Flammability limits

Flammability limits are the lowest and the highest concentrations of a fuel gas or vapour in air, or in another oxidant mixture, between which a flame can propagate through the mixture. The lower limit is called the lower flammable limit (LFL) and the upper limit the upper flammable limit (UFL). Methane in air, for example, can only carry a flame at concentrations between 5 and 15 % by volume. A mixture below the lower limit contains too little fuel, and a mixture above the upper limit too little oxygen. The same boundaries are often called the lower and upper explosion limits (LEL and UEL).

The limits describe premixed flames, in which fuel and oxidant are mixed before ignition. They are therefore basic data for assessing the fire and explosion risk of gases and vapours, including smoke gas explosions in fires. Their values depend on temperature, pressure, the direction of flame travel and the test apparatus, so a tabulated limit is a measured property rather than an exact physical constant. Safety strategies for gases usually aim to keep concentrations well below the lower limit, or to remove enough oxygen to make every mixture non-flammable.

Definition

Not every mixture of fuel, oxidant and inert gas can carry a flame. There are limits of composition outside which normal flame-type combustion cannot be sustained. Flammability limits are expressed as concentrations of the fuel in a specified oxidant and diluent mixture, at a specified temperature and pressure. For gases and vapours in air, they are normally given in percent by volume at normal temperature and pressure. NUREG-1805 defines the lower limit as the lowest concentration of fuel in air that can support flame propagation, and the upper limit as the highest.

The terms flammable limits and explosive limits are used interchangeably in the technical literature. The word explosive refers to the overpressure that results when a flame passes through a closed vessel. Zalosh draws a finer distinction. Limits measured in pressure vessels, where a pressure rise shows that a flame has spread, are strictly explosion limits. For some fuels the two kinds of limit are virtually the same, but for others they differ substantially. Introduction to Fire Safety Engineering uses the explosion terms LEL and UEL.

Between the limits, the speed of the flame varies with the composition. At the lower or upper limit, an ignited mixture burns with only an insignificant spread of flame. Between the two limits lies a point at which flame propagation is fastest. The most flammable mixture is the stoichiometric one, in which fuel and oxygen are present in exactly the proportions needed for complete oxidation. For most simple hydrocarbons, the lower and upper limits correspond to about half and three times the stoichiometric fuel-to-air ratio. Their lower limit corresponds to about 48 g of fuel per cubic metre.

Methane in air: too lean below the lower flammable limit (5 %), too rich above the upper flammable limit (15 %); the stoichiometric mixture (about 9.5 %) lies in between. Based on Beyler (2002b), p. 2-172, and Zalosh (2002), p. 3-403.

Why flames have limits

Stoichiometry describes the ratios in which substances are consumed and formed in a chemical reaction. The stoichiometric ratio is the ideal mixing ratio, at which all of the fuel and all of the oxygen take part in the reaction. The further a mixture departs from this ratio, the slower the combustion becomes. Substances that do not take part in the reaction are still heated by it. Drysdale interprets the lower limit as the point where the heat losses from the flame, mainly by radiation, exceed the rate of heat production within it. The flame then cannot sustain itself.

This idea can be made quantitative with the adiabatic flame temperature, the temperature that the products would reach if no heat were lost. For most flammable gases in air, this temperature is about 2,300 K for a stoichiometric mixture. It falls to about 1,600 K for a mixture at the lower limit. As early as 1911, Burgess and Wheeler noted that lower-limit mixtures of alkanes and air release almost the same heat per unit volume. For a wide range of fuels containing only carbon, hydrogen and oxygen, the calculated flame temperature at the lower limit is about 1,600 K, with a spread of about 150 K. Hydrogen (980 K), carbon monoxide (1,300 K) and acetylene (1,280 K) are notable exceptions, and a lower value indicates a more reactive fuel.

The limit temperature changes little with the diluent or with the initial temperature of the mixture. It therefore allows engineers to predict how the limits shift with temperature and with added inert gases. Even so, Beyler stresses that the mechanism responsible for flammable limits is not yet understood, although much has been learned that has practical value.

Mixtures outside the limits can still react if energy is supplied throughout their volume. In one experiment, a small hydrogen pilot flame burned in a stream of lean methane and air. Below 5 % methane, combustion occurred only in the wake of the pilot flame. Above 5 %, the flame could travel away from the pilot. The limits therefore describe self-sustaining propagation away from an ignition source, not whether any reaction at all can take place.

Measurement

The most widely used method was developed by the United States Bureau of Mines. A vertical tube 1.5 m long and 0.05 m in diameter is filled with the test mixture and ignited by a spark or a small pilot flame at its open lower end. The mixture counts as flammable if the flame travels at least halfway up the tube. The tube diameter is the smallest at which heat losses to the wall have little effect on the result. Limits are normally measured for upward propagation, because this gives the widest range.

The direction of flame travel matters. Limits for downward propagation are narrower than those for upward propagation. Other methods use a flask of 5 litres with a central spark, or a closed pressure vessel in which a pressure rise shows that the flame has spread. A spherical vessel is more representative of a real tank than the tube method. The lower limit is reported as the average of the lowest concentration that propagates a flame and the highest one that does not. Different national methods give broadly similar results, but some variation remains.

Hydrogen shows how large the effect of direction can be. In air at room temperature its lower limit is 4.1 % for upward propagation, 6.0 % for horizontal propagation and 9.0 % for downward propagation. In spherical vessel tests, overpressures were only measured above about 8 % hydrogen. The pressures matched theoretical values only from about 10 % hydrogen upwards. NUREG-1805 therefore notes that standards based on the 4 % value contain an implicit additional safety factor for hydrogen.

Typical values

Tabulated limits refer to a given temperature and pressure, usually mixtures with air at normal conditions. The selection below is taken from the International Fire Engineering Guidelines (2005), which draw on published tabulations. Hydrogen, carbon monoxide and acetylene have very wide ranges. The hydrocarbons in the table have much narrower ranges, with lower limits of about 1 to 5 % and upper limits of about 7 to 15 %.

Flammable ranges in air of selected gases and vapours, from the lower to the upper flammable limit. Based on National Research Council of Canada et al. (2005), pp. 3.3-2 to 3.3-3.

Flammability limits of selected gases and vapours in air

Substance	LFL (vol %)	UFL (vol %)
Methane	5.0	15
Propane	2.1	9.5
Butane	1.8	8.4
Heptane	1.05	6.7
Toluene	1.2	7.1
Methanol	6.7	36
Ethanol	3.3	19
Hydrogen	4.0	75
Carbon monoxide	12.5	74
Acetylene	2.5	100

Dusts behave differently from gases. For very fine particles, the lower limit no longer depends on the particle size. For coal dust, however, it lies well above the 48 g/m³ that is typical of gaseous hydrocarbons, because not all of the dust turns into vapour. The lower limit of such coal dust is not expected to fall below about 50 g/m³, even in pure oxygen.

Effect of conditions

Oxygen affects the two limits differently. When the oxygen concentration rises and the inert gas content falls, the upper limit increases while the lower limit hardly changes. At the lower limit, there is always more oxygen than needed for complete combustion, so extra oxygen simply replaces inert gas. At the upper limit, oxygen is short, so extra oxygen takes part in the combustion.

Pressure also affects the limits. The lower limit is insensitive to pressure, except well below atmospheric pressure. The upper limit rises when the pressure increases above atmospheric. Below about one tenth of an atmosphere, the two limits suddenly narrow and the gas becomes non-flammable.

The limits widen as the mixture temperature rises. Beyler's example uses the constant limit flame temperature of 1,600 K. Propane has a lower limit of 2.1 % at 20 °C, which falls to about 1.8 % at 200 °C. This rule only holds as long as no significant reactions take place before the flame arrives. For mixtures held near their autoignition temperature, the limit temperature is not expected to stay constant.

For liquids, the limits are linked to the vapour pressure. The flash point is the liquid temperature at which the vapour pressure is just enough to give a lower-limit mixture of vapour and air. In a closed container held above a corresponding upper limit temperature, the vapour is too rich to burn, as typically happens in a car fuel tank. In an open container, however, the vapour is diluted somewhere above the surface into the flammable range. It will then ignite if an ignition source is present.

Effect of temperature on the flammable range of a liquid's vapour in air; the flash point is where the saturated vapour reaches the lower limit. Based on Beyler (2002b), p. 2-174 (Figure 2-7.5, redraw, do not copy), and Simmons (2002), p. 1-147.

Inert gases narrow the flammable range. Flammability diagrams for methane and n-hexane show this effect for several diluents, including nitrogen and carbon dioxide. The lower limit rises in proportion to the heat capacity of the added diluent.

Beyond a certain amount of inert gas, no mixture of fuel and air can carry a flame. The oxygen concentration at this point is the limiting oxygen concentration (LOC). With nitrogen as the diluent, the LOC of most fuels lies between 10 and 12 %. With carbon dioxide, it is generally 2 to 3 percentage points higher, because carbon dioxide has a higher molar heat capacity. For propane, the oxygen content of an air and nitrogen mixture must fall below about 12 % before no mixture can burn.

Flammability diagram for methane, oxygen and nitrogen: the limit line touching the flammable region defines the limiting oxygen concentration. Based on Beyler (2002b), pp. 2-178 to 2-179 (Figure 2-7.7b, redraw, do not copy).

Mixtures of fuel gases

Real releases often contain several fuel gases, for example the pyrolysis gases of a fire. Le Chatelier's rule, an empirical rule from the late nineteenth century, estimates the lower limit of such a mixture in air from the lower limits of its components. Each gas contributes in proportion to its share of the fuel mixture, divided by its own lower limit.

$${\mathrm{L}\mathrm{F}\mathrm{L}}_m=\frac{100}{\sum _{i=1}^n{C}_{f,i}/{\mathrm{L}\mathrm{F}\mathrm{L}}_i}$$

Symbols in Le Chatelier's rule

Symbol	Description	Unit
${\mathrm{L}\mathrm{F}\mathrm{L}}_m$	lower flammable limit of the fuel mixture in air	vol %
$C_{f,i}$	share of fuel gas i in the fuel gas mixture (without air)	vol %
${\mathrm{L}\mathrm{F}\mathrm{L}}_i$	lower flammable limit of fuel gas i alone in air	vol %
$n$	number of fuel gases in the mixture	–

Beyler gives a worked example for a fuel gas of 50 % methane, 25 % carbon monoxide and 25 % hydrogen. Their lower limits in air are 5.0, 12.5 and 4.0 %. The sum of the shares divided by the limits is 10 + 2 + 6.25 = 18.25. The lower limit of the mixture is therefore 100 / 18.25 = 5.48 %. At this limit the mixture contains 2.74 % methane, 1.37 % carbon monoxide, 1.37 % hydrogen and 94.5 % air.

The rule is empirical. In the form shown here, it applies to the lower limit of a mixture of flammable gases in air. Beyler uses the same rule at the stoichiometric limit to judge whether a hot smoke layer containing several fuel gases can burn. In his example, a layer at 700 K contains 10 % hydrocarbons, 2 % carbon monoxide and 1 % hydrogen. The weighted sum comes to 1.07, which is above one, so the layer can ignite and burn when it mixes with air.

Use in fire safety engineering

Flammability limits are the starting point of every gas or vapour explosion assessment. Combustible gases, vapours and dusts present an explosion hazard when they are mixed with air in proportions between the lower and upper limits. The hazard arises when fuel and oxidant are premixed in a confined space before an ignition source appears. On ignition, the confinement prevents free expansion and the pressure rises rapidly. Protection either prevents a flammable mixture from forming, removes ignition sources or limits the pressure rise.

The International Fire Engineering Guidelines (2005) state that the normal safety strategy for gaseous fuels is to avoid concentrations within the flammable range. In practice this means keeping the concentration well below the lower limit. It is hard to make sure that a mixture above the upper limit will never be diluted into the flammable range. Within the flammable range, ignition sources of only a few millijoules can be enough. The ignition energy is lowest near the stoichiometric mixture and rises sharply as either limit is approached.

Inerting is the alternative when the fuel cannot be kept out. If the oxygen concentration stays below the LOC, premixed burning is prevented. In one of Beyler's examples, a 200 m³ room contains 30 % methane after a leak. About 167 m³ of nitrogen must be added before air can safely enter, assuming good mixing during the injection. For tanks at United States nuclear facilities, NUREG-1805 refers to NFPA 69. That standard requires the oxygen to stay at 60 % of the LOC when the LOC is above 5 %, or the fuel gas to stay below 25 % of the LEL.

In fires, flammable mixtures can form from the products of the fire itself. Introduction to Fire Safety Engineering describes a smoke explosion as the very rapid burning of flammable gases from decomposing fuel or incomplete combustion. A typical condition is that air and pyrolysis gases are reasonably premixed and lie between the LEL and UEL. The mixture can then be ignited by autoignition, by a spark from electrical equipment or by glowing particles from the fire. NUREG-1805 adds that smouldering fires, for example in polyurethane foam, can produce enough unburnt fuel and carbon monoxide for such an explosion.

The same principle applies to new hazards. In a Dutch study of car parks, gases released from electric vehicle batteries can build up in confined or poorly ventilated spaces. If their lower explosion limit is reached and the mixture is ignited, a fire gas explosion can occur. Outdoors, a cloud of vapour from a spill burns only if it meets an ignition source while part of the cloud lies within the flammable range. The regions within the limits then burn as premixed flames, followed by diffusion burning of the richer parts. NUREG-1805 describes an unconfined vapour cloud explosion as the case in which an ignition source sets off a deflagration in such a cloud, seen as a fireball.

The limits also have known weaknesses as design data. Because concentrations within a real cloud vary, fire risk analyses for United States nuclear plants commonly use half the lower limit for hydrogen clouds. Published values for the same gas also differ slightly between sources, as for the upper limit of hydrogen, given as 74 % and 75 %.

See also

• Fire triangle
• Deflagration
• Hazardous area classification
• Hydrogen safety
• Minimum ignition energy
• Equivalence ratio

References

• Beyler, C. L. (2002a). Fire hazard calculations for large, open hydrocarbon fires. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-268–3-314). National Fire Protection Association; Society of Fire Protection Engineers.
• Beyler, C. L. (2002b). Flammability limits of premixed and diffusion flames. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-172–2-187). National Fire Protection Association; Society of Fire Protection Engineers.
• Drysdale, D. D. (2002). Thermochemistry. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-90–1-98). National Fire Protection Association; Society of Fire Protection Engineers.
• Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
• National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Nowlen, S. P., Najafi, B., Wyant, F. J., Forester, J., Kazarians, M., Kolaczkowski, A., Joglar, F., Funk, D., Hannaman, G., & Anoba, R. (2005b). EPRI/NRC-RES fire PRA methodology for nuclear power facilities: Vol. 2. Detailed methodology (NUREG/CR-6850; EPRI 1011989). Electric Power Research Institute; U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research.
• Simmons, R. F. (2002). Premixed burning. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-144–1-154). National Fire Protection Association; Society of Fire Protection Engineers.
• Van der Graaf, J., Hessels, T., & Rosmuller, N. (2023). Fire safety of indoor car parks accommodating electrically powered vehicles. Nederlands Instituut Publieke Veiligheid. (Original work published 2021)
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
• Zalosh, R. (2002). Explosion protection. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-402–3-421). National Fire Protection Association; Society of Fire Protection Engineers.