Hydraulic Optimization And Structural Analysis Of The Super Low-specific-speed And Self-balancing Multistage Centrifugal Pump

Super low-specific-speed and self-balancing multistage centrifugal pump is widely used in agriculture, mining, oil chemical, electric power and other important areas for its high head and compact structure. But hydraulic components of this kind of pump is relatively long and narrow,it results relatively low efficiency and huge energy consumption, so it is necessary to optimize design high efficiency hydraulic components of Super low-specific-speed and self-balancing multistage centrifugal pump. As a result of the rotor symmetry layout structure, multi series and large span,in the process of operation, it is easily lead to deformation and failure of the rotor structure,and the resonance will occur especially when the rotational speed is similar to or simultaneously with the rotor critical speeds.Therefore, it has important practical value to analyze the statics and dynamics of the Super low-specific-speed and self-balancing multistage centrifugal pump.The main research contents and results are as follows:(1)Pump hydraulic optimization.The three-dimensional single stage model of the whole flow field of multistage centrifugal pump was established.The definition of the head and efficiency of impeller, casing and guide vane was given and the energy exchanging and the flow lost in the impeller, casing and guide vane was analyzed. The primary factors affecting the performance of the multistage centrifugal pump is the adaptation of impeller and guide vane and the secondary factors is the flow lost in the impeller. The area of the guide vane throat was calculated by making the absolute velocity in the vane throat equal to the half of the absolute velocity in the outlet of impeller.A channel radial guide vane was designed,and eliminating swirl in the impeller by adjusting the blade profile. The velocity changes in the impeller and guide vane of optimum scheme is uniform and slow, reducing the flow loss significantly.The optimum scheme model pump was manufacture and tested,showing that compared to the original model pump,the head and efficiency are significantly improved by 8.1m and 3.2% respectively, meeting the national standard requirement.Compared the simulation results and the test results,analyzing the differences between each other,providing the reference for the improvement of the hydraulic optimization of the super low-specific-speed multistage centrifugal pump.(2)Pump rotor static analysis. The torsional strength and fatigue strength checking on the shaft was carried out, the results show that maximum shear stressand maximum stress of the shaft are less than the corresponding allowable shear stress and yield strength, meeting the torsional strength and fatigue strength requirements.The pump rotor deformation was analyzed, shows that the maximum deformation occurs at the bottom edge of the outer diameter of the fourth stage of the impeller, and the amount of deformation is less than the clearance between the impeller and guide vanes, the amount of deformation of the impeller wear ring is less than the clearance between the impeller and impeller wear ring. So in the process of operation, the interference of the impeller and guide vanes, the impeller and the impeller wear ring will not occur. But the amount of deformation of the throttle sleeve are slightly more than the clearance between the sleeve and the throttle sleeve, and throttle sleeve wore after the actual operation of the pump, it is suggested wear-resistant material and tolerances should be used to ensure the leakage of the high pressure water and the pump rotor reliability.(3)Pump rotor dynamics analysis.The eight-order natural frequency of positive and negative precession of the pump rotor at dry state without prestressing, dry state with prestressing and wet state were calculated. The possible vibration form of pump rotor at each order and the differences of natural frequency of positive and negative precession at the same order between the three states were analyzed.The rate of change of the natural frequency of each order at three states were compared,it showed that the prestress of flow field can slightly increase the natural frequency of each order of the pump rotor,and it will be decreased significantly at wet state.Harmonic response of the pump rotor was calculated and analyzed on the basis of modal analysis at the dry state with prestress. Single unbalanced excitation force was applied to different impellers,shows that the unbalance of the fourth and eighth stage impeller have the greatest impact on the rotor and the amplitud responsee of the throttle sleeve is the largest,and the maximum amplitude occurs in a positive precession bending vibration at the level one critical speeds.When the unbalance excitation force is applied to rotor at different location,it does not change the critical speed,but it will change the maximum amplitude response of the rotor.Different amount of excitation force was applied to the same impeller shows that the maximum amplitude of the throttle sleeve increases significantly with the increasing of unbalanced excitation force,and the peaks occur in a positive precession bending vibration at the level one critical speeds,which shows that different amount of unbalanced excitation forceapplying to the same location does not change the critical speed.The response amplitude of the throttle sleeve increased significantly when there are two unbalanced excitation force applying to the impeller in the same direction. The response amplitude of the throttle sleeve is lower than single unbalanced excitation force applying when there are two unbalanced excitation force applying to the impeller in the reverse direction.The maximum amplitude of the response occurred in a positive precession bending vibration at the level one critical speeds at above all situation.The mode shapes of different orders at three states were compared,it showed that the prestress does not change the form of vibration and the vibration direction of positive and negative precession at each order,but the maximum amplitude position will be changed at some order.Compared with the dry state, the form of vibration and the maximum amplitude position of positive and negative precession at each order will not be changed by additional water quality, sticky and vibration damping at wet,but the vibration direction will be affect.Compared with the maximum amplitude of each order at three states,the maximum amplitude of negative precession at each order will not be affected by the prestress, but the maximum amplitude of positive precession at each order will be decreased significantly,however,the maximum amplitude of positive and negative precession at each order will be decreased by additional water quality, sticky and vibration damping at wet.Rotor bending vibration occurs the same The maximum amplitude of rotor bending vibration at each order will not be affected by the prestress and additional water quality, sticky and vibration damping at wet.Whether dry or wet state the torsional vibration maximum amplitude is the highest at each order. when pump rotor is running at rated speed,the critical speed of positive precession bending vibration and negative torsional vibration of the first order critical speed at level one at wet state are beyond the danger scope of resonance,which shows that structural design of pump rotor is reasonable safe and reliable,resonance does not exist in the operation.