Residual lifetime assessment of uPVC gas pipes

Abstract

Klis (www.loekvanderklis.com) and are used with permission. RESIDUAL LIFETIME ASSESSMENT OF UPVC GAS PIPES PROEFSCHRIFT SummaryThe Dutch gas distribution network consists of about 20% (22,500 km) of unplasticised poly(vinyl chloride) (uPVC) pipes, most of which have been installed from the mid-sixties up to the mid-seventies of the previous century and have been in service ever since. In the next decade the specified service lifetime of 50 years will be reached for these pipes. Replacing the uPVC gas pipes exactly after this specified service lifetime will lead to a costly and extremely labour intensive project. Postponing the replacement is only an option when this can be done without compromising the integrity of the network. It is therefore of great value for the network operators to have full knowledge on the condition of the pipes in their network. In this thesis the framework for a method that can determine the condition, and therewith the residual lifetime, of uPVC gas pipes is developed.Recent failure data shows that the majority of the failures in uPVC gas pipes is caused by excavation activities (third-party damage). The risk of life threatening situations after such a failure is considerably higher for a brittle fracture than for ductile failure behaviour of the pipe. Brittle uPVC gas pipes should therefore be replaced, which makes the impact behaviour the limiting factor for the service lifetime of these pipes. A review of the degradation mechanisms occurring during the lifetime of uPVC pipes shows that physical ageing is expected to be the most important mechanism that causes embrittlement. During physical ageing the polymer chains move towards their thermodynamically favoured positions, causing an increase in resistance against plastic deformation. Moreover the deformation behaviour localises, causing embrittlement on a macroscopic scale. The focus of this thesis is therefore on the influence of physical ageing on the mechanical behaviour of uPVC gas pipes. The procedure of determining the residual lifetime is based on these findings and is split into four aspects: the choice of a measure for the condition of the pipe material, characterisation of the change of the condition in time (its ageing kinetics), determining the critical condition and development of a method of measuring the current condition. Each of these aspects is individually described in consecutive chapters. i ii SummaryThe yield behaviour is selected as the measure for the condition of uPVC gas pipes, as the yield stress is a direct measure for the thermodynamic state (i.e. the age) and can also be linked to the impact behaviour of the material. The yield stress behaviour of uPVC is characterised using short-term tensile tests (at a wide range of strain rates and temperatures) in Chapter 1. The yield behaviour is accurately described by a pressure-modified Eyring relation that links the applied deformation rate to the yield stress and, vice versa, the applied stress to the plastic deformation rate. By hypothesising that failure...

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