Ignition of Flammable Mixtures

Applied Process Design for Chemical and Petrochemical Plants 

The Effect of Elevated Temperature on the Lower Flammable Limit of Combustible Solvents as Encountered in 

Lower Flammable Limit Percent Vapor by Volume et Initial Temperature, °F 

Reprinted by permission, Fire Protection Handbook, 17th Ed. (1991) p. 4-32. National Fire Protection Association [34], 

Cst = stoichiometric composition of combustionable vapor in air, expressed as a volume percent. 

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The sparking of electrical equipment, such as motors, is a major potential source of ignition, and flame proof equipment is normally specified. Electrically operated instruments, controllers and computer systems are also potential sources of ignition of flammable mixtures. 

B. Damage by confined explosions due to accumulation and ignition of flammable mixtures arising from small process leaks, e.g., flammable gas buildup in analyzer houses, in enclosed drains, around submerged tanks... 

Ignition can also be produced by a heated surface. During the process of heat transfer from a hot surface to a flammable mixture, reactions are initiated as the temperature rises and the combination of additional heat transfer from the surface and heat release by chemical reactions can lead to ignition of the mixture. 

Even if a static spark ignites a mixture of flammable vapor and air, it is not really correct to say that static electricity caused the fire or explosion. The real cause was the leak or whatever event led to the formation of a flammable mixture. Once flammable mixtures are formed, experience shows that sources of ignition are likely to turn up. The deliberate formation of flammable mixtures should never be allowed except when the risk... 

For a spark to be felt, it must have an energy of at least 1 mJ. The minimum energy required to ignite a flammable mixture is 0.2 mJ, so a spark that can be felt is certainly capable of causing ignition if flammable vapor is present. 

Figure 7-46 illustrates a typical relationship of limits of flammability and ignitibility for a methane air mixture. Note that energy required to ignite a flammable mixture (within its LET and UEL) varies with the composition, and that a 0.2 millijoule (mj) spark is inadequate to ignite even a stoichiometric mixture at atmospheric pressure at 26°C, while 1-mj spark can ignite any... 

Spontaneous heating Some flammable liquids com.bine readily with oxygen in the air at ordinary temperatures and give off heat. Wdien the heat is generated faster than it can be dissipated, the temperature rises and ignition of the mixture may occur, as with liquids on waste or rubbish or other materials [32]. 

Using the assumption of a minimum flame temperature needed for ignition of the mixture, determine the minimum fuel mass loss rate per unit surface area (m l) to cause flame propagation through the boundary layer. The heat of combustion that the volatile wood produces (Ahc) is 15 kJ/g. (Hint the adiabatic flame temperature at the lower flammable limit for the mixture in the boundary layer must be at least 1300 °C.)... 

Intrinsically Safe Circuits A circuit which any spark or thermal effect, produced either normally or in specified fault conditions, is incapable, under the test conditions prescribed in this standard, of causing ignition of a mixture of flammable or combustible material in air in its most easily ignited concentration."(3)... 

Hazardous substances present in the process are identified on the basis of their flammability, explosiveness and toxicity. The flammability of gases and vapours of flammable liquids is a great concern in the process industries. The result of an ignition can be a fire or an explosion or both. Accidental fires and explosions of flammable mixtures with air often follow the escape of combustible materials or inlet of air into process equipment. 

A further loss-limiting device which is useful where flammable dusts and vapors are used is the explosion vent. It is important to install explosion vents in areas where flammable liquids or dusts are used because of the possibility of great damage due to explosive ignition of such mixtures and air. Therefore, on a practical basis,... 

Another scenario where large quantities of flammable mixtures may be generated involves slow fuel leaks without ignition in locations where adequate ventilation is not provided. Slow leaks can occur at any of the fittings in the fuel system (from vibration, accident damage, or improper installation or repair), from defective regulators or faulty valves. Low rate fuel releases may be difficult to locate, depending on the rate of fuel release. Their existence will be readily... 

Firecrackers contain flash powder (a mixture of an oxidizer such as potassium chlorate or perchlorate and powdered aluminum or magnesium) or black gunpowder in a paper tube. An attached fuse ignites the flammable mixture, which bums explosively, producing gases that rapidly build up pressure and burst the container. Aluminum and magnesium components produce brighter flashes. 

A person typically has capacitance of 200 picofarads (pF), and if charged to 15 kilovolts (kV) could initiate a discharge of 22.5 mJ. This is enough to ignite many flammable mixtures. The energy in ordinary spark plugs is 20-30 mJ. 

The minimum ignition energy is simply the minimum energy necessary to cause the ignition of a mixture of flammable gas and oxidizing gas within the flammable range. It is usually measured by an electrical spark discharge because of the ease of the test. 

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