Supervision of District Heating Networks by Thermography
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Background
In those countries which have a long tradition of district heating, a new concern has arisen recently: certain district heating networks are approaching the end of their technical lifetimes and the heat loss in older culvert networks in increasing significantly. In order to chart the requirements and resources for maintenance measures, it is important to be able to diagnose the conditions of the piping network.
One method increasingly used for heat loss detection is based on airborne and groundborne thermography. In this method the mapping of the ground temperature can be used to give qualitative information about the network conditions, mainly with the aim of finding leaks. With the help of more refined analytical methods it is possible in the long run not only to trace leaking media pipes, but also to determine the condition of the insulation. In that way it will be possible to draw conclusions about any damage to the protective casing of the culvert by determining the amount of heat loss quantitatively.
For the last couple of years Studsvik has been working on the development of a method for quantifying heat losses from district heating pipes by means of thermography. The literature gives an overview of different parts of the earlier and present projects which have led to the heat loss interpretation model.
Aims of the project
The aim of this project was to verify a model for quantitative determination of heat losses using the temperature profile on the ground surface above a pair of hot water district heating pipes. In practical applications, this profile can be generated by means of thermographic methods.
A second aim of the project was to extend the model to include pipes for steam distribution and district cooling.
Scope of work
The work to be carried out deals with the following problems:
- refining the model by systematic sensitivity studies including second order effects due to varying ground properties.
- modification of the model for steam pipes.
- development of model extension for coolant distribution pipes.
- experimental verification of the model on a test pipe with controlled heat supply, simulating hot water and steam pipes, respectively.
- application of the model to a thermographic evaluation system.
Conclusions
- The model for quantitative thermography analyses has shown to enlarge the range of applicability compared with conventional thermography evaluation. Under ideal conditions the heat loss from district heating pipes can be analyzed within an accuracy of +/- 10%.
- The interpretation model was applied in a field experiment in Västeras showing its usefulness in the further analysis of questionable sections as detected in the course of a general thermography survey.
- The interpretation model needs an input value of the trench depth with a higher accuracy than often available from drawings. In this case the use of electronic sensor techniques for pipe detections might be a possibility.
- The thermal conductivity of the soil has - in contrast to a common opinion - only a relative small influence on the heat loss.
- The ground must be dry, otherwise evaporation of water on the ground will affect the temperature profile. However, well wetted surfaces can, under come circumstances, also be used for qualitative evaluation of the pipe standard.
- A drying surface will cause misleading results due to latent heat transport.
- Thermography during exposure to long wave and solar radiation is possible if the surface is uniformly exposed to the radiation and if it has a homogeneous emissivity within the surface area to be analyzed.
- The prevailing wind conditions affects the temperature profile for several hours. This effect can be included in the interpretation model with further analysis.
- The TX factor measured at a certain moment represents the insulation status (and temperature condition) from a period starting about one week prior. Hence, changes in the insulation status which are more recent than this, can not be correctly analyzed. Similar, if the distribution temperature changed drastically within the preceding week or so, one must be careful in interpreting the thermography results.
- The interpretation model can also be used with high accuracy for steam distribution systems. Its higher temperature level compared to hot water distribution systems facilitates the evaluation of the thermography picture.
- It is difficult to quantify the heat gain in district cooling applications by means of quantitative thermography analysis due to the small temperature differences at the ground surface.
- Quantitative thermography analysis can be used for leakage detection in district cooling systems. However, the process of leaking water soaking the ground must be further analyzed.
