Assessment of the lifetime of district heating pipes
Doctoral thesis, 2020
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.
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.
FTIR
rigid polyurethane foam
artificial ageing
{District heating pipes
natural ageing
degradation mechanisms
mechanical performance
VDL Room, Hörsalsvägen 7(A), Chalmers Univeristy of Technology
Opponent: Prof. Mikael Skrifvars, University of Borås, Sweden
Author
Alberto Vega
Chalmers, Industrial and Materials Science, Engineering Materials
District heating systems generate heat in a centralised place and distribute it to many buildings in a city. The idea is to provide energy cheaply and from renewable sources if possible. In Sweden, there is more than 24 000 km of district heating pipes. The lifetime of the pipes has become an issue for the energy companies because many of the current pipes should be close to the end of their expected service life. That's why strategies and tools to determine the status of the pipes are needed.
In this work, we studied the degradation and assessment of the lifetime of district heating pipes. The pipes were exposed to elevated temperatures to reduce considerably the time needed to obtain relevant results. The focus of the accelerated ageing was the insulation foam, which is a crucial component in a district heating pipe, but also the weakest one. Mechanical and thermal properties were followed to observe how they were affected by the accelerated ageing. The chemical structure of the foam was also studied to identify any sign of deterioration.
It was important to compare the results from the tests performed in the laboratory with results obtained from pipes that have been exposed to natural ageing for more than 30 years in service. The results provided evidence that most of the pipes were still in good shape.
In this thesis, we proposed some new guidelines for the accelerated ageing of district heating pipes, as well as a new approach to perform the accelerated ageing which better reflects the operating conditions.
In this work, we studied the degradation and assessment of the lifetime of district heating pipes. The pipes were exposed to elevated temperatures to reduce considerably the time needed to obtain relevant results. The focus of the accelerated ageing was the insulation foam, which is a crucial component in a district heating pipe, but also the weakest one. Mechanical and thermal properties were followed to observe how they were affected by the accelerated ageing. The chemical structure of the foam was also studied to identify any sign of deterioration.
It was important to compare the results from the tests performed in the laboratory with results obtained from pipes that have been exposed to natural ageing for more than 30 years in service. The results provided evidence that most of the pipes were still in good shape.
In this thesis, we proposed some new guidelines for the accelerated ageing of district heating pipes, as well as a new approach to perform the accelerated ageing which better reflects the operating conditions.
Subject Categories
Mechanical Engineering
Chemical Engineering
ISBN
978-91-7905-359-8
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4826
Publisher
Chalmers
VDL Room, Hörsalsvägen 7(A), Chalmers Univeristy of Technology
Opponent: Prof. Mikael Skrifvars, University of Borås, Sweden