Waterdiffusion, effects on preinsulated plastics pipes. Part 2

The results were hereafter compared to the theoretic and laboratoy results of the project 'Influence of Water Vapour Diffusion on Pre-Insulated Medium Pipes'. In that project a theoretical model was developed and verified by laboratory measurements. In this project the verification was done in relation to measurements in flexible pre-insulated pipes, which had been buried and in operation for several years. The investigation period stretches from April 1999 to August 2000. The investigations were to determine the influence of the pipeline on the ambient temperature and relative humidity in the ground as well as the potential water accumulation in the insulation foam and the related increase of heat losses. A pilot test was executed with the use of hygrometer/thermometer in a probe in Hvidebæk. The test showed that measurements with 'drilled down probes' do not work due to the fact that the influence of the probe on the measurements is too great. In 1999 a series of measurements were carried out on pre-insulated plastic pipes in Næstved. The results of the first measurements showed great deviations from the calculation model. After a thorough investigation of the calculation model it turned out that it is actually the medium temperature, which affects the diffusion. It appears that there may be a great deviation between the flow temperature from the district heating plant and the actual pipeline temperature, especially in service lines. In comparison, the service line temperature typically deviates up to 10 degrees. The results of this project confirms a clear connection between the theoretical model for water vapour diffusion and the actual water accumulation, which may occur in pre-insulated plastic pipes. It is thereby established that the theoretic model can determine whether water will be accumulated in the specific situation. This project has thereby lead to a tool to determine whether water vapour diffusion will be a problem for the given pipe. The most difficult requirement to fulfil for the user of the tool is to lay down the input data. The project has proved that thorough knowledge of the operational conditions in the pipeline is necessary. When the calculation model is applied on the pre-insulated plastic pipe used in this project, it will appear that the change in the heat loss even in the most critical dimensions is modest in spite of the relatively high water accumulation