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SAFETY INSTRUCTIONS
Explosive limit
The ratio between "percentage of the Lower Explosive Limit" (% LEL) and "percentage by volume"
(% v/v) varies from one gas to another. Standard IEC 60079-20-1 setting out the method for
determining explosive limits gives the following examples:
Gas
Methane
Butane
Propane
Alarm levels
In an explosive gas detection system, the alarm thresholds must be fixed relative to the Lower
Explosive Limit (LEL) of the gas detector. The alarm thresholds for explosive gases are typically
20% LEL for the first alarm level (A1) and 40% LEL for the second level (A2).
The choice of alarm levels must consider the possible effects of local climatic conditions.
Temperature
Pressure
Humidity
Climatic conditions
Temperature
Pressure
Humidity
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Never set an alarm level over 60% LEL
The LEL of a gas is affected by temperature. When the temperature increases,
the LEL decreases and explosion risk increases.
The relationship between pressure and LEL is very complex. In general, if the
pressure increases, the LEL decreases and thus the explosion risk increases.
Fluctuations in humidity rates generally found in industrial environments do not
significantly affect the LEL of a gas.
Effect on the LEL of a gas
Formula
CH
4
C
H
4
10
C
H
3
8
Effect on the explosion risk
-
100% LEL
4.4 % v/v
1.4 % v/v
1.7 % v/v
-
|
5
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