Water & Wastewater Treatment

32 | APRIL 2015 | WWT | www.wwtonline.co.uk In the know Digging deeper: plastic pipes 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 "The outcome of this investigation is vitally important for all parts of the value chain, allowing polyolefi n sewer pipes to continue to be specifi ed with increased confi dence in their asset life." 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 de… ection. 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% de… ection, 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 deŒ ned levels at the operating temperatures • Resins from which the pipes are manufactured fulŒ l the basic requirements as deŒ ned 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 defl ection 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 … exibility 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 Œ eld, 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 polyoleŒ n sewer systems all over the world. In my opinion the project has been conducted in a proper and scientiŒ cally reliable way with close cooperation between material producers and pipe and Œ tting producers with the target of demonstrating how long time service quality can be achieved in the Œ eld." PVC Sewer Systems The polyoleŒ n 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. Examples of excavated polyolefi n pipes. Plastic sewer pipes were fi rst used in the 1970s; materials have been signifi cantly improved over the years.

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