Service Life of Polypropylene and Polyethylene Sewer Pipes
“Over the years some things need improving but others don’t.”
Polypropylene and polyethylene (collectively known as polyolefin) sewer pipes have been widely used for over 40 years and have an established track record for reliability, integrity and trouble free service. However their predicted overall life expectancy has been discussed for many years without a definitive answer becoming available.
Following the conclusion of a major two year project co-ordinated by TEPPFA in conjunction with LyondellBasell, Borealis and TGM (Austria) that situation can now be changed and:
Designers, owners and operators can now be confident that polyolefin sewer systems will have an in-service life of at least one hundred years when materials, products and installation practices meet the appropriate requirements.
The outcome of this project is vitally important for material suppliers, pipe manufacturers and the designers and operators of sewer systems. Allowing polyolefin sewer pipes to continue to be specified with increased confidence in their asset life.
The basis for confidence in the conclusions from this project is explained in this guide.
Real time long term data
The project includes tests on excavated pipes which have already been in service for up to 40% of the proposed in-service lifetime. These tests have demonstrated that no excessive deterioration or degradation has occurred over this time.
Although the oldest of the excavated pipes were manufactured using “first generation” material formulations a residual lifetime of more than 50 years has been calculated. Current formulations offer superior performance to earlier materials
All excavated pipes tested were found to fulfil the minimum performance requirements of current EN product standards
Extended duration laboratory testing
Constant deflection test durations have been extended to 18 months to provide greater confidence in extrapolations to one hundred years
Correlation factors for measured data are in excess of 0.99
Use of high safety factors
Various safety factors have been used across a wide range of tests and calculations. For example:
Relaxation tests conducted at 15% deflection while the long term deflection in buried pipelines is unlikely to exceed 5.5% (Safety factor 2.7)
Tests carried out at elevated temperatures of 45⁰ C when the maximum temperature in sewer systems does not generally exceed 30⁰ C
The allowable stress at 100 years (at 30⁰ C) is 7.0 MPa while the actual stress at 8% deflection is only 2.6 (Safety factor 2.7). Even at 15% deflection the actual stress is 4.1 MPa (safety factor 1.7)
Many other factors have been investigated, results clearly demonstrating they do not adversely affect the anticipated life expectancy, these include:
The chemical composition of sewer water
The temperature profile of sewer flows
The higher stress concentrations which may be present in structured wall pipe configurations
Variations in installation practices
The methodologies used, the test results and the conclusions drawn from the project have been independently reviewed by a leading expert in this field, Professor Heinz Dragaun, TGM, Vienna
For transparency the results of his review have been made available with the project report
In this study, it has been demonstrated that a 100 years lifetime for non-pressure sewage pipes manufactured from PE and PP-B can be expected , provided that following conditions are met:
a) The pipes shall meet the requirements of European product and system standards, EN 1852 for PP, EN12666 for PE and EN13476 for Structured Wall Pipes of PE and PP, and
b) The material, pipes and installation shall meet the requirements specified in the table below.
Note: The analysis and predictions within the scope of the project are valid for virgin materials. Modified materials such as foamed materials and mineral filled materials are not covered by the outcome of this report
Designers, owners and operators can be confident that polyolefin sewer systems will have an in-service life of at least 100 years when materials, products and installation practices meet the appropriate requirements.
• Supported by real time long term data
• Proven by extended duration laboratory testing
• Reinforced by the selection of high safety factors
• Confidence from the extensive scope of the project
• Verification of results by independent assessment
TEPPFA wishes to thank Borealis and LyondellBasell for their input, expertise and use of test facilities during the course of this research project.
Also the contribution made by Professor Heinz Dragaun (TGM, Austria) during the project and in reviewing the results.