Natural gas in its natural state will not have a readily identifiable odor. To increase the detectability of natural gas, an odorant blend is added, which contains t-butyl mercaptan, triophane, or some other mercaptans to give natural gas an identifiable odor. This odorant is added at the local distribution station (natural gas in long-distance transmission pipelines usually is not odorized) where it is hoped that escaping gas will be detected before its' concentration in air reaches the combustible range (LEL is 3.9% - 4.5% and the UEL is 14.5% -15%). Enough odorant is added too make it easily recognized at concentrations of 0.5% gas in air. The stronger the odor the higher the gas concentration, however, extended periods of time being exposed to the odor will have a tendency to fatigue the sense of smell. Also, some of the odor may be removed if the gas is being past through the soil. Note: the intensity of the odor should not be used to estimate the concentration of gas. With any natural gas leak, assume the situation to be dangerous, start the evacuation and ventilation process and call for the utility company.
If your fire department has the means to monitor the concentration levels this should be started as soon as the first unit arrives on location. Natural gas will rise within structures and may use routes such as natural openings to reach the highest points within the structure. Sometimes, natural gas will build up within concealed spaces (walls) and continue to build up until an ignition source is encountered.
Sometimes leaks occur in the pipe under the ground and are referred to as "fugitive gas". Because the ground around the pipe is more porous than and not as dense as the rest of the ground in the area, natural gas leaks usually will permeate the soil, migrate upward and dissipate harmlessly into the air. However, if the ground is obstructed in any way, such as with pavement or covered with heavy snow or even frozen, this fugitive gas will start to migrate laterally until it fines a channel to follow. This channel can be a sewer line, electrical or telephone conduit, or even the gas line. The natural gas will follow this path directly to a basement wall. This movement of the fugitive gas can extend for hundreds of feet before entering a structure. If well approaching a structure, your monitor should activate the alarm it's telling you to stay out and start some form of ventilation from the outside. Remember vent high.
Unlike most fires, which should be extinguished as soon as possible, the general rule for natural gas is to allow the gas to burn and protect all exposure until the supply can be shut off. If the shut off valve is readily available, but is being impinged on by the fire, it would then be the proper practice to extinguish the gas flame to permit reaching and operating the shut off valve. Gas flames are best extinguished by separating the flame from the source of fuel with a dry chemical or some other method. If the valve is unable to be closed off and natural gas continues to flow, it is likely to form an explosive mixture with the air and possibly reach some source of ignition; the explosion which follows will probable cause much more damage than the original gas flame if it had been allowed to burn.
The use of water as a cooling agent is preferred for extinguishing smoldering embers, however, if water is used, avoid directing solid streams at the escaping gas. Injecting water into the gas line would create an additional hazard and make repairs slower and harder for the utility company.
What if there is no fire only escaping gas (outside)? Many times an excavator will accidentally dig up a natural gas line. A safe area surrounding the location should be cleared and barricaded. Have someone assigned to securing any and all open flames and prohibit smoking in the area as well as checking all surrounding buildings, particularly in basements for the odor or presence of natural gas. If need be have the traffic in the area rerouted.
How should we handle escaping gas which is burning within a structure? First determine if the gas supply line can be shut off at the stopcock ahead of the meter. If this is not possible, keep surrounding combustibles wet with a fog stream until the utility company can respond and shut off the supply. Should the gas flame be extinguished accidentally, immediate relighting is preferable and frequently, material burning in the vicinity of the escaping gas will relight the gas. This will be an extremely hazardous tactic and should be avoided if at all possible! Once a stopcock valve has been shut by the fire department it remains shut and only the utility company is to reopen it.