Flammability limit

Last updated

Flammability limits or explosive limits are the ranges of fuel concentrations in relation to oxygen from the air. Combustion can range in violence from deflagration through detonation.

Contents

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

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 :

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 1050 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.

SubstanceFlammability limit (%vol.) NFPA
class
Flash
point
Minimum ignition energy (mJ)
@ proportion in air at which achieved [a] [5]
Autoignition
temperature
LowerUpper
Acetaldehyde 4.057.0IA−39 °C0.37175 °C
Acetic acid (glacial)419.9II39–43 °C463 °C
Acetic anhydride II54 °C
Acetone 2.6–312.8–13IB−17 °C1.15 @ 4.5%465 °C, 485 °C [6]
Acetonitrile IB2 °C524 °C
Acetyl chloride 7.319IB5 °C390 °C
Acetylene 2.5100 [7] IAFlammable gas0.017 @ 8.5%; 0.0002 @ 40%, in pure oxygen305 °C
Acrolein 2.831IB−26 °C0.13
Acrylonitrile 3.017.0IB0 °C0.16 @ 9.0%
Allyl chloride 2.911.1IB−32 °C0.77
Ammonia 1528IIIB11 °C680651 °C
Arsine 4.5–5.1 [8] 78IAFlammable gas
Benzene 1.27.8IB−11 °C0.2 @ 4.7%560 °C
1,3-Butadiene 2.012IA−85 °C0.13 @ 5.2%
Butane, n-butane 1.68.4IA−60 °C0.25 @ 4.7%420–500 °C
n-Butyl acetate, butyl acetate1–1.7 [6] 8–15IB24 °C370 °C
2-Butanol 1.79.829 °C405 °C
Isobutanol 1.710.922–27 °C415 °C
n-Butanol 1.4 [6] 11.2IC35 °C340 °C
n-Butyl chloride, 1-chlorobutane1.810.1IB−6 °C1.24
n-Butyl mercaptan 1.4 [9] 10.2IB2 °C225 °C
Butyl methyl ketone, 2-hexanone 1 [10] 8IC25 °C423 °C
Butylene, 1-butylene, 1-butene1.98 [8] 9.65IA−80 °C
Carbon disulfide 1.050.0IB−30 °C0.009 @ 7.8%90 °C
Carbon monoxide 12 [8] 75IA−191 °C Flammable gas609 °C
Chlorine monoxide IAFlammable gas
1-Chloro-1,1-difluoroethane 6.217.9IA−65 °C Flammable gas
Cyanogen 6.0–6.6 [11] 32–42.6IAFlammable gas
Cyclobutane 1.811.1IA−63.9 °C [12] 426.7 °C
Cyclohexane 1.37.8–8IB−18 – −20 °C [13] 0.22 @ 3.8%245 °C
Cyclohexanol 19IIIA68 °C300 °C
Cyclohexanone 1–1.19–9.4II43.9–44 °C420 °C [14]
Cyclopentadiene [15] IB0 °C0.67640 °C
Cyclopentane 1.5–29.4IB−37 – −38.9 °C [16] [17] 0.54361 °C
Cyclopropane 2.410.4IA−94.4 °C [18] 0.17 @ 6.3%498 °C
Decane 0.85.4II46.1 °C210 °C
Diborane 0.888IA−90 °C Flammable gas [19] 38 °C
o-Dichlorobenzene, 1,2-dichlorobenzene2 [20] 9IIIA65 °C648 °C
1,1-Dichloroethane 611IB14 °C
1,2-Dichloroethane 616IB13 °C413 °C
1,1-Dichloroethene 6.515.5IA−10 °C Flammable gas
Dichlorofluoromethane 54.7Non flammable, [21] −36.1 °C [22] 552 °C
Dichloromethane, methylene chloride1666Non flammable
Dichlorosilane 4–4.796IA−28 °C0.015
Diesel fuel 0.67.5IIIA>62 °C210 °C
Diethanolamine 213IB169 °C
Diethylamine 1.810.1IB−23 – −26 °C312 °C
Diethyl disulfide 1.2II38.9 °C [23]
Diethyl ether 1.9–236–48IA−45 °C0.19 @ 5.1%160–170 °C
Diethyl sulfide IB−10 °C [24]
1,1-Difluoroethane 3.718IA−81.1 °C [25]
1,1-Difluoroethylene 5.521.3−126.1 °C [26]
Difluoromethane 14.4 [27]
Diisobutyl ketone 1649 °C
Diisopropyl ether 121IB−28 °C
Dimethylamine 2.814.4IAFlammable gas
1,1-Dimethylhydrazine IB
Dimethyl sulfide IA−49 °C
Dimethyl sulfoxide 2.6–342IIIB88–95 °C215 °C
1,4-Dioxane 222IB12 °C
Epichlorohydrin 42131 °C
Ethane 3 [8] 12–12.4IAFlammable gas, −135 °C515 °C
Ethanol, ethyl alcohol3–3.319IB12.8 °C365 °C
2-Ethoxyethanol 31843 °C
2-Ethoxyethyl acetate 2856 °C
Ethyl acetate 212IA−4 °C460 °C
Ethylamine 3.514IA−17 °C
Ethylbenzene 1.07.115–20 °C
Ethylene 2.736IA0.07490 °C
Ethylene glycol 322111 °C
Ethylene oxide 3100IA−20 °C
Ethyl chloride 3.8 [8] 15.4IA−50 °C
Ethyl mercaptan IA
Fuel oil No.1 0.7 [8] 5
Furan 214IA−36 °C
Gasoline (100 octane)1.47.6IB< −40 °C246–280 °C
Glycerol 319199 °C
Heptane, n-heptane1.056.7−4 °C0.24 @ 3.4%204–215 °C
Hexane, n-hexane1.27.5−22 °C0.24 @ 3.8%225 °C, 233 °C [6]
Hydrogen 4/18.3 [28] 75/59IAFlammable gas0.016 @ 28%; 0.0012, in pure oxygen500–571 °C
Hydrogen sulfide 4.346IAFlammable gas0.068
Isobutane 1.8 [8] 9.6IAFlammable gas462 °C
Isobutyl alcohol 21128 °C
Isophorone 1484 °C
Isopropyl alcohol, isopropanol2 [8] 12IB12 °C398–399 °C; 425 °C [6]
Isopropyl chloride IA
Kerosene Jet A-1 0.6–0.74.9–5II>38 °C, as jet fuel210 °C
Lithium hydride IA
2-Mercaptoethanol IIIA
Methane (natural gas)ISO101565.014.3IAFlammable gas0.21 @ 8.5%580 °C
IEC60079-20-14.417
Methyl acetate 316−10 °C
Methyl alcohol, methanol6–6.7 [8] 36IB11 °C385 °C; 455 °C [6]
Methylamine IA8 °C
Methyl chloride 10.7 [8] 17.4IA−46 °C
Methyl ether IA−41 °C
Methyl ethyl ether IA
Methyl ethyl ketone 1.8 [8] 10IB−6 °C505–515 °C [6]
Methyl formate IA
Methyl mercaptan 3.921.8IA−53 °C
Mineral spirits 0.7 [6] 6.538–43 °C258 °C
Morpholine 1.810.8IC31–37.7 °C310 °C
Naphthalene 0.9 [8] 5.9IIIA79–87 °C540 °C
Neohexane 1.19 [8] 7.58−29 °C425 °C
Nickel tetracarbonyl 2344 °C60 °C
Nitrobenzene 29IIIA88 °C
Nitromethane 7.322.235 °C379 °C
Octane 1713 °C
iso-Octane 0.795.94
Pentane 1.57.8IA−40 – −49 °C0.18 @ 4.4%, as 2-pentane 260 °C
n-Pentane 1.47.8IA0.28 @ 3.3%
iso-Pentane 1.32 [8] 9.16IA420 °C
Phosphine IA
Propane 2.19.5–10.1IAFlammable gas0.25 @ 5.2%; 0.0021, in pure oxygen480 °C
Propyl acetate 2813 °C
Propylene 2.011.1IA−108 °C0.28458 °C
Propylene oxide 2.936IA
Pyridine 21220 °C
Silane 1.5 [8] 98IA<21 °C
Styrene 1.16.1IB31–32.2 °C490 °C
Tetrafluoroethylene IA
Tetrahydrofuran 212IB−14 °C321 °C
Toluene 1.2–1.276.75–7.1IB4.4 °C0.24 @ 4.1%480 °C; 535 °C [6]
Triethylborane −20 °C−20 °C
Trimethylamine IAFlammable gas
Trinitrobenzene IA
Turpentine 0.8 [29] IC35 °C
Vegetable oil IIIB327 °C
Vinyl acetate 2.613.4−8 °C
Vinyl chloride 3.633
Xylenes 0.9–1.06.7–7.0IC27–32 °C0.2
m-Xylene 1.1 [6] 7IC25 °C525 °C
o-Xylene IC17 °C
p-Xylene 1.06.0IC27.2 °C530 °C
  1. Note that for many chemicals it takes the least amount of ignition energy halfway between the LEL and UEL.

ASTM E681

Image of a flame of R-32 (Difluoromethane) near its LFL in a 12 L ASTM E-681 apparatus. [27]

In the U.S. the most common method of measuring LFLs and UFLs is ASTM E681. [27] 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

References

  1. Bee, A.; Börner, M. (2023). "Laminar Burning Speeds and Flammability Limits of CH4/O2 Mixtures With Varying N2 Dilution at Sub-Atmospheric Conditions" (PDF). Combustion Science and Technology. 195 (8). Taylor & Francis: 1910--1929. doi:10.1080/00102202.2021.2006191.
  2. "Gases - Explosion and Flammability Concentration Limits".
  3. "Current Intelligence Bulletin #66: Derivation of Immediately Dangerous to Life or Health (IDLH) Values" (PDF). The National Institute for Occupational Safety and Health (NIOSH). November 2013. Retrieved 11 February 2018.
  4. Morrell, Robert W. (1931). Oil Tankers (Second ed.). New York: Simmons-Boardman Publishing Company. pp. 305&306.
  5. Britton, L. G "Using Material Data in Static Hazard Assessment." as found in NFPA 77 - 2007 Appendix B
  6. 1 2 3 4 5 6 7 8 9 10 Working with modern hydrocarbon and oxygenated solvents: a guide to flammability Archived June 1, 2009, at the Wayback Machine American Chemistry Council Solvents Industry Group, pg. 7, January 2008
  7. Matheson Gas Products. Matheson Gas Data Book (PDF). p. 443. Archived from the original (PDF) on 30 September 2019. Retrieved 30 October 2013.
  8. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 "Gases - Explosive and Flammability Concentration Limits" . Retrieved 9 September 2013.
  9. "ICSC 0018 - n-BUTYL MERCAPTAN". www.inchem.org. Retrieved 18 March 2018.
  10. "2-HEXANONE ICSC:0489". oit.org. Retrieved 18 March 2018.
  11. "IPCS INTOX Site Closed". www.intox.org. Retrieved 18 March 2018.
  12. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 211
  13. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 216
  14. "ICSC 0425 - CYCLOHEXANONE". www.inchem.org. Retrieved 18 March 2018.
  15. "MSDS Cyclopentadiene". ox.ac.uk. Archived from the original on 7 December 2010. Retrieved 18 March 2018.
  16. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 221
  17. "ICSC 0353 - CYCLOPENTANE". www.inchem.org. Retrieved 18 March 2018.
  18. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 226
  19. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 244
  20. Walsh (1989) Chemical Safety Data Sheets, Roy. Soc. Chem., Cambridge.
  21. "Encyclopedia.airliquide.com" (PDF). Archived from the original (PDF) on 26 May 2020. Retrieved 25 June 2023.
  22. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 266
  23. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 281
  24. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 286
  25. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 296
  26. Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 301
  27. 1 2 3 Kim, Dennis K.; Klieger, Alexandra E.; Lomax, Peter Q.; Mccoy, Conor G.; Reymann, Jonathan Y.; Sunderland, Peter B. (14 September 2018). "An improved test method for refrigerant flammability limits in a 12 L vessel". Science and Technology for the Built Environment. 24 (8): 861–866. Bibcode:2018STBE...24..861K. doi:10.1080/23744731.2018.1434381. ISSN   2374-4731. S2CID   139489210.
  28. "Periodic Table of Elements: Hydrogen - H (EnvironmentalChemistry.com)". environmentalchemistry.com. Retrieved 18 March 2018.
  29. "Combustibles" (PDF). afcintl.com. Archived from the original (PDF) on 3 March 2016. Retrieved 18 March 2018.

Further reading