Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | APRIL 2015 | 31 In the know Digging deeper: 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 fi gure at 100 years P olypropylene and polyethylene (collectively known as polyole n) 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 de nitive 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 di• erent temperatures allow us, with Arrhenius extrapolations, to make a reliable estimate of the predicted lifetime at speci c 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 de ection 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 TONY CALTON GENERAL MANAGER, THE EUROPEAN PLASTIC PIPES & FITTINGS ASSOCIATION (TEPPFA) operators can now be con dent that polyole n 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 polyole n sewer pipes to continue to be speci ed with increased con dence 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 suˆ cient 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 de ection at long term. • The possible long-term eff ects of sewer water composition and the temperature of the sewer water have been evaluated • The eff ect of potentially high localised stress concentrations which may be present in structured wall pipe con gurations • Excavation projects have been undertaken across various European countries to provide pipe samples for an analyse of their condition a‰ er 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 A plastic pipe is placed under a 30% defl ection test as part of the study. This signifi cantly exceeds the 8% defl ection such a pipe would be expected to face in service.

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