Numerical Investigation on Water Hammer Pressure in Long-Distance and Large-Diameter Gravity Flow Water Pipelines in Hilly Areas

The closure strategy of the downstream end valve is one of the important factors affecting the safety of long-distance, large-diameter gravity flow water supply pipelines. This paper takes the large-diameter gravity flow water supply pipeline of the Siqiao Reservoir Irrigation District Project in Quzhou, China, as its research object. Based on the basic principles of transient water hammer theory, numerical simulation methods are used to analyze the influence of different valve closure strategies on the water hammer pressure in long-distance gravity flow water supply pipelines under three conditions: linear (uniform rate) closure, two-stage (fast–slow) closure, and abrupt (fast) closure. The research results indicate that extending the linear closure time can effectively reduce the maximum peak pressure drop in the pipeline by up to 0.74 MPa. The two-stage closure is more effective than linear closure in reducing the pressure before the valve, with a maximum reduction of 0.233 MPa compared with the linear closure. The initial fast closure phase primarily governs the maximum pressure amplitude, whereas the subsequent slow closure stage dominates the amplitude and frequency of pressure oscillations after the valve is fully closed. The entire closure process collectively determines the dynamic characteristics of pipeline pressure changes. This study provides theoretical basis for the design of long-distance, large-diameter gravity flow water supply pipelines in hilly areas and research on water hammer pressure.