| Long distance water diversion projects are constructed to solve the problem of severely unbalanced distribution of water resources in China. By transferring water from resourceful regions to areas facing water shortage, water diversion projects optimize the allocation of water resources and thus the water resources can be fully and rationally utilized. In pressure conduit of water diversion projects, both valve opening and closing, pump starting and stopping would cause sudden change of velocity of flow in pipeline, leading to water hammer, which means hydraulic pressure rapidly increase and decrease alternatively. Either positive or negative hydraulic pressure is harmful to pipes. To ensure the safe operation of projects, appropriate measures should be taken to prevent water hammer. Therefore, research on the theoretical analysis and predictive control of water hammer has great significance and practical value. Air valve is used to defend negative pressure. It is an economic and safe water hammer protection measure which has been widely used in projects. However, high positive hydraulic pressure will happen when conventional air valve is used, because of the fast closing at the end of exhausting phase. It does great harm to pipes and is not expected. For best protection result, non-slam air valve is exploited. Nowadays, theoretical research on non-slam air valve is scarce; the numerical simulation is based on the model of conventional air valve, using smaller exhausting coefficient distinguishes non-slam air valve from conventional air valve. This measure is not in accordance with the working principle of non-slam air valve. Besides, most design of non-slam air valve is according to experience, its theoretical research is far below application. Therefore, establishing mathematical model of non-slam air valve and research on the protection efficiency and the influence factors has great significance. Moreover, research result can provide a reference for the optimization design of non-slam air valve. The major contents of this paper are as follows:(1) The background and significance of research are presented. The cause, classification, damage and prevention of water hammer are discussed. The research progress of air valve at home and abroad is summarized and the problems of current research and application are pointed out. (2) The basic differential equations of water hammer and the method of characteristics are described in detail. Using the method of characteristics to establish the common boundary condition in water diversion projects, including upriver, downriver, valve, pump and air valve. The solutions of boundary conditions are studied and the corresponding FORTRAN computer programs are built.(3) An improved mathematical model of non-slam air valve at exhausting phase is set up according to its two-phase exhausting working principle. It is based on the mathematical model of conventional air valve. Calculations are carried out for comparing the protection efficiency between conventional air valve and non-slam air valve. The influence factors of protection efficiency by non-slam air valve are analyzed and optimum values of factors are recommended.(4) Using the method of characteristics to establish the numerical model of practical project, and non-slam air valve is used to defend water hammer. The water hammer caused by stopping of pumps is calculated. The influence to water hammer and pump running by closing regulation of valve behind pumps is discussed. Secure and reasonable non-slam air valve protection measures are selected and the influence of number and location of non-slam air valve to protection efficiency are analyzed.μ...
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