Mixture Flammability Limits (Le Chatelier) Calculator

Understanding the mixture flammability limits calculator — concept and principle

A gas mixture can only burn when the fuel concentration in air lies between the lower explosive limit (LEL) and the upper explosive limit (UEL). Natural gas, LPG and off-gases are mixtures, so this calculator estimates the LEL and UEL of a blend from the limits of its components using Le Chatelier's mixing rule.

How it is calculated

• LEL_mix = 100 ÷ Σ(yᵢ ÷ LELᵢ)
• UEL_mix = 100 ÷ Σ(yᵢ ÷ UELᵢ)

yᵢ is the percentage of component i in the fuel part of the mixture (the fractions should add up to 100), and LELᵢ and UELᵢ are its limits in volume percent in air.

The principle

Le Chatelier's rule treats each fuel as contributing its share of the "flammability" needed to sustain a flame. At the lower limit, a component present at concentration cᵢ in air supplies the fraction cᵢ/LELᵢ of what is needed, and the mixture is at its limit when these fractions add up to one. The rule works best for chemically similar fuels such as light hydrocarbons, because their limit flames have similar temperatures. It is less accurate at the upper limit and for mixtures containing hydrogen or unusual components.

Worked example

1. Defaults resemble methane, ethane and propane: y = 80, 15, 5 %; LEL = 5.0, 3.0, 2.1 %; UEL = 15, 12.4, 9.5 %.
2. Lower limit sum: 80/5.0 + 15/3.0 + 5/2.1 = 16 + 5 + 2.381 = 23.381.
3. LEL_mix = 100 ÷ 23.381 = 4.28%.
4. Upper limit sum: 80/15 + 15/12.4 + 5/9.5 = 5.333 + 1.210 + 0.526 = 7.069.
5. UEL_mix = 100 ÷ 7.069 = 14.15%.

How to read the result

Between about 4.3% and 14.1% fuel in air, this blend can ignite. Heavier components pull the LEL down, so even small amounts of propane or butane make a gas easier to ignite at low concentration. Gas detectors usually display readings as a percentage of LEL, and alarm points are set at a fraction of it; use the mixture LEL rather than a pure component's value to interpret such readings.

Common mistakes

• Entering fractions of the total mixture including air or inert gas instead of fractions of the fuel only.
• Mixing up the order of the lists so a fraction is paired with the wrong limit.
• Applying ambient-condition limits at high temperature or pressure, or in oxygen-enriched atmospheres, where the flammable range widens.

Frequently asked questions

Is LEL the same as LFL?

Yes. Lower explosive limit and lower flammability limit are used for the same quantity, as are UEL and UFL.

Can a mixture above the UEL be safe?

It will not burn as it is, but dilution with air during a leak or purge passes through the flammable range, so it is not inherently safe.

Where do the component limits come from?

From measured data compiled in references such as the US Bureau of Mines reports; always check the conditions of the source values.

Sources: Zabetakis, M. G. (1965). Flammability Characteristics of Combustible Gases and Vapors. US Bureau of Mines Bulletin 627. · Crowl, D. A. & Louvar, J. F. Chemical Process Safety: Fundamentals with Applications. Prentice Hall.