Natural Gas Distribution Main Mileage
The 10 states with the most cast or wrought iron distribution main mileage in 2013 account for 83 percent of all such mileage. The iron distribution main mileage in these states decreased 17 percent between 2004 and 2013 .
The 10 operators or companies reporting the most cast or wrought iron main mileage in 2013 account for over 57 percent of such main mileage. Collectively, these operators reduced their iron main mileage by nearly 10 percent between 2004 and 2013 .
Natural Gas Service Line Count
The 10 states with the most cast or wrought iron service lines in 2013 account for 98.5 percent of all such services. Collectively, the number of iron service lines in these states decreased by nearly 43 percent between 2004 and 2013 .
The 10 operators or companies reporting the most cast or wrought iron service lines in 2013 account for 96 percent of all such services. Collectively, these operators decreased their cast/wrought iron service line count by 71 percent between 2004 and 2013 .
Recent Incidents Involving Cast Iron Pipelines
Even though the amount of cast iron pipelines is declining, there have been a number of recent incidents caused by cast iron gas distribution main failures, resurging attention to the risks associated with cast and wrought iron pipelines.
- January 9, 2012 – A home exploded on Payne Ave in Austin, Texas, resulting in one fatality and one injury. The leak originated at a break in a four-inch cast iron gas main installed in 1950. The cast iron main break occurred after rainfall that followed extended drought conditions.
February 9, 2011 – A tragic explosion occurred on North 13th Street in Allentown, Pa. Local emergency responders worked to limit the spread of the fire while the operator cut through reinforced concrete to access the gas main. A preliminary investigation found a crack in a 12-inch cast iron main that was installed in 1928 and was operating at less than 1 psig at the time of incident. As a result of the explosion and ensuing fire, five people lost their lives, three people required in-patient hospitalization, and eight homes were destroyed.
January 18, 2011 – An explosion and fire caused the death of one gas utility employee and injuries to several others while gas utility crews were responding to a natural gas leak in Philadelphia, Pa. A preliminary investigation found a circumferential break on a 12-inch cast iron distribution main that was installed in 1942 and was operating at 17 psig.
Incident and Consequence Analysis
PHMSA regulations require gas distribution operators to submit incident reports when a leak causes an injury or fatality, property damage in excess of $50,000, or the unintentional release of more than three million standard cubic feet of gas. Gas distribution incident reports (excluding those caused by leaks beyond the customer meter) for 2005 through 2013 show the following:
- 10.5 percent of the incidents occurring on gas distribution mains involved cast iron mains. However, only 2.5 percent of distribution mains are cast iron.
- In proportion to overall cast iron main mileage, the frequency of incidents on mains made of cast iron is more than four times that of mains made of other materials.
- 38 percent of the cast/wrought iron main incidents caused a fatality or injury, compared to only 20 percent of the incidents on other types of mains.
- 12 percent of all fatalities and 8 percent of all injuries on gas distribution facilities involved cast or wrought iron pipelines.
What Causes Iron Pipe Leaks?
The biggest threat to cast or wrought iron pipe is earth movement. If these pipelines are disturbed by digging, seasonal frost heave, or changes in ground water levels, leakage may occur.
Another serious threat called graphitization is a natural process in which iron degrades to softer elements, making iron pipelines more susceptible to cracking. The extent of graphitization depends on many factors, but gas may leak from the joints or through cracks in the pipe if graphitization has occurred.
When leaks occur on low-pressure systems with cast or wrought iron distribution lines, the volume of gas escaping through the failure point is much less than what might escape through the same size failure in a system operating at higher pressures. However, even a relatively small volume of natural gas leakage can have catastrophic consequences.
Cast and wrought iron pipelines were originally constructed to transport manufactured gas beginning in the 1870s and 1880s, with cast iron becoming more popular in the early 1900s.
In 1970 PHMSA began collecting data about gas pipelines mileage categorized by pipe material type. In 1983, gas distribution pipeline operators reported 61,536 miles of cast iron and 4,371 miles of wrought iron pipe. Operators began submitting merged data for the two beginning in 1984.
Wrought iron pipelines were joined end-to-end using either threaded or compression couplings, while cast iron pipelines were linked using bell and spigot joints with packing material stuffed in the bell to form a gas tight seal. Since these pipelines transported wet, manufactured gas, the packing material absorbed moisture and generally did not leak.
As dry, natural gas began supplanting manufactured gas use in the mid-20th century, the packing material sealing the joints dried out, causing leaks. A variety of clamping and encapsulation techniques have been implemented over the years to repair the joints.
Distribution Integrity Management Programs
In late 2009, PHMSA implemented pipeline safety regulations for managing the integrity of gas distribution pipelines. Operators were required to create and implement Distribution Integrity Management Programs (DIMP) by August 2011. Operators are required to know the specific characteristics of their system and operating environment to identify threats, evaluate the risk, and take measures to reduce the risk.
Specifically for cast/wrought iron, operators must have knowledge of the specific characteristics of the pipe and environments where graphitization could be severe. Evaluating past leak history and monitoring cast/wrought iron pipe during excavations are also key components of maintaining integrity.
National Transportation Safety Board Recommendations
The National Transportation Safety Board is an independent federal agency that conducts investigations to determine the probable causes of transportation accidents.
In 1986, the NTSB investigated an explosion at a restaurant in Derby, Conn., that killed six people and injured 12. The NTSB issued recommendations for corrective action only to the pipeline operator. In 1990, a natural gas explosion and fire killed one person, injured nine, destroyed two homes, and damaged two adjacent houses in Allentown, Pa. The NTSB report found that a water main leak eroded support under a 4-inch cast iron gas main. This ground disturbance results in a circumferential crack in the gas main. Natural gas migrated through the soil and into the basement of one of the homes where it ignited, exploded, and burned. The cast iron gas main was significantly weakened by graphitization.
In 1991, the NTSB recommended that PHMSA – then called the Research and Special Projects Agency – require pipeline operators to implement a program to identify and replace cast iron pipelines that may threaten public safety. PHMSA issued two Advisory Bulletins related to cast iron replacement programs.
RSPA Alert Notice 91-02 Encourages operators to develop procedures to identify segments of cast iron pipe that may need replacement. Reminds operators that pipeline safety regulations require generally graphitized cast iron pipe to be replaced and protect excavated cast iron pipe from damage.
RSPA Alert Notice 92-02 Reminds operators that pipeline safety regulations require operators to have a procedure for continuing surveillance of pipeline facilities to identify problems and take appropriate action concerning failures, leakage history, corrosion, and other unusual operating and maintenance conditions. This procedure should also include surveillance of cast iron to identify problems and take appropriate action concerning graphitization.