PVC pipes in gas distribution: still going strong!

In the Netherlands (impact-modified) PVC is the preferred material for low-pressure (30 and 100 mbar) gas distribution systems. More than 50% of the total length (about 122,000 km) of this system is rigid PVC or impact-modified PVC. The installation of rigid PVC (uPVC) pipelines started about 50 years ago. Presently, about 22,500 km of rigid PVC is still in operation. In this paper the good experiences with rigid PVC gas distribution systems in the Netherlands will be illustrated by results of regular leak surveys and test results on pipe samples taken from the gas grid. It will be shown that the leakage rate of uPVC pipe systems is very low and about equal to that of PE and steel pipe systems. Impact tests show no significant decrease in ductility with respect to time of use. Furthermore, the ductility of the PVC pipeline materials which have been in use for many years is shown to be mainly dependent on the (initial) quality (degree of gelation). The good performance of PVC gas pipeline systems is also proven by modelling studies. It will be shown that the long-term failure behaviour of uPVC is determined by the ability to yield. Failure will occur if a certain critical value of the plastic strain is surpassed. Using this model the long-term behaviour under internal pressure of rigid PVC pipes can be predicted quantitatively

Lifetime Assessment of Load-Bearing Polymer Glasses: The Influence of Physical Ageing

The timescale at which ductile failure occurs in loaded glassy polymers can be successfully predicted using the engineering approach presented in a previous publication. In this paper the influence of progressive physical ageing on the plastic deformation behaviour of unplasticised poly(vinyl chloride) (uPVC) is characterised and incorporated in the existing approach. With the modification it is possible to quantitatively predict long-term failures which show a so-called endurance limit. The predictions are compared with failure data of uPVC specimens which were subjected to constant or dynamic loads. In dynamic loading conditions a second type of failure mode was observed: fatigue crack growth. A brief study on the influence of the frequency and stress ratio of the applied stress signal shows that crack growth failure is not expected to occur within experimentally reasonable timescales for constant loading conditions