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Plastic Pipes - Built to Last?

The life expectancy of plastic sewer pipes has been a matter of some debate, but a recent study has placed the figure at 100 years

by Tony Calton, General Manager, The European Plastic Pipes & Fittings Association (TEPPFA)

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.

The prediction of the life expectancy of plastic pipe systems is well documented for pressure applications, where stresses in the pipe wall are acting continuously. Hydrostatic tests at different temperatures allow us, with Arrhenius extrapolations, to make a reliable estimate of the predicted lifetime at specific pressure and temperature ranges. However limited data is available for non-pressure applications in sewage and drainage, where pipes are installed and operate without internal pressures, and a continuous deflection and subsequently a constant strain loading applies.

Following the conclusion of a major two year project co-ordinated by TEPPFA in conjunction with two leading material suppliers (LyondellBasell and Borealis) this knowledge gap has now been addressed.

As a direct result of this project, sewer 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 investigation is vitally important for all parts of the value chain; raw material suppliers, pipe manufacturers and designers and operators of sewer systems as it will allow polyolefin sewer pipes to continue to be specified with increased confidence in their asset life.

Project Scope

To demonstrate the long-term performance of PP and PE sewer pipes, of both solid wall and structured-wall construction, the following areas have been analysed and investigated as part of the project:

• Collection of evidence and determination of material properties to provide sufficient long term resistance to thermo-oxidative degradation.
• The long-term behaviour under constant strain loading of non-pressure sewer pipes has been analysed and based on relaxation tests, calculations have been made for the prediction of the long-term stresses and evaluation of the risk of pipe failure under constant deflection at long term.
• The possible long-term effects of sewer water composition and the temperature of the sewer water have been evaluated
• The effect of potentially high localised stress concentrations which may be present in structured wall pipe configurations
• Excavation projects have been undertaken across various European countries to provide pipe samples for an analyse of their condition after long periods of operation (in some cases up to 40 years) and tests conducted to predict their remaining life expectancy.

Thermo-oxidative degradation

To achieve a 100 year service lifetime it has to be proven that non-pressure pipes resist premature (brittle) failures caused by thermo-oxidative degradation. To prove this a test method was required.

It is common practice to perform durability tests at elevated temperatures to accelerate ageing and to estimate the theoretical lifetime at service conditions based on an Arrhenius equation. An example of such a lifetime prediction methodology is given in ISO 9080, pos. 5.1.4 and 5.2 for pressure pipes. It seems practicable to adopt the extrapolation rules postulated in ISO 9080 for a 100 years lifetime prediction of non-pressure pipes. As a consequence the accelerated ageing tests for non-pressure pipes should be performed at minimum 50 °C higher temperatures than the service temperature and for a minimum duration of 8760 hours (= 1 year) without brittle failures to achieve an extrapolation time factor of 100 (years) as described in ISO 9080, pos. 5.2.

Long-term behaviour under constant strain

The long-term behaviour under constant strain loading of non-pressure sewer pipes been analysed by the use of relaxation tests, measuring the relaxation modulus as a function of loading time. The relaxation tests have been assessed based on 3 possible basic shapes for relaxation (compliance) curves. The data is then used to calculate the long term expected stresses by means of extrapolation to 100 years. The values found should be lower than the allowed stresses to prevent the risk of pipe failure under constant long term deflection.

Testing has been completed on pipes manufactured from virgin material and on the excavated pipes. The constant load applied during the test was 15%, which is far above the maximum allowed load of 8% deflection, as generally indicated by installation requirements.

Project Outcome

The project has demonstrated that a 100-year lifetime for non-pressure sewage pipes manufactured from PE and PP can be expected, provided that some basic requirements are met related to the raw materials, the pipe construction and installation practices:

• The resistance to thermo-oxidative degradation of the resins has been determined
• The allowable stresses at long term are not continuously exceeding the defined levels at the operating temperatures
• Resins from which the pipes are manufactured fulfil the basic requirements as defined in the EU products standards
• Pipes are manufactured within accepted levels of production practices and meet all test requirements of the relevant EU product standards
• Pipes are installed in accordance with recognised industry or EU standards
• The long term pipe deflection in service shall be below 8%.
• The design of structured-wall pipes needs to avoid excessive localised stress concentrations. In order for this to be demonstrated it is recommended to require compliance with the 30 % ring flexibility test in the EU product standard for structured wall pipes.

Independent Assessment of Results

Following completion of the project the methodology used, the tests conducted, the results achieved and the conclusions drawn were independent assessed by a leading expert in this field, Prof. Dr. Heinz Dragaun of TGM-Versuchsanstalt - Federal Institute of Technology, Department Plastics Technology and Environmental Engineering, Austria.

He concluded: “I consider this project to be very relevant to improving knowledge of the in-service performance of polyolefin sewer systems all over the world. In my opinion the project has been conducted in a proper and scientifically reliable way with close cooperation between material producers and pipe and fitting producers with the target of demonstrating how long time service quality can be achieved in the field.”

PVC Sewer Systems

The polyolefin sewer lifetime expectancy project follows a similar investigation conducted several years ago by TEPPFA on PVC sewer systems which demonstrated similar results. The full technical reports on both projects can be downloaded from www.teppfa.eu.

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