Energy ignition

The minimum ignition energy, denoted, and expressed in J is the minimal energy required to initiate the combustion of a fuel by any means (e.g., thermal, electrical, mechanical). Usually ignition energy of most fuels are in the order of a millijoule (mj). 

Eigure 3-5.4.1 was constructed from data in Table A-4-1.3. It shows that hydrogen s LMIE is found for small diameter spark electrodes using a circuit having a small capacitance and hence a small RC time constant. The same general trend is also found for methane and is primarily due to reduced 

FIQURE 3-5.4.1 Effect of electrode diameter and circuit capacitance on MIE of hydrogen in 

Test methods reporting the decrease of stored energy during discharge of a capacitor use the relation 

This can be measured by injecting dust into a premixed flammable gas-air mixture so that the final gas concentration is known. Provided the gas does not condense when pressurized it is possible, with appropriate precautions, to inject the dust using a pressurized gas-air mixture identical to the mixture in the test vessel. This avoids concentration gradients. Because it is a non-standard technique the possibility of estimating the hybrid mixture MIE (HMIE) should be considered (6-1.3). 

See 2-3.1. Typical laboratory conductivity meters have insufficient sensitivity to measure semiconductive and nonconductive liquids, in Appendix B, some tabulated conductivities appear as suggesting that the instmment used was inappropriate. Some liquids listed as conductive might fall instead into the semiconductive category (e.g., cymene). Eor conductivities less than 100 pS/m especially, highly sensitive picoammeters are required to measure the small currents involved and great care is needed to avoid contamination of both the sample and the test cell. Several ASTM methods are available according to the conductivity range involved [143-146]. 

1 Data from I. Glassman, Combustion, 3d ed. (New York Academic Press, 1996). 

The minimum ignition energy (MIE) is the minimum energy input required to initiate combustion. All flammable materials (including dusts) have MIEs. The MIE depends on the specific chemical or mixture, the concentration, pressure, and temperature. A few MIEs are given in Table 6-4. 

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For local, short-duration heat sources, such as electrostatic sparks, the reported ignition energies for different pressures are on the orders of magnitude given in Table 2 (48—50). 

Table 2. Ignition Energy of Gaseous Acetylene at Various Pressures...

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. 

Materials having low minimum spark ignition energies, or autoignition temperatures... 

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]. 

The effective energy of the static discharge must exceed the ignition energy of the local mixture. 

FIQURE 2-5. Ignition energies (mJ) of various materials and types of ignition source that may ignite them. 

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