Ignition source, flame propagation

The autoignition temperature is the minimum temperature required for self-sustained combustion in the absence of an external ignition source. The value depends on specified test conditions. Tht flammable (explosive) limits specify the range of concentration of the vapor in air (in percent by volume) for which a flame can propagate. Below the lower flammable limit, the gas mixture is too lean to burn above the flammable limit, the mixture is too rich. Additional compounds can be found in National Fire Protection Association, National Fire Protection Handbook, 14th ed., 1991. 

Thermoplastic Fibers. The thermoplastic fibers, eg, polyester and nylon, are considered less flammable than natural fibers. They possess a relatively low melting point furthermore, the melt drips rather than remaining to propagate the flame when the source of ignition is removed. Most common synthetic fibers have low melting points. Reported values for polyester and nylon are 255—290°C and 210—260°C, respectively. 

The lower flammable limit (LEL) or lower explosive limit (LEL) is the minimum concentration of vapor in air below which a flame is not propagated when an ignition source is present (61—64). Below this concentration, the mixture is considered too lean to bum. The lower flammable limit and the flash point of a flammable Hquid are closely related by the Hquid s vapor pressure characteristics. 

Methyl bromide is nonflammable over a wide range of concentrations in air at atmospheric pressure and offers practically no fire hazard. With an intense source of ignition, flame propagation within a narrow range from 13.5 to 14.5% by volume has been reported. The material has no flash point. Thermal decomposition in a glass vessel begins somewhat above 400°C. 

Detonation arresters are typically used in conjunction with other measures to decrease the risk of flame propagation. For example, in vapor control systems, the vapor is often enriched, diluted, or inerted, with appropriate instrumentation and control (see Effluent Disposal Systems, 1993). In cases where ignition sources are present or pre-dic table (such as most vapor destruct systems), the detonation arrester is used as a last-resort method anticipating possible failure of vapor composition control. Where vent collec tion systems have several vapor/oxidant sources, stream compositions can be highly variable and... 

Definition of Dust E losion A dust explosion is the rapid combustion of a dust cloud. In a confined or nearly confined space, the explosion is characterized by relatively rapid development of pressure with a flame propagation and the evolution of large quantities of heat and reaction products. The required oxygen for this combustion is mostly supphed oy the combustion air. The condition necessaiy for a dust explosion is a simultaneous presence of a dust cloud of proper concentration in air that will support combustion and a suitable ignition source. 

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... 

As mentioned previously, in order to initiate flame propagation, an ignition source of adequate energy and duration is required. Ignition sources can include ... 

The location and energy level of an ignition source has a profound effect on flame propagation and the ability of a DDA to quench the flame. 

Flammable Limits The minimum and maximum concentration of fuel vapor or gas in a fuel vapor or gas/gaseous oxidant mixture (usually expressed in percent hy volume) defining the concentration range (flammable or explosive range) over which propagation of flame will occur on contact with an ignition source. See also Lower Flammable Limit and Upper Flammable Limit. 

Deflagration initiation. A relatively weak energy source, such as an electric spark, ignites the mixture and a laminar flame is first formed. The mechanism of laminar flame propagation is via molecular transport of energy and free radicals from the reaction zone to the unburnt mixture ahead of it. 

LEL, LOWER FLAMMABLE (oR EXPLOSIVE) LIMIT The lowest Concentration of vapour/gas in air at a given pressure and temperature that will propagate a flame when exposed to an ignition source. 

Flashback Propagation of a flame through a gaseous mixture from the ignition source back to the release point of the flammable material. 

Lower Explosive Limit (LEL) - The minimum concentration of combustible gas or vapor in air below which propagation of flame does not occur on contact with an ignition source. 

Most early studies of flammability limits at reduced pressures indicated that the rich and lean limits converge as the pressure is reduced until a pressure is reached below which no flame will propagate. However, this behavior appears to be due to wall quenching by the tube in which the experiments were performed. As shown in Fig. 4.26, the limits are actually as wide at low pressure as at 1 atm, provided the tube is sufficiently wide and an ignition source can be found to... 

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