The second aspect of the classical fire
triangle is air—or more specifically, the availability
of oxygen. Dry air contains 20.95
vol.% of oxygen, even up to 35 miles above
the earth, and this is quite sufficient to support
combustions unless it is significantly
diluted with non-flammable gases. In practical
terms, the air can be considered to be a
rather constant aspect of the fire triangle,
and one that does not affect the LEL value
of any flammable gas.
The final element involves the ignition
source. While numerous origins are available,
the most common gas house source
Figure 3
is a sufficiently energetic spark: electrical,
static or mechanical. In contrast, the usual
source for consumer incidents is a flame,
followed by electrical sparks.
Flames can be found in gas-fired hot
water heaters, gas stoves, furnaces and
propane barbecues and fish cookers. Sparks
arise from electric toasters, on/off wall
switches, electric motors under refrigerators
or deep-freezers, washing machines, driers,
coffee pots and other appliances. Many
household fires—due to either aerosols or
other products—are initiated at essentially
the floor level. This is because the products
are often used at floor level, such as certain
insecticides, solvent cleaners and so forth.
Also, nearly all flammable gases and vapors
have densities from about 1.5 to 4.0 times
that of air, enabling them to invisibly pour downward to the floor level, after which they can spread prodigiously.
In fact, some states have recognized this aspect and have issued regulations that gas-fired hot water heaters must be
elevated about 16" (406mm) above the floor for safety reasons.
The ignition source must confer sufficient energy to the super-LEL gas-air mixture to overcome the reaction
threshold and initiate combustion. For example, electrical equipment of the micro-ampere type is considered intrinsically
safe in hazardous atmospheres because any sparks would be too “cold” to cause an ignition. While some
experts contend that sparks from dropping a steel wrench or similar object onto a gas house floor could cause an
ignition, the costly 3% beryllium, 97% copper tools will not. They do, in fact, create tiny sparks on a hard surface,
but these are not sufficiently energetic to incite combustion.
From hot-wire experiments, it is thought that temperatures of over about 2,550°F (1,400°C) are required to
ignite an LEL gas-air mixture. (However, within the flammable range—and particularly at the stoichiometric composition—
lower temperatures are sufficient to cause auto-ignition.) Flames, household 110V sparks and many static
sparks are hotter than this and thus are sufficiently energetic to start fires.
Fire engineers (including Fire Marshals), insurance inspectors and other experts generally have a very good
grasp of these basic flammability aspects. They are applied to safety designs (ventilation, detection, dousing) and
safety precautions (no smoking, no use of cotton or wool clothing, minimal occupancy) of gas houses. Beyond this,
however, they are also considered in such matters as product labeling, home construction, home safety devices and
so forth.
LEL determinations
Until about 1960, the determination of LEL values was a relatively inexact science. This was due to the proliferation
of experimental equipment and test methods, the unavailability of highly purified flammable gases, deviations
in atmospheric pressure and other factors. Today, for a specific method, repeatability is about ±0.5% and reproducibility
is about ±1.0%. The equipment is usually quite complex, and most parts are customized. The fact that
different instruments and techniques have been used to obtain LEL data has been a source of concern and some
confusion. For example, a major specialty gas supplier reported the LEL of n-butane in his catalogs as 1.6 vol.% in
1970 and as 1.9 vol.% in the year 2000. (The LEL of isobutane was given as 1.8 vol.% in both editions.)
During the 1960s, the LEL of isobutane was reported as 1.6 vol.% (2000 mL globe) and also as 2.2 vol.% (downward
flame travel in a tube). The current value is often given as 1.83 vol.%. Some authors have elected to “err on
40 Spray January 2019