Explosive and Flammable Atmospheres

Explosive or Flammable Atmospheres

Proportionally increases the rate of many chemical reactions
Can cause ordinary combustible materials to become flammable or explosive
Many standards (including USA 29 CFR 1910.146) Specify 23.5 % as oxygen enriched
- Other codes (such as USA 29 CFR 1915 and NFPA guidelines) are more stringent
- More conservative approach is to use 22.5 % as hazardous condition threshold

Minimum concentration of a combustible gas or vapor in air which will ignite if a source of ignition is present

Most but not all combustible gases have an upper explosive limit
- Maximum concentration in air which will support combustion
- Concentrations which are above the U.E.L. are too "rich" to burn

The range between the L.E.L. and the U.E.L. of a combustible gas or liquid
Concentrations within the flammable range will burn or explode if a source of ignition is present

LEL sensor only designed to detect 0-100% LEL concentration of flammable gas
If O2 concentration less than 10% O2, LEL sensor will not read properly
Also, sensor may be damaged by exposure to higher than 100% LEL concentrations
To prevent damage, sensor is switched OFF, the alarms are activated, and instrument shows an "OL" message (Over Limit)
CSA 22.2 stipulates latched "OL" alarm cannot be set higher than 60% LEL

A combustible hazard exists whenever the combustible gas concentration exceeds 10% LEL
This is the general hazardous condition threshold, NOT the concentration that should always be used for the LEL alarm set-point
According to the original preamble to 1910.146, if Alternate Entry Procedures are used, the hazard condition threshold is 5% LEL
In some cases it may be necessary to use an even lower alarm setting to allow workers adequate time to escape