Denver Water Conduit 12 Relocation
Commercial weld methods on steel pipe were not available until the early 1930s. Earlier steel pipes relied on rolled plates joined together by a series of longitudinal and girth riveted seams. In the 1920s, the predecessor of Denver Water, now the largest treated water provider in the State of Colorado, constructed several large-diameter transmission mains using riveted steel pipe. One such 54-inch diameter main, designated as Conduit 12, remains in service to present day.
In 2015, a major highway expansion project in Central Denver required the re-routing of a portion of Conduit 12. As the riveted pipeline was still considered in good condition, only a 700-foot section would be necessary to move. Conventional AWWA C200 sprialweld steel pipe with cement mortar lining and polyurethane coating was selected for the re-routed section, as of course, riveted pipe is no longer manufactured. The re-routed section included a 60-inch diameter casing for the new 54-inch diameter casing pipe. The pipe within the casing was furnished in 25 foot lengths and double lap welded and air tested prior to being jacked into the tunnel.
Modern steel spiralweld pipe has much higher yield strength than pipe manufactured in the 1920s, with the Conduit 12 replacement section using 42,000 psi yield material. As a result, the wall thickness of the replacement pipe was only 0.285 inch, or about 40% less than the original pipe. The construction team verified the original pipe could sustain field-welding procedures.
We manufactured a modified butt strap to connect the new pipe. Because the original welded steel pipe is essentially uncoated, we fabricated an insulated flange to electrically isolate the old sections from the new. The high dielectric strength polyurethane coating on the new pipe could result in a galvanic differential, leading to a potential corrosion issue with the original pipe.
In a unique twist on history, the original riveted pipe was furnished by Thompson Pipe & Steel Company of Denver, Colorado, which was acquired by Northwest Pipe Company in 1996. The replacement pipe was manufactured 90 years later in the same facility.