Thermal fatigue analysis of district heating pipeline under variable frequency regulation of circulating water pump

The use of variable frequency control of circulating water pumps is an important energy-saving operation strategy to cope with changes in heating load. However, it can lead to low cycle fatigue failure of the pipeline when the cyclic stress generated by frequent water temperature fluctuations reaches the fatigue limit. Accurately prediction of the maximum total temperature difference cycles of pipelines is of great significance for ensuring the safe operation of centralized heating networks with frequent water flow rates regulation. Based on the actual operating data of 360,000 heat exchange stations, the stress amplitude and corresponding temperature difference cycles of pipelines at different operating temperatures were calculated, the maximum total temperature difference cycle of pipelines for 30 years was predicted. We calculated the stress limit of pipelines at different safety levels, and evaluated the impact of temperature on the fatigue life of pipelines under different building types and working conditions. The results show that the number of temperature difference cycles in primary network pipelines is three times that in secondary network pipelines, and the number of temperature difference cycles in pipelines of different building types varies greatly. The lower the operating temperature of the pipeline, the smaller the temperature difference fluctuation and the greater the contribution of pipeline fatigue damage when regulating the variable flow rate of the circulating water pump. The stress limit values of primary network supply and return water pipelines are 650 MPa and 759 MPa, while the stress limit values of secondary network supply and return water pipelines are 811 MPa and 946 MPa.