Mixture flammability limits
LFL and UFL of a gas mixture by Le Chatelier's rule, with temperature and pressure effects, percent of LFL and the oxygen (LOC) check.
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How this calculator works
The lower flammable limit of a fuel mixture comes from Le Chatelier's rule: LFLmix = 100 / Σ(Ci/LFLi), where Ci is each gas as a percentage of the fuel only — without air or inerts (Perry's §26, Eq. 26-43). The upper limit is approximated the same way. LFL falls about 8 % of its value for each 100 °C rise and UFL rises about 8 %. At raised pressure the UFL is estimated by UFLP = UFL + 20.6·(log₁₀P + 1), P in MPa absolute (Eq. 26-44); the LFL hardly changes.
The example is Perry's Table 26-11 mixture in air: 0.8 % hexane, 2.0 % methane and 0.5 % ethylene. Each is below its own LFL, yet the 3.3 % of fuel is above the mixture LFL — the mixture can burn. With the limits used here (methane 4.4 %, from IEC 60079-20-1) the mixture LFL is 2.36 %; Perry, using older US Bureau of Mines limits (methane 5.0 %, ethylene 2.7 %), gets 2.5 %.
Rules of thumb from Perry's §26: keep fuel at or below 25 % of the LFL with instruments and interlocks (NFPA allows up to 60 % LFL outside ovens and furnaces); when inerting, keep O₂ below 60 % of the LOC, or 2 points below it with continuous monitoring.
References: Perry's Chemical Engineers' Handbook §26 (process safety); Zabetakis, US Bureau of Mines Bulletin 627; Coward and Jones, Bulletin 503; IEC 60079-20-1; NFPA 69.