Assessment of the lifetime of district heating pipes

Sammanfattning: This thesis is concerned with the degradation and assessment of the lifetime of district heating pipes, where accelerated ageing is used to study the degradation in a short time frame. The project was divided into four interrelated parts. In the first part, polyurethane foam obtained from district heating pipes was aged at an elevated temperature in two different environments, and the effects of the accelerated ageing on the physical properties and chemical structure were studied. In the second part, district heating pipes were aged using an accelerated method to examine directly the degradation of the polyurethane foam in the pipes. The adhesion strength and thermal conductivity were followed during the entire ageing process. The chemical structure of some polyurethane samples was also analysed to identify any sign of deterioration. In the third part, several naturally aged pipes were collected and their remaining adhesion strength measured, as well as their chemical structure to compare the natural and accelerated ageing. Finally, an alternative method for accelerated ageing was proposed, applying thermal and cyclic mechanical loads on the pipes at the same time and exposing the pipes to more realistic conditions than the traditional accelerated ageing. The results confirmed that oxygen plays a central role in the activation of the degradation process, which is faster at higher temperatures. The adhesion strength and thermal conductivity measurements showed that the temperature of accelerated ageing should be chosen carefully to avoid the activation of degradation processes that are not relevant for the specific application. The studies of natural ageing presented evidence that some pipes have lost only approximately 20% of their initial adhesion strength after 30 years in service. These pipes could probably be used for a longer time. Finally, the combination of thermal and cyclic mechanical loads is a promising method of accelerated ageing because mechanical loads affected both the adhesion strength and the chemical structure of the tested pipes, leading to more rapid ageing. This methodology should be adopted in accelerated ageing testing to avoid overestimating the lifetime of DH pipes.