One example of the rebuilding process is the relining of the Metropolitan Street sanitary sewer force main in New Orleans East, the area of the city hardest hit by Hurricane Katrina.To refurbish the steel force main without having to dig trenches, the Sewerage and Water Board of New Orleans specified cured-in-place
pipe (CIPP) technology.
CIPP was specified over alternatives such as inserting polyvinyl chloride liners inside the host pipe or centrifugal casting a liner of fiberglass/mortar onto the inner wall. Either slipliner technique would have reduced the inside diameter of the force main and brought about an unwanted loss of wastewater flow capacity.
The pipe rehabilitation required a team effort led by the Sewerage and Water Board, whose primary team partner was contractor Boh Bros. Construction Co. LLC, New Orleans. Playing vital supporting roles were Compliance EnviroSystems LLC, Baton Rouge, La.; Garner Environmental Systems Inc., Pasadena, Texas; National Envirotech Group, Houston;Visu-Sewer Clean and Seal Inc., Pewaukee, Wis.; Applied Felts Inc., Martinsville, Va.; AOC, Collierville, Tenn., and CIPPCON, Ponte Vedra Beach, Fla.Gravity sewers won’t work because almost all of New Orleans is below sea level, force mains are needed to convey sewage to a higher elevation for treatment. Sewer force mains move wastewater under pressure between lift stations that are equipped with pumps. When New Orleans East lost power during Hurricane Katrina, lift station pumps shut down to create a system failure. However, from this misfortune, the Sewerage and Water Board of New Orleans found serendipity.
“While inspecting our facilities after the flood waters receded, we discovered small pinholes along a length of the Metropolitan Sewer Force Main,” said Joseph Becker, who oversaw the project for the Sewerage and Water Board.“Further investigation found that this line was one-half to two-thirds filled with sediment and debris, which was mostly sand.”
The Metropolitan force main is 50-in. diameter steel pipe that is part of a system that was installed in the 1940s. To restore the pipe for maximum ongoing use, the board hired construction specialist Boh Bros. to install a new CIPP liner. It was the second phase of a project that followed rehabilitation of 30- and 36-in. diameter pipe.
The critical task of preparing the interior of the decades old metal pipe was subcontracted to Compliance EnviroSystems. CES had already played a major role in post- Hurricane Katrina recovery. The company was in charge of extensive debris and mud removal that included cleaning out New Orleans’ overburdened storm drainage system.
For high efficiency and low-to-no odor generation, CES cleared the sanitary sewer force mains using a Sewer Hog/Grit Gator system from Garner Environmental Systems. The system uses water-jet power to dislodge debris, highpower vacuum to remove it and a large container to collect debris so it can be shipped away. CES crews removed more than 1,250 tons or 1,575 cu yds of debris from the Metropolitan force main during the cleaning process. “After the debris was removed, we discovered there was more work to do before the pipe was ready for a new liner,” said Brad Dutruch, vice president of CES. “Rust stalactites that had formed inside the pipe did not come free under the force of the Sewer Hog.Wearing proper gear, workers had to enter the pipe and manually scrape the stalactites off the top of the pipe.” Resins
When the force mains were originally installed, the interiors were coated with coal tar epoxy. Using National Liner technology licensed from National Envirotech Group, Boh Bros. lined the pipe with a more advanced material system based on non-woven felt tubes and thermoset polymer resin.
The resin was supplied in a liquid state that allowed the felt to be wet-out before the tube was inserted into the pipe. Because the Metropolitan Street force main does not have manholes, access pits needed to be excavated to accomplish the insertions.The Metropolitan Street relining project involved 4,660 ft of pipe that was accessed through five pits strategically located to minimize surface disruption and facilitate insertion.Applied Felts supplied the 50- in. diameter tube sections. Each section length corresponded with the length of a planned insertion. The longest single insertion was 1,120 ft.
The resin for the job was Vipel L010 bisphenol A epoxy vinyl ester, engineered by AOC for CIPP. Most CIPP liners are fabricated with isophthalic polyesters, a resin family with proven resistance to the chemical environment of municipal wastewater. However, the internal pressure of the New Orleans force main required the vinyl ester to take advantage of the resin’s higher tensile properties.
“One of the most important resin attributes for resisting the force main pressure is tensile elongation,” explained David Treadwell, CIPP resin product leader for AOC. “Using test method ASTM D638, a typical cast specimen of Vipel L010 vinyl ester exhibits a tensile elongation 4.6 percent, compared to 3 percent for our Vipel L704 isopolyester.”
To keep wastewater moving forward, the pressure in the Metropolitan force main generally operates in the 30- to 40- psi range.To ensure sufficient performance, the pipe with liner was designed to withstand an internal pressure of at least 65 psi.
Using equipment designed by Visu-Sewer, workers resin impregnated the felt inside a protective tent. The felt was then conveyed to a crane, which lifted the tube over the hole for insertion.“The Vipel L010 vinyl ester offered good handling characteristics and processed very well in the field,” said Marlin Gonzales, operations manager for Boh Bros.“The AOC resin was provided through specialty distributor CIPPCON. As part of his great technical support, CIPPCON president Jim McCormick came out to observe the first inversion.”
As the resin-impregnated felt was inserted into the access points, water was pumped into the interior of the tube.This created pressure that moved the felt forward and inverted it against the inner wall of the host pipe.
At the end of each inversion cycle, the water was heated to initiate the chemical reaction that caused the resin to cure into a solid state in which the resin’s molecular structure is irreversibly crosslinked. At the conclusion, a new seamless liner was formed against the host pipe inner wall to seal off the pinholes and strengthen the pipe for efficient operation.
The pipe rehabilitation included a minor system redesign to improve flow. Bends in the original system that were no longer needed were removed and replaced with new ductile iron pipe. Application of the CIPP liner included the new pipe sections.
Rebuilding the infrastructure is essential to bringing a sense of normalcy back to the residents of The Crescent City. While much of the activity is happening where all can readily see, essential underground sewer enhancement technologies are just as much a part of the post-Katrina rebirth.
Ben R. Bogner, P.E., C. Eng., is corrosion and infrastructure market development specialist for AOC.