The gas-air temperature is a complicating factor. Raising the temperature to 100°C (212°F) acts to reduce the
LEL value by about 9%. At 400°C (752°F), the LEL is reduced by about 32%. This means that large ignition sources
can cause the combustion of gas-air mixtures where the gas concentration is below the LEL level for ambient
temperatures, especially in quiescent mixtures. Most flammability limit testers use either a 2–4mm induction coil
spark or a very small flame as the preferred ignition source. In a few cases, a tuft of guncotton is fired by a spark or
hot platinum wire to produce an instantaneous large flame. The use of electrically heated, white-hot platinum wire
has been attempted; however, the results are unreliable. Rhodium wire, which melts higher, at 3,560°F (1,960°C),
gives better results.
The effect of testing conditions for propane-air mixtures is shown in Table 1.
Table 2
Propane Isobutane
43.2 vol.% Nitrogen 40.4 vol.% Nitrogen
29.8 vol.% Carbon dioxide 28.1 vol.% Carbon dioxide
13.4 vol.% CFC-12 13.2 vol.% CFC-12
7.8 vol.% Halon 1301 7.5 vol.% Halon 1301
January 2019 SPRAY 39
Table 1
If propane is added to an “air” mixture
containing 11.6 vol.% oxygen or less,
combustion is not possible. For the
butanes, this figure is 12.1 vol.%. Similarly,
if propane is added to air that
is under 98mm of absolute pressure
(24.8" Hg. of vacuum), combustion
cannot occur. In another experiment,
propane gave an LEL of 2.32 vol.%
in atmospheric air (0.0 psig). When
the air pressure was increased to 12
atmospheres (or 176psig), the LEL only
decreased slightly—to 2.01 vol.%.
Many gas houses in the U.S. still
have Halon 1301 receptacles near the ceiling to almost instantly extinguish any possible gas-air fire. At least one has
carbon dioxide receptacles, for the same purpose, although the extinguishing process is somewhat slower. In the
case of propane and isobutane, experiments have
shown that there can be no LEL (or flammability)
when certain amounts of gas diluents are
added (Table 2). In practice, larger amounts are
needed to ensure that all susceptible areas of the
gas house are quenched—even though diluent
concentrations will vary from locale to locale—and
to positively control the quickly expanding ball of
potentially explosive fire (typically within about
0.12 seconds for the Halon units). On that basis, the engineering target is about 12.5 vol.% of Halon 1301 as a
total room average. The Halon atomized gas-liquid is primarily directed at the most likely flammability sites, and
typically suppresses the fire ball by the time it reaches a diameter of about 13 to 18" (33 to 46mm).
In the case of nitrogen and carbon dioxide, huge amounts of gas would have to be very rapidly released, producing
a suffocating atmosphere, and even then the flame suppression would probably be too slow for worker safety.
The fire triangle
Perhaps the most fundamental concept in fire technology is the relationship between fuel (gas), air (oxygen) and an
ignition source. These elements can conveniently occupy the corners of what is termed the “fire triangle” or “flammability
triangle” (Figure 3). Each is subject to certain parameters—outside of which combustion cannot take place.
First, the fuel must be gaseous and within the flammability range; i.e. between the LEL and UEL. The gaseous
aspect may sound strange, if one considers a piece of wood or a chunk of coal. However, these objects must first be
heated to the point where they produce a flammable vapor at the surface before combustion can proceed. In the
case of liquids, the temperature at which this occurs is called the flash point.
The volatile aliphatic hydrocarbons, in liquid form, have flashpoints below room temperature, ranging from
propane (-156°F or -104°C) to n-octane (56°F or 13°C). Isobutane, which is the most important for the aerosol
industry, flashes at any temperature above approximately -117° F (-83°C). The importance of flash points can be
demonstrated using a small quantity of liquid n-butane (-101°F or -74°C). If as little as 1.0 gram of liquid n-butane
(boiling point 31°F) is placed in a 1oz. bottle and a flame is passed over the top, an immediate flare is produced,
up to about 10" (254mm) high. Likewise, if that 1.0 gram is poured onto a concrete floor, it will very quickly evaporate,
producing about 414mL of invisible gas. The gas is cool, and about 2.1 times as heavy as air. It spreads very
rapidly (and surprisingly widely). In doing so, it will produce virtually all possible mixtures with the air.
Just taking the LEL concentration for calculating purposes, it will generate about 22.3 liters (5.9 U.S. gallons)
of flammable gas-air mixture. This is sufficient to create a brief fire of about 10 U.S. gallons in volume if ignited.
Obviously, such experiments should not be done or else be conducted with extreme precautions.