Flammability – More Things You Should Know – LFL and UFL

Although, fortunately, we seldom encounter flammability issues in cleaning, it’s a good idea to be familiar with some of the terms like Upper and Lower Flammability and Explosive Limits for the sake of safety.

In the preceding blog, it was explained that three things were needed to produce a fire – Fuel, Oxidizer, and a Source of Ignition.  In fact, it is a little more complicated than that.  We all know, for example, that just because we can smell gas in our home from a small leak or a pilot light that has gone out doesn’t mean that the house is going up in flames imminently.  It’s a good WARNING that there is something wrong that needs to be corrected and that shutting off the gas as soon as possible would be a very good idea.  If, on the other hand, the leak is major, there is a good chance that the potential for fire exists at least in the immediate vicinity of the leak.  Most humans can detect the smell of natural gas at concentration levels of just a few parts per billion.

Interesting Note – Natural Gas is really methane and has no smell.  Before the gas enters the pipeline, a chemical, mercaptan, is added by the gas company to give the gas its smell for safety reasons.

A mixture of a few parts per billion of natural gas in air, although it has a detectable odor, will not ignite.  The ratio of gas to air that will burn is bracketed by the Lower Flammability Limit (LFL) and the Upper Flammability Limit (UFL).  These terms are commonly used interchangeably with the terms Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL).  There may be some technical differences but for all practical purposes a flame is an explosion if the ratio of fuel to oxidizer is within the proper range and a source of ignition is present.  It’s a little like the story of the Three Little Bears – too much, too little and just right.

Although the terms LFL and UFL usually apply to a gas, there are some cases in which a suspended solid in a powdered form (flour is one example) or an atomized liquid (gasoline, for example) may also exhibit lower and upper flammability limits when disbursed in air.  The ratio of fuel to air that will support combustion is different for different materials.  Charts giving the limits for a wide range of materials are readily available.

In the case of atomized liquids and suspended powders, the LFL and UFL may, in part, be influenced by temperature as well as concentration.  As discussed in the preceding blog, most solids and liquids must go through the transformation to a gas before a flame will occur due to the introduction of an ignition source.  This is exactly why early internal combustion engines (and even today many small engines with carburetors) incorporated a “choke” to provide a richer mixture of fuel to air to begin operation when the engine was cold.  More fuel was required to produce sufficient vapor to mix with the available air to produce a combustible mixture of vapor and air.  The excess fuel that was not vaporized and burned exited the exhaust.  Once the engine had heated up, the engine heat, of course, increased the rate of vaporization of the atomized liquid.  This reduced the ratio of raw fuel to air that was required for combustion.  The ratio was reduced by turning the “choke” off.  Without a choke, these engines either won’t start (especially in cold weather) or will consume excess fuel.  In extreme cases, the vapor to air ratio may exceed the UFL or UEL and the engine won’t run because the mixture is too rich.

–  FJF  –


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