OpenSpecEngineering

Finds the lower and upper flammability limits (LFL and UFL) in air of a fuel made of up to three gases or vapors. Enter a concentration of the mixture in air and it also tells you how close that is to the lower limit. Process safety engineers use it for vent headers, dryer exhausts, tank vapor spaces and gas detector settings.

Equations

LFL_mix = 1 / Σ(y_i/LFL_i)     UFL_mix = 1 / Σ(y_i/UFL_i)

Fraction of LFL = 100·C/LFL_mix

This is Le Chatelier’s mixing rule [1], [2]. y_i is the mole fraction of component i in the fuel alone, not counting air or inert gas. LFL_i and UFL_i are that component’s limits in air, in vol %. C is the concentration of the whole fuel mixture in air, in vol %.

Inputs

• Choosing a gas fills in its limits in air at about 25 °C and 1 atm, from Zabetakis [4]. Choose Custom to enter your own.
• The mole percentages need not add up to 100. The page scales them to fractions of their sum. So you can enter each gas’s own concentration in air, and enter their total as the mixture concentration.
• Set a component’s mole percentage to 0 to leave it out. Leave the mixture concentration at 0 to skip the margin.

The status flags a concentration below 25 % of the LFL. NFPA 69 uses that level as its usual upper limit for a space kept below the LFL [3].

Limits and common mistakes

• Don’t enter nitrogen, carbon dioxide or water vapor as a component. Leave inerts out of the fuel fractions.
• The rule works best for similar hydrocarbons. It is less exact for the UFL, and for mixtures with hydrogen or carbon monoxide [2].
• Limits widen as temperature and pressure rise, and they narrow as oxygen falls. The tabulated values hold only near room conditions in air.

Worked example

A solvent dryer’s exhaust holds 0.8 vol % hexane, 2.0 vol % methane and 0.5 vol % ethylene, 3.3 vol % fuel in all. Is the exhaust flammable?

1. Enter Component 1 — mol % of fuel 0.8 and choose Component 1 — gas or vapour n-Hexane. The limits fill in as 1.2 and 7.4.
2. Enter Component 2 — mol % of fuel 2.0 and choose Methane (5 and 15).
3. Enter Component 3 — mol % of fuel 0.5 and choose Ethylene (2.7 and 36).
4. Enter Mixture Concentration in Air 3.3 and press Solve. The Solution shows: Mixture LFL: 2.63609 vol %
   Mixture UFL: 12.9244 vol %
   Flammable range width: 10.2883 vol %
   Fraction of LFL: 125.185 % LFL
   Status: Within flammable range

Check by hand: the fuel fractions are 0.242, 0.606 and 0.152. Then 0.242/1.2 + 0.606/5 + 0.152/2.7 = 0.2020 + 0.1212 + 0.0561 = 0.3793, so the LFL is 1/0.3793 = 2.636 vol %. The exhaust at 3.3 vol % is flammable. Bringing it down to 25 % of the LFL, 0.659 vol %, means diluting it about fivefold.