Sierra Monitor Corporation TR-002 - Manual

Technical Reprint TR-002
Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: 408-262-9042
Visit our Web Site at: http://www.sierramonitor.com E-Mail:
[email protected]
Rev. A1
100%
Too rich
for combustion
Upper Explosive
Limit (U.E.L.)
CON
C
ENT
RATI
O
N
(% g
as in
the a
ir)
Will support
combustion
Lower Explosive
Limit (L.E.L.)
0%
Too lean
for combustion
Fig. 1 – Explosive Limits
Gas Risk Management – A Safer Approach to
Monitoring for Hazardous Gases
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