# Flammability limit

(Redirected from Explosive limit)

Mixtures of dispersed combustible materials (such as gaseous or vaporised fuels, and some dusts) and oxygen in the air will burn only if the fuel concentration lies within well-defined lower and upper bounds determined experimentally, referred to as flammability limits or explosive limits. Combustion can range in violence from deflagration through detonation.

Limits vary with temperature and pressure, but are normally expressed in terms of volume percentage at 25 °C and atmospheric pressure. These limits are relevant both to producing and optimising explosion or combustion, as in an engine, or to preventing it, as in uncontrolled explosions of build-ups of combustible gas or dust. Attaining the best combustible or explosive mixture of a fuel and air (the stoichiometric proportion) is important in internal combustion engines such as gasoline or diesel engines.

The standard reference work is still that elaborated by Michael George Zabetakis, a fire safety engineering specialist, using an apparatus developed by the United States Bureau of Mines.

## Violence of combustion

Combustion can vary in degree of violence. A deflagration is a propagation of a combustion zone at a velocity less than the speed of sound in the unreacted medium. A detonation is a propagation of a combustion zone at a velocity greater than the speed of sound in the unreacted medium. An explosion is the bursting or rupture of an enclosure or container due to the development of internal pressure from a deflagration or detonation as defined in NFPA 69.

## Limits

### Lower flammability limit

Lower flammability limit (LFL): The lowest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in the presence of an ignition source (arc, flame, heat). The term is considered by many safety professionals to be the same as the lower explosive level (LEL). At a concentration in air lower than the LFL, gas mixtures are "too lean" to burn. Methane gas has an LFL of 5.0%. If the atmosphere has less than 5.0% methane, an explosion cannot occur even if a source of ignition is present. From the health and safety perspective, the LEL concentration is considered to be Immediately Dangerous to Life or Health (IDLH), where a more stringent exposure limit does not exist for the flammable gas.[1]

Percentage reading on combustible air monitors should not be confused with the LFL concentrations. Explosimeters designed and calibrated to a specific gas may show the relative concentration of the atmosphere to the LFL—the LFL being 100%. A 5% displayed LFL reading for methane, for example, would be equivalent to 5% multiplied by 5, or approximately 0.25% methane by volume at 20 degrees C. Control of the explosion hazard is usually achieved by sufficient natural or mechanical ventilation, to limit the concentration of flammable gases or vapors to a maximum level of 25% of their lower explosive or flammable limit.

### Upper flammability limit

Upper flammability limit (UFL): Highest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in the presence of an ignition source (arc, flame, heat). Concentrations higher than UFL or UEL are "too rich" to burn. Operating above the UFL is usually avoided for safety because air leaking in can bring the mixture into combustibility range.

### Influence of temperature, pressure and composition

Flammability limits of mixtures of several combustible gases can be calculated using Le Chatelier's mixing rule for combustible volume fractions ${\displaystyle x_{i}}$ :

${\displaystyle LFL_{\text{mix}}={\frac {1}{\sum _{i}{\frac {x_{i}}{LFL_{i}}}}}}$

and similar for UFL.

Temperature, pressure, and the concentration of the oxidizer also influences flammability limits. Higher temperature or pressure, as well as higher concentration of the oxidizer (primarily oxygen in air), results in lower LFL and higher UFL, hence the gas mixture will be easier to explode. The effect of pressure is very small at pressures below 10 millibar and difficult to predict, since it has only been studied in internal combustion engines with a turbocharger.

Usually atmospheric air supplies the oxygen for combustion, and limits assume the normal concentration of oxygen in air. Oxygen-enriched atmospheres enhance combustion, lowering the LFL and increasing the UFL, and vice versa; an atmosphere devoid of an oxidizer is neither flammable nor explosive for any fuel concentration. Significantly increasing the fraction of inert gases in an air mixture, at the expense of oxygen, increases the LFL and decreases the UFL.

## Controlling explosive atmospheres

### Gas and vapor

Controlling gas and vapor concentrations outside the flammable limits is a major consideration in occupational safety and health. Methods used to control the concentration of a potentially explosive gas or vapor include use of sweep gas, an unreactive gas such as nitrogen or argon to dilute the explosive gas before coming in contact with air. Use of scrubbers or adsorption resins to remove explosive gases before release are also common. Gases can also be maintained safely at concentrations above the UEL, although a breach in the storage container can lead to explosive conditions or intense fires.

### Dusts

Dusts also have upper and lower explosion limits, though the upper limits are hard to measure and of little practical importance. Lower flammability limits for many organic materials are in the range of 10–50 g/m³, which is much higher than the limits set for health reasons, as is the case for the LEL of many gases and vapours. Dust clouds of this concentration are hard to see through for more than a short distance, and normally only exist inside process equipment.

Flammability limits also depend on the particle size of the dust involved, and are not intrinsic properties of the material. In addition, a concentration above the LEL can be created suddenly from settled dust accumulations, so management by routine monitoring, as is done with gases and vapours, is of no value. The preferred method of managing combustible dust is by preventing accumulations of settled dust through process enclosure, ventilation, and surface cleaning. However, lower flammability limits may be relevant to plant design.

### Volatile liquids

Situations caused by evaporation of flammable liquids into the air-filled void volume of a container may be limited by flexible container volume or by using an immiscible fluid to fill the void volume. Hydraulic tankers use displacement of water when filling a tank with petroleum.[2]

## Examples

The flammable/explosive limits of some gases and vapors are given below. Concentrations are given in percent by volume of air.

• Class IA liquids with a flash point less than 73 °F (23 °C) and boiling point less than 100 °F (38 °C) have a NFPA 704 flammability rating of 4
• Class IB liquids with a flash point less than 73 °F (23 °C) and a boiling point equal to or greater than 100 °F (38 °C) and class IC liquids with a flash point equal to or greater than 73 °F (23 °C), but less than 100 °F (38 °C) have a NFPA 704 flammability rating of 3
• Class II liquids with a flash point equal to or greater than 100 °F (38 °C), but less than 140 °F (60 °C) and class IIIA liquids with a flash point equal to or greater than 140 °F (60 °C), but less than 200 °F (93 °C) have a NFPA 704 flammability rating of 2
• Class IIIB liquids with a flash point equal to or greater than 200 °F (93 °C) have a NFPA 704 flammability rating of 1
Substance LFL/LEL in %

by volume of air

UFL/UEL in %

by volume of air

NFPA Class Flash point Minimum ignition energy in mJ

expressed at percent by volume in air
(Note that for many chemicals it
takes the least amount of
ignition energy halfway between
the LEL and UEL.)[3]

Autoignition
temperature
Acetaldehyde 4.0 57.0 IA −39 °C 0.37 175 °C
Acetic acid (glacial) 4 19.9 II 39 °C to 43 °C 463 °C
Acetic anhydride II 54 °C
Acetone 2.6–3 12.8–13 IB −17 °C 1.15 @ 4.5% 465 °C, 485 °C[4]
Acetonitrile IB 2 °C 524 °C
Acetyl chloride 7.3 19 IB 5 °C 390 °C
Acetylene 2.5 100[5] IA Flammable gas 0.017 @ 8.5% (in pure oxygen 0.0002 @ 40%) 305 °C
Acrolein 2.8 31 IB −26 °C 0.13
Acrylonitrile 3.0 17.0 IB 0 °C 0.16 @ 9.0%
Allyl chloride 2.9 11.1 IB −32 °C 0.77
Ammonia 15 28 IIIB 11 °C 680 651 °C
Arsine 4.5–5.1[6] 78 IA Flammable gas
Benzene 1.2 7.8 IB −11 °C 0.2 @ 4.7% 560 °C
1,3-Butadiene 2.0 12 IA −85 °C 0.13 @ 5.2%
Butane, n-butane 1.6 8.4 IA −60 °C 0.25 @ 4.7% 420–500 °C
n-Butyl acetate, butyl acetate 1–1.7[4] 8–15 IB 24 °C 370 °C
2-Butanol 1.7 9.8 29 °C 405 °C
Isobutanol 1.7 10.9 22–27 °C 415 °C
n-Butanol 1.4[4] 11.2 IC 35 °C 340 °C
n-Butyl chloride, 1-chlorobutane 1.8 10.1 IB −6 °C 1.24
n-Butyl mercaptan 1.4[7] 10.2 IB 2 °C 225 °C
Butyl methyl ketone, 2-hexanone 1[8] 8 IC 25 °C 423 °C
Butylene, 1-butylene, 1-butene 1.98[6] 9.65 IA −80 °C
Carbon disulfide 1.0 50.0 IB −30 °C 0.009 @ 7.8% 90 °C
Carbon monoxide 12[6] 75 IA −191 °C Flammable gas 609 °C
Chlorine monoxide IA Flammable gas
1-Chloro-1,1-difluoroethane 6.2 17.9 IA −65 °C Flammable gas
Cyanogen 6.0–6.6[9] 32–42.6 IA Flammable gas
Cyclobutane 1.8 11.1 IA −63.9 °C[10] 426.7 °C
Cyclohexane 1.3 7.8–8 IB −18 °C to -20 °C[11] 0.22 @ 3.8% 245 °C
Cyclohexanol 1 9 IIIA 68 °C 300 °C
Cyclohexanone 1–1.1 9–9.4 II 43.9–44 °C 420 °C[12]
Cyclopentadiene[13] IB 0 °C 0.67 640 °C
Cyclopentane 1.5–2 9.4 IB −37 to −38.9 °C[14][15] 0.54 361 °C
Cyclopropane 2.4 10.4 IA −94.4 °C[16] 0.17 @ 6.3% 498 °C
Decane 0.8 5.4 II 46.1 °C 210 °C
Diborane 0.8 88 IA −90 °C Flammable gas[17] 38 °C
o-Dichlorobenzene, 1,2-dichlorobenzene 2[18] 9 IIIA 65 °C 648 °C
1,1-Dichloroethane 6 11 IB 14 °C
1,2-Dichloroethane 6 16 IB 13 °C 413 °C
1,1-Dichloroethene 6.5 15.5 IA −10 °C Flammable gas
Dichlorofluoromethane 54.7 Non flammable,[19] -36.1 °C[20] 552 °C
Dichloromethane, methylene chloride 16 66 Non flammable
Dichlorosilane 4–4.7 96 IA −28 °C 0.015
Diesel fuel 0.6 7.5 IIIA >62 °C (143 °F) 210 °C
Diethanolamine 2 13 IB 169 °C
Diethylamine 1.8 10.1 IB −23 to −26 °C 312 °C
Diethyl disulfide 1.2 II 38.9 °C[21]
Diethyl ether 1.9–2 36–48 IA −45 °C 0.19 @ 5.1% 160–170 °C
Diethyl sulfide IB −10 °C[22]
1,1-Difluoroethane 3.7 18 IA −81.1 °C[23]
1,1-Difluoroethylene 5.5 21.3 −126.1 °C[24]
Diisobutyl ketone 1 6 49 °C
Diisopropyl ether 1 21 IB −28 °C
Dimethylamine 2.8 14.4 IA Flammable gas
1,1-Dimethylhydrazine IB
Dimethyl sulfide IA −49 °C
Dimethyl sulfoxide 2.6–3 42 IIIB 88–95 °C 215 °C
1,4-Dioxane 2 22 IB 12 °C
Epichlorohydrin 4 21 31 °C
Ethane 3[6] 12–12.4 IA Flammable gas -135 °C 515 °C
Ethanol, ethyl alcohol 3–3.3 19 IB 12.8 °C (55 °F) 365 °C
2-Ethoxyethanol 3 18 43 °C
2-Ethoxyethyl acetate 2 8 56 °C
Ethyl acetate 2 12 IA −4 °C 460 °C
Ethylamine 3.5 14 IA −17 °C
Ethylbenzene 1.0 7.1 15–20 °C
Ethylene 2.7 36 IA 0.07 490 °C
Ethylene glycol 3 22 111 °C
Ethylene oxide 3 100 IA −20 °C
Ethyl chloride 3.8[6] 15.4 IA −50 °C
Ethyl mercaptan IA
Fuel oil No.1 0.7[6] 5
Furan 2 14 IA −36 °C
Gasoline (100 octane) 1.4 7.6 IB < −40 °C (−40 °F) 246–280 °C
Glycerol 3 19 199 °C
Heptane, n-heptane 1.05 6.7 −4 °C 0.24 @ 3.4% 204–215 °C
Hexane, n-hexane 1.1 7.5 −22 °C 0.24 @ 3.8% 225 °C, 233 °C[4]
Hydrogen 4/18.3[25] 75/59 IA Flammable gas 0.016 @ 28% (in pure oxygen 0.0012) 500–571 °C
Hydrogen sulfide 4.3 46 IA Flammable gas 0.068
Isobutane 1.8[6] 9.6 IA Flammable gas 462 °C
Isobutyl alcohol 2 11 28 °C
Isophorone 1 4 84 °C
Isopropyl alcohol, isopropanol 2[6] 12 IB 12 °C 398–399 °C; 425 °C[4]
Isopropyl chloride IA
Kerosene Jet A-1 0.6–0.7 4.9–5 II >38 °C (100 °F) as jet fuel 210 °C
Lithium hydride IA
2-Mercaptoethanol IIIA
Methane (natural gas) 5.0 (ISO10156) / 4.4 (IEC60079-20-1) 14.3 (ISO10156) / 17 (IEC60079-20-1) IA Flammable gas 0.21 @ 8.5% 580 °C
Methyl acetate 3 16 −10 °C
Methyl alcohol, methanol 6–6.7[6] 36 IB 11 °C 385 °C; 455 °C[4]
Methylamine IA 8 °C
Methyl chloride 10.7[6] 17.4 IA −46 °C
Methyl ether IA −41 °C
Methyl ethyl ether IA
Methyl ethyl ketone 1.8[6] 10 IB −6 °C 505–515 °C[4]
Methyl formate IA
Methyl mercaptan 3.9 21.8 IA −53 °C
Mineral spirits 0.7[4] 6.5 38–43 °C 258 °C
Morpholine 1.8 10.8 IC 31–37.7 °C 310 °C
Naphthalene 0.9[6] 5.9 IIIA 79–87 °C 540 °C
Neohexane 1.19[6] 7.58 −29 °C 425 °C
Nickel tetracarbonyl 2 34 4 °C 60 °C
Nitrobenzene 2 9 IIIA 88 °C
Nitromethane 7.3 22.2 35 °C 379 °C
Octane 1 7 13 °C
iso-Octane 0.79 5.94
Pentane 1.5 7.8 IA −40 to −49 °C as 2-Pentane 0.18 @ 4.4% 260 °C
n-Pentane 1.4 7.8 IA 0.28 @ 3.3%
iso-Pentane 1.32[6] 9.16 IA 420 °C
Phosphine IA
Propane 2.1 9.5–10.1 IA Flammable gas 0.25 @ 5.2% (in pure oxygen 0.0021) 480 °C
Propyl acetate 2 8 13 °C
Propylene 2.0 11.1 IA −108 °C 0.28 458 °C
Propylene oxide 2.9 36 IA
Pyridine 2 12 20 °C
Silane 1.5[6] 98 IA <21 °C
Styrene 1.1 6.1 IB 31–32.2 °C 490 °C
Tetrafluoroethylene IA
Tetrahydrofuran 2 12 IB −14 °C 321 °C
Toluene 1.2–1.27 6.75–7.1 IB 4.4 °C 0.24 @ 4.1% 480 °C; 535 °C[4]
Triethylborane −20 °C −20 °C
Trimethylamine IA Flammable gas
Trinitrobenzene IA
Turpentine 0.8[26] IC 35 °C
Vegetable oil IIIB 327 °C (620 °F)
Vinyl acetate 2.6 13.4 −8 °C
Vinyl chloride 3.6 33
Xylenes 0.9–1.0 6.7–7.0 IC 27–32 °C 0.2
m-Xylene 1.1[4] 7 IC 25 °C 525 °C
o-Xylene IC 17 °C
p-Xylene 1.0 6.0 IC 27.2 °C 530 °C

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