the side of caution,” and have
decided to use the lowest experimental
figure.
The accompanying graph
(Figure 4) provides a plot of LEL
values from methane to n-decane
homologous series, also showing
LEL ranges for those compounds
where this data is available. At
first glance, the higher hydrocarbons
would seem to have lower
LELs and therefore be more
flammable than the lower ones.
This is not really the case for two
reasons. First, if we calculate the
LEL values on a weight-% basis
(almost never done), all the numbers
become very similar. Some
representative data will illustrate
this (Figure 5).
Chemical reactions, such as
the burning (oxidation) of hydrocarbons,
are always on a weight
basis, as are the calories/gram
or kiloJoules/gram heat outputs. The second reason is that the vaporization rate of the higher hydrocarbons is
much slower, usually allowing ventilation to waft their vapors away soon after they are produced.
The graph does not differentiate between isomers, such as n-butane and isobutane, n-pentane, isopentane and
neopentane, the five hexanes, the nine heptanes and so forth. Except for the butanes, there is a scarcity of LEL
data on these compounds.
For instance, pure neopentane
is an extremely rare
commodity, quite possibly
unobtainable commercially.
There is no incentive
to determine an LEL value
for it. Within experimental
error, it is probable that the
LEL values for every set of
isotopes are either identical
or very close together.
Significance of LEL values
The gaseous hydrocarbons, typically propane, n-butane and isobutane, have extremely low LEL values (Figure
6). Consequently, a small amount can produce a large volume of potentially flammable gas-air mixture. An often
used example is where about 9.2 grams of liquid n-butane or isobutane (roughly16mL) is poured into a standard
55 U.S. gallon (204 liter) open-top drum. The liquid will almost immediately evaporate, producing about 3.81
liters of pure gas. If this is stirred up, using a canoe paddle or some similar device, to give an LEL (1.87 vol.%)
volume of 204 liters, then the entire gas-air content of the drum will become potentially flammable. If ignited,
the flame volume will typically be about 300 liters (80 U.S. gallons) with fire tongues easily reaching heights of
5–6 feet.
To complete the example, if twice the amount of butane is added (18.4 grams), then stirred into the air
mass in the drum, the composition will closely approach the stoichiometric concentration needed for complete
combustion. The consequences of ignition will then be much more dramatic, including a shock wave, faster
flame propagation, and easily three times the LEL flame volume.
For propane, n-butane and isobutane, the problem of low LEL values is exacerbated by their gas densities,
which are 1.5503, 2.076 and 2.011 times as heavy as air (at 60°F or 15.6°C). Once released, the gases tend to sink
slowly in air. However, if they are sprayed or otherwise atomized, dilution with air quickly follows. The density of
the gas-air mixtures will vary, but at the LEL of 1.87 vol.% for isobutane, the mixture is only 1.9% heavier than
air. The settling rate in tranquil air then becomes miniscule or even debatable.
It is possible to take photographs, and even motion pictures, of hydrocarbon gas-air clouds, presumably with
coated plates sensitive to ultraviolet molar extinction coefficients at about 265 millimicrons. In one moving
January 2019 SPRAY 41
Figure 4
Figure 5