Development of a non-destructive testing method for thermal assessment of a district heating network

Licentiate thesis, 2020

This thesis presents the development of a non-destructive testing (NDT)method for thermal assessment of pre-insulated district heating (DH) pipes with high accuracy, in which the development process from literature review to its present stage is presented and discussed.

Pre-insulated DH pipes have been in use for more than 40 years. The thermal performance of these pipes decreases over time as a result of thermal aging, which leads to higher heat  losses. Present methods are unable to assess these heat losses with a high accuracy.

The main idea with the method is to perform a temporary shutdown of a selected part of a network for less than two hours, which enables temperature measurements during the cooling phase. Measured temperatures are then used for analyzing the thermal performance of the pipes. The accessibility for temperature measurements on the pipes depend on the conditions in field. Thus, the methodology for the development of this cooling method involves different measuring points during different conditions in field.

This thesis covers three conducted field tests during maintenance, which allowed for temperature measurements on the service pipe, the casing pipe, and connected valves. Furthermore, the method utilizes the copper wire, which is already embedded in the polyurethane insulation for detection of water leakage, as a sensor for measuring the mean temperature at copper wire position along the pipe under assessment.

This thesis presents the possibilities and uncertainties with the cooling method at its present stage. The method shows good potential to meet the aim as an NDT method with high accuracy, and to be a future tool for the network owners.