Thermal-Fatigue Analysis of District-Heating Pipelines in China

The temperature variation of the district heating (DH) pipelines during the operation is the most severe threat to the lifetime of the systems. The number of cycles used in the fatigue analysis in the European standard were based on the lEA projects, and there were no sufficient studies conducted in China. This paper aims to make reasonable estimations of the maximum number of temperature load cycles for DH pipelines in China. Based on the data, the rain flow cycle-counting algorithms were applied to calculate the number of full temperature cycles. The regression models of design lines between full temperature cycles and temperature variations were established. The maximum values for supply pipelines and return pipelines were obtained by using the design lines. The equivalent full action cycles ($b=4$ and $\mathrm{\Delta }{T}_{\mathrm{ref}}=110°\mathrm{C}$) are about 200 cycles for supply pipelines and 136 cycles for return pipelines which are both less than 250 cycles recommended in the European standard. Using 200 for the supply pipelines and 136 for the return pipelines, the stress limits could increase from 707 to 750 MPa and from 707 to 823 MPa respectively.