Analysis and Research on Hydraulic Transients and Optimum Operation Dispatching in Cascade Water Delivery Systems
|Course||Fluid Machinery and Engineering|
|Keywords||Hydraulic transient Water hammer prevention Pumping system Cascade pumping stations Water delivery project Dynamic planning Optimum dispatching|
With the increasing contradiction in supply and demand of water resources, more and more large scale water delivery cascade pumping stations are built. Such a kind of project has not only long distance, large discharge, and high lift, but also various types of hydraulic structures and versatile flow patterns. The hydraulic machinery and the control components as well as the water delivery systems consist of very complex fluid network construction. For such kind of systems, the major problems that should be solved urgently are safety and economical operation. That is, the hydraulic transients in the system should be forecasted correctly, and appropriate measures should be adopted to protect it on the basis of the calculations. As for the economical operation of the pumping systems, the water delivery cost should be decreased through proper operational dispatching, since water is lifted and delivered to the objective regions by the use of electrical energy.Hydraulic transients in pump systems are refered to as the transition state that the flow and the pump unit change from a steady state to another. The reasons resulting transients are either the disturbance by human beings, such as valve stroking, regulation of the vane angles of the pumps, regulation of the rotational speed of the pumps,even the regulation of the gates in the open channels, or the disturbance of the system itself due to accidental events, such as power failure to the pumps. Since the hydraulic transients caused by power failure to pumps are the most serious in most cases, and are the major research topics for the control of them, this thesis puts stresses on the hydraulic transients in complex pumping systems, so as to probe into the physical nature, and to lay a foundation for the control of the transient processes.In the research of the hydraulic transients, correct analysis and study of the transient flow properties of the pump system are of importance for the design and the operation of the devices and the water delivery channel. The calculation of the hydraulic transients in light of the real structure of the pump system and the components can forecast the limit of a lot of important parameters, the discharge capacity of the system,the regulation schemes in the transition processes of the operating conditions, and the influence of the accidents on the system, so that the optimal emergency measures can be chosen. For water delivery projects, calculations of the hydraulic transients according to the real structure of the pump system and the components can also reach at the optimal structure scheme and the design parameters for the system, so as to reduce the project cost, and to promote the safety reliability and the operational stability during the system operation.In the thesis, mathematical models for the hydraulic transients in complex pumping systems are developed based on the thoery of unsteady flows and the computationalmethod in the light of large scale cascade pumping stations, which include those of two-phase hydraulic trasients in long pipeline water delivery systems, complex pumping systems consisted of pumps, pipes, canals and ponds, and the jointly operating cascade pumping systems. By the use of the mathmatical models as a computational platform, detailed calculations and analyses of typical projects are carried out on the basis of the establishment of the boundary conditions of air valves, surge tanks/one-way surge tanks, and the special boundary of water column separation and rejoining, and the water hammer protective stratergy for long-distance water delivery systems is put forward, which supplies technical bases for the perfection and the development of hydraulic transient computation thoery, and for the optimum design and the economical operation of the large scale water delivery projects.Utilizing Guangdong Dayawan Water Supply Project as an objective, the protective functions of different water hammer protective measures are studied in light of the conditions and the properties for the occurrence of the column separation in long distance piping systems, and the water hammer protective strategy that multi one-way surge tanks with air valves operate jointly is put forward, which will not only promote the quality of the water hammer protection, but also reduce the project investment on it. They are of great importance for the design and the safe operation of the large scale water delivery engineering.Due to the restrictions of the topography and the construction conditions, large scale water delivery systems with cascade pumping plants often consist of various hydraulic structures, such as pipes, open channels, tunnels and reservoirs, and so on, so the hydraulic transients in such kind of systems should also be studied urgently, in which, besides the unsteady flows in pressurized pipes, there are open channel flow processes and the interaction between them and the pressure transients. By open channel unsteady flow, it means that the velocity and the water depth in the channel will be varified, which ofen occurs in water transportation systems of a pumping plant, the diversion and draft-tube pipes of a hydropower plant, rivers in flood periods, and tidal estuary. For which, the thesis establishes an intergrated calculation model for the open channel flow and the pressurized flow, and constructs a united computational platform for the hydraulic transients of the whole system of the large scale water delivery project based on the study of various different computational methods, on which, it is realised that pump-pipe-canal-pond in multi-stage pumping systems and the alternate transient flows can be simulated jointly.For the startup of the large scale pumping station with a siphon outflow channel, the two-phase transient model for the calculation of starting hydraulic transients is set up, which is necessary for the forecast of the maximum pump head and the load of theelectrical motors, and will supply a technical basis for simplifying the startup procedure and decreacing the investment in the priming devices, at the same time, it supplies the boundary conditions for the calculation of the unsteady flow process during the start of the cascade pumping stations.Computational results show that, since the rotational speed of the pump unit rises linearly, the flowrate in the discharge tank supplied by the pumps has a tendency of linear increase. Thus, the boundary condition of the pumping station for the simulation of the hydraulic transient in the whole hydraulic system may be taken as that of a linearly-increasing discharge. The calculation results accord to the experimental one, which demonstrates the correctness of the mathematical models.In order to realize the dynamic discharge balance in large scale long-distance water delivery systems, a dynamic regulaton model based on the hydraulic transient simulation for the cascade pumping project is set up. By the use of Guangdong Dongshen Water Supply Project as an objective, hydraulic transients in every operating condition have been simulated and analyzed, which include the unsteady flows in the delivery channels during the startup of the pumping stations, the unsteady flows caused by the power failure to a single pumping station and the countermeasures of stopping the pump units at the upper/downstream pumping plants, and the hydraulic transient processes and their controls if power failure occurs to the whole pumping system.In the calculations, the method of startup in the open channels from a virtual flow state and the mathematical model of the alternative transient flow in the open channels has been applied, which further demonstrates the availability and the practicality of the computational platform for the hydraulic transients in large scale cascade pumping systems. At the same time, the calculating results supply a foundation for the dynamic regulation and the dynamic simulation of the cascade water delivery projects, and supply a basis for the suppression of the surges in the transportation channels. It also lays a foundation for the continuous online regulation of the pumping stations installed with vane adjustable pumps, so that water waste can be decreased and energy and water savings, as well as the optimum dispatching of the pump systems can be realised.By the line flowrate balance, it means that, under the prerequisite that the pumped flowrate at the inlet of the 1st stage pumping station or that at the end of the last stage pumping station be given, taking into account of the division flowrates and their variations, the inlet and the outlet flowrates at every stage pumping station are equal through the appropriate regulations of the number of the pump units and the vane angles, so that the target of stable operation in the whole line, no water waste or empty bottom can be reached at, so as to avoid the waste of water and energy resources.In order to realize the economical operation of the large scale cascade water deliveryprojects, the thesis sets up optimum dispatching models for the pumping stations based on the fitness of curved surface and the dynamic planning method, constructs the relationship between the installation efficiency, the vane angle,and the static head and the discharge, by the utilization of the principle of dynamic planning, establishes the optimum dispatching model, which can be used for the optimum dispatching of the pumping station taking into account of time,vane regulation, speed regulation, as well as different types of pumps. At the same time, the thesis puts forward the optimum dispatching model for the cascade pumping stations based on the dynamic planning method and the decomposition-coordination principle, which may take into consideration of the influences of the unit prices of the electricity at different regions in different times on the optimum dispatching. Since the optimization for the subsystems is apart from the requirement that the pump types should be the same, the method can be used to the calculation of the optimum dispatching of the internal pump units in the pumping stations, as well as the calculation of that of different intervals. Through which, it can be realised that the real simulation and the optimization of the operational dispatching for the pumping systems with variable rotational speeds and adjustable vanes in large scale cascade pumping stations.Although the thesis have made a lot of studies on the hydraulic transients in large scale cascade pumping water delivery systems, the versatility and the restriction of the models for the two-phase transient flows, hinder their development, especially, the studies are often not harmonous with the fluid-structure interaction. The further research will be given to the liquid-solid coupling to perfect and develop the column separation mechanism and mathematical models.At the mean time, since the real control and the optimal dispatching have a strong demand on the real-time of the transient modelling, while for unsteady flows in the long-distance open channels, the requirement in the stability of the simulation models will slow down the calculating speed of the computer, the next work of the author’s will also be stressed on setting up models with high speed of convergence, as well as the establishment of the parallel computation of the transient flows and the increase of the data transmission speed in numerical simulations.