimum Ignition Energy (mJ) Variation with Capacitance and Electrode Diameter... 

Minimum ignition energy (mj) Maximum explosion pressure (psi) " Maximum rate of pressure rise. (psi/s) Maximum oxygen concentration to preyent ignition (% by volume)... 

Important flammability characteristics are the lower and upper flammability limits, the flash point, the minimum ignition energy, the minimum oxygen concentration, and the autoignition temperature. Values of some of these properties are published for many compounds (NFPA, 1994). These numbers have typically been developed under standardized test conditions. Process conditions may influence their values. 

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