Paraffins flammability limits

Figure 15. Flammable limits for paraffin hydrocarbons, with nitrogen and carbon dioxide.

When a combustible substance is mixed with air, the mixture will explode only when it is neither too rich nor too lean. The lower explosion limit (LEL) is the minimum volume percent of the substance in air with flammability, which is separated from the upper explosion limit (UEL) by the explosive concentration range. The tabulations in handbooks are based on experimental data, and sometimes derived from estimation methods based on the elemental composition of the fuel as CmEtxOy. Figure 6.11 shows the LEL for the series of normal paraffins and of 1-alcohols versus the number of carbon atoms. There are two ways to plot the results, which show that, for paraffins, the volume percent shows a steeply declining trend, but the weight percent shows a mildly increasing trend. One may conclude that a smaller volume percent of higher paraffin... 

In vinyl plasticized with various levels of a readily flammable ester, for example DOP, LOI will rise gradually with each part of antimony trioxide, up to about 5-7 parts, at which point it will tend to level off. At about 75 phr flammable plasticizer, this will limit compositions to about 25 percent LOI, usually sufficient to pass horizontal but not vertical flame tests. At 30-50 phr plasticizer, 5-7 phr antimony trioxide will provide an LOI of 30-35, adequate for simple vertical tests. Substitution of zinc borate for antimony trioxide in such systems will result uniformly in lower LOI, but use instead of a 1 1 blend will generally duplicate results with antimony trioxide alone because of their synergistic effect. Zinc borate does not appear to interfere with gas-phase action of antimony plus chlorine. One-third replacement of flammable plasticizer with chlorinated paraffin will result in a greater increase in LOI per part antimony trioxide added. The effects of replacement with FR phosphate plasticizers are discussed in Chapter 9. 

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