Degradation mechanisms in PUR foam of district heating pipes after accelerated ageing

Other conference contribution, 2017

Modern societies demand the use of non-fossil and sustainable energy resources. In this sense, district heating (DH) systems have been playing an important role in the last years. However, some questions still remains unanswered such as technical life time prediction, heat losses challenge or status assessment. These questions are the driving force of DH system development, especially for the DH pipe manufacturers.

Pre-insulated heating pipes include a HDPE jacket, rigid polyurethane (PUR) foam and steel service pipe, and are used to transport the heat from a central point out to the customers and then back again for reuse. These pipes must withstand axial mechanical loads and have good long-term thermal properties. Therefore, it is important to have reliable methods to evaluate the current status of a DH system at any time.

The main aim of the project is to determine the degradation mechanisms during natural and accelerated aging of DH pipes. In this project, the pipes have been aged using an accelerated thermal ageing and both mechanical and thermal properties were investigated. Mechanical property was evaluated using the SP plug method, which determined the remaining adhesion force between the PUR and the steel service pipe [1]. This method has also been designed for application in field for a quick status check. At the same time, the thermal property has also been measured in real-time using a transient plane source (TPS) technique instead of a steady-state method [2]. Characterization of the PUR material has been performed using a three-point flexural test, Fourier transform infra-red spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC-MS) analysis [3].

Preliminary results suggest three different phases where the degradation of DH pipes is a combination of physical and chemical phenomena. In the early aging period, the changes in both mechanical and thermal properties are caused by physical effects. After that, an upturn is observed due to changes in the PUR chemical structure observed in the FTIR analysis. Finally, the chemical degradation takes over which is an effect of thermo-oxidation.

At the end a better model comprising all three phases for the entire degradation process is needed and will be proposed in order to determine the real technical life time of DH pipes.