Study On Secury Operation Of Hydraulic Transients In Long-distance Water Transmission

Many projects of long-distance water transmission have been built to resolve problem of the shortage of urban water resources, But most of them are installed by single or double pipelines, thus, the operation security is important problem in the management. However, the accidents of operation always result from the hydraulic transient including two kinds, hydraulic transient of open channel and pressurized flow.The hydraulic transient is the process between two different stable flows, which is the main reason for the water transmission accident. Then, establishing a stable hydraulic transient is a feasible method to keep the projects safety.An experimental platform was built to simulate the pipeline of water transmission by the observing and measuring the parameter of the process of hydraulic transient, and the measurement value was verified by the theories of volume arithmetic for U-style pipe, fluid transient of pressurized flow provided in the paper. Subsequently, the theories validated by experimental data were applied in the process of filling water and operation in a project case.The U-style pipe is defined as a style that the high level of pipe center shows as up-low-up, and the value that the high level at the lowest pipe subtracts from the pipe center of high level at downstream pipe is more than the length of diameter, owing to the characteristic of open channel free surface flow. And the pipelines are embedded along the high level of ground; the long-distance water transmission projects are composed by a series of U-style pipes, which can divide complex fluid transient into simple the stations of open channel flow and pressurized flow. The equations were deduced by the theories of hydraulic transient, such as arriving time of water head in open channel of U-style, the water level rising of upstream and downstream, the stable time of overfall, filling water into pipe by adding flow at upstream pipe or decreasing flow at downstream pipe with valve, and the equations were verified by the experimental data. The theories of the arriving time of water head was developed to keep pipeline safety by monitoring the leakage conditions. The process of filling water at first time was divided into 3 steps according to the target of operation in project, filling water, excluding air and enhancing flow, which were benefit for the application of study result and guide of operation.The valve operation model was formulated as an inverse transient problem to analysis the segmentation point of two-step valve closure and the constraints of optimal time of valve operation, and the model was solved by the optimization method. The valve should be closed as soon as possible when the pipe is leaking, but too quick valve closure will produce huge water hammer. So, it is necessary to guide the operation of valve by the optimal time of valve closure. The constraints of the problem is meeting pressure requirements including maximal water hammer pressure less than the pipeline bearing capacity, and the minimal pressure greater than the saturated vapour pressure for all pipelines in the project.The theories of safe long-distance water transmission were educed from the analysis of open channel flow and pressurized flow, which had been applied to simulate the two processes, filling water at first time and operation. In the first process, the concept of U-style pipe was provided to simulate the conditions of filling water, excluding air and enhancing flow so as to ensure the security and order of the process of filling water at first time. In the second process, the segmentation point of two-step valve closure was adopted as the 5% of the flow coefficient range owing to the characteristics of the butterfly valve. While the time of the two-step valve closure is shorter than linear valve closure by the optimal method for the inverse problem of hydraulic transient. Thus, the results of optimal two-step valve closure can be applied in project directly.The system of numerical simulation and analysis created by the study of hydraulic transient has guided the operation security of long-distance water transmission project. The results of experiments and theories applied can popularized to the processes of filling water at first time and operating in the long-distance water transmission projects in security.