Limits of flammability

Mixtures become inflammable when the vapour concentration lies between two limits called the lower explosive limit (LEL) and the upper explosive limit (UEL). These limits depend on numerous factors, especially temperature and pressure. 

LEL is the most important of the two limits. It is mostly useful when inflammable substances are handled in confined spaces (reservoirs, painting cabins, ovens etc). Detaiis of limits of inflammability are kept by chemical substance manufacturers who are required to mention them on safety sheets that have to be put at clients disposal. When compared with the equilibrium concentration determined as indicated before, LEL aiiows determination of whether a working environment presents a risk of explosion in the presence of a source of ignition. 

Despite ail of this, LEL is not regarded as an important inflammability parameter. In fact, as will be seen in paragraph 1.3, flashpoints are considered to be more convenient in the evaluation of fire hazard of chemical substances. 

But as will be seen in paragraphi. 3.3 LEL presents an indirect factor of interest in the estimation of flashpoints and in this respect will be considered as an essential parameter in the study of evaluation criteria of fire hazard. This point of view will be reinforced when using LEL as a parameter in the calculation of the 11 code of fire hazard presented in paragraph 1.5.5. 

The object of these comments is, first of all, to draw attention to the fact that experimental limits of inflammability present a big experimental error and have to be handled with caution. The methods enable an estimation of the relevance of these experimental values to be made, and when these are not sufficiently reliable or are unknown - an estimation based on calculation models is made. 

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Based on Coward and Jones, Limits of Flammability of Gases and Vapors, U.S. Bureau of Mines Bulletin 503 (USNTIS AD-701575), 1952,... 

Flammability Limits There are both upper (or rich) and lower (or lean) limits of flammability of fuel-air or fuel-oxygen mixtures. Outside these hmits, a self-sustaining flame cannot form. Flammability limits for common fuels are listea in Table 27-18. 

Table 3. Limits of flammability of gases and vapors, percent in air ...

Explosivity limits for various pure components are given in Table 3. The limits of flammability ( concentration, C) for a mixture of gases can be computed from the following expression ... 

Below atmospheric pressure there is no effect on the limits of flammability of natural gas-air mixtures and most other gas-air mixtures. Below about 25 mm absolute pressure, carbon monoxide-air mixtures are not flammable. 

From atmospheric pressure, up to 2170 kPa, the lower limit of flammability is not affected, but the upper limit rises as the pressure on the mixture is increased. This widens the limits of flammability as the pressure increases, as shown in Table 4. Above 2170 kPa the lower limit will be reduced. 

Before the size of the flammable portion of a vapor cloud can be calculated, the flammability limits of the fuel must be known. Flanunability limits of flammable gases and vapors in air have been published elsewhere, for example, Nabert and Schon (1963), Coward and Jones (1952), Zabetakis (1965), and Kuchta (1985). A summary of results is presented in Table 3.1, which also presents autoignition temperatures and laminar burning velocities referred to during the discussion of the basic concepts of ignition and deflagration. 

Coward, H. F., and G. W. Jones. 1952. Limits of flammability of bases and vapors. Bureau of Mines Bulletin 503. 

Lean mixture A mixture of flammable gas or vapor and air in which the fuel concentration is below the fuel s lower limit of flammability (LFL). 

TABLE 7.2.1 Flammability Limits of Flammable Compounds Under Normal Pressure, Room Temperature ... 

The violence of the confined explosions (flammable v apor, gas, and dust) can be calculaled by the e.xplosion pressme mid energy eqmitions introduced earlier. The behavior of flammability (or e.xplosivity) limits of flammable mixtures and dusts is also covered in preceding sections. 

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... 

Figure 7-45. Effect of temperature on limits of flammability of a combustible vapor in air at constant initiai pressure. By permission, U.S. Bureau of Mines, Bulletin 627 [43].

Figure 7-48 [43] shows the effects of temperature on limits of flammability at a constant initial pressure. As temperature is increased, the low er limit decreases and the upper limit increases. This is extremely important in... 

F igure 7-50 shows the effect of pressure on the limits of flammability of an ethane system. 

Limits of flammability (% by volume to form combustible mixture) 5-15 2-10 1.8-9... 

When tested in accordance with the procedure described in Appendix B, the odor of the gas shall be distinctive and unpleasant, and the odor in a gas/air mixture shall be such that it is detectable down to a concentration of 20 per cent of the concentration corresponding to the lower limit of flammability for the hydrocarbon mixture concerned. 

When mixed with air, LPG can form a flammable mixture. The flammable range at ambient temperature and pressure extends between approximately 2 per cent of the vapor in air at its lower limit and approximately 10 per cent of the vapor in air at its upper limit. Outside this range, any mixture is either too weak or too rich to propagate flame. However, over-rich mixtures resulting from accidental releases can become hazardous when diluted with air. At pressures greater than atmospheric, the upper limit of flammability is increased but the increase with pressure is not linear. 

The limits of flammability for propane and butane are much narrower than most other gaseous fuels, making LPG safer in this respect. 

Codes require sufficient stench to be added so that the odor of the gas can be detected in air at concentrations down to one-fifth of the lower limit of flammability, i.e. about 0.4 per cent gas in air. 

The flash pt for IPN using the Abel closed cup method is 53°F while the Cleveland open cup method gives a value of 72°F. Min spont ign temp range in air on a heated brass, Al stainless steel or mild steel plate is 245—60° The following data from Ref 6 present, in tabular form, the limits of flammability of IPN vapor/air mixts over a press and temp range ... 

Sung C.J. and Law C.K., Extinction mechanisms of nearlimit premixed flames and extended limits of flammability, Proc. Combust. Inst., 26 865-873,1996. 

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