Coulomb operator

Mixtures of dispersed combustible materials (such as gaseous or vaporised fuels, and some dusts) and 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, to explosion.

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 that by Zabetakis using an apparatus developed by the US Bureau of Mines.

Violence of combustion

Combustion can be less or more violent. 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 explosive limit

Lower explosive limit (LEL): The lowest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). The term is considered by many safety professionals to be the same as the lower flammable limit (LFL). At a concentration in air lower than the LEL, gas mixtures are "too lean" to burn. Methane gas has a LEL of 5%. If the atmosphere has less than 5% methane, an explosion cannot occur even if a source of ignition is present.

Percentage reading on combustible air monitors should not be confused with the LEL concentrations. Explosimeters designed and calibrated to a specific gas may show the relative concentration of the atmosphere to the LEL—the LEL being 100%. A 5% displayed LEL reading for methane, for example, would be equivalent to 5.1% 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. The maximum of a lower explosive limit is also known as the "top LEL."

Upper explosive limit

Upper explosive limit (UEL): Highest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source (arc, flame, heat). Concentrations higher than UFL or UEL are "too rich" to burn.

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:

$LE{L}_{mix}=\frac{1}{\sum \frac{x_i}{LE{L}_i}}$

and similar for UEL.

Temperature, pressure, and the concentration of the oxidizer also influences flammability limits. Higher temperature results in lower LFL and higher UFL, while greater pressure increases both values. 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, raises the LFL and decreases the UFL.

Controlling explosive atmospheres

Gas and vapor

Controlling gas and vapor concentrations outside the explosive 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 explosive 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.

Explosion 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 explosion limits may be relevant to plant design.

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 (flash point less than 73 °F (22.8 °C); boiling point less than 100 °F (37.8 °C) are NFPA 704 flammability rating 4
• Classes IB (flash point less than 73 °F (22.8 °C); boiling point equal to or greater than 100 °F (37.8 °C)) and IC liquids (flash point equal to or greater than 73 °F (22.8 °C), but less than 100 °F (37.8 °C)) are NFPA 704 flammability rating 3
• Classes II (flash point equal to or greater than 100 °F (37.8 °C), but less than 140 °F (60 °C) and IIIA liquids (flash point equal to or greater than 140 °F (60 °C), but less than 200 °F (93.3 °C)) are NFPA 704 flammability rating 2
• Class IIIB liquids (flash point equal to or greater than 200 °F (93.3 °C) are NFPA 704 flammability rating 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 as percent by volume in air (Note, for many chemicals it takes the least amount of ignition energy midpoint between the LEL and UEL.)	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
Acetonitrile			IB	2 °C		524 °C
Acetyl chloride	7.3	19	IB	5 °C		390 °C
Acetylene	2.5	100	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	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	8–15	IB	24 °C		370 °C
Butyl alcohol, butanol	1	11	IC	29 °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	10.2	IB	2 °C		225 °C
Butyl methyl ketone, 2-hexanone	1	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	Flammable gas
1-Chloro-1,1-difluoroethane	6.2	17.9	IA	-65 °C Flammable gas
Cyanogen	6.0–6.6	32–42.6	IA	Flammable gas
Cyclobutane	1.8	11.1	IA	-63.9 °C		426.7 °C
Cyclohexane	1.3	7.8–8	IB	-18 °C to -20 °C	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
Cyclopentadiene			IB	0 °C	0.67	640 °C
Cyclopentane	1.5–2	9.4	IB	-37 to -38.9 °C	0.54	361 °C
Cyclopropane	2.4	10.4	IA	-94.4 °C	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		38 °C
o-Dichlorobenzene, 1,2-dichlorobenzene	2	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, -36.1 °C		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
Diethyl ether	1.9–2	36–48	IA	-45 °C	0.19 @ 5.1%	160–170 °C
Diethyl sulfide			IB	-10 °C
1,1-Difluoroethane	3.7	18	IA	-81.1 °C
1,1-Difluoroethylene	5.5	21.3		-126.1 °C
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	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	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 (−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
Hydrogen	4/18.3	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	9.6	IA	Flammable gas		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 (100 °F) as jet fuel		210 °C
Lithium hydride			IA
2-Mercaptoethanol			IIIA
Methane (natural gas)	4.4–5	15–17	IA	Flammable gas	0.21 @ 8.5%	580 °C
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
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 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	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	2.3	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	Flammable gas
Trinitrobenzene			IA
Turpentine	0.8		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	7	IC	25 °C		525 °C
o-Xylene			IC	17 °C
p-Xylene	1.0	6.0	IC	27.2 °C		530 °C

See also

• Flammability
• Limiting oxygen concentration
• Minimum ignition energy

References

43 year old Petroleum Engineer Harry from Deep River, usually spends time with hobbies and interests like renting movies, property developers in singapore new condominium and vehicle racing. Constantly enjoys going to destinations like Camino Real de Tierra Adentro.

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