L_t = 1.02 x L x (1-7.75 x10^-4 x T), where L is the laboratory value of LEL, T is the elevated temperature and L_t is the LEL at T. The UEL value increases with increase in mixture temperature, also by about 8% with 180^oF increase in T.

Increasing the mixture pressure above atmospheric affects the LEL very little. The UEL, however, increases greatly. One source reports that for several saturated hydrocarbons, the UEL increases in proportion to the logarithm of the pressure. Another source reports an opposite effect for some hydrocarbons and warns that pressure effects vary according to the hydrocarbon considered.

Inert gases play no part in combustion reactions, but absorb heat when present in a hydrocarbon/air mixture. For that reason, adding inerts to a mixture tends to reduce the spread between LEL and UEL until finally the mixture is no longer flammable. Reference 2 provides the graph below showing specific effects for hydrogen, carbon monoxide and methane when inerted with nitrogen and carbon monoxide. For example, methane in air with no inerts (Ratio = 0) has LEL = 5 and UEL = 15. But if 3.5 mols of CO2 are added to a mixture containing 1 mol of CH4 (in air), the mixture is no longer flammable.

Another way to look at the presence of inerts is to calculate the Minimum Oxygen for Combustion (MOC). The table below from Reference 4 provides values for some compounds. The data came from Reference 5.

The flammability limits of a mixture of various hydrocarbons can be calculated using Le Chatelier’s law, which states that a mixture at the lower limit of flammability mixed with other mixtures which are also at the lower limits of flammability will yield a resulting mixture at the lower limit of flammability. For example, to calculate the LEL of a hydrocarbon mixture which is 70% CH₄, 20% C₂H₆ and 10% C₃H₈,

LEL = 100% / (70/5.0 + 20/3.0 + 10/2.1) = 3.9%

This mixture has a LEL of 3.9% hydrocarbons in air. The mixture UEL is calculated the same way.

If the refractory lining a burner or furnace is hot enough to bring the hydrocarbon/air mixture to the autoignition temperature, rapid combustion will start. Heat transfer from the hot surface depends on gas velocity and turbulence, explaining why "swirl" type burners often seem more stable than more linear types.

Flames from pilot burners are the typical means of initiating combustion of a hydrocarbon/air mixture. Nozzle mix burners (where the fuel mixes with the combustion air within the furnace) have zones that are too lean or too rich for combustion, so the pilot flame must be positioned to heat a volume of well mixed gas. Large pilot flames can overcome poor positioning of the pilot tip.

Sparks are used to ignite pilot burners and also main burners in some cases. Occasionally small sparks (static electricity) are capable of initiating combustion, but the extra energy in a large spark helps insure lightoff. Undesired sparks, such as those resulting from debris moving through steel ducting or fans, can initiate combustion and require careful design to avoid.

By adding natural gas or other hydrocarbon, the mixture can be brought above the UEL, preventing combustion.

By adding inerts such as N2 or CO2, the mixture can be brought to a nonflammable state.

Flames progress at a defined rate through a flammable mixture. Considerable lab testing with non-turbulent mixtures has been done – data for a few gases is listed below from Reference 7. For instance, the "maximum flame velocity" of a methane/air mixture is 1.48 ft/sec under lab conditions. If this mixture flows through a pipe at 1.5 ft/sec, any flame will be unable to propagate against the flow. This fact is used in designing flare tips, burner nozzle and some flame arrestors – by designing for gas velocity above the flame velocity, the flame can be prevented from moving upstream from the point of ignition. NOTE: as a flame front moves through a vessel or pipe, the flame velocity increases. With long enough piping the velocity can increase to detonation levels, which are supersonic.

Mechanical flame arrestors stop flame propagation into or through a pipe (more information below). By placing a flame arrestor at the end of the flammable mixture pipe feeding a flare or burner, flame can be prevented from moving into the pipe regardless of the mixture velocity.

Mechanical flame arrestors may be attached to vent pipes on hydrocarbon storage tanks to allow passage of potentially flammable hydrocarbon/air mixtures but preventing passage of flame into the tank from outside. This protects storage tanks from explosions triggered by lightning ignition of vented gases outside the tank.

For a flashback to progress into equipment, combustion heat must be transferred into the combustible mixture. By passing a potentially flammable mixture through a water spray chamber or some sort of heat sink, a flashback can be stopped. Mechanical inline flame arrestors and detonation arrestors are common heat sinks (see below).

Many methods to stop flashbacks have been devised. "Active" methods require maintenance of certain parameters, such as liquid level or gas velocity. "Passive" methods require only routine inspection and typically have no moving parts or instrument requirements.

Venturi flame arrestors simply create a restriction in the hydrocarbon/air mixture delivery pipe so that the gas velocity is faster than the flame speed, preventing progression of a flashback upstream. Flashback in the direction of flow can still happen. Even a partly closed valve can create a high velocity for flashback prevention, but a venturi shape creates much lower pressure drop. If gas flow stops, the venturi is no longer effective, so methods to measure flow and add makeup gas (nitrogen, for instance) are often included. Note: initially flame velocity is limited to the values in the literature, but extended pipe runs and fittings act to increase the velocity, eventually reaching detonation velocities. A venturi arrestor must be located close to the point of ignition to avoid problems.

Group B - butadiene, ethylene oxide, hydrogen, manufactured gases containing more than 30% hydrogen by volume and propylene oxide

Group C - acetaldehyde, cyclopropane, diethyl ether, ethylene, and unsymmetrical dimethyl hydrazine

Group D – acetone, acrylonitrile, ammonia, benzene, butane, butyl alcohol, secondary butyl alcohol, n-butyl acetate, isobutyl acetate, ethane, ethyl alcohol, ethyl acetate, ethylene dichloride, gasoline, heptanes, hexanes, isoprene, methane (natural gas), methanol, isobutyl alcohol, methyl isobutyl ketone, isobutyl alcohol, tertiary butyl alcohol, petroleum naphtha, octanes, pentanes, amyl alcohol, propane, propyl alcohol, isopropyl alcohol, propylene, styrene, toluene, vinyl acetate, vinyl chloride, xylenes.

Note that a detonation arrestor certified for Group B hydrocarbons is also suitable for Group C and D hydrocarbons. There is some evidence that methanol belongs in Group C or B, but recertification is not complete.

Currently the European Union is defining separate standards for testing and certification of detonation arrestors acceptable in that jurisdiction.

This type of flame arrestor works by bubbling the hydrocarbon/air mixture upwards through a liquid bath (usually water), forming discrete bubbles. The gas exits above the liquid to the ignition source. A flash back is stopped when flame is unable to move from bubble to bubble in order to reach the upstream pipe. Some certification work has been done in Europe on this type arrestor, but so far there are no certified models on the market. A common liquid seal flame arrestor design is shown below. Note the water level must be maintained safely above the level of the sparger at all times to insure bubbles.

When a flash back is detected, one way of stopping it is to temporarily modify the hydrocarbon/air mixture so that it is no longer in the explosive range. Often this is done by injecting an inert gas such as nitrogen. Using this method, the mixture is diluted so that the final oxygen concentration is below the Minimum Oxygen for Combustion (defined above). This approach is sometimes used when a standing flame at a flame arrestor is detected – inert gas is added upstream, snuffing the flame. The system then returns to normal operation. It can also be used for routine operation, in order to prevent the possibility of any flashbacks.

1. Perry’s Chemical Engineers’ Handbook, 4^th Edition

2. Bureau of Mines Bulletin 503 "Limits of Flammability of Gases and Vapors"

3. Flammability Properties of Hydrocarbon Fuels, Wilbur A Affens, Journal of Chemical and Engineering Data, Vol. 11, No. 2, April 1966.

4. Industrial Explosion Prevention and Protection

5. National Fire Protection Association, Standard on Explosion Prevention Systems, NFPA 69, Boston, 1973.

6. Flammability Calculations for Gas Mixtures, W.M. Heffington and Gaines, W.R., Oil & Gas Journal Nov. 16, 1981.

7. North American Combustion Handbook, Second Edition.