Sierra Monitor Corporation TR-002 manual Hazards of Gas, Concentration % gas in the air

Technical Reprint TR-002

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.

100%		Too rich
		for combustion
CONCENTRATION (% gas in the air)			Upper Explosive
			Limit (U.E.L.)
		Will support
		combustion	Lower Explosive
			Limit (L.E.L.)
		Too lean
0%		for combustion
	Fig. 1 – Explosive Limits

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=		C1T1 + C2T2 + C1T1+ ………………CnTn
			8
where	Ci=	Concentration in period I where concentration
	Ti=	remains constant
		Period of duration in hours at concentration Ci

Sierra Monitor Corp. 1991 Tarob Ct., Milpitas, California 95035 USA 408-262-6611, 800-727-4377 FAX: 408-262-9042

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