Insight to Monitoring for Explosive Gas Mixtures
Visitors to our websites often search for details on combustible or flammable gases since leaks and exposures of this gas type have created some of the most catastrophic industry events. We will post a series of articles helping to better explain the differences between flammability and combustibility, explosive limits, as well as present a gas tables for reference.
Understanding Explosive Limits
Three elements are necessary to create an explosion; an oxidizer such as oxygen or air, a fuel such as gas or dust, and energy the ignition source. These elements are also known as the Explosion Triangle.
- Air is naturally occurring in most environments and in engineering a gas detection system, it is necessary to plan for the presence of air/oxygen.
- Industry creates many fuels as byproducts of a process or they may be intentionally present.
- Ignition sources are present in many forms from an open flame and mechanically generated sparks to hot surfaces and electrostatic discharge.
Despite the presence of these three elements, their presence alone is not enough to create an explosion. Present mixtures of fuel can be too little to sustain a flame or explosion. But the fuel mixture can also be too rich. This brings us to the difference between LEL and UEL.
The lower explosive level (LEL) is the lowest concentration of a gas in air capable of producing a flash of fire when met with a source of ignition. Concentrations below the LEL will not fuel or continue an explosion, these concentrations are considered too “lean”.
The upper explosive limit (UEL) is the highest concentration of a gas in air capable of producing a flash of fire when met with a source of ignition. Concentrations above the UEL will not fuel or continue an explosion, these concentrations are considered too “rich”.
The LEL and UEL of gases vary greatly depending on the chemical composition of the gas or vapor. However, safety applications monitor the LEL as rich explosive atmospheres are more common in the process industry. Gas detectors monitoring LEL measure the percent LEL of the target gas and typically scale 0-100% LEL. At 100% LEL the monitored gas has reached the minimum concentration that can sustain combustion.
The chart below provides references for the lower explosive limit, upper explosive limit, and NFPA classification codes.