For combustible gases, the Lower Explosive Limit (LEL) or Lower Flammable Limit (LFL) is the minimum concentration of vapor in air below which the propagation of the flame does not occur when it contacts an ignition source, such as a spark or flame. In a specific area or space, if the LEL of a gas is 4 percent, it is less than 4 percent of the total air/gas mixture, and it will not ignite.
According to the U.S. Department of Labor Occupational Safety and Health Administration (OSHA), atmospheres with a concentration of flammable vapors at or above 10 percent of the lower explosive limit (LEL) are considered hazardous when located in confined spaces. While atmospheres with flammable vapors below 10 percent of the LEL atmospheres are too lean to burn, they are not necessarily safe.
At the other end of the spectrum is the Upper Explosive Limit (UEL) or Upper Flammable Limit (UFL). This is the maximum concentration of vapor or gas in the air above which the propagation of flame will not occur. The Explosive Range or Flammable Range exists between the lower and upper flammable limits, and it is expressed as a percentage of vapor or gas in the air by volume.
Depending on the gas, the LEL will vary considerably. For example, LEL values for common gases (expressed as a percentage of the gas in air by volume) are:
- Methane (CH4): 5%
- Propane (C3H8): 2.1%
- Hydrogen (H2): 4%
- Ethylene (C2H4): 2.7%
- Acetylene (C2H2): 2.5%
- Gasoline: 1.4%
- Carbon Monoxide (CO): 12.5%
In addition to OSHA requirements to monitor the LEL in industrial and other environments, other countries have safety laws that require pre-entry testing before a worker enters a confined space. Organizations such as the National Fire Protection Association (NFPA) provide guidance/requirements, such as the NFPA 69 Standard on Explosion Prevention Systems.
Sensing tools for LEL
Since LEL requires compliance with OSHA regulations, monitoring can be achieved using portable or fixed gas detectors.
One portable measurement product that personnel can hold or attach to a belt detects when they enter a confined space and measures combustibles (LEL), as well as oxygen (O2), carbon monoxide (CO), and hydrogen sulfide (H2S).
A pellistor (or catalytic bead) sensor that consists of a pair of heated metal oxide beads inside a flameproof housing is commonly used in a Wheatstone bridge to measure LEL. In the pellistor, one element has a catalyst (such as platinum) and the other (the reference or compensator) bead does not. Both elements are heated to a temperature that normally would not support combustion, for example 500°C. Since the catalyst “burns” gas at a low level, it heats up relative to the element without the catalyst and has a higher resistance. The Wheatstone bridge measures the difference in resistance between the two elements, which correlates to the LEL.
Besides measurements in confined spaces, other applications for this portable sensor include:
- tank rooms
- vent lines
- air monitoring
- sea vessels
- food processing
- chemical plants
- refrigeration systems
- emergency response
Some environments require continuous monitoring. In these instances, a permanently installed LEL fixed gas detection system can monitor the concentration of flammable gases or vapors in the air and provide warnings when the concentration reaches or exceeds a potentially dangerous level.
References
Image source: Lower and Upper Explosive Limits for Flammable Gases and Vapors
Pellistor (Catalytic Bead) Sensor Technical Notes
NFPA 69 Standard on Explosion Prevention Systems
Understanding Lower Explosive Limits (LeLs) in Workplace Gas Detection
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