In the Netherlands PVC is the pipe material that is most used: more than half of the drinking water distribution network is made of PVC pipes. Being able to assess and predict the condition of the pipes is vital for good asset management. The objective of this project is to numerically describe the current knowledge of the failure and degradation behaviour of PVC, and to embed it in the Comsima calculation tool. The water companies can then use the knowledge to support and underpin their replacement decisions, inspection programmes, lifespan predictions, exit assessments, and to develop new condition-assessment techniques.
Develop knowledge about PVC
PVC pipes are designed with an expected lifespan of 50 years under nominal loads. This means that the oldest pipes (about 10,000 km) are already at the end of their once anticipated life expectancy. However, research has shown that for good-quality PCV pipes that have not been loaded above standard levels a lifespan of 100 years can be expected. This research also identified and described the failure and degradation mechanisms that occur. Between 2007 and 2015 research was carried out indicating that there are in practice great variations in the quality and loading of PVC pipes. A basic understanding of the failure of PVC is therefore key to determining, and explaining, which pipes fail ‘prematurely’, but also to allowing for better predictions of the number of failures in an aging PVC pipe network.
By combining the mathematical description of different failure, degradation and loading mechanisms in a single model, one acquires more insight into their interrelation and into the individual contribution of each to the resulting total process. The tool has been developed with this objective in mind. Comsima combines physical load models, design standards and a parameterized description of the pipe and its surrounding, so as to approach the pipe condition with numerical simulations. The tool is also equipped with phenomenological degradation models for the purpose of anticipating, on the basis of available pipe data, the moment at which the pipe condition will no longer be satisfactory. In this way general knowledge about degradation can to applied to specific parts of the network, so that network measurement data and specifications can be translated into information about the condition.
Extension of Comsima
The objective of this project is to extend Comsima using current knowledge about the PVC material. Not all the relevant processes for this material have yet been implemented in Comsima. Mechanical aging of the material due to crack-growth, physical aging of the material and ring stiffness are lacking. Physics models have been described in the literature for each of these processes. The project will identify, supplement or describe these mechanisms, and embed them in Comsima.
A sensitivity analysis will be conducted on the completed model to determine which degradation mechanisms are dominant, and which system parameters are most important to be known. In this way, the current model knowledge concerning the failure of PVC as a pipe material will become accessible and clear. The knowledge will thereby be usable to support and underpin replacement decisions, inspection programmes, lifespan predictions, exit assessment, and to develop new condition-assessment techniques.
Literature on PVC
Breen J. (2006). Levensduurverwachting van bestaande PVC leidingen, Management samenvatting MT-RAP-06-18659/mso. TNO