The“LEL”: What is it? technical editor
Montfort A. Johnsen, Ph.D.
Three potent little letters: LEL. Not even an acronym. It is sometimes called LFL. Yet, they hold tremendous
significance for the aerosol industry. They are a crucial factor in our flammability test methods,
in formula development, in plant safety and, ultimately, in consumer safety. As an estimate, while
aerosol-related lawsuits are very rare, probably more than 50% of them involve LELs in some way.
LEL is “quick-speak” for Lower Explosive Limit. Similarly, LFL stands for Lower Flammability Limit. The
terms were originally called the “Lower Limit of Flammability,” as used in the watershed book titled Limits
of Flammability of Gases & Vapors by H.F. Coward & G.W. Jones of the Bureau of Mines (U.S. Dept. of the
Interior). It was first published in 1952 as 155 data-packed pages called Bulletin 503 for 70¢ per copy. In it, the
authors pulled together and codified 368 references dating back to at least 1886.
As the name suggests, the LEL is the lowest concentration of gas or vapor which, when mixed with air and
touched with an ignition source, will propagate a flame. The term erroneously suggests that an explosion will
always follow; however, this is only the case when the containment vessel is damaged due to the pressure buildup.
All combustions produce heat and many also produce additional molecules—typically, carbon dioxide and
water vapor.
As an example, a 3.0 v.% mixture of ethane in air, uniformly distributed in a pressure-resistant, four liter glass
globe, then ignited, will produce a pressure of 4.3 atmospheres, or 61.5 psi-g. The pressure will quickly subside
as the gases cool, and the water vapor condenses. Even if this mixture occupied only 1% of a closed vessel,
the almost instantaneous pressure increase would be about 0.615 psi-g—or 88.5 psf-g (pounds per square foot—
gauge)—discounting any slight cooling effect of unburned gas (oxygen) in the container. This is enough pressure
to do very serious damage to cars, rooms or houses. For hydrocarbon gases in air, flame propagation proceeds
at the typical rate of about 12.5 feet per second (3.8 m/s) in all available directions from the source. In almost
all cases, this means that all the hydrocarbon gas-air mixtures within the flammable range will have combusted
within less than a second, often sending a shock wave or pressure front outward at very high speed, which can
extend far beyond the range of the fire itself. Tests conducted by Factory Mutual Research Corp. have shown
that the instantaneous release of isobutane, next to an ignition source, will result in about 80% of the gas being
combusted. The remainder is outside the flammable range of about 1.86 to 8.4 vol.%.
The LEL for a given gas or vapor is not a fixed volume-% figure; it depends upon many factors. The propagation
of flame depends upon the transfer of energy from the burned to the unburned gas. In a limit mixture, the
amount of energy available for transfer is only just enough to maintain propagation. Anything that reduces the
available energy will affect the LEL. The lowest LEL values will result from the use of vessels large enough that
cooling from their walls is negligible. Large pressure-resistant globes, of at least two liters capacity, will give lower
LEL results than smaller containers.
Closed glass tubes, containing LEL mixtures, will often show vibratory flame front movements. This is due
to pressure increases, resulting in combustion at one end, followed by cooling at that end as the flame moves
onward. In many cases, the flame will simply go out before reaching the far end. The direction of flame travel,
in vertical tubes, is also important. Upward proliferation produces lower LEL results than downward travel. The
gentle convection air currents, set up by rising flames, assist in their propagation. Conversely, for flames initiated
at the top, the rise of heated air inhibits downward flame travel, and the LEL increases.
Atmospheric changes in air pressure have a small effect on LEL values. They tend to increase LEL and
decrease UEL (upper explosive limit) values, but only when the initial pressure is about 8 to 10% lower than
the average sea-level pressure of 760mm Hg absolute. If the atmospheric pressure increases, the LEL value will
decrease. In some extreme situations, “non-flammable” gases will develop an LEL and UEL. For example,
methylene chloride vapors are flammable near the Dead Sea (bordering Israel), or at the bottom of a 1500-foot
deep mine shaft. In the same way, mixtures of HFC-134a and air, at about 15psi-g, will develop an LEL at about
12.5 vol.% gas and 87.5 vol.% air. (The UEL will be very slightly higher). Similarly, humidity conditions have
almost no effect on the LEL, although the difference between very dry (0% R.H.) and very wet (100% R.H.) air
can change the UEL of hydrocarbon and air mixtures by 0.4 to 0.5 vol.%. For aerosols, this effect is academic.
Extremely dry LEL mixtures are typically harder to ignite—also academic.
38 Spray January 2019
Montfort A. Johnsen
revisited
This article originally appeared in the June 2005 issue of SPRAY.
Because it is still relevant information we are offering it again to our readers.
It has been reviewed by the author and independent sources for accuracy.