Field experimental investigation of the insulation deterioration characteristics of overhead pipeline for steam heating network

The thermal insulation performance of pipeline is significant for the safety and economical operation of vapor steam network. In order to investigate the long-time insulation performance of pipeline, a field test was carried out in 12 different heating networks with the operating time ranging from 1 to 11 years. Two different type of insulation coating layer were included. The total heat flux and surface heat flux in pipeline were measured to obtained the equivalent thermal conductivity of the insulation coating layer, respectively. And a deterioration coefficient was applied to analyze the insulation deterioration characteristics. The results show that the one-dimensional thermal conduction method is feasible to calculate the heat flux of the pre-manufactured pipeline with hard insulation materials. And an error less than 2% for pipeline 1 was obtained. Further, a proper size of slip in the insulation coating layer has improved the insulation performance by 13.6% in this work. For the soft type insulation coating, the structure has been changed under the long-term effect of gravity. Accordingly, a single thermal conductivity cannot accurately characterize the heat transfer process within the insulation layer. The surface heat flux only account for less than 30% of the total heat loss. The insulation deterioration trend with operating time cannot exactly obtained by the surface heat flux. In addition to the thickness reduction at top and hollow at bottom, the convection heat leakage from the gaps should also be concerned for long time operation.