Degradation mechanisms and long-term performance of PUR insulation in district heating pipes

Licentiatavhandling, 2018

District heating networks are used worldwide, especially in Scandinavian countries and also in many other countries, e.g. Germany, Russia, United States, South Korea and China. From an environmental point of view, district heating networks are a good alternative to more traditional building heating systems.

In 1970th, the third generation of district heating systems was established on the market, where pressurised hot water was used as a heat carrier. The heat is transported with help of pre-insulated pipes, which should have good mechanical and thermal performance in order to withstand both external and internal loads, and minimize the energy loss. These performances are normally evaluated in accordance with the European Standard EN 253. To estimate the lifetime of pipes, an accelerated thermal ageing is normally performed at higher temperatures than at normal operation.

The aim of the studies included in this thesis was to determine the parameters affecting the lifetime of district heating pipes during accelerated ageing tests. In the first part, the focus was on determining the most relevant degradation mechanisms in the insulation foam of district heating pipes. Physical and chemical properties were studied with different methods to establish the most significant degradation processes. After that, district heating pipes were subjected to accelerated ageing at three different temperatures. The results of ageing were obtained after various periods and interpreted using the key findings obtained.

It was confirmed that oxygen plays a main role in the activation of the degradation processes, being faster at higher temperatures. The most interesting results were found in the mechanical strength measurements, where the pipe aged at 130~${}^\circ$C behaved quite different in comparison with the pipes at 150 and 170~${}^\circ$C. These measurements are often used to evaluate the performance of the pipe and in the prediction of lifetime. The degradation pattern of pipes was further confirmed by the results collected from thermal conductivity measurements and analyses of the chemical structure.

Our investigation points out that accelerated ageing of pipes at too high temperatures may cause other degradation mechanisms rather than acceleration of the natural processes. This may lead to erroneous estimation of pipe lifetime, leading to incorrect assessment of the district heating networks.