Lower Explosive Limit – LEL

Posted: Jun. 15, 2019 • By Kevin Kohler

Lower Explosive Limit – LEL

Do I need to worry about flammable and explosive atmospheres?

The short answer is yes, consider the following event amongst many investigated by the US Chemical Safety and Hazard Investigation Board:

“Police say four workers have been taken to hospital after a propane explosion outside a strip mall in Mississauga, Ont. (A) Peel regional police Const. says five roofers showed up for work … and opened the door to a shipping container that held their tools and propane tanks. He says the explosion occurred when they opened the door. …”

Tragically these types of events recur every year in a wide variety of industrial, municipal and agricultural settings.

What does the LEL have to do with it?

From our fire triangle we know that at a given temperature and pressure, mixtures of combustible and flammable substances in air (an oxygen source) will only burn (or a flame propagate) if the concentration of fuel is in a range bounded by its upper and lower flammability (or explosive) limits. Too much fuel and the mixtures is too “rich” to burn, and too little fuel and the mixture is too “lean” to burn. These limits are often referred as the lower and upper explosive limits or LEL and UEL.

When we are working with or near combustible and flammable materials, be they liquids, gases or combustible dusts (grain, flour) in enclosed, constrained, still or low lying areas we have to be aware that even low concentrations of these substances can exceed the LEL and create a potentially explosive atmosphere.

So I’m safe to work above the LEL as long as there is no source of ignition?

No. The danger is simply too high that a source of ignition, whether previously identified or not, will “find” the explosive atmosphere. Investigations into explosions are often unable to pinpoint an ignition source that can be as simple as friction heat or static electricity. The risk to life and property posed by an explosion is considered to be so high that the permissible level for entering or working is limited to 10% or 20% of the LEL concentration, depending upon the jurisdiction and circumstances of the work. Where the potential for an explosive atmosphere may exist, and particularly in confined spaces, many technical safety measures will be required as a fail safe should conditions change.

How do I make sure that the LEL is not exceeded?

From a risk management standpoint processes and operations that involve working in the vicinity of flammable and combustible substances must be assessed for the hazards that they might present under:

  • Normal operations;
  • Upset operations (ruptures, spills, process upsets);
  • Maintenance operations (cleaning, repairs, draining, flushing).

Where there is the possibility of an explosive atmosphere, written plans should be developed based on the hazard and risk analysis.

I can measure the LEL with a meter, so what’s the big deal?

Measuring the % LEL either continuously or with hand held devices is often a necessary requirement to verify that the hazard control measures are effective or that unanticipated conditions have not arisen. There are numerous considerations when using instruments to measure %LEL, one of the foremost of which is being sure that the meter has been properly calibrated for the gases likely to be encountered and that the limitations of gas testing and the gas testing equipment are well understood.

Often gas testing is required because conditions are abnormal (a spill, a release, maintenance), we shouldn’t count on the conditions being the same at all times and in all areas we are working in. The gas meter should primarily be used to verify and to warn that conditions are not as we anticipated or they have changed.

Don’t get caught in a hazardous atmosphere, identify and control the hazards first


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