Techniques to Determine the Remaining In-Service Life of Polymer Pipes for the Water Industry

Abstract

Polymer pipes have been used in water distribution networks in the UK since the 1950’s. Prior to this, the water system implemented in the Victorian era had served the country for over 100 years. In more recent times, the Victorian pipe system has undergone extensive renovation. Much of this has required the replacement of the old network using polymer pipes, which are expected to have a working life of 50-100 years. The value of using polymer water pipes has been studied for many decades. Specifically, research has used set conditions to determine overall pipe in-service lifetime. However, these set conditions often do not mirror reality; in-service pipes experience and must cope with, different loads, varying soil types and changeable chemical composition of water additives. As a result, water companies currently have no methodology to accurately determine the remaining in-service life of their polymer pipes. Currently, a water pipe is only replaced when it has failed, or is deemed to have reached the end of its in-service life. The ability to do small scale tests to obtain accurate service life information would greatly benefit the planning of works and the locating of sites where pipes are no longer fit for service. This in turn would improve cost effectiveness of pipe replacement works and importantly, maintain good customer-relations. This thesis aimed to review how different techniques could be utilised to predict the overall lifetime of polymer pipes under various, more realistic conditions, and to critically assess each for suitability and accuracy. To thoroughly investigate each of these techniques, tests were carried out on polyethylene pipes. Two different polyethylene grades currently used by Thames Water were tested: PE100 used mostly as the central main pipeline and PE80 used to connect the mains to the customer. Accelerated ageing was employed to artificially age pipe material for varying periods of time and under different stresses and temperatures. In order to quantify the incurred effects upon the polymer pipe, specific markers pertaining to polymer carbonyl content, crystallinity, density and susceptibility to thermal oxidation were subsequently investigated. Extrapolation methods were then used to identify suitable markers for determining remaining in-service lifetime of polymer pipe. The results presented in this thesis are of numerous chemical evaluations carried out on various aged polyethylene samples. The identification and subsequent use of appropriate chemical evaluation techniques allowed the generation of a method that ranked pipes in terms of replacement priority. Furthermore, these results indicate that polyethylene degrades via a different mechanism in Arkopal as compared with water; it appears that Arkopal promotes polymer chain disentanglement, not oxidation-related embrittlement.