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Flammability limit

 
Sci-Tech Dictionary: flammability limits
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(′flam·ə′bil·əd·ē ′lim·əts)

(chemistry) The stoichiometric composition limits (maximum and minimum) of an ignited oxidizer-fuel mixture what will burn indefinitely at given conditions of temperature and pressure without further ignition.

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Flammability limits, also called flammable limits, or explosive limits give the proportion of combustible gases in a mixture, between which limits this mixture is flammable. Gas mixtures consisting of combustible, oxidizing, and inert gases are only flammable under certain conditions. The lower flammable limit (LFL) (lower explosive limit) describes the leanest mixture that is still flammable, i.e. the mixture with the smallest fraction of combustible gas, while the upper flammable limit (UFL) (upper explosive limit) gives the richest flammable mixture. Increasing the fraction of inert gases in a mixture raises the LFL and decreases UFL.

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.

Contents

Limits

Flammability limits of mixtures of several combustible gases can be calculated using Le Chatelier's mixing rule for combustible volume fractions xi:

LFL_{mix}=\frac{1}{\sum \frac{x_{i}}{LFL_{i}}}

and similar for UFL.

Temperature and pressure 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 hardly been studied.


Lower Explosive Limit(LEL): The explosive limit of a gas or a vapor is the limiting concentration(in air) that is needed for the gas to ignite and explode. 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(arch, flame, heat). At concentration in air below the LEL there is not fuel to continue an explosion. Concentrations lower than LEL are "too lean" to burn. eg: Methane gas has a LEL of 4.4% (at 138 degrees C) by volume, meaning 4.4% of the total volume of the air consists of methane. At 20 degrees C the LEL is 5,1 % by volume. If the atmosphere has less that 5.1% methane, an explosion cannot occur even if a source of ignition is present. When methane(CH4)concentration reaches 5% an explosion can occur if there is an igniton source. Each combustible gas has its own LEL concentration.

These percentages should not be confused with LEL instrumentation readings. Instruments designed and calibrated to read LEL also read as percent values. A 5% displayed LEL reading for methane for example would be equivalent to 5% divided by 5,1% methane by volume at 20 degrees C.

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 (arch, flame, heat). Concentration higher than UEL are "too rich" to burn. Also called UFL

Gases and vapours

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 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) 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 LEL in %

by volume of air

 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.) [1]
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 [2]
Acetonitrile IB 2°C 524°C
Acetyl chloride 7.3 19 IB 5°C 390°C
Acetylene 2.5 82 IA -18°C 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 [3] 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
Butyl alcohol, Butanol 1 11 IC 29°C
n-Butanol 1.4 [5] 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 [6] 10.2 IB 2°C 225°C
Butyl methyl ketone, 2-Hexanone 1 [7] 8 IC 25°C 423°C
Butylene, 1-Butylene, 1-Butene 1.98 [8] 9.65 IA -80°C
Carbon disulfide 1.0 50.0 IB -30°C 0.009 @ 7.8% 90°C
Carbon Monoxide 12 [9] 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 [10] 32 - 42.6 IA Flammable gas
Cyclobutane 1.8 11.1 IA -63.9°C [11] 426.7°C
Cyclohexane 1.3 7.8 - 8 IB -18°C - -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 to -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 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 12°C
Diethylamine 1.8 10.1 IB -23°C to -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]
Diisobutyl ketone 1 6 49°C
Diisopropyl ether 1 21 IB -28°C
Dimethylamine 2.8 14.4 IA Flammable gas
1,1-Dimethyl hydrazine 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 [26] 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 [27] 15.4 IA −50°C
Ethyl Mercaptan IA
Fuel oil No.1 0.7 [28] 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 [29]
Hydrogen, Deuterium 4 75 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 [30] 9.6 IA Flammable gas 462°C
Isobutyl alcohol 2 11 28°C
Isophorone 1 4 84°C
Isopropyl alcohol, Isopropanol 2 [31] 12 IB 12°C 398 - 399°C; 425°C [32]
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 [33] 36 IB 11°C 385°C; 455°C [34]
Methylamine IA 8°C
Methyl Chloride 10.7 [35] 17.4 IA -46 °C
Methyl ether IA −41 °C
Methyl ethyl ether IA
Methyl ethyl ketone 1.8 [36] 10 IB -6°C 505 - 515°C [37]
Methyl formate IA
Methyl mercaptan 3.9 21.8 IA -53°C
Mineral spirits 0.7 [38] 6.5 38-43°C 258°C
Morpholine 1.8 10.8 IC 31 - 37.7°C 310°C
Naphthalene 0.9 [39] 5.9 IIIA 79 - 87 °C
Neohexane 1.19 [40] 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 [41] 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
Silane 1.5 [42] 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 [43]
Triethylborane -20°C -20°C
Trimethylamine IA Flammable gas
Trinitrobenzene IA
Turpentine 0.8 [44] 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 [45] 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

References

  1. ^ Britton, L. G “Using Material Data in Static Hazard Assessment.” as found in NFPA 77 - 2007 Appendix B
  2. ^ [http://www.americanchemistry.com/s_acc/sec_solvents.asp?CID=1488&DID=5735 WORKING WITH MODERN HYDROCARBON AND OXYGENATED SOLVENTS:A GUIDE TO FLAMMABILITY] American Chemistry Council Solvents Industry Group, pg. 7, January 2008
  3. ^ Gases - Explosive and Flammability Concentration Limits
  4. ^ [http://www.americanchemistry.com/s_acc/sec_solvents.asp?CID=1488&DID=5735 WORKING WITH MODERN HYDROCARBON AND OXYGENATED SOLVENTS:A GUIDE TO FLAMMABILITY] American Chemistry Council Solvents Industry Group, pg. 7, January 2008
  5. ^ WORKING WITH MODERN HYDROCARBON AND OXYGENATED SOLVENTS:A GUIDE TO FLAMMABILITY American Chemistry Council Solvents Industry Group, pg. 7, January 2008
  6. ^ n-BUTYL MERCAPTAN ICSC: 0018
  7. ^ 2-HEXANONE ICSC:0489
  8. ^ Gases - Explosive and Flammability Concentration Limits
  9. ^ Gases - Explosive and Flammability Concentration Limits
  10. ^ Cyanogen
  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. ^ CYCLOHEXANONE ICSC: 0425
  14. ^ MSDS Cyclopentadiene
  15. ^ Yaws, Carl L.; Braker, William; Matheson Gas Data Book Published by McGraw-Hill Professional, 2001 pg. 221
  16. ^ CYCLOPENTANE ICSC: 0353
  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. ^ [1]
  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. ^ Gases - Explosive and Flammability Concentration Limits
  27. ^ Gases - Explosive and Flammability Concentration Limits
  28. ^ Gases - Explosive and Flammability Concentration Limits
  29. ^ [http://www.americanchemistry.com/s_acc/sec_solvents.asp?CID=1488&DID=5735 WORKING WITH MODERN HYDROCARBON AND OXYGENATED SOLVENTS:A GUIDE TO FLAMMABILITY] American Chemistry Council Solvents Industry Group, pg. 7, January 2008
  30. ^ Gases - Explosive and Flammability Concentration Limits
  31. ^ Gases - Explosive and Flammability Concentration Limits
  32. ^ [http://www.americanchemistry.com/s_acc/sec_solvents.asp?CID=1488&DID=5735 WORKING WITH MODERN HYDROCARBON AND OXYGENATED SOLVENTS:A GUIDE TO FLAMMABILITY] American Chemistry Council Solvents Industry Group, pg. 7, January 2008
  33. ^ Gases - Explosive and Flammability Concentration Limits
  34. ^ [http://www.americanchemistry.com/s_acc/sec_solvents.asp?CID=1488&DID=5735 WORKING WITH MODERN HYDROCARBON AND OXYGENATED SOLVENTS:A GUIDE TO FLAMMABILITY] American Chemistry Council Solvents Industry Group, pg. 7, January 2008
  35. ^ Gases - Explosive and Flammability Concentration Limits
  36. ^ Gases - Explosive and Flammability Concentration Limits
  37. ^ [http://www.americanchemistry.com/s_acc/sec_solvents.asp?CID=1488&DID=5735 WORKING WITH MODERN HYDROCARBON AND OXYGENATED SOLVENTS:A GUIDE TO FLAMMABILITY] American Chemistry Council Solvents Industry Group, pg. 7, January 2008
  38. ^ [http://www.americanchemistry.com/s_acc/sec_solvents.asp?CID=1488&DID=5735 WORKING WITH MODERN HYDROCARBON AND OXYGENATED SOLVENTS:A GUIDE TO FLAMMABILITY] American Chemistry Council Solvents Industry Group, pg. 7, January 2008
  39. ^ Gases - Explosive and Flammability Concentration Limits
  40. ^ Gases - Explosive and Flammability Concentration Limits
  41. ^ Gases - Explosive and Flammability Concentration Limits
  42. ^ Gases - Explosive and Flammability Concentration Limits
  43. ^ [http://www.americanchemistry.com/s_acc/sec_solvents.asp?CID=1488&DID=5735 WORKING WITH MODERN HYDROCARBON AND OXYGENATED SOLVENTS:A GUIDE TO FLAMMABILITY] American Chemistry Council Solvents Industry Group, pg. 7, January 2008
  44. ^ Combustibles
  45. ^ [http://www.americanchemistry.com/s_acc/sec_solvents.asp?CID=1488&DID=5735 WORKING WITH MODERN HYDROCARBON AND OXYGENATED SOLVENTS:A GUIDE TO FLAMMABILITY] American Chemistry Council Solvents Industry Group, pg. 7, January 2008

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