The Dynamic Characteristics Study On Multi-stage Centrifugal Pump Rotor Coupled System

The multi-stage centrifugal pump, as one of the key devices in fluid transport, has been widely applied in industries of petroleum, chemical engineering, steel, thermal power and sea water desalinization. At the present stage, the multi-stage centrifugal pump is developing towards the target of high flow rate, high rotational speed and high head of delivery, but the issues of vibration, noise and stability are also becoming more obvious at the same time. These issues, which are caused by the imperfect calculation model of the multi-stage centrifugal pump, has become the critical factors restricting the development of the multi-stage centrifugal pump rotor system. In fact, the issues of vibration and stability can be attributed to the fluid-solid interaction in rotor dynamics, and there exists not only rotor-bearing coupling issue but also rotor-seal coupling issue in the rotor system of the multi-stage centrifugal pump. However, the current rotor vibration model of the multi-stage centrifugal pump is still not sufficiently comprehensive, and the coupling effect of the multiple factors of sealing ring, bearing and axial forces has not been taken into account, so it’s not capable of the calculation under conditions of high flow rate, high rotational speed and high head of delivery. Therefore, the establishment of a more sophisticated coupling vibration model for the multi-stage centrifugal pump rotor system and more accurate prediction of rotor system dynamics are of vital importance to the research and engineering application.In this thesis, the research object is the multi-stage centrifugal pump rotor interaction system. The steady-state model of lateral motion, the transient model of lateral-axial interactive motion as well as the nonlinear motion model were established and the dynamic characteristics were calculated respectively. The dynamic behavior of multi-stage centrifugal pump was analyzed with different working conditions and structure parameters, and the optimum design method was explored. Meanwhile, the feasibility and accuracy of the numerical calculation method were verified by corresponding experiments. The main contents of this thesis can be summarized in the following four sections:Firstly, the static equilibrium position of the sliding bearing was solved by hybrid iteration of bisection method and secant method, and the dynamic characteristic coefficient of the sliding bearing was solved by perturbation method. The simplified solution model for the dynamic coefficients of self-aligning ball bearing with pure radial load was derived based on Hertz point contact theory and EHL theory, and the comprehensive stiffness dynamic coefficients including the effects of contact stiffness and oil film stiffness were calculated. The dynamic coefficients of finite-length annular seal were solved by Child’s theory of finite length and shooting method and then verified by experimental tests. The steady-state lateral motion model for the coupled rotor-bearing-seal system of the multi-stage centrifugal pump was established based on Lagrange equation, finite element method and matrix operation method.Secondly, the first domestic ’wet’ rotor system test rig for the testing of rotor coupling dynamic response considering the supporting effect the seal was designed and improved by the optimization of critical components in order to cope with higher rotational speed; The dynamic responses of multi-stage rotor coupling system under different conditions of annular seal pressure difference and seal gap were tested and the first-order critical speed of the rotor system was obtained. The reliability of the steady-state lateral motion model for the coupled rotor-bearing-seal system of the multi-stage centrifugal pump was verified, and the effects of seal pressure difference and seal gap on the steady-state dynamic characteristics and stability of the coupled multi-stage rotor system were investigated.Thirdly, the lateral motion model and axial motion model for the coupled multi-stage centrifugal pump rotor system including the effects of axial forces were established by Lagrange’s equation and finite element method, the axial motion model was verified by the critical speed of free longitudinal vibration of multi-stage centrifugal pump shaft system from the related literature. The interactive lateral-axial vibration model for the rotor-bearing-seal system of the multi-stage centrifugal pump was established by coupling the independent lateral and axial motion models using_matrix integration method. The transient response solution program for the coupled lateral-axial rotor system vibration was written by implicit Newmark iterative method, and the detailed investigation of the effects of operating conditions and structure parameters on the transient dynamic characteristics of the coupled lateral-axial vibration of the multi-stage centrifugal pump rotor system was performed.Lastly, the analytical nonlinear motion model for the single-stage centrifugal pump coupling system was established based on the classical Capone nonlinear bearing oil film force model and Muszynska nonlinear seal flow physical model and solved by self-designed fourth-order Runge-Kutta method. The nonlinear dimensionless differential motion equation was established for the coupled multi-stage centrifugal pump system by finite element method on the basis of the single-stage rotor coupling system, and the dynamic characteristics were analyzed. The effects of rotational speed, axial force, bearing length and seal gap on the nonlinear dynamic behavior of the multi-stage centrifugal pump rotor system influenced by various exciting forces were analyzed in detail.The research result reveals that the hybrid method possesses higher calculation speed and efficiency than bisection method and secant method. As to the first-order critical rotational speed, the maximum relative error of the calculation result of the multi-stage centrifugal pump rotor model introduced in this thesis is only 5.5% compared with the experimental result, with the rest of the errors less than 5%. The annular seal pressure difference, seal gap, axial force, rotational speed and bearing length all have great influence on the stability and dynamic characteristics of the multi-stage rotor system. Various nonlinear motion forms like periodic motion, quasi-periodic motion and multiple-periodic motion exist in the coupled rotor system. The research in this thesis can provide theoretical guidance and reference for the structure design and dynamic response optimization of the coupled multi-stage centrifugal pump rotor system.