LEL & Flash Point Principles

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.

Limits

 Lower Explosive Limit

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 (arc, 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. e.g.: 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% multiplied by 5, or 0.25% methane by volume at 20 degrees C.

 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). Concentration higher than UEL are "too rich" to burn. Also called UFL

 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 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.

 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.

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

 

Flash point

The flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a liquid's flash point requires an ignition source. At the flash point, the vapor may cease to burn when the source of ignition is removed.

The flash point is not to be confused with the autoignition temperature, which does not require an ignition source.

The fire point, a slightly higher temperature, is defined as the temperature at which the vapor continues to burn after being ignited. Neither the flash point nor the fire point is related to the temperature of the ignition source or of the burning liquid, which are much higher.

The flash point is often used as a descriptive characteristic of liquid fuel, and it is also used to describe liquids that are not normally used as fuels. “Flash point” refers to both flammable liquids and combustible liquids. There are various international standards for defining each, but most agree that liquids with a flash point less than 43°C are flammable, while those having a flash point above this temperature are combustible.

Mechanism

Every liquid has a vapour pressure, which is a function of that liquid's temperature. As the temperature increases, the vapour pressure increases. As the vapour pressure increases, the concentration of evaporated flammable liquid in the air increases. Hence, temperature determines the concentration of evaporated flammable liquid in the air.

Each flammable liquid requires a different concentration of its vapour in air to sustain combustion. The flash point of a flammable liquid is the lowest temperature at which there will be enough flammable vapour to ignite when an ignition source is applied

 Measuring flash points

There are two basic types of flash point measurement: open cup and closed cup.

In open cup devices the sample is contained in an open cup which is heated, and at intervals a flame is brought over the surface. The measured flash point will actually vary with the height of the flame above the liquid surface, and at sufficient height the measured flash point temperature will coincide with the fire point. The best known example is the Cleveland Open Cup (COC).

There are two types of Closed cup testers: non-equilibrium, such as Pensky-Martens where the vapours above the liquid are not in temperature equilibrium with the liquid, and equilibrium, such as Small Scale (commonly known as Setaflash) where the vapours are deemed to be in temperature equilibrium with the liquid. In both these types the cups are sealed with a lid through which the ignition source can be introduced. Closed cup testers normally give lower values for the flash point than Open cup (typically 5-10 °C) and are a better approximation to the temperature at which the vapour pressure reaches the lower flammable limit (LFL).

The flash point is an empirical measurement rather than a fundamental physical parameter. The measured value will vary with equipment and test protocol variations, including temperature ramp rate (in automated testers), time allowed for the sample to equilibrate, sample volume and whether the sample is stirred.

Methods for determining the flash point of a liquid are specified in many standards. For example, testing by the Pensky-Martens closed cup method is detailed in ASTM D93, IP34, ISO 2719, DIN 51758, JIS K2265 and AFNOR M07-019. Determination of flash point by the Small Scale closed cup method is detailed in ASTM D3828 and D3278, EN ISO 3679 and 3680, and IP 523 and 524.

 Examples of flash points

Fuel Flash point Autoignition
temperature
Ethanol 12.8 °C (55 °F) 365 °C (689 °F)
Gasoline (petrol) <−40 °C (−40 °F) 246 °C (475 °F)
Diesel >62 °C (143 °F) 210 °C (410 °F)
Jet fuel >60 °C (140 °F) 210 °C (410 °F)
Kerosene (paraffin oil) >38–72 °C (100–162 °F) 220 °C (428 °F)
Vegetable oil (canola) 327 °C (620 °F)[1]
Biodiesel >130 °C (266 °F)

Gasoline (petrol) is designed for use in an engine which is driven by a spark. The fuel should be premixed with air within its flammable limits and heated above its flash point, then ignited by the spark plug. The fuel should not preignite in the hot engine. Therefore, gasoline is required to have a low flash point and a high autoignition temperature.

Diesel fuel flash points vary between 52 °C and 96 °C (126 °F to 204 °F). Diesel is designed for use in a high-compression engine. Air is compressed until it has been heated above the autoignition temperature of diesel; then the fuel is injected as a high-pressure spray, keeping the fuel-air mix within the flammable limits of diesel. There is no ignition source. Therefore, diesel is required to have a high flash point and a low autoignition temperature.

Jet fuel flash points also vary greatly. Both Jet A and Jet A-1 have flash points between 38 °C and 66 °C (100 °F to 150 °F), close to that of off-the-shelf kerosene. Yet both Jet B and FP-4 have flash points between -23 °C and -1 °C (-10 °F to +30 °F ).

 Standardization

Flash points of substances are measured according to standard test methods. These test methods define the apparatus required to carry out the measurement, key test parameters, the procedure for the operator or automated apparatus to follow, and the precision of the test method. Standard test methods are written and controlled by a number of national and international committees and organizations. The three main bodies are the CEN / ISO Joint Working Group on Flash Point (JWG-FP), ASTM D02.8B Flammability Section and the Energy Institute's TMS SC-B-4 Flammability Panel.

 References

  1. ^ MSDS for Refined, Bleached and Deodorized Canola Oil, issued by Avatar Corp, 16 Nov 2001, accessed 22 March 2008

 

Source and  Terms of Use:

http://en.wikipedia.org/wiki/Flash_point

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