Influence of Branch Pipe Arrangement on Hydraulic Transition Process in Water Conveyance Systems

[Objective] Appropriately reducing the diameter of branch pipes is an effective way to lower the superimposed water hammer pressure in water conveyance systems. However, there is limited research on how replacing a single large-diameter branch pipe with multiple smaller parallel branch pipes affects the hydraulic transient process in such systems. Moreover, there is a lack of research on how to design the diameters of these parallel branches to minimize hydraulic transients while maintaining flow requirements and ensuring economic efficiency. Based on the Qitai high-head gravity-flow water conveyance project, we investigated the impact of branch pipe arrangement on hydraulic transients and proposes principles for determining branch pipe diameters. [Methods] Using numerical simulation, we compared the hydraulic transient behavior in complex pipelines with a single branch pipe versus multiple small-diameter parallel branches, and examined the impact of parallel arrangements on water hammer pressure and pressure fluctuation duration, summarized the design method for branch pipe diameters in parallel schemes, and verified the engineering applicability of the conclusions using a real-world case. [Results] When the number of parallel branch pipes increased from 2 to 5, the maximum positive pressure head in both main and branch pipes significantly decreased under various scenarios. Under the most critical condition, the maximum water hammer pressure was reduced by 10.32% in the main pipe and 48.75% in the branch pipes, respectively, compared to the single large-diameter branch layout. Simultaneous valve closures in the branches did not result in additional pressure increases in the main pipe, and water hammer waves between branches did not interfere with one another. Moreover, replacing a single large branch with multiple smaller branches effectively eliminated negative pressure heads in the branches, thus preventing negative-pressure-induced water hammer, and shortened the duration of pressure fluctuations. The maximum reductions in pressure fluctuation duration at key points in the main and branch pipes were 63.16% and 46.15%, respectively. For stable hydraulic transitions and economic feasibility, the ratio of branch to main pipe diameter at the connection point should be between 0.12<β<0.28. In the Qitai project, using two parallel branch pipes (β=0.226) increased construction costs by 33.33% compared to the single large branch solution, but reduced investment in water hammer protection by 39.10%. [Conclusions] Under the premise of safe water delivery, a water supply scheme that replaces a single large-diameter branch pipe with multiple smaller parallel branches can effectively reduce valve-closing water hammer pressure and shorten the duration of pipeline pressure fluctuations.