Buried Polyethylene Pipe: An Excellent Choice for Water Service
Buried steel pipe is prone to pitting due to external (soil-side) corrosion and internal (water-side) corrosion and biofouling. Preventive and mitigative measures include periodic inspection, cleaning, chemical and biocide treatments, and repairs.
A cost-effective alternative consists in replacing the buried steel pipe with a non-metallic pipe, in particular high density polyethylene (HDPE). This alternative, widely used for buried waterworks and gas distribution piping, has been used at several nuclear power plants for non-safety related piping.
Through ASME Code Case N-755, an alternative is now available for safety-related Class 3 piping.
The first industrial polymerization of the ethylene monomer CH2=CH2 into polyethylene (-CH2-CH2-)n dates back to the 1930’s. Today’s polyethylene pipe is designated by its ASTM D 3350 name, such as PE3408 where 3 stands for the cell density, 4 is its resistance to slow crack growth and 08 reflects its hydrostatic design stress at room temperature (allowable stress in service) of 800 psi.
When preparing to replace buried steel pipe with polyethylene pipe, the project must address six areas:
- System thermo-hydraulic review to verify flow rates, pressure drops, pumps and valves sizing for the new pipe.
- Selection of a suitable material for the range of operating temperatures, pressures and design life.
- Detailed design for normal operating loads (pressure, temperature, soil and surface loads) and postulated accidents (seismic and waterhammer).
- Fabrication, typically by butt fusion, with flanges at the HDPE-steel transitions.
- Non-destructive examination, through in-process controls of the fusion process, with supplementary examinations.
- Pressure testing, in the form of hydrostatic test of the new pipe.
Code Case N-755 is essential in implementing these steps in an orderly and competent manner. The engineer will also find important technical information in EPRI Report 1011628, Technical Support for Proposed Polyethylene Pipe Code Case, December 2005 (Wes Rowley/TWC and Jack Spanner/EPRI, Principal Investigators), and EPRI Report 1011836, Design and Qualification of High-Density Polyethylene for ASME Safety Class 3 Piping Systems, December 2005 (George Antaki/Consultant, Principal Investigator).
Other valuable resources include the American Waterworks Association (AWWA) standards for HDPE pipe, the Plastic Pipe Institute (PPI) standards, pipe manufacturers engineering guides, and textbooks such as Piping & Pipeline Engineering (Chapters 14 Buried Pipe and 24 Plastic Pipe) by George Antaki (published by Dekker), and Plastics Pipes for water Supply by Lars-Eric Janson (published by Borealis).
HDPE pipe with butt fused joints
HDPE Pipe has outstanding elongation at rupture under short-term hydrostatic pressure Note the high degree of hoop deformation before rupture
Becht Engineering has the expertise and practical experience in HDPE piping engineering to help you develop and implement a successful HDPE pipe replacement project.
3 thoughts on “Buried Polyethylene Pipe: An Excellent Choice for Water Service”
one of the major attributes of HDPE along the Gulf coast is it’s flexibility. down here underground facilities are always moving (settling, etc.). HDPE is very useful in this area. additionally, the corrosive nature of some of the soils along the coast (lots of our facilities are within eye site of the Gulf) has practically no affect on HDPE.
finally the use of HDPE promotes use of products of the petro-chemical industry
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