LEL / UEL MEANING

In this article, we explain what does LEL / UEL (lower and upper explosive limit) mean and provide the LEL / UEL chart for most explosive / combustible lel gas (such as hydrogen LEL and methane LEL values). We also explain what is a LEL meters and detectors and

LEL / UEL IMPORTANCE

Explosion is one of the key risks associated with the handling of combustible gases. It is therefore important to know under which conditions specific gases may explode when in contact with air and an ignition element. To light a fire or have an explosion, three things should occur at the same time:

  1. Presence of a combustible gas, the fueling element, in specific concentration

  2. Presence of Oxygen

  3. Existence of a sparking element (that ignites the two elements)

The proportion of fuel and the oxygen needed to generate an explosion depends on the type of combustible gas (each gas has a specific ratio). Each gas will ignite only when mixed with air within a specific concentration range. If the gas in mixed with oxygen with too low or too high concentrations, the gas will not ignite (there will be no explosion). The lower and the upper explosion values define exactly the required level of concentration by type of gas. Explosions will occur for gas concentrations within the LEL and the UEL value, not above or below, and the maximum explosive power will be for concentration at the midpoint of the flammable range. 

WHAT DOES LEL MEAN?

LEL definition: The lower explosive limit (LEL) is the minimum concentration of a specific combustible gas required to generate its combustion when in contact with oxygen (air). If the concentration of the gas is below the LEL value, the mix between the gas itself and the air is too weak to spark.

Note that LEL is sometimes also called LFL (lower flammable limit).

Examples of LEL definition for two common explosive gas:

LEL for Hydrogen: 4.0

LEL for Methane: 5.0

WHAT DOES UEL MEAN?

UEL definition: The upper explosive limit (UEL) is the maximum level of concentration of the gas that will burn when mixed with oxygen; when the gas concentration is above the UEL value for the gas / vapor, the mix is too “fat” to ignite or explode. Note that UEL is sometimes also called UFL (upper flammable limit).

FLAMMABLE RANGE MEANING

The range between the lower and the upper explosive limit (LEL / UEL %) is defined as the flammable range of a specific explosive and flammable gas.

LEL UEL Combustible Gases

LEL / UEL BY GAS (LEL CHART):

(Note: LEL / UEL values are based on room temperature and atmospheric pressure, ignition fired by a tube of 2 inch diameter. As the temperature, the pressure and the ignition increase, the explosive limits by gas vary. The values are determined empirically and may change depending on the source of the information). The lower and the upper explosive limits by gas are:

LEL Gas

LEL %

 

UEL %

Acetone

2.6

 

13.0

Acetylene

2.5

 

100.0

Acrylonitrile

3.0

 

17

Allene

1.5

 

11.5

Ammonia

15.0

 

28.0

Benzene

1.3

 

7.9

1,3-Butadiene

2.0

 

12.0

Butane

1.8

 

8.4

n-Butanol

1.7

 

12.0

1-Butene

1.6

 

10.0

Cis-2-Butene

1.7

 

9.7

Trans-2-Butene

1.7

 

9.7

Butyl Acetate

1.4

 

8.0

Carbon Monoxide

12.5

 

74.0

Carbonyl Sulfide

12.0

 

29.0

Chlorotrifluoroethylene

8.4

 

38.7

Cumene

0.9

 

6.5

Cyanogen

6.6

 

32.0

Cyclohexane

1.3

 

7.8

Cyclopropane

2.4

 

10.4

Deuterium

4.9

 

75.0

Diborane

0.8

 

88.0

Dichlorosilane

4.1

 

98.8

Diethylbenzene

0.8

 

1,1-Difluoro-1-Chloroethane

9.0

 

14.8

1,1-Difluoroethane

5.1

 

17.1

1,1-Difluoroethylene

5.5

 

21.3

Dimethylamine

2.8

 

14.4

Dimethyl Ether

3.4

 

27.0

2,2-Dimethylpropane

1.4

 

7.5

Ethane

3.0

 

12.4

Ethanol

3.3

 

19.0

Ethyl Acetate

2.2

 

11.0

Ethyl Benzene

1.0

 

6.7

Ethyl Chloride

3.8

 

15.4

Ethylene

2.7

 

36.0

Ethylene Oxide

3.6

 

100.0

Gasoline

1.2

 

7.1

lower / upper explosive limits by gas, continued...

Gas

LEL

 

UEL

Heptane

1.1

 

6.7

Hexane

1.2

 

7.4

Hydrogen

4.0

 

75.0

Hydrogen Cyanide

5.6

 

40.0

Hydrogen Sulfide

4.0

 

44.0

Isobutane

1.8

 

8.4

Isobutylene

1.8

 

9.6

Isopropanol

2.2

 

Methane

5.0

 

15.0

Methanol

6.7

 

36.0

Methylacetylene

1.7

 

11.7

Methyl Bromide

10.0

 

15.0

3-Methyl-1-Butene

1.5

 

9.1

Methyl Cellosolve

2.5

 

20.0

Methyl Chloride

7.0

 

17.4

Methyl Ethyl Ketone

1.9

 

10.0

Methyl Mercaptan

3.9

 

21.8

Methyl Vinyl Ether

2.6

 

39.0

Monoethylamine

3.5

 

14.0

Monomethylamine

4.9

 

20.7

Nickel Carbonyl

2.0

 

Pentane

1.4

 

7.8

Picoline

1.4

 

Propane

2.1

 

9.5

Propylene

2.4

 

11.0

Propylene Oxide

2.8

 

37.0

Styrene

1.1

 

Tetrafluoroethylene

4.0

 

43.0

Tetrahydrofuran

2.0

 

Toluene

1.2

 

7.1

Trichloroethylene

12.0

 

40.0

Trimethylamine

2.0

 

12.0

Turpentine

0.7

 

Vinyl Acetate

2.6

 

Vinyl Bromide

9.0

 

14.0

Vinyl Chloride

4.0

 

22.0

Vinyl Fluoride

2.6

 

21.7

Xylene

1.1

 

6.6

 

LEL METER DEFINITION

To operate safely in hazardous environments, i.e. closed spaces with possible presence of combustible gases, the concentration of the gas should be monitored closely. As the concentration of the gas exceeds 20% of the gas LEL, is considered unsafe.

To monitor gas concentration value in closed and hazardous environments, operators may use LEL meters (also called, LEL meters / detectors) which are designed with catalytic bead and infrared sensing elements to measure the lower explosive limit of gases. These gas detectors give warnings to the operators whenever the combustible gas is present in the environment at levels around 10%. LEL meters are rather sophisticated devices, that feature microprocessors based modular design with self-calibration and digital display of the information.

PID METER 

The most used LEL meter is the Wheatstone bridge type, which is effective for most applications and environments. However, the Wheatstone bridge LEL detector may not be effective for specific conditions, or gases, that require higher sensitivity sensors. The PID detectors (“Photoionization detectors”) are an option when a more accurate LEL measurement is required in hazardous environments. PID can measure the concentration of inflammable gases and other toxic gases even at very low levels (from ppb, i.e. parts per billion, up to 10k ppm, i.e. 1%). PIDs are way more sensitive tools then common LEL meters, and are generally more expensive. PIDs are suited to measure the following organic compounds:

 

  • Alcohol

  • Aromatics

  • Amines & Amides

  • Chlorinated hydrocarbons

  • Ketones & Aldehydes

  • Sulfur compounds

  • Unsaturated hydrocarbons

  • Saturated hydrocarbons - like butane and octane.

The inorganic compounds that can be measured by photoionization detectors are:

  • Ammonia

  • Bromine

  • Iodine

  • Hydrogen sulfide

  • Nitric Oxide

  • Semiconductor gases

 

 

Looking for additional info? Contact the pipingonline expert team

Useful links: ANSI | ASME | ASTM | API | MSS | BS | DIN | ISO