Effects of mechanical loads on ageing of district heating pipes

The first goal of this project is to investigate whether prefabricated district heating pipes in sliding zones, which are subjected to cyclical mechanical load, degrade faster than unloaded ones. The second goal is to put the failure mechanism with loss of adhesion between insulation and service pipe in perspective to other failure mechanisms.

It is likely that today's district heating networks could be a resource and supply heat for many years to come. It would require extremely large reinvestments and lead to negative resource and environmental impacts to replace them. We need data and methods to determine where and when it is time to replace parts of the networks, so that the remaining parts can function for many more years. In the project, we focus on pre-manufactured district heating pipes with service pipes of steel, insulation of polyurethane and casing of polyethylene.

A recently acquired knowledge is that mechanically loaded district heating pipelines degrade significantly faster than unloaded ones. In the project, we will investigate the degradation of old pipelines in the field in and outside the slip zones. The failure mechanism studied is loss of adhesion between the insulation and the service pipe, which can lead to fatigue of steel service pipes and crushing of polyurethane foam at bends and, by extension, leakage.

Experience from other failure mechanisms and associated investigation methods will also be compiled in the project. In a parallel project (funded by the Swedish Energy Agency), better methods are developed for lifetime assessment of district heating pipes, which, together with status assessments, are the key components to smart maintenance and selective replacement of worn-out parts. In that project, extensive laboratory investigations will be carried out under well-controlled conditions. Ultimately, a calculation model for estimating the remaining life should be developed, which better matches actual conditions in the field.