Knowledge-based rules for PVC mains

As a result of the large-scale use of PVC by the Dutch drinking water sector since the 1950s, 52% of the current drinking water distribution network is made of this pipe material. Under normal circumstances, PVC is not affected by drinking water or the soil, but it is definitely susceptible to degradation mechanisms like chemical, mechanical and physical aging.

Because of changes over the period that PVC has been used (manufacturing methods, PVC composition, requirements), there is a great variety in the mains’ quality and associated failure behaviour. Despite their low frequency, mains failures are apparently unpredictable. This makes it difficult for water companies to make the right decisions in their asset management concerning PVC mains and their efforts to limit the number of failures. In this project, the existing knowledge about PVC mains is collected and embedded into knowledge-based rules to allow for the assessment of the failure behaviours and remaining lifespan of PVC mains.

Knowledge-based rules formulated from existing information

Over the past few years, a lot of information about PVC mains and their behaviour in water distribution networks has been generated by materials research (regular and failure research), failure registration and research into failure mechanisms. We collected the results of this work, analysed them closely, placed them in perspective and elaborated them into knowledge-based rules. We also described the history and standardisation of the material, so that research into an existing main can be related to the time of its manufacture and the standards prevalent at the time.

Failure behaviour is predictable with new set of knowledge-based rules

The key formulated knowledge-based rules are:

PVC mains produced before 1980 have, for PVC, a relatively high failure frequency.
When the main’s stress is known, and when research has defined its ‘slow crack-growth resistance’, the lifespan of a PVC main can be predicted with sufficient accuracy.
The ‘slow crack-growth resistance’ can vary greatly and is difficult to predict.
Physical aging occurs in the studied pipe sections, but is not directly related to parameters such as DCMT or age.

Still, the condition of a PVC water main cannot be established on the basis of a single rule. It requires a combination of research into the material properties of the main in question and the loading circumstances, which are a product of the surroundings. For mains whose failure would have a significant impact, failure risk can be determined through a combination of such research. Mains with a high failure frequency merit extra research to determine whether the cause relates to a low value in a material property or to a high loading. When PVC mains are installed, preference should be given to a high pressure class, so as to keep down the material stress during the mains’ use.

Extrapolated 50-year strength from 28 different research projects. The slow crack-growth resistance, with the 95% confidence interval per measurement, is sorted by age (the most recent pipe sections are on the left).