Water & Wastewater Treatment Magazine
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Water pipelines – is there life a er design life? Understanding the residual life of pipelines is crucial to making effective investment decisions, writes Guy Cleveland, pipelines specialist lead at Stantec www.wwtonline.co.uk | WWT | NOVEMBER 2019 | 23 The Knowledge Pipeline management appropriate decisions on rehabilitation or replacement. The primary cause of degrada- tion of cast-iron water pipe is corrosion of the pipe wall. Predicting failure The starting point for the pre- diction of the failure of a pipe- line must be identification of the possible failure modes. For iron pipelines, these include through wall perforation, bursting, crushing, longitudi- nal bending and joint leakage. Most dangerous amongst these other failure modes are circumferential fracture in smaller diameter pipes, and longitudinal fracture in larger diameters. Resistance to all of these, other than joint leakage, depends on the residual pipe- wall thickness, but whereas T he bulk of water supplies are carried by major supply aqueducts and distribution pipelines, which suffer deterioration with age. The water companies owning these ageing assets will firstly look to explore how they can enhance and improve their existing asset before consider- ing investment in complete new-build projects. This is where asset management, maintenance and renovation to promote additional design life becomes an attractive proposition. Cast-iron pipes have been used for many hundreds of years for the conveyance of water. Despite iron no longer being the predominant mate- rial for the construction of new pipelines, it still accounts for the greater part of the exist- for most failure modes the relationship is linear, in the case of the crushing mode, resistance depends on the square of wall thickness, and the effect of corrosion is thus greatly amplified. Unfortunately, the reli- ability of what seems to have become the traditional ap- proach to condition assess- ment can be questioned, on the grounds that it addresses only one of the potential, and unlikely, failure modes of cast iron water mains. The current, and widely used, remaining asset life calculation consists only of looking at the risk of through wall corrosion. Not only does this ignore other failure modes that are o€en observed, but it seems frequently to be done in a questionable way. ing water supply networks. Maximising the benefit derived from the enormous asset value of these existing cast iron mains is essential to the successful management of distribution and transmission systems. Proactive renovation must be properly targeted if all the benefits are to be realised. This targeting requires a reliable es- timate of the remaining asset life. The service life of cast iron water mains is dependent on the type of cast iron, the type of soil the pipe is buried in, and the loads imposed on the pipe during its service life. Whilst many cast-iron pipes used for UK water supply have already comfortably exceeded their intended design life, it is vital to understand their residual life in order to make