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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

Any mixture of combustibles has specific lower and upper flammability limits. These limits are a function of the pressure, temperature and composition. These limits are often shown in flammability diagrams for which an example can be found in the work by Bee and Börner.¹

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%.² 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.³

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 {\displaystyle x_{i}} :

L F L mix = 1 ∑ i x i L F L 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.

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.⁴

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		Flammability limit (%vol.)		NFPA class	Flash point	Minimum ignition energy (mJ) @ proportion in air at which achieved56	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 °C7
Acetonitrile				IB	2 °C		524 °C
Acetyl chloride		7.3	19	IB	5 °C		390 °C
Acetylene		2.5	1008	IA	Flammable gas	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		4.5–5.19	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		1–1.710	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		1.411	11.2	IC	35 °C		340 °C
n-Butyl chloride, 1-chlorobutane		1.8	10.1	IB	−6 °C	1.24
n-Butyl mercaptan		1.412	10.2	IB	2 °C		225 °C
Butyl methyl ketone, 2-hexanone		113	8	IC	25 °C		423 °C
Butylene, 1-butylene, 1-butene		1.9814	9.65	IA	−80 °C
Carbon disulfide		1.0	50.0	IB	−30 °C	0.009 @ 7.8%	90 °C
Carbon monoxide		1215	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		6.0–6.616	32–42.6	IA	Flammable gas
Cyclobutane		1.8	11.1	IA	−63.9 °C17		426.7 °C
Cyclohexane		1.3	7.8–8	IB	−18 – −20 °C18	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 °C19
Cyclopentadiene20				IB	0 °C	0.67	640 °C
Cyclopentane		1.5–2	9.4	IB	−37 – −38.9 °C2122	0.54	361 °C
Cyclopropane		2.4	10.4	IA	−94.4 °C23	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 gas24		38 °C
o-Dichlorobenzene, 1,2-dichlorobenzene		225	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,26 −36.1 °C27		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		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 °C28
Diethyl ether		1.9–2	36–48	IA	−45 °C	0.19 @ 5.1%	160–170 °C
Diethyl sulfide				IB	−10 °C29
1,1-Difluoroethane		3.7	18	IA	−81.1 °C30
1,1-Difluoroethylene		5.5	21.3		−126.1 °C31
Difluoromethane		14.432
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		333	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		3.834	15.4	IA	−50 °C
Ethyl mercaptan				IA
Fuel oil No.1		0.735	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 °C36
Hydrogen		4/18.337	75/59	IA	Flammable gas	0.016 @ 28%; 0.0012, in pure oxygen	500–571 °C
Hydrogen sulfide		4.3	46	IA	Flammable gas	0.068
Isobutane		1.838	9.6	IA	Flammable gas		462 °C
Isobutyl alcohol		2	11		28 °C
Isophorone		1	4		84 °C
Isopropyl alcohol, isopropanol		239	12	IB	12 °C		398–399 °C; 425 °C40
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	Flammable gas	0.21 @ 8.5%	580 °C
	IEC60079-20-1	4.4	17
Methyl acetate		3	16		−10 °C
Methyl alcohol, methanol		6–6.741	36	IB	11 °C		385 °C; 455 °C42
Methylamine				IA	8 °C
Methyl chloride		10.743	17.4	IA	−46 °C
Methyl ether				IA	−41 °C
Methyl ethyl ether				IA
Methyl ethyl ketone		1.844	10	IB	−6 °C		505–515 °C45
Methyl formate				IA
Methyl mercaptan		3.9	21.8	IA	−53 °C
Mineral spirits		0.746	6.5		38–43 °C		258 °C
Morpholine		1.8	10.8	IC	31–37.7 °C		310 °C
Naphthalene		0.947	5.9	IIIA	79–87 °C		540 °C
Neohexane		1.1948	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	0.18 @ 4.4%, as 2-pentane	260 °C
n-Pentane		1.4	7.8	IA		0.28 @ 3.3%
iso-Pentane		1.3249	9.16	IA			420 °C
Phosphine				IA
Propane		2.1	9.5–10.1	IA	Flammable gas	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		1.550	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 °C51
Triethylborane					−20 °C		−20 °C
Trimethylamine				IA	Flammable gas
Trinitrobenzene				IA
Turpentine		0.852		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		1.153	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 E681.⁵⁴ 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

• Chemistry portal

• ATEX
• 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; ISBN 0-8493-0565-9

References

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