Thermal Fatigue Analysis of the Secondary Network of District Heating Systems in China

Central heating is the main heating method in cities in northern China in winter, and it is also an important part of urban utilities. The safety of the heating pipe network operation is an important livelihood issue. Temperature fatigue damage of the central heating pipeline has a severe impact on the safe operation of the system, and reasonable estimation of the number of fatigue cycles during the service life of the pipeline is an essential link in the pipeline design of a district heating (DH) system. The temperature data of five typical secondary networks of heat exchanger stations in Jiancao District of Taiyuan City in four heating seasons were collected. The frequency of temperature change during operation was counted, and the maximum total temperature difference cycle number of pipelines was estimated. In this paper, the regression analysis method is adopted to fit the design equation of the number of temperature cycles of water supply and return pipelines in 30 years of service life, which provides a way of thinking for predicting the number of temperature difference fatigue cycles of pipelines in the design stage. In the calculation of the number of maximum total temperature difference cycles, the different values of temperature difference defining parameter $b$ and reference temperature difference $\mathrm{\Delta }{T}_{\mathrm{ref}}$ are analyzed. The results show that the selection of temperature difference defining parameter $b$ can effectively distinguish the effect of large and small temperature difference on the number of fatigue cycles of pipelines; at the same time, the cyclic temperature difference is normalized within a specific range of design temperature difference, which overcomes the problem of simplifying the guidance of the operation parameters of the central heating system and makes the predicted maximum total temperature difference cycle have universal guiding significance in a specific heating area.