Analyses of Axial Displacement Measurements from a Monitored District Heating Pipeline System

Abstract The need for understanding the response of district heating piping systems on temperature-driven loads to improve life-time analysis has led to the development of a monitoring programme. This programme includes the design of the connection of an instrumented section of piping within an operating district heating network. The design complies with the current European district heating recommendations and standards [1, 4]. There are different conditions under testing and minor differences in design of the sections causing different friction resistance in the bedding soil. The pipes´ axial displacements are recorded in six positions along each of the sections of the pipeline. For the first time axial displacements of four similar buried preinsulated bonded pipes were measured under defined on-site conditions. This experimental setup allowed preliminary results of axial displacements, which are relevant for the district heating design, operation and maintenance. The contribution focusses on the results of the first period of operation, where a defined start-up procedure is performed (temperature increase up to 90 °C, held constant for 60 days). Afterwards the operating temperature was reduced down to the ambient temperature. The start-up procedure was repeated once before starting the cyclic-operating procedure up to 140 °C. The measured maximum pipes´ axial displacements at operating temperature of 90 °C were around 25 mm in the bows of the monitored pipelines. The measurement results are between estimated values using current recommendation procedures (23 mm) and commercial computer programs (31 mm). In addition, residual displacements at ambient temperature level were measured. The impact of unloading-reloading cycles on pipe deformation behaviour will be a future research focus of the ongoing monitoring programme. The results of the four sections will be presented in a comparative way and a forecast on the cyclic-operating procedure will be given here.