Development of a non-destructive field testing method for thermal assessment of district heating pipes
Doktorsavhandling, 2022

The thermal state of a district heating (DH) network governs the heat losses. It is a parameter considered if a change is to be made in the network. Heat losses are costly and economic aspects are important when planning for the maintenance or replacement of DH pipes. In addition, knowledge of the DH network, and of the parts of the network that may contribute to high heat losses, is important for system control. Pre-insulated DH pipes have been in use for over 40 years. The thermal performance of these pipes decreases over time as a result of thermal aging, leading to higher heat losses. Present methods cannot assess these heat losses with high accuracy. This thesis proposes a developed non-destructive cooling method, the main purpose of which is to perform a temporary shutdown of a selected part of a network, and where temperature measurements are performed during the cooling phase.

This thesis presents the development process and the final method to use for thermally assessing pre-insulated DH pipes with high accuracy. The main research questions of this work were linked to the accessibility and measurability of the buried pipe or its connected parts. The methodology for developing the method is based on laboratory tests, field tests with several measurement points, and mathematical models of DH pipes and connected valves.

The work resulted in a method and a user guide that can be used by network owners to assess parts of a DH network. A method that by a shorter shutdown, in the range of a few hours, can be used to capture the temperature decline in a DH pipe. Results indicate that drainage valves, which are directly connected to the underlying DH network, were suitable measurement points where the temperature-decline phase of the DH pipe could be captured. The method allowed a prediction of the thermal conductivity of a buried DH pipe in operation with 2% deviation from the reference value.

heat losses

thermal status



cooling method

non-destructive testing

thermal conductivity

district heating network

Sven Hultins gata 6, in lecture hall SB-H5
Opponent: Ass. Prof. Toke Rammer Nielsen, DTU, Denmark


Peter Lidén

Chalmers, Arkitektur och samhällsbyggnadsteknik, Byggnadsteknologi

District heating (DH) is water heated in a power station, transported through kilometres of underground pipe networks, to finally supply residential and commercial buildings with heat. DH is often produced from what would otherwise go to waste in a specific location. It can be waste from the forest industry, household waste, or surplus heat from, for example, industries or data centres. It is economical and sustainable due to its resource efficiency.

Underground DH networks of pre-insulated pipes have been constantly expanded since their introduction in the late 1960s, and many of the old pipes are still in use. However, sooner or later the pipes need to be replaced. There are many possible reasons for pipe replacement, and increased heat losses due to aged and degraded insulation is one parameter to be considered when planning for a change in a pipe network. Statistics describing produced versus delivered heat energy show that approximately 10% of the heat on average is lost during distribution in the pipe network. Unfortunately, the age of a pipe alone does not reveal its present insulation status, as even very old pipes may be of good quality. Present methods for status assessment are unable to locate and assess these heat losses with sufficiently high accuracy. This thesis aims to fill that research gap and contribute with a method that can be used by network owners.

This thesis presents the development of a non-destructive field testing method for thermal assessment of part of a DH network. The method is based on field measurements conducted at several measurement points, and on mathematical models of a DH pipe and its connected valves. The results of this work are partly presented as a user guide that provides network owners with simple instructions for using the method, and convenient graphs for evaluating the thermal state of the examined network part.









Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5107



Sven Hultins gata 6, in lecture hall SB-H5


Opponent: Ass. Prof. Toke Rammer Nielsen, DTU, Denmark

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