Franklin Fueling Systems NANOVAPOR NV-SS ST-1000, Installation And Operation Manual, Page: 9 / 60

5
The Lower Explosive Level (LEL) is the lowest concentration of a flammable gas in air capable of
producing a fire when contacted by an ignition source. Concentrations below the LEL will not
propagate a flame front to create a fire (or cause an explosion if in a confined space) as these
concentrations are considered too “lean.”
The Upper Explosive Limit (UEL) is the highest concentration of flammable gas in air capable of
producing a fire when contacted by an ignition source. Concentrations above the UEL will not
propagate a flame front to create a fire (or cause an explosion if in a confined space), as these
concentrations are considered too “rich.”
The LEL and UEL of flammable gases varies greatly depending on chemical composition. In fuel
tank maintenance and inspection operations, most safety procedures are designed to monitor
the flammable vapor level as a percent LEL. This is because explosive atmospheres can occur in
these industrial operations.
As a safety process, a combustible gas meter is used to monitor the % of LEL of an underground
storage tank while work is being performed that requires a gas-free environment. Measurements
of the environment are usually expressed in a percentage scale of 0-100%. This scale is
interpreted on the high side of 100% to mean that at 100% LEL, the monitored fuel vapor/air
mixture has reached the minimum concentration that can ignite and sustain combustion.
Preventing explosions
As you have seen, the three elements necessary to create an explosion are:
• An oxidizer such as air or oxygen.
• A fuel such as natural gas or petroleum-based products
• An ignition source which is some kind of added energy, such as a spark.
These elements comprise the Explosion Triangle shown below:
When working on or in fuel tanks, fuel and oxygen are always present. Fuel vapors exist in the
tank, and oxygen is present in the tank headspace and as part of the surrounding atmosphere.
Consequently, when you are conducting tank work, ALL IGNITION SOURCES MUST BE IDENTIFIED
AND REMOVED FROM THE HAZARD AREA as shown below.