Study And Protection On End Valve-closure Water Hammer Of Tree Gravitational Flow Pipe Network

Gravity flow tree pipe network is an important form of long distance water transportation,which has the advantages of economy,reliability and safety.However,the water use conditions of each branch of the tree-shaped pipe network are complicated,and the water hammer phenomenon in the pipe seriously threatens the safety of the pipe network.In order to ensure the safety of tree pipe network and the comprehensiveness of water hammer calculation,this paper mainly analyzes the water hammer simulation and protection of gravity flow tree pipe network under various operating conditions.First of all,according to the basic theory of water hammer,the boundary conditions of pipe network system are derived.An experimental platform was set up to compare and analyze the valve closing water hammer installed at the end of gravity flow pipe with float ball type water level control valve and high proportion nozzle control valve.It is verified that the hydraulic automatic control of flow and pressure and the good effect of preventing the water hammer from closing the valve with high proportion spray hole control valve are proved.At the same time,the model simulation data and the experimental data are compared and analyzed.The results show that the software simulation results are basically consistent with the measured data.It lays a theoretical and computational foundation for the simulation calculation of valve-closing water hammer under multiple working conditions.Secondly,the calculation of single valve-closing and combined valve-closing at the end of gravity flow tree pipe network under the design conditions is carried out.Simulation calculation of valve closure for each branch pipe running separately is also carried out.The results show that the pressure fluctuation in the tree-like pipe network increases with the number of shutoff valves in the branch pipes.Under the design condition,the maximum pressure of water hammer is 233.27 m.When a branch pipe is running alone,the pressure of water hammer is 10 ～ 50 m higher than that of the water hammer at the end of the branch pipe when the valve is closed.The longer the length of the branch pipe,the larger the flow,the greater the impact of the water hammer on the whole pipe network.The branch pipes with close distance and little difference in diameter are prone to superpose of water hammer wave.The water hammer pressure of each branch pipe will obviously decrease when it is transmitted to the main pipe.Thirdly,by controlling the initial closing time of the valves at the end of each branch pipe,the maximum pressure of the direct water hammer produced by the valves is superimposed at each branch point,and the pressure process lines at each branches point are compared.It is found that the water hammer pressure rise generated by the superimposed valve closing of each branch of the tree gravity flow pipe is 10 ～ 30 m larger than the water hammer pressure generated by the simultaneous valve closing of each branch.The maximum water hammer pressure rise generated by each branch pipe being superimposed and closing the valve in turn is 246.37 m.For pipelines with different pressure values of each branch,the safety of the pipeline when the valve is superimposed on each branch should be checked.Finally,through the calculation and analysis of water hammer protection of engineering examples,it is found that the maximum water hammer boost of gravity flow tree pipe network generally occurs at the valves at the end of each branch pipe.Installing a certain number of box-type two-way surge tanks on each branch pipe can reduce the impact of valve closing at the end of the branch pipe on pressure fluctuations in the main pipe and prevent superimposed water hammer in the main pipe.Installation of high proportion nozzle control valve at the end of the branch pipe can effectively control the pressure of water hammer in the pipe network.