Study On Dynamic Characteristics Of Bearing-ring Seal-rotor System In Seawater Desalination High-pressure Pump

In the reverse osmosis desalination system,segmental multistage centrifugal pump is often used as seawater desalination high-pressure pump,and the rotor is supported by the water lubricated bearings at both ends.As the only supporting part of the rotor system,the running state of the rotor system is affected by the dynamic characteristics of the water lubricated bearing directly.If the water lubricated bearing is not designed properly,the dynamic stability of the rotor system will be reduced,and even the critical speed of the rotor will break through the safety range,resulting in the resonance of the whole pump,which will bring irreparable loss to the seawater desalination system.Therefore,on the basis of considering the dynamic characteristics of water lubricated bearings,the establishment of bearing rotor system for critical speed analysis is of great significance to the reliability design of seawater desalination high-pressure pump.The structure of the multi-stage rotor of the seawater desalination high-pressure pump is relatively complicated.There are various ring seals in the rotor system,mainly including the ring seal,the inter-stage seal and the balance drum seal.When the high-pressure pump is running,the exciting force in the ring seal is produced by the fluid in the pump,which changes the dynamic characteristics of the rotor system and has an important influence on the critical speed and stability of the rotor system.Therefore,the dynamic characteristics of the ring seal should be included in the calculation of the critical speed and stability analysis of the rotor system,and the bearing-ring seal-rotor system should be established for research.The study belongs to the project which is supported by the 13 th Five-Year National Key Project "Development of key equipment and components for large-scale membrane seawater desalination"(2017YFC0403703).Based on the rotor system of the seawater desalination high-pressure pump,the dynamic characteristics of the water-lubricated bearings in the rotor system,the dynamic characteristics of the ring seal,the critical speed and stability of the rotor system are calculated and analyzed respectively.The main research contents and achievements are as follows:1.Seven groups of water lubricated bearing test schemes with differentcombinations of length diameter ratio and radius clearance are designed.According to the measured axial trajectory and circumferential liquid film pressure characteristics,the optimal combination of bearing radius clearance and length-to-diameter ratio is selected,and the water-lubricated bearing structure in the seawater desalination prototype pump is determined according to the equal ratio conversion of the clearance ratio parameter.2.Based on the static characteristic calculation theory of hydrodynamic sliding bearing,the capacity,eccentricity and deflection angle of water lubricated bearing are calculated by MATLAB.Based on the static characteristic of the bearing,according to the calculation theory of the dynamic characteristic coefficient of the hydrodynamic sliding bearing,the dynamic characteristic coefficient of the water lubricated bearing is solved by MATLAB programming.The internal relationship between the dynamic characteristic coefficient and bearing radius clearance,length-to-diameter ratio,rotor span and impeller series is analyzed,which provides bearing dynamic parameters for the calculation and analysis of the critical speed of the rotor system.3.According to the characteristics of the flow field of the ring seal,a quasi steady three-dimensional model of the ring seal is established.According to the dynamic model of the ring seal,the dynamic coefficients of the ring seal,the interstage seal and the balance drum seal in the seawater desalination high-pressure pump are calculated by CFD.The dynamic parameters of the ring seal are provided for the calculation of the critical speed and the stability analysis of the bearing-ring seal-rotor system.4.Without considering the annular seal,the critical speed of the bearing rotor system of the prototype pump is obtained by the finite element modal based on ANSYS Workbench.And the effects of bearing radius clearance,length diameter ratio,rotor span and stage on the critical speed are compared and analyzed.The results show that: the rotor system of the prototype pump is a rigid rotor,the first critical speed meets the design requirements of safety margin;when the radius gap is between0.12 and 0.24 mm,with the increase of the radius gap,the critical speed decreases in varying degrees,however it is still within the safe range;when the length-to-diameterratio is between 1.0 and 1.4,the critical speed increases with the increase of the length to diameter ratio;when the span is between 939 mm and 1179 mm,the first critical speed increases with the reduction of span,when the number of stages is 9,the rigid rotor is transformed into the flexible rotor,and the critical speed breaks through the safety margin.when the ring seal is considered,the critical speed of the rotor system of the prototype pump increases,and the first critical speed increases greatly.Compared with the rigid rotor,the critical speed of the multi-stage flexible rotor is more easily affected by the ring seal.From the analysis of rotor dynamic stability,the real part of the characteristic values of the prototype pump bearing-ring seal-rotor system are all negative,and the minimum logarithmic decay rate is 0.215.The stability of the rotor system is good.