New Measurement Data Has Implications For Quantifying Natural Gas Losses From Cast Iron Distribution Mains
As of May 2009, Comgas had sampled 912 segments and calculated its average leak rate to be 1.614 Liters/meter-hour (803,548 scf/mile-year) from cast iron pipelines at average operating pressure. In arriving at this average, Comgas took a conservative approach and excluded data points showing leak rates higher than 4 Liters/meter-hour (1,991,444 scf/mile-year), which represent 15.4% of the sample measurements. This approach was taken given that, in some cases, it was thought that the blocks at each end of the isolated segments were leaking or that gas may have escaped into unmapped service lines, thus creating inaccurately high leak estimates. Seeking reliable results that represent the current condition of leaks, Comgas updates the study monthly. To incorporate new data to its average leak factor, Comgas weights recent measurements over the past year.
Figure 3 illustrates the results obtained from Comgas and EPA/GRI studies. With 912 data points, Comgas provides a larger sample size and Comgas’ average leak rate is nearly double the EPA/GRI factor. To put this in context, it should be noted that, Comgas’ cast iron network is comparable to or younger than most U.S. cast iron networks. However, until 1993, Comgas’ system supplied town gas whereas U.S. lines initially delivered coal gas, later switching to natural gas. Both town gas and coal gas were wetter than the natural gas that replaced them.
Comgas Leak Mitigation Activities
During the first five years of the program, Comgas spent US$82 million to renew 250 km (155 miles) by inserting polyethylene pipes into the existing cast iron network – eliminating the equivalent of 125 MMcf per year of gas losses. Its goal is to continue replacing 30 km each year from 2005 to 2010.
Each pipeline segment is renewed over three days. The first day, trenches are dug for launching and receiving ports for the plastic inserts. During the second day, service is interrupted to customers while the inserts are launched through the segment, and the segment is brought back online by connecting the internal plastic inserts and welding the cast iron skeleton to the adjacent cast iron segments. By the third day, service is renewed and trenches are refilled. Figure 2 shows Comgas employees inserting plastic liner into a cast iron segment. Comgas evaluated several mitigation options, but ultimately decided on plastic inserts for the following advantages:
- Elimination of leaks,
- Mitigation of risks during operations,
- Minimization of excavated trenches,
- Lower cost with improved productivity,
- Facilitated renewal of service lines,
- Polyethylene pipes are protected by old cast iron pipe, with possible use of pipe locator, and
- Ability to accommodate higher operating pressures and stabilization during peak demand periods.
Other Mitigation Options - Plastic Inserts
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- Coatings, pipe joint
- Compressor components
- Contractor, pipeline
- Contractor, river crossing/ directional drilling
- Directional drilling rigs, large
- Fittings, valves: plastic
- Meters, flow
- Pigs, cleaning
- Pigs, intelligent
- Pigs, scraper/ sphere launchers/ traps
- Scada systems
- Ultrasonic inspection
- Vacuum excavators/ potholing
- Valves, ball
- Welding systems, automatic