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Lower explosive limits

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Lower explosive limits mean flammable gases, vapours from flammable or combustible liquids and solids can ignite through an ignition source if they are at specific concentrations represented on a scale named lower explosive limits.

It is the concentration between which a gas mixture is explosive or flammable. These mixtures are expressed as percentages of the volume of gas or vapour in the air and are determined at normal pressure and temperatures for each substance.

There are two levels you should pay attention to when measuring flammable gases.

  1. Lower Explosive Limits (LEL): the lowest concentration of a gas in air that can cause combustion or generate a flame when combined with an ignition source.
  2. Upper Explosive Limit (UEL): It is the highest concentration of a gas in air that can cause combustion or generate a flame when combined with an ignition source.

If the concentration of a gas is below its LEL, it cannot be ignited and the confined space or other area is considered safe. If the concentration of a gas is higher than its UEL, the gas is too abundant and there is not enough oxygen for combustion.

According to OSHA (Section 1915.12(b)(3) Flammable atmospheres. Atmospheres with a concentration of flammable vapours at or above 10 per cent of the lower explosive limit (LEL) are considered hazardous when located in confined spaces. However, atmospheres with flammable vapours below 10 per cent of the LEL are not necessarily safe

Lower Explosive Limits definition

LEL, short for Lower Explosive Limits, is defined as the lowest concentration (by percentage) of a gas or vapour in the air that is capable of producing a flash of fire in the presence of an ignition source (arc, flame, heat). Or the Lower Explosive limit (LEL) is the minimum concentration of flammable gases, vapours or mists in air below which the mixture is not explosive. Concentrations below the Lower Explosive Limits are ‘too lean to burn; those above the Upper Explosive Limit (UEL) are too rich to burn.

The lower explosive limits of zero per cent (0% LEL) denotes an atmosphere free of flammable gas. The lower explosive limits of one hundred per cent (100% LEL) indicate an atmosphere in which the gas is at its lower explosive limits. The relationship between per cent LEL and per cent by volume differs from gas to gas.

Lower exposure limits (LEL) and upper exposure limits (UEL) vary by gas. Methane, for example, has an LEL of 5% and a UEL of 17%. Combustion of methane is possible when the gas level is 5% or more, but less than 17%. Gas detectors display the presence of gas as a percentage of its LEL. An atmosphere free of methane will give a 0% LEL on the gas detector, but an atmosphere with 5% methane will give a 100% LEL.

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Lower explosive limits

The above example demonstrates the flammability of methane (natural gas) in the air. At concentrations of 0-5% methane in the air, the mixture is too lean to ignite or burn. Methane concentrations between 5% and 17% favour ignition and are considered highly flammable. At levels above 17%, the atmosphere is too rich for methane to ignite.

What is the range between LEL and UEL?

The Lower Explosive Limits (LEL) is the minimum concentration of gas in the air below which fire is not possible. … Upper Flammability Limit (UFL) is the maximum concentration of gas in the air above which fire is not possible. Lower explosive limits 

For an explosive atmosphere to form, the concentration of flammable gases, vapours or mists in the air must be within a certain range. Said range of the explosive limits of a flammable substance is delimited by lower and upper explosive limits. These limits are usually expressed as volume percentages of the gas or vapour in the volume of the mixture. Lower explosive limits 

The range between the LEL and UEL is known as the ‘flammable rich mixture range’. A mixture below the lower limit is said to be “poor” and a mixture above the upper limit is said to be “rich”. Both the “rich” and the “poor” mixture are off-limits to burn or explode.

If it is intended that a certain mixture of flammable gases or vapours in the air does not produce an explosive atmosphere, it will be necessary to maintain a concentration that remains below the Lower Explosive Limits or above the Upper Explosive Limit.

Explosion prevention can be achieved by operating outside the flammability range in air processes. However, processes that take place below the lower flammability limit are safer. For this reason, in general, whenever possible, the first option is chosen, through the use of suitable means of ventilation or extraction on the outside of the equipment, for example, with the use of compressors etc.

Lower explosive limits

Sometimes it is necessary to choose the second option. Then the preventive measures that are taken are usually used inside the equipment, such as tanks or reservoirs, pipes, etc.

Flash Point

The flash point is the set of conditions (pressure, temperature, concentration, etc.) of a substance that favours its ignition when faced with a heat source. Examples of flash points at normal pressure: Ethanol 70%: 16 ºC, Diesel: 50ºC, Gasoline: -43ºC etc.

Flash point is the lowest temperature at which fuel gives off enough vapour to form a flammable mixture from an external source of heat. By flammable mixture, to obtain the flash point is understood as the amount of gas or vapour mixed with atmospheric air sufficient to ignition in contact with a source of heat.

The flash point should not be confused with the auto-ignition temperature, which does not require an ignition source, the temperature at which vapour continues to burn after it has been ignited. Lower explosive limits 

Acetylene, ammonia, hydrogen, propane, propylene, and methane are flammable gases, also known as fuel gases. They burn when mixed with an oxidant and if provided with an ignition source.

LEL Gases

The following are the lower explosive limits (LEL) of the selected gases by volume
Acetone2.5%hexane1.1%
Acetylene2.5%Hydrogen4.0%
Ammonia15.0%Isopropyl alcohol (isopropanol)2.0 l%
Benzene1.2%Methane5.0%
Butane1.9%Methyl alcohol (methanol)6.0 %
Butyl alcohol1.4%methyl ethyl ketone1.4%
Carbon monoxide12.5% ​​n-pentane1.4%
Diethyl ether1.9%Propane2.1%
ethane3.0%Propylene2.0%
Ethyl alcohol (ethanol) –3.3%styrene0.9%
Ethylene2.7%toluene1.1%
Ethylene oxide2.7%xylene1.1%

Example of flammable substances

Flammable gases and gas mixtures:

  • Liquefied gas (butane, butene, propane, propene)
  • Natural gas
  • Combustion gases (eg carbon monoxide or methane)
  • Gaseous chemicals (eg acetylene, ethylene oxide or vinyl chloride)

Flammable liquids:

  • Solvents
  • Fuels
  • Crude oil
  • Fuel oils
  • Lubricant oils
  • Used oils
  • Water-soluble and non-water-soluble chemical substances

Flammable solid powders:

  • Coal
  • Wood
  • Food for human or animal consumption (for example, sugar, flour or cereals)
  • Synthetic materials
  • Metals
  • Chemical products

What are ignition sources

The analysis of possible sources of ignition should focus not only on ignition sources of electrical origin but also on those of a mechanical nature, as well as those caused by static electricity (for example, that originating in work clothes ), as well as those derived from human behaviour (maintenance work, fundamentally). According to standard EN 1127-1, the ignition sources to be considered are the following:

  • Hot surfaces
  • Flames and hot gases
  • Mechanically generated sparks
  • Electrical sparks
  • Dispersion electrical currents
  • Protection against cathode corrosion
  • Static electricity
  • Lightning
  • Gas flows and chemical reactions

What is the definition of UEL?

The Upper Explosive Limit (UEL) is the maximum concentration of flammable gases, vapours or mists in the air above which the mixture is not explosive.

Why is LEL important?

The values of the lower and upper limits of flammability define the so-called Range or Field of Flammability or Explosively. It is defined as the minimum concentration of vapour or gas mixed with air, below which there is no flame propagation when in contact with an ignition source. Lower explosive limits 

What is LEL & UEL of natural gas?

Methane is a gas, commonly known as natural gas. The lower explosive limits is 5%. This means that a mixture with enough natural gas to be flammable is: 5% methane and 95% air, or in other words, 5% methane, 19.95% oxygen and 75.05% nitrogen.

For example, if you take 2.5% methane in the air and divide it by 100% LEL methane (5%), the result is:

50% LEL. (2.5% ÷ 5% = 50%)

The same theory can be applied to any of the gases listed here. Instead, you can multiply the %LEL of the unknown concentration by 100%LEL to get the % by volume:

(50% LEL Methane x 5% = 2.5% by volume).

What is 10% of the LEL?

As an example, hydrogen has a lower limit of 4% in volume, if the gas is 2%, the detector will be reading 50% LEL, because %LEL is a scale proportional to the explosiveness of the gas. However, It is considered that confined space with concentrations of gas, steam or flammable liquid greater than 10%LEL is a risky atmosphere.

Lower explosive limits

Taking this into account, it is recommended that the alarm of a flammable gas detector be set at 10%LEL or more restrictive, thus guaranteeing a quick response to the evacuation.

OSHA and ANSI/ASSE standards state that the following must be measured: Lower Explosive Limits (LEL), Oxygen (O2) and other toxic gases (normally H2S and CO are measured). These same standards also establish that the LEL must be less than 10%; oxygen must be above 19.5%, and the other toxic gases (or dust) must be below their maximum permissible limits. Lower explosive limits 

If not, controls should be implemented. NIOSH (1979) establishes a series of classes to define the levels. It is important to clarify that the classes do not determine the levels; on the contrary, the levels determine the classes. This rating is not of much importance and was established to help competent persons select the protection system.

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Lower explosive limits 

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