The Definition of Lower Explosive Limit (LEL)

Definition of Lower Explosive Limit (LEL)

The lower explosive limit (LEL) is the lowest concentration of a flammable gas or vapor in air that can ignite and burn or explode when it meets an ignition source. Below the LEL, the mixture is too lean to burn because there is not enough fuel. The term lower flammable limit (LFL) means the same thing.

Every flammable gas and vapor has its own LEL, usually given as a percentage by volume in air. For example, methane has an LEL of about 5 percent, propane about 2.1 percent, gasoline vapor about 1.4 percent and hydrogen about 4 percent. The LEL for a chemical is listed in section 9 of its safety data sheet (SDS).

Above the LEL, the mixture can burn until it reaches the upper explosive limit (UEL), where there is too much fuel and not enough oxygen. The range between the LEL and UEL is called the flammable or explosive range.

LEL in Gas Detection

Percent LEL readings

Combustible gas detectors usually display readings as a percentage of the LEL, not as a percentage of the gas in air. A reading of 100 percent LEL means the atmosphere has reached the lower explosive limit. For methane, 10 percent LEL equals about 0.5 percent methane by volume.

Alarm settings and action levels

• Many four-gas monitors have a low alarm at 10 percent LEL and a high alarm at 20 percent LEL.
• OSHA's permit-required confined space standard treats a flammable gas above 10 percent of its LEL as a hazardous atmosphere.
• Hot work permits commonly require readings at or near 0 percent LEL, and work must stop if levels rise.

Calibration and sensor limits

Catalytic and infrared sensors are calibrated to a specific gas, often methane or pentane. When measuring a different gas, the reading may be higher or lower than the true value, so correction factors from the manufacturer should be used. Monitors should be bump tested before each use and calibrated on schedule. Some sensors can be poisoned by silicones, lead or sulfur compounds, and catalytic sensors need oxygen to work properly. See gas detection for more.

Controlling Flammable Atmospheres

• Test the atmosphere before entry and continuously during work in tanks, pits, trenches and other enclosed areas.
• Use ventilation to keep concentrations well below the LEL.
• Remove or isolate ignition sources and use intrinsically safe equipment in hazardous areas.
• Control leaks and spills of flammable liquids promptly.
• Use grounding and bonding to prevent static sparks during transfers.
• Train workers to understand LEL readings and to evacuate when alarms sound.

Example of Lower Explosive Limit

A maintenance crew was preparing to weld a bracket inside a pump room at a fuel terminal. Before issuing the hot work permit, the supervisor tested the room with a calibrated four-gas monitor. The reading near the floor drain was 14 percent LEL, caused by gasoline vapor from a small leak at a valve.

Hot work was not allowed to start. The crew isolated the line, repaired the valve, cleaned up the residue and ran a portable explosion-proof fan. A retest showed 0 percent LEL throughout the room, and the permit was issued with continuous monitoring and a fire watch.

Frequently Asked Questions: Lower Explosive Limit

What does 10 percent LEL mean?

It means the atmosphere contains a flammable gas at one-tenth of the concentration needed to ignite. It is a common warning level that calls for action before conditions become dangerous.

Is LEL the same as LFL?

Yes. Lower explosive limit and lower flammable limit are used interchangeably.

Can an atmosphere above the UEL be safe?

No. An atmosphere above the UEL is too rich to burn, but adding air through ventilation or opening a door can bring it back into the flammable range. It is also likely to be oxygen deficient and toxic.