As a service provider to the general public, fire departments are called upon to investigate gas odors these days. Most of the time these incidents can be classified as minor in nature and very little effort on the fire department's behalf will be required. Other times, however, we can find the situation to be more involved and require a great deal more of our resources.
As a service provider to the general public, fire departments are called upon to investigate gas odors these days. These odors more than likely come from a natural gas appliance or the pipelines supplying the appliance. Most of the time these incidents can be classified as minor in nature and very little effort on the fire department's behalf will be required. It can be as simple as the pilot light on an appliance being out. Generally, the local utility company will have a service representative respond and check the service as well as relight the pilot. Other times, however, we can find the situation to be more involved and require a great deal more of our resources.
Transmission Pipelines are used to convey natural gas from storage or production facilities to the local utility company. The pressure within the Transmission Pipeline has the potential of reaching 1200psi in most cases. Once the utility company receives the natural gas for distribution, the pressure will be lowered to accommodate its intended use. Under normal usage pressures seldom exceed 150 psi for high pressure distribution and generally are more around the 60 psi range, found in residential settings. A grid system is used by the utility company for distribution of the natural gas throughout their service area. This grid system is known as the Distribution Pipeline or Main Pipeline. Off this grid system, Service Laterals or Lines, deliver the natural gas to the customer.
Newer homes and small businesses have a low pressure gas meter located on the outside of the structure; however, it is not uncommon to find gas meters inside some of the older homes and businesses. Inside meters will be equipped with a vent pipe extending outside the structure. In the event of a malfunction, the gas is vented to the outside atmosphere. Every meter, will have a shut-off valve located on the supply side which is known as a stopcock, requiring one-quarter turn to shut off the flow of gas. Some larger services employ a standard valve as a shut off. Most low pressure service lines and all high pressure services will have an underground shut off valve at the street. On high pressure installations, a regulator is installed ahead of the meter to reduce the pressure for suitable use by the customer. Firefighters should have the knowledge and skills to shut off valves at the meter, however, only the utility company should shut down the valve at the street location.
Natural gas is not poisonous as it contains no significant amounts of carbon monoxide or for that matter any other toxic gases. Therefore, we can safely say that it has no toxic effect on human beings. However, if the concentration levels of natural gas start to exceed 25% in a confines space area it will start too displace most if not all the air in that space along with the oxygen. Due to the lack in oxygen, one can suffocate.
Natural gas is naturally occurring, largely hydrocarbon gas, recovered from wells. Natural gas is mostly methane, with lesser amounts of nitrogen, ethane, propane, and traces of butane, pentane, carbon dioxide and oxygen. The mixture can range from 72% - 95% methane and 3% - 13% ethane, less than 1% - 4% propane and less than 1% - 18% nitrogen. This combination makes natural gas lighter than air; having a vapor density that ranges between 0.59 and 0.72.
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.
Not all responses for natural gas leaks and fires will occur in a structure or excavation sites some will be in manholes or vaults. These responses will require the barricading off of the area to assure the safety of everyone. Fires in manholes should not be extinguished; however, fog spray should be used to cool the immediate as is the area around the manhole or vault. If vaults are open and gas lines or valves are visible, a fine spray on equipment in the vault is helpful, but do not extinguish the flames and avoid flooding of the vault or manhole. Never enter a manhole or vault the utility company's responsibility as they know what to expect once inside. Have nearby basements checked for odors and possible seepage. Keep all forms of ignition away, including our apparatus, if there is no fire and never park our fire apparatus over the top of a manhole in the immediate vicinity as they have been known to blow from time to time.
Treat all natural gas incidents with the same respect we would other incidents; wear all the proper personal protection equipment and SCBA as other hazards can be present from time to time at these types of incidents. Incomplete combustion of gaseous fuels will elevate the carbon monoxide levels and displace oxygen in the area, creating a highly toxic environment within which to work. Asphyxiation can occur either from suffocation due to the lack of oxygen or from the toxic fumes. A broken or plugged vent pipe from an appliance or furnace can cause toxic effects. Be prepared!