Study On Optimum Hydraulic Design For Vertical Pump System With Low Head

The water resources in our country are distributed very unevenly in both time and space. The East Route Project of South to North Water Diversion Project is an inter-basin and across province large water diversion project to resolve the serious shortage of water resources of the north areas in China, the main project of which is pumping station, and the pump system is the core of the pumping station. With the accelerating of China’s urbanization, developing of regional economic, a number of large pumping stations will be or plan to be built to improve the water enviorment of the city. The common feature of these pumping stations is lower head.Vertical axial-flow pump system, as a mature type, has been applied widely by hundreds of large pumping stations. Compared with other types of pump system, it has many advantages, such as running stably, high dependability, installing and maintaining easily, low capital investment, less maintenance charge, etc. Furthermore, a wealth of experience has been accumulated in designing, manufacturing, running, managing, etc. The hydraulic losses of the inlet and outlet conduits for a low head pump system account for a larger proportion in its head, the lower the head, the larger the proportion. At the same time, the flow in a vertical pump need to turn90degrees twice, while the water-head between the upper and lower reaches for a low head pumping station is smaller. This leads to the arrangement of inlet and outlet conduits in elevation of the direction more haste, which leads to a bigger conduit hydraulic loss. So, it is difficult to improve the pumping system efficiency for a low head vertical pumping station when the two factors act together.The pump system efficiency depends on pump efficiency and conduit efficiency. To increase the hydraulic performance of low head vertical pump system, focus on the optimal hydraulic design of inlet and outlet conduit and pump type selection, the main works and main conclusions including:(1) In order to study influence of the velocity circulation at the guide vane outlet on the hydraulic performance of outlet conduit for a large pump system quantitatively, it was put forward that the method to calculate velocity circulation and the method to measure the average angular velocity for the flow at the guide vane outlet of the pump system. The methods of both numerical simulation and model test were applied to research the circulation influence of circulation on the hydraulic losses for both siphon and straight outlet conduits. The results indicate that the hydraulic losses of the outlet conduits were obviously influenced by the circulation at the guide vane outlet, the hydraulic losses of the siphon and straight conduit under the condition of the optimal circulation were0.126m and0.180m lower than those under the condition of zero circulation, respectively; there is optimal circulation for the hydraulic loss of the outlet conduit to be minimized, the optimal circulations for siphon and straight outlet conduit are0.97m2/s and1.31m2/s respectively. The research results above could be helpful to optimal hydraulic design both for outlet conduit and guide vane of a axial-flow pump.(2) The uniformity devices with different vane numbers are designed to change the uniformity degree of velocity circulation at the guide vane, the model test method is applied to study the influence of the distributing of velocity circulation at the guide vane outlet on the hydraulic loss of outlet conduit for axial-flow pump quantitatively. The results indicate that with the vane number increasing, the changing range of hydraulic loss for outlet conduit becomes smaller and smaller. This account for that the vane number has effect on the distributing of flow velocity circulation and the distributing of flow velocity circulation has effect on the outlet conduit hydraulic loss, the influence degree are both small. The hydraulic loss discrepancy between uniformity device with12vanes and natural state (uniformity device with0vanes) for siphon and low-hump outlet conduits are3.85%and3.73%respectively. This account for that the flow velocity circulation obtained under normal state is close to even distribution. According to the research result, it is feasible to preset an average circulation at the inlet of outlet conduit when numerically simulating the outlet conduit, and the average circulation could be expressed by an average angular velocity. The angular speed could be measured by relevant model test.(3) In order to improve the hydraulic performance of a low head pump system, the design thinking of reducing nD value for the pump system was discussed. The consistency between reducing nD value and increasing impeller diameter while reducing pump speed was derived. The influence of reducing nD value on pump type selection, energy performance and cavitation performance for the pump system was discussed. Meanwhile, examination index for.the cavitation performance of the pump system and the influence of increasing impeller diameter while reducing pump speed on conduit control size and equipment investment was also discussed. The research results indicate that under the condition of a certain design discharge to decrease nD value suitably is beneficial for pump type selection and improvement of both energy and cavitation performance for the pump system. Therefore it is more suitable for a large low head pumping station with long running operation time to adopt a lower nD value.(4) The method of3D turbulent numerical calculation is applied to study the influence of the middle division pier on hydraulic performance for elbow inlet conduit, siphon outlet conduit and straight outlet conduit respectively, and the calculation results were verified by conduit model experiments. The research results indicate that the flow patterns in both inlet and outlet conduits observed by the model test are consistent with those by the numerical simulation method; the middle division pier almost has no influence on the flow patterns for the elbow inlet conduit, and the hydraulic losses obtained by numerical calculation will be increased by0.005m under the condition of design discharge; the middle pier is helpful to improve the flow patterns at the outlet of inlet conduit when the flow direction is defluxion at the inlet; due to the flow circulation at the outlet of the pump guide vane, the flow patterns in the outlet conduit are obviously asymmetrical and easily affected by the middle division pier, as a result, the hydraulic losses obtained by numerical calculation will be increased by0.024m and0.033m respectively under the condition of design discharge. The model test results are0.006m,0.023m and0.023m for elbow inlet conduit, siphon outlet conduit and straight outlet conduit respectively, under the condition of design discharge. Therefore, the middle division pier is not conducive to the hydraulic performance of the inlet and outlet conduits. The middle division pier for a large pumping station with low head could be cancelled if its structural design condition may be satisfied.(5) The design head and average head of Changgou and Denglou pumping stations are both lower than4m, the vertical pump system has been adopted by the two pumping stations. To obtain better hydraulic performance under the condition of low head, numerical simulation method is applied to optimal hydraulic design the inlet and outlet conduits for the two pumping stations. The conduit model test method was applied to verify the optimal result. The pump system efficiency was forecasted based on the test data for pump models on the same test bed and research results of the optimum hydraulic design for conduits. The pump system efficiency may be obtained by multiplying pump segment efficiency with conduit efficiency, where the pump segment efficiency in the pump system may be modified according to the results obtained through the pump model tests on the same test bed, and the conduit efficiency may be calculated according to the conduit hydraulic loss and pump system head. The model test of pump system was carried out. The research results indicate that the excellent hydraulic performance are obtained for the vertical pump systems of Changgou and Denglou pumping station under the condition of low head of less than4m. The efficiencies of the two pump systems reach78%and the NPSHc are lower than6m at both design head and average head working points. The vertical pump system also has a wide application prospect in low head pumping stations.