Influence Of Different Shaft Section Lengths On Rotor Dynamics Characteristics Of Vertical Long Shaft Fire Pumps

The vertical long shaft fire pump is mainly used in fire-fighting places far away from land and lacking large amounts of water supply,such as offshore platforms and wharfs,taking the seawater as the fire-fighting water source.It has the characteristics of small footprint,large flow,high lift,fast start-up and stable operation.The difference between the vertical long shaft fire pump and other ordinary pumps is that the pump shaft is very long,and the length of the drive shaft can be adjusted according to the height of the sea level.When the sea level is lower than the installation of the pump system,the vertical long shaft fire pump can invert the water,which can avoid problems such as water diversion and cavitation caused by the higher suction height.The vertical long shaft fire pump is a large vertical rotating machine,and the stability of its rotor system is the key to the safety of the pump system.The thesis takes XBC18-178-240LC3 model of the vertical long shaft fire pump as the research object,studying the static and dynamic characteristics of shafting rotor connected by different shaft lengths.The CFD simulation method and the Workbench module are applied to study whole flow field simulation,fluid-structure interaction,strength check,modal analysis and the critical speed on the shafting rotor of the pump.The main research contents and results are as follows:(1)Analysis of hydraulic performance of the model: First,the numerical calculation and experiment of the single pump were carried and the results of simulation were compared to the experimental data of the hydraulic performance on the single pump to obtain the reliable numerical method for single pump.Then,the full flow field numerical simulation of the multistage pump used the same set with the single pump.Last,the simulation results obtained were applied to analysis of the static characteristics of shafting rotor.(2)Analysis of static characteristics of the shafting rotor connected by different shaft lengths: First,three different shaft connection schemes were set up,and then the total deformation of the shafting rotor(Z4,Z5,Z6)and its equivalent stress change law under the three schemes were studied.Finally,when the total length of the intermediate shaft remains unchanged,the influence of changing the length and number of the single-segment intermediate shaft on the total deformation and equivalent stress of the shafting rotor was analyzed.The results showed that Increasing or reducing the number of single-stage intermediate shafts will affect the deformation and equivalent stress of the rotor model.(3)Analysis of modal of the shafting rotor connected by different shaft lengths: First,the first 6-order natural frequencies and mode patterns of the single-stage impeller were analyzed,and then the dry and wet modes of the single-stage impeller were compared.Finally,shafting rotors' first 12 natural frequencies and vibration shapes of the Z4,Z5(original model)and Z6 models were analyzed.The results showed the additional mass of water will reduce the natural frequency and amplitude of the impeller in the water;increasing or reducing the length and number of the single-stage intermediate shaft has a greater influence on the natural frequency and modes of the shafting rotor.(4)Analysis of the critical speed of the shafting rotor connected by different shaft lengths: First,the influence of the shaft length on the critical speed under the same setting of boundary conditions was analyzed.Then the change law of the critical speed of shafting rotors of Z4,Z5(original model)and Z6 models in the 12 th order rotor mode was analyzed.Finally,the critical speed of the shafting rotor in the first 12 th order mode when the impellers of the Z4,Z5(original model)and Z6 models were located at different positions were analyzed.The results showed that the shafting rotors connected by different shaft lengths correspond to different orders of the critical speed;the critical speed of the shafting rotor increases as the length of single-section intermediate shaft decreased when the total length of the intermediate shaft remains unchanged.(5)Analysis of critical speed of shafting rotor under different positions of impeller:First,9 different impeller arrangement spacing schemes were established;then,the critical speed change law of the shafting rotor in the first 12 modes under different schemes was studied.The results showed that different positions of the impeller on the shaft have a great impact on critical speed of the shafting rotor.With the increase of the distance between the impeller stages,the low-order critical speed(6th order and within)of the shafting rotor with fewer shaft sections decreases;however,the low-order critical speed(5th order and within)of the shafting rotor with more shaft sections decreases with the increase of the distance between the impeller stages,and the high-order critical speed doesn't satisfy this law.Combining the above analysis,considering the statics characteristics of the shafting rotor,critical speed changes,the spacing distribution of impeller stage and actual engineering applications,the thesis selects the Z5-stage spacing 800 mm shafting rotor model as the best case,providing a theoretical support for the subsequent vertical long shaft fire pump design optimization.