Water Loss Solved; Delicate Environment Protected
CAVE CITY, Ky – The Caveland Environmental Authority (CEA) has found the solution to reduce its 40 percent unaccounted water loss and protect the area’s sensitive subterranean ecology by replacing aging and leaking water lines with high-density polyethylene (HDPE) pipe. Completed in May 2016, the project used more than 3.5 miles of PE 4710 pipe.
“During the past five or six years they’ve been replacing old water lines,” explained Roger M. Boyers, P. E. of Water Management Services, LLC, the Nashville, Tennessee-based firm that designed the project. “This includes replacing PVC, some cast iron, asbestos concrete and even clay pipes. Everything they’ve been putting in now is HDPE. Our company first started working on this years ago with the Mammoth Cave National Park project, which was replacing all the water lines in the park with HDPE pipe. They were concerned with chlorinated water possibly getting into the cave structures and causing a problem down there.
Cave City operates and maintains the Mammoth Cave National Park water and sewer system.
“When all of the water mains were replaced within the park, the National Park Service required CEA to install only HDPE pipe as this was the best choice for preventing water leaks and line breaks,” he continued. “Prior to this, the water loss was very high. Now, the park’s unaccounted water is less than 10 percent, and most of this is due to air release valve failures, not the pipe itself. HDPE pipe systems have a zero allowance for water loss per AWWA M55 Manual which is part of the design with these improvements.”
For a karst area such as Cave City, finding water leaks is difficult because the water usually does not come to the surface but goes down into one of the sinkholes. For this reason the Cave City system went to HDPE pipe to have a fully-restrained-pipe, and joint-free pipe system. Heat fusing the HDPE sections makes a monolithic pipeline, which minimizes water loss and means that the pipe will not separate at the joint if there is ground movement or a seismic shift.
“With bell and spigot joined pipe systems this is not the case unless restraints are used,” Boyers explained. “Fused HDPE pipe is just about impossible to pull apart. This has been proven in numerous earthquakes around the world. Once again this is part of the design for this project. The fused HDPE pipe also significantly reduces any chance for water loss from the joints and prevents separation of the pipe due the karst nature of the topography.
“We based our design on a working pressure of 160 psi which requires a DR 13.5 HDPE (PE 4710 with Pressure Class, PC 160) pipe. To meet the same design requirements, PVC C900 DR 14 pipe with a PC 305 would be required to handle working pressures, surge pressures and the 100 year fatigue life. While PVC pipe has a higher static hoop stress capability, our focus was on the pipe’s dynamic strength and not its static strength. In our design, we had to take into account the proposed water booster station. While we were trying to minimize the pumps turning on and off, there was the potential for water hammer and therefore, the overall long-term fatigue of the pipe had to be taken into consideration. This resulted in increasing the PVC pipe pressure class to account for the flow velocities and number of cycles from the booster station.”
According to the Plastics Pipe Institute, Inc. (PPI), the major North American trade association representing all segments of the plastic pipe industry, PE 4710 is the highest performance classification of HDPE piping material for potable water applications.
“PE 4710 compounds offer utilities and designers a greater level of performance,” stated Camille George Rubeiz, P.E., F. ASCE, senior director of engineering for PPI’s Municipal and Industrial Division. “This translates to the ability to utilize HDPE with increased flow capacities plus increased resistance to surge pressure, fatigue and slow crack growth when compared to previous PE 3608 compounds. The updated ANSI/AWWA C906-15 standard includes PE 4710 for sizes up to 65 inches and recognizes the increased durability and reliability of HDPE pressure pipe used in water systems.”
According to Rubeiz many engineers select the pipe’s pressure class based on the system’s working pressure and on the pipe’s static hoop stress capability. “However, and as noted by Mr. Boyers, Water Management Services provided its client, Cave City, with solutions and accounted for the static and dynamic strength of each pipe material. To simplify these preliminary calculations, eTrenchless Group developed and maintains a web- based calculator called PPI PACE. The free program allows users to complete pressure pipe designs including water distribution lines, transmission main systems and force mains in accordance with existing AWWA and ASTM published standards.
Since PPI PACE compares HDPE, PVC and Ductile Iron pipe, relevant standards used include AWWA C900, AWWA C901, AWWA C905, AWWA C906, C150/151 along with AWWA manuals such as M23, M55 and M41. PPI PACE also contains extensive information on the selection of design parameters and typical input valves for all three major materials used in water systems.
Rubeiz also discussed this essential and major difference in industry standards for pipe. “The AWWA defines pressure class differently for all three of the different materials. For example, the pressure class for PVC and Ductile Iron pipe is not equivalent or relevant for HDPE pipe. For this project and to meet the design requirements of 160 psi, the required flow velocities and fatigue life, PVC pipe with a DR 14 and pressure rating of 305 psi would be required, while PE 4710 pipe with a DR 13.5 and rated at 160 psi met the project design requirements. For a project like this it would be common for a PVC pipe with DR 25 (PC165) or DR 18 (PC235) to be specified to satisfy the 160 psi design pressure requirement. If this was done, the system would be under-designed by having a fatigue life that is less than 100 years. For Ductile Iron pipe, the fatigue life calculations are not available in AWWA C150/151 and M41.”
The Cave City project used 18,500 feet of eight and twelve inch PE 4710 pipe from PPI member company, WL Plastics, Inc. (Ft. Worth, Texas). Installation was done using open trench, cut and cover. Jack and bore was used to go under several state roads and railroad tracks. According to Boyers, any new lines will also use PE 4710 pipe
For additional information, please visit the PPI website: www.plasticpipe.org.
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The Plastics Pipe Institute Inc. (PPI) is the major North American trade association representing all segments of the plastic pipe industry and is dedicated to promoting plastics as the material of choice for pipe applications. PPI is the premier technical, engineering and industry knowledge resource publishing data for use in development and design of plastic pipe systems. Additionally, PPI collaborates with industry organizations that set standards for manufacturing practices and installation methods.