Development of a 'Nol Ring' test to study polyethylene pipe degradation and its implementation on field house connection pipes

摘要

In Europe, since 2003, premature brittle failures of polyethylene house connection pipes have occurred on some water networks using chlorine dioxide as a disinfectant. These failures are caused by the appearance and development of longitudinal cracks in the internal pipe wall. Inspired by the ASTM D2290-04 standard, a new test, called "Nol ring" (NR) was developed by the Laboratoire national de métrologie et d'essais (LNE) with the partnership of Suez- Environnement, to characterise the PE pipe degradation. Tensile tests were also performed on specimens taken along the pipe axis. It appears that the Nol ring test is more relevant for characterising pipe damage than the tensile test. Moreover, severe degradation can be revealed for pipes that still comply with the new pipe requirements. Then, a wide range of degradation states was observed and led to the definition of five categories of degradation, from new pipes to pipes with brittle failures. The comparison with the Quick Burst Test showed that the hoop stresses generated by the Nol ring Test were similar to those obtained with the Pressure Test. Finally, the nol ring test was implemented to characterise the pipe degradation for 115 samples exhumed from field under different operating conditions. This validation on field samples allowed to define three degradation categories and a new degradation index, called NRR. This newly classification applied directly to the Nol Ring's parameters, I.e the sample appareance and the elongation at break value, provide information regarding to the chemical oxidation and the initial material quality of the analysed samples. Thus, the developed test is relevant for characterising the level of degradation of polyethylene house connection pipes under usual operating conditions.