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

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\ua0 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

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

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

Development of a Non-destructive Testing Method for Assessing Thermal Status of District Heating Pipes

Pre-insulated district heating (DH) pipes have been in use for more than 40\ua0years. The thermal performance of these pipes decreases over time as a result of thermal and mechanical aging, which leads to higher heat energy losses. A non-destructive method based on a cooling method is under development for assessing the thermal performance of a DH network. 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 temperature at the position of the copper wire. The method involves a shutdown of a selected part of a network for less than 2\ua0h and evaluating the cooling process by measuring the supply pipe temperature and the temperature of the copper wire. The method was applied in a DH network during heating season. The thermal conductivity calculated by the method [0.027\ua0W/(m\ua0K)] was in good agreement with the reference value [0.026\ua0W/(m\ua0K)]. The cooling of the network can be measured at a valve or at other more easily accessible steel parts. It was also shown that the method is applicable for DH networks without a copper wire. However, a temperature measurement is needed on the casing pipe

Diagnostic protocol for thermal performance of district heating pipes in operation. Part 2: Estimation of present thermal conductivity in aged pipe insulation

Buried and operating district heating (DH) pipes are exposed to thermal degradation of their polyurethane (PUR) insulation over time, and their status is hard to assess without excavation. By using DH pipe valves in manholes as measurement points during a shutdown with an ensuing cooling period, non-destructive assessments can be performed. This study compares new improved field measurements with numerical simulations of the temperature decline in drainage valves and shutdown valves. The drainage valve measurements were used to thermally assess part of a buried DH network. Results indicate that by using the drainage valves as measurement points in a cooling method, the thermal conductivity of the buried DH network could be predicted with an accuracy of >95%. In addition, a general diagnostic protocol has been established for assessing the thermal status of a DH network, ready for network owners to use

Diagnostic protocol for thermal performance of district heating pipes in operation. Part 1: Estimation of supply pipe temperature by measuring temperature at valves after shutdown

This study evaluates temperatures measured at district heating (DH) valves in manholes and their usability for non-destructively assessing the thermal performance of buried DH pipes. The study was conducted as a field test in which part of a DH network was shut down and the temperature decline in the valves was analysed in terms of absolute temperature and thermal response time from the DH pipe to the top of the valve. The calculated and measured supply pipe temperatures by the drainage valves were in good agreement, with 1% deviation. The valve measurement analysis from this study shows that the drainage valve has good potential to serve as a measurement point for assessing the thermal status of a DH network. However, the shutdown valve measurements were greatly affected by the manhole environment

Non-destructive methods for assessment of district heating pipes: a pre-study for selection of proper methods

Many energy companies are facing renewal of their district heating and cooling (DHC) network. However, there are no non destructive methods to determine the performance of existing pre-insulated pipes during operation. A pre-study has led to a selection of a couple of non-destructive methods that will be further evaluated and tested in field and in laboratory. Two non-destructive methods, individually or in combination, are considered interesting for further studies. A cooling method and a method for evaluation of the temperature coefficient of resistance (TCR), which aims at using the existing copper wire in pre-insulated pipes