Application Data Sheet
Gas Sensor Placement Guidelines
Selecting the location for placement of gas sensors involves several different factors. You must first evaluate the gas risk in the facility. The risk assessment process is critical in both identifying the potential consequences of a gas leak to personnel, facilities and the environment and establishing the liability to the firm of a significant leak. Based upon the risk assessment you can decide what sensors are required. Second, you must evaluate the physical location to determine the number of sensors required and their placement.
Qualification:
The guidelines presented below are for reference only. They are not exhaustive and should not be substituted for a professional analysis of the gas risk
problem. These guidelines do not address related subjects such as a comprehensive hazard assessment, data logging, system operation, system alarm response and alarm procedures at the facility.
A. Use engineering judgment
There are no complete and definitive regulations or guidelines published by ISA, NFPA, UL, FM or other agencies that tell users where or how many gas sensors to use. 1 Each gas leak possibility must be evaluated as a unique problem to assess the risk to people and property. The object of monitoring system design is to reduce the risk to people and property by responding to the gas leak.
Note 1: NFPA has published NFPA 73E Standard on Automatic Fire Detectors, Appendix A, which suggest that a diffusion sensor used for smoke detection be placed every 900 square feet of ceiling space.
B.Consider
∙Probability of gas leaks (i.e. equipment or material failure,
operating error, changes in composition of materials used, etc.)
∙Quantity of gas that could be present
∙Environmental conditions (i.e. wind speed, wind direction, blowers/fans in room, temperature, etc.
∙Dilution rates and convection currents (as affected by natural diffusion, forced air ventilation procedure, etc.)
∙Density of gas (heavier or lighter than air
– see Chart 1)
∙Interfering gases
Chart 1: Gas Weight in Relation to Air for Typical Gases
Ammonia | Lighter |
Butane | Heavier |
Carbon Dioxide | Heavier |
Carbon Monoxide | Slightly Lighter |
Methane | Lighter |
Chlorine | Heavier |
Ethane | Slightly Heavier |
Ethylene | Slightly Lighter |
Heptane | Heavier |
Hydrogen | Lighter |
Hydrogen Chloride | Heavier |
Hydrogen Cyanide | Lighter |
Hydrogen Sulfide | Heavier |
Methyl Alcohol | Heavier |
Nitric Oxide | Slightly Heavier |
Nitrogen Dioxide | Heavier |
Pentane | Heavier |
Propane | Heavier |
Sulfur Dioxide | Heavier |
Toluene | Heavier |
Sierra Monitor Corporation 1991 Tarob Court Milpitas, California 95035 (408)
