Flammability Limits

Flammability Limits

Lower flammable limit (LFL) or Lower Explosive Limit (LEL) is minimum vapor concentration in air which a mixture will burn when an ignition source is present.

Upper flammable limit (UFL) or Upper Explosive Limit (UEL) is maximum vapor concentration in air which a mixture will burn when an ignition source is present.

Concentration of mixture of vapor in air below LFL/LEL (too lean) or above UFL/UEL (too rich), mixture will not burn even an ignition source is present. Therefore, flammable range or explosive range is concentrations between LFL/UFL and UFL/UEL.

LFL/LEL and UFL/UEL for some common gases are indicated in table below. Some of the gases are commonly used as fuel in combustion processes.

Fuel Gas	(LFL/LEL) (%)	(UEL/UFL) (%)
Acetaldehyde	4	60
Acetone	2.6	12.8
Acetylene	2.5	81
Ammonia	15	28
Arsine	5.1	78
Benzene	1.35	6.65
n-Butane	1.86	8.41
iso-Butane	1.80	8.44
iso-Butene	1.8	9.0
Butylene	1.98	9.65
Carbon Disulfide	1.3	50
Carbon Monoxide	12	75
Cyclohexane	1.3	8
Cyclopropane	2.4	10.4
Dimethyl Ether	3.4	27
Diethyl Ether	1.9	36
Ethane	3	12.4
Ethylene	2.75	28.6
Ethylene Oxide	3.6	100
Ethyl Alcohol	3.3	19
Ethyl Chloride	3.8	15.4
Fuel Oil No.1	0.7	5
Hydrogen	4	75
Isobutane	1.8	9.6
Isopropyl Alcohol	2	12
Gasoline	1.4	7.6
Kerosine	0.7	5
Methane	5	15
Methyl Alcohol	6.7	36
Methyl Chloride	10.7	17.4
Methyl Ethyl Ketone	1.8	10
Naphthalene	0.9	5.9
n-Heptane	1.0	6.0
n-Hexane	1.25	7.0
n-Pentene	1.65	7.7
Neopentane	1.38	7.22
Neohexane	1.19	7.58
n-Octane	0.95	3.20
iso-Octane	0.79	5.94
n-Pentane	1.4	7.8
iso-Pentane	1.32	9.16
Propane	2.1	10.1
Propylene	2.0	11.1
Silane	1.5	98
Styrene	1.1	6.1
Toluene	1.27	6.75
Triptane	1.08	6.69
p-Xylene	1.0	6.0

Note : The limits indicated are for component and air at 20^oC and atmospheric pressure.

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Minimum Oxygen Concentration (MOC)

Standard air composition is the bulk average air composition encountered at or around sea level. Typically it contains the following composition.

Component	Abbr.	%Wt	%Vol.	MW
Nitrogen	N2	75.47	78.08	28.01
Oxygen	O2	23.20	20.95	32.00
Carbon Dioxide	CO2	0.0590	0.038	44.01
Hydrogen	H2	0	0.00005	2.02
Argon	Ar	1.28	0.93	39.95
Neon	Ne	0.0012	0.0018	20.18
Helium	He	0.00007	0.0005	4.00
Krypton	Kr	0.0003	0.0001	83.80
Xenon	Xe	0.00004	8.7x10^-6	131.30

From standard air composition at sea level, the oxygen content is approximately 20.95% by volume. Oxygen is the main component in fire triangle in generating of fire. From chemistry, without any one of the element, a fire will NOT form.

Although Oxygen is the major component in generating fire, there is still a minimum oxygen concentration (MOC) required present combustible mixture so that a fire can be initiated. Below this limit, a fire will not form. Base on this principle, hydrocarbon mixture is purge with inert gas such as Nitrogen, Carbon dioxide, etc so that the MOC is not reach and therefore combustible mixture is not form.

Common unit used for MOC is % oxygen in air plus fuel. Following table listed several

Component	MOC (%O2)
Hydrogen	4.0
Acetylene	6.2
Methane (C1)	12
Ethane (C2)	11.2
Ethylene (C2=)	9.9
Propane (C3)	11.6
Propylene (C3=)	11.5
Butane (C4)	12.3
1-Butene (C4=)	11.0
Pentane (C5)	11.8
Hexane (C6)	11.8
Benzene	11.5
Methyl Alcohol	9.9
Ethyl Alcohol	10.5
Dimethyl Ether	10.5
Diethyl Ether	10.2
Methyl Acetate	11.0
Methyl Ether Ketone	11.0
Carbon Disulfide	5.0

Generally a mixtures of Hydrocarbon vapor MOC is around 10% Vol.

API STD 520 recommends MOC of 6 %Vol. for flare stack design to avoid combustible mixture present in flare system. Hydrogen rich flare gas shall consider lower MOC i.e. 3-4% vol.

Ref : "Application of Flammability Diagram For Evaluation of Fire & Explosion Hazards of Flammable Vapor", C.V. Mashuga, D.A. Crowl


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Conversion from MMSCFD to kg/s

How to convert MMSCFD to Kg/s ?

Standard condition (S) measure at 1.01325 bara & 15.56 degC (60 degF)
Normal condition (N) measure at 1.01325 bara & 0 degC

1.0 MMSCFD
= 1,000,000/(24 x 3600) Sft³ /s
= 1,000,000/(24 x 3600)/35.3152 Sm³/s
= 1,000,000/(24 x 3600)/35.3152 x (273.15/288.71) Nm³/s
= 1,000,000/(24 x 3600)/35.3152 x (273.15/288.71)/22.414 kmol/s
= 1 / 72.2826

1 MMSCFD = 1 / 72.2826 kmol/s

or

1 MMSCFD = MW / 72.2826 kg/s

or

1 kg/s = 72.2826 / MW MMSCFD

MW = Molecular weight

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