FATIGUE RESISTANCE AND FLEXIBILITY HDPE
Pipe can be field bent to a radius of 30 times the nominal pipe diameter or less depending on wall thickness (12” HDPE pipe, for example, can be cold formed in the field to a 32-foot radius). Willoughby, D. A. (2002). Plastic Piping Handbook, McGraw-Hill Publications, New York.
SEISMIC RESISTANCE
The physical attributes that allow HDPE pressure pipe to safely ac commodate repetitive pressure surges above the static pressure rating of the pipe, combined with HDPE’s natural flexibility and fully restrained butt fusion joints, make it well suited for installation in dynamic soil environments and in areas prone to earthquakes or other seismic activity.
CONSTRUCTION ADVANTAGES HDPE
Pipe’s combination of lightweight, flexibility and leak-free, fully restrained joints permits unique and cost-effective installation methods that are not practical with alternate materials. Installation method such as horizontal directional drilling, pipe bursting, slip lining, plow and plant, and submerged or floating pipe, can save considerable time and money on many installations. At approximately one-eighth the weight of comparable steel pipe, and with integral and robust joining methods, installation is simpler, and it does not need heavy lifting equipment. Polyethylene pipe is produced in straight lengths up to 50 feet and coiled in diameters up through 6”. Coiled lengths over 1000 feet are available in certain diameters. Polyethylene pipe can withstand impact better than PVC pipe, especially in cold weather installations where other pipes are more prone to cracks and breaks.
DURABILITY OF POLYETHYLENE
Polyethylene pipe installations are cost-effective, have long-term cost advantages due to the pipe’s physical properties, leak-free joints, and reduced maintenance costs. The polyethylene pipe industry estimates a service life for HDPE pipe to be, conservatively, 50-100 years if the system has been properly designed, installed and operated in accordance with industry established practice and the manufacturer’s recommendations. This longevity confers savings in replacement costs for generations to come. Properly designed and installed PE piping systems require little on-going maintenance. PE pipe is resistant to most ordinary chemicals and is not susceptible to galvanic corrosion or electrolysis.
HYDRAULICALLY EFFICIENT
For water applications, HDPE pipe’s Hazen Williams C factor is 150 and does not change over time. The C factor for other typical pipe materials such as PVC or ductile iron systems declines dramatically over time due to corrosion and tuberculation or biological build-up. Without corrosion, tuberculation, or biological growth HDPE pipe maintains its smooth interior `all and its flown capabilities indefinitely to insure hydraulic efficiency over the intended design life.
TEMPERATURE RESISTANCE
PE pipe’s typical operating temperature range is from -40°F to 140°F for pressure service. Extensive testing at very low ambient temperatures indicates that these conditions do not have an adverse effect on pipe strength or performance characteristics. Many of the polyethylene resins used in HDPE pipe are stress rated not only at the standard temperature, 73° F, but also at an elevated temperature, such as 140°F. Typically, HDPE materials retain greater strength at elevated temperatures compared to other thermoplastic materials such as PVC. At 140°F, polyethylene materials retain about 50% of their 73°F strength, compared to PVC which loses nearly 80% of its 73°F strength when placed in service at 140°F .
As a result, HDPE pipe materials can be used for a variety of piping applications across a very broad temperature range. The features and benefits of HDPE are quite extensive, and some of the more notable qualities have been delineated in the preceding paragraphs.
DUCTILITY
Ductility is the ability of a material to deform in response to stress without fracture or, ultimately, failure. It is also sometimes referred to as trainability and it is an important performance feature of PE piping, both for above and below ground service. For example, in response to earth loading, the vertical diameter of buried PE pipe is slightly reduced. This reduction causes a slight increase in horizontal diameter, which activates lateral soil forces that tend to stabilize the pipe against further deformation. This yields a process that produces a soil-pipe structure that is capable of safely supporting vertical earth and other loads that can fracture pipes of greater strength but lower strain capacity. With its unique molecular structure, HDPE pipe has a very high strain capacity thus assuring ductile performance over a very broad range of service conditions. Materials with high strain capacity typically shed or transfer localized stresses through deformation response to surrounding regions of the material that are subject to lesser degrees of stress. Because of this transfer process, stress intensification is significantly reduced or does not occur, and the long-term performance of the material is sustained. Materials with low ductility or strain capacity respond differently. Strain sensitive materials are designed based on a complex analysis of stresses and the potential for stress intensification in certain regions within the material. When any of these stresses exceed the design limit of the material, crack development occurs which can lead to ultimate failure of the part or product. However, with materials like polyethylene pipe that operate in the ductile state, a larger localized deformation can take place without causing irreversible material damage such as the development of small cracks. Instead, the resultant localized deformation results in redistribution and a significant lessening of localized stresses, with no adverse effect on the piping material. As a result, the structural design with materials that perform in the ductile state can generally be based on average stresses, a fact that greatly simplifies design protocol. To ensure the availability of sufficient ductility (strain capacity) special requirements are developed and included into specifications for structural materials intended to operate in the ductile state; for example, the requirements that have been established for “ductile iron” and mild steel pipes. Similar ductility requirements have also been established for PE piping materials. Validation requirements have been added to PE piping specifications that work to exclude from pressure piping any material that exhibits insufficient resistance to crack initiation and growth when subjected to loading that is sustained over very long periods of time, i.e. any material that does not demonstrate ductility or strain ability. The PE piping material validation procedure is described in the chapter on Engineering Properties of Polyethylene.
