Справочник Пользователя для Sierra Monitor Corporation TR-002

 

 

 

 

100% 

 

 

Too rich  
for combustion 

 

 

Upper Explosive 
Limit (U.E.L.) 
 

 

 

Will support 
combustion 

Lower Explosive 
Limit (L.E.L.) 

0% 

 

Too lean  
for combustion 

 
Monitoring for gas leaks has historically consisted of the use of dosimeters, or fixed detectors.  Little has been done to 
use these measurement methods within a system’s concept; and, collectively, these technologies do not encompass all 
the ingredients required to comprehensively manage the risk of personnel or equipment exposure to gas leaks. 
 
Essential to the goal of protecting people and facilities from the hazards of exposure to gas are the selection of the 
appropriate gas measurement techniques, timely analysis of monitoring data and a plan to respond to a leak.  This 
article discusses issues and options to be considered in formulating a gas risk management program with a focus on 
area monitoring and system capability. 

 
Hazards of Gas 
 

The hazards of gas exposure are generally categorized as combustible, toxic, or the special category of oxygen 
deficiency.  These hazards are typically found in-plant at the source, at landfills, and in incinerators. 
 
Combustible Gas.  In industrial facilities, methane, natural gas and Hydrogen are the combustible gases of primary 
concern.  Methane and natural gas are used in the facility for fuel and can be present due to pipeline leaks, or poor 
maintenance.  Solvents, propane, and other combustible gases may also be present and require monitoring. 
 
The lowest concentration at which a gas will support combustion is called the Lower Explosive Limit (LEL).  Below this 
concentration, the gas is too “lean” to support combustion.  There is a corresponding Upper Explosive Limit above 
which the concentration is too “rich” to support combustion, Figure 1.  It is important that the concentration of gas in a 
facility remains below the LEL and that appropriate action is executed to insure the LEL is not reached. 
 
Although the concentration at which different gases will combust varies widely, the Lower Explosive Limit is used as a 
common reference in setting alarm levels of gas monitoring systems.  The approach employs alarm levels that are 
determined by how close the gas concentration is to the LEL and not an arbitrary absolute concentration of the gas.  In 
this fashion, all gases can be compared to their specific explosive limit and relative comparisons of risk can be made.  
For example, the LEL of methane and Hydrogen are approximately 5.0 and 4.0 percent, respectively, of the gas in air.  
A safe reference such as 20 percent of the LEL can be used as an alert to the danger of the gas leak (which 
corresponds to 1.0 and 0.8 percent of methane and Hydrogen, respectively, in air).  Figures 2 and 3 demonstrate this 
approach. 
 
Toxic Gases.  Toxic gases create both an immediate and long-term risk to personnel and include such gases as 
Carbon Monoxide, Chlorine, Nitric Oxide, Sulfur Dioxide, Hydrogen Chloride, Hydrogen Cyanide, Ammonia, Hydrogen 
Fluoride and many others. 

 
Toxic gases are often hazardous at low concentrations and are 
usually characterized in terms of the Threshold Limit Value (TLV).  
TLVs are the maximum 8-hour time-weighted average 
concentration permitted of an airborne contaminant.  The time 
weighted average (TWA) is calculated as follows: 
 
 

 

 

 

TWA= C

1

T

1

 + C

2

T

2

 + C

1

T

1

+ ………………C

n

T

n 

 8 

where C

i

= 

Concentration in period I where concentration 
remains constant 

 

T

i

= 

Period of duration in hours at concentration C

i