BY BRIAN L. PARSLEY, CSP, CFPS, and GREGORY F. SCHWAB
Natural gas (NG) is one of the dominant energy sources in North America. Sixty million U.S. homes use NG for home and water heating, cooking, outdoor lighting, clothes drying, and air-conditioning. NG is also extensively used in commercial, industrial, and power-generation processes. According to the American Gas Association, a 1.3 million-mile network of underground pipe transports NG to more than 175 million U.S. customers.
The U.S. Department of Transportation (DOT) characterizes natural gas and petroleum liquids pipelines as the safest method of transporting energy. Even with the gas industry’s excellent safety record, however, you likely will be called to respond to a natural gas emergency.
Emergencies may be the result of damage to or the failure of the NG pipeline system or human error in the operation or installation of NG-fueled appliances or equipment or may be an indirect result of another emergency such as a structure fire or motor vehicle crash.
Regardless of the cause, you can apply similar strategies and tactics to all NG emergencies to ensure a safe conclusion. In addition to discussing the key strategies and tactics, this article includes a summary of NG chemical properties, an overview of the “wellhead to burner tip” NG pipeline system, and a discussion of NG-powered vehicle safety. This information, along with a cooperative working partnership with your local gas utility companies, will enable you to safely resolve natural gas emergencies.
CHEMICAL PROPERTIES OF NATURAL GAS
Because it is the result of the natural decay of organic matter, NG is not a single substance but rather is made up of a number of hydrocarbon gases. Methane (CH4) is the predominant gas, making up 90 to 95 percent of the total volume. Other flammable gases such as ethane, propane, and butane and nonflammable gases such as nitrogen and carbon dioxide account for the remaining volume.
NG does not contain carbon monoxide (CO), which is a by-product of incomplete combustion that can result from improper venting of NG appliances or malfunctioning and improperly adjusted burners and equipment. A common myth is that gas utility companies inject air into the pipelines to “thin” the gas to save money. That would create an extremely dangerous condition for both the consumer and gas company employees. Utility companies take special precautions to ensure that gas and air are mixed only at the point of combustion immediately before ignition.
During prolonged or unusually cold weather, gas utilities may inject propane into the pipeline to supplement the supply of natural gas. This is known as “peak shaving.” To compensate for differing chemical and physical properties, such as Btu content and specific gravity, it is necessary to mix air with propane prior to injection into the pipeline to create a mixture that simulates natural gas burning characteristics at the burner tip. The resulting mixture in the pipeline is approximately 50 percent natural gas, 30 percent propane, and 20 percent air-the mixture is not within its flammable range. Even in the most quiescent environments, the NG-propane-air mixture remains homogeneous and possesses chemical and physical properties similar to natural gas, including its tendency to rise and dissipate.
Because it may be odorless in its natural state, federal and state laws often require the addition of a small amount of odorant to NG. Utility companies evaporate a hydrocarbon liquid compound (frequently from the “mercaptan” family) into the gas stream to give it the characteristic “skunk-like” smell. Only a trace amount of odorant is necessary to create this warning property, and the presence of this substance does not change the chemical properties or hazards of NG. A fire or spill at an odorizing facility where liquid odorant is stored, however, will require emergency strategies that this article does not address.
Other characteristics of NG that may be important to emergency responders include the following (see Figure 1): NG is nontoxic but will displace oxygen, making NG a simple asphyxiant. NG is lighter than air and will rise and harmlessly dissipate unless contained in a structure or when natural air currents are affected by strong downdrafts or thermal inversions. The lower explosive limit (LEL) of NG is 4 percent gas in air; the upper explosive limit (UEL) is 17 percent. The DOT classifies “Natural Gas, Compressed UN1971” as a Division 2.1 hazardous material (flammable gas). Natural gas is stable and will not readily react with most other chemicals; however, like all fuels, NG will become more readily ignitable and will burn with more intensity in the presence of strong oxidizers (oxygen, bromine and fluorine, and their compounds, for example).
FROM THE WELLHEAD TO THE BURNER TIP
Natural gas makes its journey from the wellhead to the burner tip through a network of pipelines, compressors, conditioners, meters, regulators, and valves. The NG system has three major segments: production, transmission, and distribution. As a result of this industry structure, you may need to develop partnerships with more than one utility company in your area.
“Production” refers to the effort of drilling and extracting NG from the rock formations where it formed and was trapped. Ninety-nine percent of the NG used in the United States is from North America. Most domestic production is located in the Gulf States, primarily in Texas and offshore in the same region. As one can imagine, the Gulf States region is not the primary NG market.
Using a system of large, high-pressure, long-haul pipelines, the “transmission” segment transports gas from production areas to market areas. Transmission pipelines are typically between eight and 36 inches in diameter and commonly operate at pressures exceeding 500 psi. Compressor stations along transmission pipelines facilitate the movement of gas and maintain adequate delivery pressures. It takes approximately two days for gas produced in the Gulf States region to move to market in the Northeast along this system.
Transmission companies deliver gas to local distribution companies at town border stations (TBS). The TBS have two functions: (1) to measure the amount of gas being put into the local system, and (2) to cut the gas pressure to a level safe for distribution pipelines.
Although rare, emergencies involving transmission pipelines are the most catastrophic and most dangerous because of the high operating pressures and large volumes of escaping gas. Failures and “dig-ins” on transmission pipelines will almost always result in the escaping gas’ excavating a large crater at the point of damage. If the escaping gas ignites during the resulting pressure-release explosion, the heat release will be tremendous, creating life-safety and fire-exposure problems.
NG use is not constant throughout the year. To compensate for seasonal demand fluctuations and to enable the system to operate with smaller transmission pipelines, transmission companies have devised a system to continuously transport NG throughout the year at an average flow rate and store gas in rock formations closer to the markets. Known as storage wells, these geological formations are depleted production wells. Transmission companies pump NG into storage wells in the summer months, in effect creating a local reserve close to the consumer, and release gas from the wells in winter months to supplement the normal, steady flow from production.
Storage fields are not large underground caverns but instead are porous rock formations, such as sandstone, hundreds of feet below the surface. Porous rock allows the gas to travel through and be stored in the formation similar to the way water is stored in a bucket of sand. Nonporous geological formations define the perimeter of the storage field and contain the gas. Storage fields may cover thousands of acres. NG storage wells create no unusual risks and few use restrictions for the surface land other than in areas close to wellheads and in pipeline right-of-ways.
Emergencies related to storage wells are similar to those associated with the transmission segment, except that the operating pressures on the well field pipelines can be much higher. At the peak of storage, operating pressures in storage fields can exceed 2,000 psi.
The final segment of the NG system is “distribution.” This is the segment most familiar to the end-user of NG. Distribution companies operate in local areas and focus on the needs of the consumer. Distribution systems begin at TBS and operate at pressures from 150 psi down to a few ounces at the point of delivery to a residential customer. A system of regulators ensures that NG is safely transported within the pressure limits of the pipeline and delivered at a usable pressure to the customer.
Regulators are located along the distribution pipeline network in fenced lots, small outbuildings, or underground vaults or pits. Underground facilities may be equipped with a ventilation stack for gas that may be released as part of the regulator’s normal operation. It is not uncommon to have a slight odor of NG in the area of a regulator installation. Ask your local utility representative what is considered “normal.”
Although very rare, there have been cases of regulator failure resulting in NG being delivered to a customer, appliance, or process at higher than normal pressures. In addition to the common incident management strategies discussed later, the best strategy for overpressure emergencies is to shut off the flow of gas to each customer at the respective meter.
Small regulators may also be found immediately before a meter. Meters are located at a point of delivery to measure the volume of gas being used by a customer.
Emergencies in the distribution segment can occur from damage to pipelines by excavating equipment, improper handling of NG at the point of use, damage to system components from motor vehicle crashes, structure fires and collapses, improperly installed piping, and malfunctioning NG-fueled equipment and appliances.
COMMON OPERATIONAL AND SIZE-UP CONSIDERATIONS
Establishing a working partnership with your local gas utility in advance of an NG incident is perhaps the best emergency management strategy the fire service can employ. A working partnership is characterized by the following: (1) establishing protocols for agency-company interaction at various types of NG incidents; (2) communicating key contact telephone numbers; (3) seeking technical support when developing standard operating procedures and response guidelines; (4) establishing protocols for the fire department’s role in operating street and curb valves; (5) collaborating on preincident planning at large commercial and industrial facilities; and (6) participating in joint training exercises.
The fundamental operational priorities of companies responding to NG emergencies are to make every reasonable attempt to effect rescues, evacuate at-risk occupants, properly shield evacuees and bystanders, eliminate ignition sources, and ventilate the structure. The level of attention and resources that each of these priorities may demand must be assessed based on a thorough size-up. Cues to the severity and life-safety risks of an NG incident may come from (1) details available from the dispatcher, (2) preincident planning data, (3) an obvious odor of NG in the area, (4) an obvious sound of escaping gas, (5) reports from utility company or law enforcement personnel on the scene, and (6) the behavior of and comments from occupants and bystanders.
Also during the initial size-up, an attempt should be made to identify the point closest to the leak at which the flow of NG can be shut off. Remember that it is always okay to shut off the gas at the meter. Even if you believe that you may have made a mistake, never reopen a valve that you have closed. Valves must only be reopened when the building or area has been made safe and the leak has been repaired or isolated from the rest of the system, and then only by the gas company or a qualified, authorized contractor.
ODOR OF NG IN A BUILDING
Fire companies often overlook the fire and explosion potential of NG accumulation in structures. Therefore, a full structure assignment should be the standard response to emergencies involving the odor of NG.
The first-arriving company should stop short of the dispatch location. Doing so will limit the possibility of the vehicle’s becoming an ignition source. If the truck must be positioned closer to the structure, select a location that is in line with the corners of the structure and not in the collapse zones of walls. This will help to protect the apparatus and responders from flying debris should an explosion occur. The first-arriving company’s establishing incident command, identifying and communicating a safe staging location for incoming companies, and conducting a scene size-up will set the stage for an orderly, safe resolution to the incident.
To perform a thorough size-up of building conditions, incident command can use a two-firefighter reconnaissance team to investigate the structure or site. Until the extent of the emergency is assessed and safe conditions verified, recon team members must wear full protective clothing, be breathing air from an SCBA, and have their PASS devices in the “on” position. Additional equipment for the recon team includes a direct-reading atmospheric monitor (DRAM) (combustible gas detector/indicator), portable radios, spanner wrenches (with gas meter key slots) or a halligan tool, and intrinsically safe (or explosionproof) flashlights. Unless portable radios are intrinsically safe, they should be left in the “off” position. Turn on flashlights before approaching the scene, and leave them on.
Prior to the recon team’s approach, a backup team consisting of at least two firefighters with full protective clothing, SCBA, and a charged hoseline should be readied. The backup team should position itself where it has good visibility of the recon team and it is shielded from potential flying debris.
The recon team’s safest method for approaching a suspect structure is at a 45-degree angle to the walls-in other words, in line with the corners of the building, similar to the consideration given to truck placement earlier. Prior to entering a suspect structure, the team can make an initial atmospheric assessment using the DRAM through a partially opened window or door.
On entering the structure, the recon team needs to make a more thorough assessment of gas concentrations (see “Effective Use of Direct-Reading Atmospheric Monitors” on page 74), evacuate at-risk occupants; identify and, if possible, eliminate potential ignition sources; and give an initial size-up report to the incident commander.
The recon team must advise occupants against operating light switches, thermostats, and circuit breakers and unplugging appliances. All of the actions are potential ignition sources. The recon team must work quickly to complete a primary search, ensuring that all at-risk occupants are safely evacuated.
If atmospheric monitoring confirms that the NG concentration is near or above the LEL, the following operations need to begin:
- Shut off the gas supply to the structure as close to the leak as possible. Attempt to shut off leaking gas inside the building only if it can be done without unreasonable risk to responders and civilians. An example of an unreasonable risk would be accessing an interior gas meter or valve in a room, basement, or crawl space where atmospheric monitoring indicates the presence of NG concentrations above the LEL and ignition sources have not been completely eliminated or controlled.
It is much safer to shut off gas to a building at an outside valve. As previously stated, it is always okay to shut off gas at the meter. Meters serve a single customer or building, which is often easily identifiable by the location of or markings on the meter.
The safest way to shut off the gas supply is to close a curb or street valve-a task that must only be done by the local gas utility company unless a protocol has been established between the local gas utility company and the fire department. Street or curb valves are frequently difficult to locate and operate. The location of the valve may be misleading, and the valve may not control gas to the structure it appears to service. Also, valves in streets and along curbs may control gas in pipelines that serve several, a hundred, or perhaps thousands of customers. As noted: Never reopen a valve of any kind that you have closed, even if you believe that you have made a mistake. Contact the electric company to shut off power to the structure from the outside to further eliminate ignition sources.
After the source of gas has been shut off and ignition sources have been controlled, it is safe to begin ventilating the structure. Open windows and doors (including interior doors) throughout the structure. It is important to open all interior building compartments such as closets, small storage rooms, cabinets, and cupboards that can trap concentrated gas. When using fans to assist with ventilation, use positive pressure ventilation (PPV) techniques, starting fans away from the structure and moving them into position while running. Place PPV fans outside the structure and low in doors and windows.
PPV has the potential to encourage gas migration into building voids and compartments. To minimize the associated risks, monitor these spaces to ensure safe conditions before returning building control to the occupants. Even with this potential, PPV is the preferred ventilation technique because it works with NG’s natural tendency to rise and dissipate.
LEAKING NG AT RESIDENTIAL OR COMMERCIAL METERS
A meter setting may involve several meters. Each address will usually have its own 1/4-turn shutoff valve immediately before its respective meter on that setting.
Meter sets are the means by which the utility company regulates the delivery pressure and measures the flow of natural gas to the customer. A typical residential or small commercial meter set consists of a 1/4-turn valve, a meter, a small regulator, and the associated piping (see photo above). A regulator may not be present on low-pressure pipeline systems. In nearly all cases, the meter set is the last point of control owned by the utility company in the “wellhead to burner tip” system.
Meter sets may be located inside or outside the structure. Outside meter sets may be adjacent to or remote from a building. It is not uncommon for large commercial and industrial NG customers to have meter sets located at or near a property line in dedicated buildings or fenced areas. Meter sets may be located on any side of a customer’s property or building, not necessarily on the same side or near pipeline right-of-ways. Keep in mind that not all meters are well-identified in a meter set. Preincident planning will provide you with meter locations and will pay off during an emergency.
For emergency calls involving leaking or burning NG at a customer’s meter set, basic incident management goals remain the same. The following tactics for handling meter-related emergencies may not apply to large industrial customers that have very high delivery pressures and volumes. Preincident planning with your utility company is important for successfully resolving emergencies at such facilities.
The following also describes emergencies that are still primarily related to meter settings and have not propagated into full structure fires:
- If NG is leaking or burning at a meter, establish charged handlines. Determining the “location” of the leak during incident size-up is important to properly and safely handling the emergency. If this cannot be done from a safe distance using binoculars, firefighters-under the protection of a water fog from handlines and wearing full protective clothing, including breathing air from SCBA-may be able to safely approach the meter set to more closely assess the damage. If the escaping gas is burning, do not extinguish the fire with the fog spray. Burning gas escaping from pipelines will not explode; allow burning NG to burn, and protect the exposures.
If the leak or damage is “after” or on the “consumer side” of the 1/4-turn valve, shutting off the valve will usually resolve the emergency. It will still be necessary to thoroughly assess gas concentrations in affected buildings before relinquishing control of the scene to the owner or occupant.
If the gas is burning, close the valve under the protection of water fog, again being careful not to extinguish the fire. This action will stop the flow of gas and extinguish the gas fire by eliminating the fuel source. Once the residual gas in the piping is burned away, extinguish any remaining structure, vehicle, or brush fire.
If the leak is “before” or on the “supply side” of the valve, protect exposures, wait for the utility company to shut off the gas supply at the street, and then proceed to extinguish surrounding fires.
DAMAGE TO NATURAL GAS DISTRIBUTION PIPELINES
Goals and tactics for dig-ins or damage to NG pipelines again remain very much the same as other types of NG incidents. Be mindful, however, that it may be necessary to place first-arriving and staging companies farther from the incident. Because of the potential volume of escaping gas when a pipeline is damaged, it is critical to identify a safe location upwind and at a safe distance from the leak.
Inform the utility company of the exact location/address of the escaping gas, if identifiable, and the best upwind approach or access for its responding crews and equipment. Secure the scene from access, and identify a safe refuge area upwind and away from the incident area for a command post and for evacuees and bystanders.
If leaking gas is not burning, eliminate ignition sources, and monitor gas concentrations in nearby structures starting with buildings located in the immediate area and downwind. If unsafe gas levels are detected in any structure, handle the emergency using the guidelines provided above for the odor of NG in buildings. If the escaping gas is burning, do not extinguish the fire except when absolutely necessary for viable rescues or at the request of the utility company. NG fires are most effectively extinguished using a B-C- or A-B-C- rated dry chemical agent.
In either case, secure a water supply, and lay handlines as necessary to effect rescues and protect exposures. It is not necessary for emergency responders to flow water into an excavation where NG is escaping from a damaged pipeline. Several problems that make resolving the incident more difficult can result from this good-intentioned act:
- The air currents created by the water stream can slow the safe, natural dissipation of NG up and into the atmosphere.
- Water in an open excavation can destabilize trench walls or loosen shoring equipment, resulting in a trench collapse that may require re-excavating the site to resolve the emergency.
- Water in excavations can seep into pipelines and flow away from the incident area. Water infiltration can block the gas flow through the pipeline system, causing outages weeks, days, or even months later.
- Unstable trench walls, mud, and accumulating water (and in cold weather, ice) will greatly slow the efforts of utility workers as they attempt to stop the leaking gas and repair the damaged pipeline. This delay could result in extended gas outages to customers, which, of course, would be critical during cold weather.
DIG-IN OR FAILURE OF AN NG TRANSMISSION PIPELINE
NG transmission lines are high-volume, high-pressure delivery systems. NG fires on transmission lines are extremely large, hot fires that release large amounts of energy in the form of radiant heat. These incidents by their nature are always defensive operations and may last many hours, or perhaps days. The goals of managing emergencies on transmission pipelines start with the basics: establish command and a safe staging area, effect viable rescues, eliminate ignition sources (if gas has not ignited), and protect exposures. In populated areas, anticipate exposure fires; in rural areas, anticipate wildfires.
The damage to the immediate area, including structures, can be severe even when no ignition of escaping gas occurs. If access to these exposures can be made without unreasonable risk to firefighters, conduct a primary search and attempt to make these structures safe by eliminating ignition sources and the possibility of secondary emergencies such as “food on the stove” fires left by evacuating occupants.
Do not expect to extinguish fires on transmission pipelines. The only safe way of extinguishing such a fire is to remotely eliminate the fuel source by isolating the damaged section of the pipeline-a task that only the pipeline operator can perform. However, it remains important to secure a water supply and stretch handlines where appropriate to effect viable rescues and protect savable exposures.
Do not enter the immediate fire area until the pipeline operator has indicated that it is safe to do so. Most pipelines today operate with automatic valves that open when timers and pressure sensors on the line indicate the need. The large loss in pressure that results from a pipeline failure may be read by automatic systems as an increased demand, causing valves to operate, sending a rush of gas toward the breach in the line, resulting in a violent increase in the size of the fire.
COMPRESSED NATURAL GAS VEHICLE EMERGENCIES
A product to meet society’s search for cleaner fueled cars and trucks has arrived, the compressed natural gas (CNG) vehicle. Many fleet vehicles for governments, public transportation, universities, and gas utility companies are fueled by CNG.
CNG cars and trucks display on the rear of the vehicle a royal blue, reflective, diamond-shaped label with the letters “CNG” in white. Some vehicles are dual fueled (natural gas and gasoline) and also display this identifier. CNG vehicles do not display the DOT hazardous materials warning label. DOT labels, and sometimes placards, are required only for NG carried on the vehicle as cargo and not for the quantity carried as part of the fuel system.
CNG is stored in a high-pressure (usually 3,000 to 3,600 psi) cylinder located in the trunk or on the underside of the vehicle. Single-fuel and larger vehicles, such as transit buses, may have several cylinders manifolded together for greater capacity. All of these fuel assemblies are designed and tested to pass government crash and safety tests.
Some of the safety features included on CNG vehicle fuel systems are cylinder overpressure disks, individual cylinder shutoff valves, and an emergency 1/4-turn fuel line shutoff valve-clearly labeled and usually located near the driver’s rocker panel-to stop the flow of gas toward the engine compartment.
In the case of a vehicle fire, a CNG-fueled vehicle has all of the same fire hazards as other vehicles (high Btu plastics, foams, synthetic fabrics, and rubber) with the addition of the high-pressure cylinder filled with flammable gas. During a vehicle fire, CNG cylinders will most likely be exposed to fire completely enveloping them. CNG is not liquefied natural gas (LNG); there is no vapor-liquid line below which the cylinder is less vulnerable to weakening by heat. With CNG, the entire cylinder may be vulnerable. Overpressure disks are designed to fail and release increasing pressure before the cylinder walls fail.
CNG vehicles, however, have an important safety feature. If the system leaks or an overpressure disk operates, the system will empty quickly. And unlike gasoline, escaping NG will most likely safely dissipate up and away from the emergency scene before first-due companies are on the scene. It will not puddle under the vehicle.
Remembering and adhering to the following summary of facts and guidelines will help to ensure firefighter and civilian safety in emergencies involving natural gas:
- NG is nontoxic but displaces oxygen and is, therefore, considered to be a simple asphyxiant.
- The most effective way to extinguish NG fires is to eliminate the fuel source. This can be done at the meter or by having the utility company operate a street valve. Remember that it is always okay to shut off the gas at the meter. Never reopen a valve you have closed, even when you believe that you have made a mistake.
- The key strategies that apply to all natural gas emergencies are
- establish command and a safe staging area,
- secure the scene,
- evacuate at-risk occupants and bystanders,
- effect viable rescues,
- eliminate ignition sources, and
- notify the local utility company.
- The best overall strategy, however, includes developing a working partnership with your local gas utility companies and conducting preincident planning sessions at large NG customers in your response area.
BRIAN L. PARSLEY, CSP, CFPS, is the safety and health compliance manager for Columbia Gas of Ohio & Kentucky. He has more than 18 years of fire service and occupational safety experience. Parsley is a professional member of the American Society of Safety Engineers and a member of the American Gas Association’s Safety & Occupational Health Operating Committee. He has a BS in fire protection administration from Eastern Kentucky University and an AAS in fire prevention from Columbus (OH) State Community College. The views expressed in this article should not be taken as representing the position of either Columbia Energy Group, including its operating companies, or the American Gas Association.
GREGORY F. SCHWAB is a captain with the City of Grandview Heights Division of Fire, adjacent to Columbus, Ohio. In his 15 years of fire service experience, he has served as a platoon commander, training officer, safety officer, hazardous materials officer, and fire marshal. He is a fire science instructor at Columbus State Community College and has taught for the Ohio Fire Academy. He has a BS in fire protection administration from Eastern Kentucky University.