New Measurement Data Has Implications For Quantifying Natural Gas Losses From Cast Iron Distribution Mains

By Carey Bylin, Luigi Cassab, Adilson Cazarini, Danilo Ori, Don Robinson and Doug Sechler | September 2009 Vol. 236 No. 9

Comgas is an international partner in the U.S. EPA’s Natural Gas STAR Program, a voluntary partnership between the EPA and the oil and natural gas industry, designed to promote implementation of cost-effective methane emission reduction activities. Partner companies report methane mitigation activities to the EPA in order to promote technology transfer and capacity building throughout the industry. In addition to replacing networks with plastic pipeline, partners have reported inserting plastic liners. Thin-walled plastic liners take advantage of structural support offered by the cast iron parent pipe and provide the low leakage factors of plastic piping. Plastic liners can be pulled through long lengths of buried piping and bonded at joints to minimize leakage. Partners have reported on three lining methods: Starline, Rolldown, and Subline.

Since 1991, the Starline method has rehabilitated more than 250 miles of 4 to 24 inch distribution mains. This “cured in place” liner consists of a polyurethane adhesive mix outer layer enveloping a polyester woven liner and polyurethane internal coating. The pipeline system is first grit-blasted to clean it and create an adequate bonding surface. Adhesive is mixed and applied to the liner above ground, after which the inverted liner is propelled through the pipeline by compressed air or water so that as the liner unrolls inside out, the adhesive side is forced against the cleaned cast iron walls, creating a bond. The method takes approximately 1 hour to line sections up to 1,000 feet.

In the Rolldown method, individual polyethylene pipe liner lengths are fused onsite into appropriate lengths to suit particular site conditions and installation lengths. The pipe liner is drawn through roller dies to concentrically reduce the liner diameter, by about 10%, so that long, continuous lengths can then be inserted in a single operation. A single pull can insert liner for up to 5,000 feet of cast iron pipeline; however, a large insertion trench must be dug to allow the polyethylene insert to be drawn into the pipeline at a grade which will not compromise the liner. This method allows renewal of pipeline with diameters of 4 to 20 inches, and bends up to 11¼˚ can be negotiated. Once inserted, the ends of the liner are sealed off, and the liner is reverted to a close-fit by filling it with cold water and pressurizing it. The process requires excavations to reconnect pipeline segments.

The Subline process was developed to allow lining of large diameter pipes and improve the ability to negotiate bends. Polyethylene pipeline is folded in on itself along its central axis, making a “heart” shaped cross-section, so that it can be inserted into the cast iron pipeline. Similar to Rolldown method, long lead-in trenches are required for welded plastic strings and local excavations are required to reconnect the segment to adjacent sections. Subline allows for up to 3,300 feet of liner to be inserted in pipeline segments ranging from 3 to 64 inches; and the folded shape of the liner allows for bends up to 22½˚ to be negotiated. Once the folded polyethylene liner is inserted, pressurized cold water is used to revert it to a tight fit inside the pipeline with minimal reduction of capacity.

Other Mitigation Options - CISBOT