Static Electricity 2000 Edition

7.2.2* Flammable Limits.
Vapors or gases in air are flamma-
ble only between certain concentrations — the lower flamma-
ble limit (LFL) and the upper flammable limit (UFL). The
concentrations between these limits constitute the flammable
range. Below the LFL, vapors are too lean to burn and above
the UFL, they are too rich to burn. Increased pressure (above
atmospheric pressure) and increased temperature both widen
the flammability range of typical hydrocarbons.
7.2.3 Ignition Energy.
The energy needed to ignite a vapor–
air mixture varies with the concentration. (See Figure 7.2.3 for a
typical relationship between ignition energy and concentration.) For
most materials, the lowest ignition energy value occurs at a
concentration near the midpoint between those for the LFL
and UFL. The lowest value is referred to as the minimum igni-
tion energy (MIE). Some MIEs are given in Appendix B.
FIGURE 7.2.3
Minimum ignition energy of benzene as a 
function of concentration. (Adapted from Britton, 1992,
pp. 56–70.)
7.2.4* Oxidant Concentration.
Combustibility is normally
determined for atmospheric air, which contains 21 percent
oxygen. With an oxygen-enriched atmosphere, the flammable
range expands; that is, the LFL decreases and the UFL
increases. If the oxygen concentration is sufficiently reduced
by inerting, however, an oxygen concentration below which
no ignition is possible is reached. This concentration is
referred to as the limiting oxygen concentration (LOC). By effec-
tively inerting to below the LOC, the hazard of ignition can be
eliminated, as explained in NFPA 69, Standard on Explosion Pre-
vention Systems. Other oxidants, if present in the mixture,
should be addressed similarly. Laboratory testing might be
required to evaluate the hazard.
Concentration volume (%)
1000
100
10
1
0.1
Minimum ignition energy (mJ)
0
1
2
3
4
5
6
7
8
9
Lower flammable limit
Stoichiometric
Lowest
minimum
ignition
energy
Upper flammable limit

---