Flammability limit explained

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 in 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 4.4%.^[1] If the atmosphere has less than 4.4% 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.^[2]

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 4.4%, or approximately 0.22% 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

x _{i} :LFL _{mix} = (1)/(\sum _{i} (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.

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 (except for gases that can energetically decompose even in the absence of an oxidizer, such as acetylene). 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.^[3]

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 73F and boiling point less than 100°F have a NFPA 704 flammability rating of 4
• Class IB liquids with a flash point less than 73F and a boiling point equal to or greater than 100°F and class IC liquids with a flash point equal to or greater than 73F, but less than 100°F have a NFPA 704 flammability rating of 3
• Class II liquids with a flash point equal to or greater than 100°F, but less than 140F and class IIIA liquids with a flash point equal to or greater than 140F, but less than 200F have a NFPA 704 flammability rating of 2
• Class IIIB liquids with a flash point equal to or greater than 200F have a NFPA 704 flammability rating of 1

Substance	Flammability limit (%vol.)		NFPA class	Flash point	Minimum ignition energy (mJ) @ proportion in air at which achieved^[4]	Autoignition temperature
Lower	Upper
Acetaldehyde		4.0	57.0	IA	−39 °C	0.37	175 °C
Acetic acid (glacial)		4	19.9	II	39–43 °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^[5]
Acetonitrile				IB	2 °C		524 °C
Acetyl chloride		7.3	19	IB	5 °C		390 °C
Acetylene		2.5	100^[6]	IA		0.017 @ 8.5%; 0.0002 @ 40%, in pure oxygen	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^[7]	78	IA
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	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	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^[8]	10.2	IB	2 °C		225 °C
Butyl methyl ketone, 2-hexanone		1^[9]	8	IC	25 °C		423 °C
Butylene, 1-butylene, 1-butene		1.98	9.65	IA	−80 °C
Carbon disulfide		1.0	50.0	IB	−30 °C	0.009 @ 7.8%	90 °C
Carbon monoxide		12	75	IA	−191 °C Flammable gas		609 °C
Chlorine monoxide				IA
1-Chloro-1,1-difluoroethane		6.2	17.9	IA	−65 °C Flammable gas
Cyanogen		6.0–6.6^[10]	32–42.6	IA
Cyclobutane		1.8	11.1	IA	−63.9 °C^[11]		426.7 °C
Cyclohexane		1.3	7.8–8	IB	−18 – −20 °C^[12]	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^[13]
Cyclopentadiene^[14]				IB	0 °C	0.67	640 °C
Cyclopentane		1.5–2	9.4	IB	−37 – −38.9 °C^[15] ^[16]	0.54	361 °C
Cyclopropane		2.4	10.4	IA	−94.4 °C^[17]	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^[18]		38 °C
o-Dichlorobenzene, 1,2-dichlorobenzene		2^[19]	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,^[20] −36.1 °C^[21]		552 °C
Dichloromethane, methylene chloride		16	66
Dichlorosilane		4–4.7	96	IA	−28 °C	0.015
Diesel fuel		0.6	7.5	IIIA	>62 °C		210 °C
Diethanolamine		2	13	IB	169 °C
Diethylamine		1.8	10.1	IB	−23 – −26 °C		312 °C
Diethyl disulfide		1.2		II	38.9 °C^[22]
Diethyl ether		1.9–2	36–48	IA	−45 °C	0.19 @ 5.1%	160–170 °C
Diethyl sulfide				IB	−10 °C^[23]
1,1-Difluoroethane		3.7	18	IA	−81.1 °C^[24]
1,1-Difluoroethylene		5.5	21.3		−126.1 °C^[25]
Difluoromethane		14.4
Diisobutyl ketone		1	6		49 °C
Diisopropyl ether		1	21	IB	−28 °C
Dimethylamine		2.8	14.4	IA
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	12–12.4	IA	Flammable gas, −135 °C		515 °C
Ethanol, ethyl alcohol		3–3.3	19	IB	12.8 °C		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	15.4	IA	−50 °C
Ethyl mercaptan				IA
Fuel oil No.1		0.7	5
Furan		2	14	IA	−36 °C
Gasoline (100 octane)		1.4	7.6	IB	< −40 °C		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		1.2	7.5		−22 °C	0.24 @ 3.8%	225 °C, 233 °C
Hydrogen		4/18.3^[26]	75/59	IA		0.016 @ 28%; 0.0012, in pure oxygen	500–571 °C
Hydrogen sulfide		4.3	46	IA		0.068
Isobutane		1.8	9.6	IA			462 °C
Isobutyl alcohol		2	11		28 °C
Isophorone		1	4		84 °C
Isopropyl alcohol, isopropanol		2	12	IB	12 °C		398–399 °C; 425 °C
Isopropyl chloride				IA
Kerosene Jet A-1		0.6–0.7	4.9–5	II	>38 °C, as jet fuel		210 °C
Lithium hydride				IA
2-Mercaptoethanol				IIIA
Methane (natural gas)	ISO10156	5.0	14.3	IA	0.21 @ 8.5%	580 °C
	IEC60079-20-1	4.4	17
Methyl acetate		3	16		−10 °C
Methyl alcohol, methanol		6–6.7	36	IB	11 °C		385 °C; 455 °C
Methylamine				IA	8 °C
Methyl chloride		10.7	17.4	IA	−46 °C
Methyl ether				IA	−41 °C
Methyl ethyl ether				IA
Methyl ethyl ketone		1.8	10	IB	−6 °C		505–515 °C
Methyl formate				IA
Methyl mercaptan		3.9	21.8	IA	−53 °C
Mineral spirits		0.7	6.5		38–43 °C		258 °C
Morpholine		1.8	10.8	IC	31–37.7 °C		310 °C
Naphthalene		0.9	5.9	IIIA	79–87 °C		540 °C
Neohexane		1.19	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 – −49 °C		260 °C
n-Pentane		1.4	7.8	IA		0.28 @ 3.3%
iso-Pentane		1.32	9.16	IA			420 °C
Phosphine				IA
Propane		2.1	9.5–10.1	IA		0.25 @ 5.2%; 0.0021, in pure oxygen	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	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
Triethylborane					−20 °C		−20 °C
Trimethylamine				IA
Trinitrobenzene				IA
Turpentine		0.8^[27]		IC	35 °C
Vegetable oil				IIIB	327 °C
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	7	IC	25 °C		525 °C
o-Xylene				IC	17 °C
p-Xylene		1.0	6.0	IC	27.2 °C		530 °C

ASTM E681

In the U.S. the most common method of measuring LFLs and UFLs is ASTM E681. This standard test is required for HAZMAT Class 2 Gases and for determining refrigerant flammability classifications. This standard uses visual observations of flame propagation in 5 or 12 L spherical glass vessels to measure the flammability limits. Flammable conditions are defined as those for which a flame propagates outside a 90° cone angle.

See also

• Flammability
• Limiting oxygen concentration
• Minimum ignition energy

Further reading

• David R. Lide, Editor-in-Chief; CRC Handbook of Chemistry and Physics, 72nd edition; CRC Press; Boca Raton, Florida; 1991;

Notes and References

1. Web site: Gases - Explosion and Flammability Concentration Limits.
2. Web site: Current Intelligence Bulletin #66: Derivation of Immediately Dangerous to Life or Health (IDLH) Values. November 2013. The National Institute for Occupational Safety and Health (NIOSH). 2018-02-11.
3. Book: Morrell, Robert W. . Oil Tankers . Simmons-Boardman Publishing Company . Second . 1931 . New York . 305&306 .
4. Britton, L. G "Using Material Data in Static Hazard Assessment." as found in NFPA 77 - 2007 Appendix B
5. http://www.americanchemistry.com/s_acc/sec_solvents.asp?CID=1488&DID=5735 Working with modern hydrocarbon and oxygenated solvents: a guide to flammability
6. Book: Matheson Gas Products . Matheson Gas Data Book . 443 . 2013-10-30 . 2019-09-30 . https://web.archive.org/web/20190930132307/https://www.mathesongas.com/pdfs/products/Lower-%28LEL%29-%26-Upper-%28UEL%29-Explosive-Limits-.pdf . dead .
7. Web site: Gases - Explosive and Flammability Concentration Limits . 2013-09-09.
8. Web site: ICSC 0018 - n-BUTYL MERCAPTAN. www.inchem.org. 18 March 2018.
9. Web site: 2-HEXANONE ICSC:0489. oit.org. 18 March 2018.
10. Web site: IPCS INTOX Site Closed. www.intox.org. 18 March 2018.
11. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 211
12. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 216
13. Web site: ICSC 0425 - CYCLOHEXANONE. www.inchem.org. 18 March 2018.
14. Web site: MSDS Cyclopentadiene. ox.ac.uk. 18 March 2018. 7 December 2010. https://web.archive.org/web/20101207162619/http://msds.chem.ox.ac.uk/CY/cyclopentadiene.html. dead.
15. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 221
16. Web site: ICSC 0353 - CYCLOPENTANE. www.inchem.org. 18 March 2018.
17. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 226
18. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 244
19. Walsh (1989) Chemical Safety Data Sheets, Roy. Soc. Chem., Cambridge.
20. Web site: Encyclopedia.airliquide.com. 25 June 2023. 26 May 2020. https://web.archive.org/web/20200526073305/https://encyclopedia.airliquide.com/sds/en/042_AL_EN.pdf. dead.
21. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 266
22. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 281
23. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 286
24. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 296
25. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 301
26. Web site: Periodic Table of Elements: Hydrogen - H (EnvironmentalChemistry.com). environmentalchemistry.com. 18 March 2018.
27. Web site: Combustibles. afcintl.com. 18 March 2018. https://web.archive.org/web/20160303191927/http://www.afcintl.com/pdf/combustibles.pdf. 3 March 2016. dead.