The Path to Proper Pipe Selection

Proper Pipe SelectionPipe selection is a key component, if not the keystone in this decision making process. Done incorrectly, the ramifications can be catastrophic. Pipe selection must be based on ?best practices? using industry standards such as ASTM and AWWA design, installation and inspection methods and performance. The goal is to install a system that can deliver 100-year design life with reduced lifecycle costs, water loss and failure rates. High-density polyethylene (HDPE) pipe has been proven to deliver all these hallmarks.

HDPE pipe?s combination of flexibility and leak-free, fully restrained fused joints makes it ideal for underground installation methods such as open cut and trenchless construction. Because heat-fused HDPE pipe joints are as strong as the pipe itself, sections can be confidently joined into long runs by following various industry guidelines and procedures such as ASTM F2620, PPI TR-33, PPI TN-34, PPI TN-41 and PPI TN-42.

Numerous life prediction studies have been conducted on HDPE and can be found at http://plasticpipe.org/municipal_pipe/water_publications.html. HDPE materials are predicted to last a minimum of 100 years in most environmental situations prevalent within U.S. and Canadian water systems. Since HDPE is a highly ductile material and has exceptional impact strength, it provides a pipe system that offers the lowest susceptibility to damage during regular operation and natural disasters such as earthquakes, hurricanes and tsunamis.

Proper Pipe SelectionEvolving Standards

Today?s newer, high-performance resins such as PE 4710, allow for higher pressures and greater hydraulic capacities (increase in the pipe ID) due to its greater resistance to slow crack growth and other requirements as defined in PPI TR-3. PE 4710 pipes are designed for water and wastewater applications meeting AWWA C901, AWWA C906 or ASTM F714 standards and others. On July 30, 2013, the AWWA Executive Committee completed its comprehensive review of all the technical and procedural appeals and unanimously approved the C906-12 revisions as was previously approved by the AWWA Standards Council.?

The intent of the revisions to the AWWA C906 standard is to enable users to take advantage of the enhanced performance properties of PE 4710 materials and increase the design and operating safety of water pipelines.?The Safety Factors of PE 4710 relative to multiple conditions exceed 2.0 as demonstrated to the AWWA Executive Committee. For example, the Safety Factors for PE 4710 exceed 10 for predicted lifetime relative to 100-year service life and is 3.6 for yield stress (3,625 psi) relative to hydrostatic design stress (1,000 psi). In addition, a properly designed HDPE system that includes criteria such as pressure, recurring and occasional water hammer and 100-year fatigue life,?results in a larger pipe ID, higher DR and longer fatigue life than PVC, as calculated in industry software programs such as PPI PACE.

Also for the long-term, HDPE pipe provides and maintains the required flow rates over the lifespan of the system. HDPE pipe has a Hazen Williams C factor of 150 which does not change over time. It is typical, unfortunately, for this factor to be diminished in other pipe systems due to corrosion and tuberculation, which does not occur in HDPE pipes.?

Cost Effectiveness

Cost and performance should always be prime drivers when considering infrastructure projects, and HDPE pipe offers several advantages. The Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia?s national science agency, is one of the largest and most diverse research agencies in the world. According to CSIRO, in its Life Cycle Analysis of Water Networks, ?polyethylene networks show significantly lower costs throughout their lifetime, and the combined benefits of low failure and water loss rates can potentially result in long term cost savings.??

If the decision process is solely based on the purchased price of the pipe materials, then money is being wasted. The project cost analysis also needs to include the installed cost and long-term performance considerations. On a typical pipeline project, pipe material costs are about 15 percent of the total job costs associated with installation. Basing a decision solely on the purchase price of materials represents poor engineering and business practice. It is also alarming since the water industry ranked water infrastructure, capital costs and availability of funds among the top current industry concerns. Project planners need to consider the total cost and performance over the entire lifetime of the pipe system. This will ensure efficient use of funds and proper selection of materials that have at least a 100-year design life and furnish a leak-free system.?

According to Professor Stewart Burn, a leading scientist with CSIRO, ?The United States? preference to look strictly at lowest bid price could lead to serious consequences in the future with (water pipe) networks experiencing higher costs for repair and water loss and requiring faster renewal frequency?as a result of the lowest upfront price approach.?

Design Tools and Resources

By using industry-authoritative sources such as the free PPI PACE software program that was developed by experts from the University of Waterloo, engineers and industry professionals can easily perform plastic pressure pipe design calculations in accordance with current published AWWA and ASTM industry standards. The web-based program assists professionals in the selection of appropriate pipe class for the required design life by providing an easy-to-use tool that is consistent with those existing standards and practices. For flow analysis, it also calculates the proper selection of pipe ID, fatigue and design life.

PPI PACE can be used with the PPI Design and Engineering Calculator, another tool that assists in greater understanding of HDPE pipe capabilities. The calculator will assist a design engineer in performing multiple computations relating to internal and external pressure ratings, pressure and gravity water flow, water hammer and low pressure, plus automates other assessments for above and underground systems. For using horizontal directional drilling (HDD), there is the PPI-BoreAid program.

Installation

Because HDPE pipe is flexible, reliable, highly ductile and durable, it is the product of choice for trenchless applications. The survey in Trenchless Technology magazine (Aug. 2012) showed that HDPE was used in 66 percent of water pipe trenchless construction. When compared to open cut, HDD and trenchless methods will also significantly reduce cost and interruption to residents and business.

In addition, HDPE has an exceptional bending radius (11.2 x outside diameter for DR 13.5; refer to PPI PE Handbook, Chapter 10, Table 3) that allows it to adapt to challenging site conditions. HDPE can be installed per AWWA M55, ASTM F2620 (for butt fusion), ASTM F1962 (for Horizontal Directional Drilling), PPI PE Handbook Chapter 7 for sliplining and Chapter 16 for pipe bursting. Besides HDPE, none of the previously listed materials have ASTM standard methods for pipe fusion and/or directional drilling.

Pipe selection is critically affected by multiple factors, foremost of which is material application. Properly designed and?installed?PE 4710?pipe systems require minimal maintenance and can?provide 100-plus years of service life using superior fused joints to produce a monolithic pipeline that protects our water?resources, resists?earthquakes, hurricanes and fatigue, prolongs water supply and extends system service life.?HDPE pipe offers low initial and overall life cycle costs and has a low failure rate among pipe materials.
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HDPE water pipe systems have been in use for numerous years globally and have been studied, tested, analyzed, and evaluated in the lab and in the field. The Plastics Pipe Institute has technical reports, third-party evaluations, manuals and?tools including software and calculators for proper design and installation of HDPE water pipe systems to achieve optimal performance?and an extensive life. All PPI design and installtion tools can be found on the institute?s website, www.plasticpipe.org.

Camille Rubeiz is director of the engineering, municipal and industrial division for the Plastics Pipe Institute, Inc. (PPI).

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