Study On Vibration Characteristics Of Shafting System With Continuous Three Braces(N+1 Braces)

With the continuous growth of national energy demand and the optimization of energy structure,thermal power generation is developing towards efficient and environmentally friendly ultra-supercritical units.The secondary reheat ultra-supercritical unit has better thermal economy and safety and is an effective way to further improve the thermal efficiency of thermal power plants.With the increase of unit parameters,the shafting structure becomes more and more complex.Some models of ultra-supercritical secondary reheat units adopt a new type of continuous three support shafting structure.Although the new shafting structure can effectively shorten the length of shafting and improve the critical speed of rotors at all levels,it also brings some problems,such as unstable vibration of some bearing bushes,sensitive vibration and installation process of the bushes,which affect the safety and stability of the unit operation.Variable support based on multi span rotor experiment system to simulate the secondary reheat ultra supercritical unit shaft system,through the vibration testing and finite element analysis,studies the three support the vibration of the shafting structure features,unbalanced problem such as frequency response and the support stiffness,vibration data analysis and combined with the actual site,for the safe operation of such unit and provide a basis for the vibration fault management.In this paper,a multi-span variable-braced rotor experimental system,which is in line with the dynamic characteristics and shaft structure characteristics of each rotor in a practical unit,is built to simulate the continuous three-braced shaft structure of a secondary reheat ultra-supercritical unit.Then,multi-group vibration tests and experimental studies are carried out under the double braced structure and continuous three-braced structure respectively.The vibration characteristics of the rotor and the transmission law of unbalanced frequency response under different shafting structures were analyzed based on the data obtained from vibration test.At the same time,the field start and stop data of large steam turbine are analyzed.The results show that with the increase of rotor mass,the vibration coupling effect is basically that the rear rotor affects the front rotor.At the same time,near the critical speed of adjacent rotors,the vibration of the unbalanced rotor will also be excited.In this paper,a geometric finite element model was established based on the rotor experiment system.By comparing the modal analysis results of the traditional double-braces shafting structure and the continuous three-braces shafting structure,and verifying each other with the experimental results,the coupling vibration characteristics of the shafting system under different shafting structures and the vibration transfer law between adjacent rotors were studied.Combined with the finite element analysis,the first-order critical speed of the middle and high pressure rotors under three continuous support structures decreases obviously with the change of shafting structure.The first-order mode of each rotor will produce vibration transmission.In practical engineering,the co-direction and reverse vibration components excited by adjacent rotors should be taken into account when the vibration fault management of large unit with continuous three-braces shafting structure is carried out.In this paper,based on the finite element model of continuous three-braced shafting structure,harmonic response analysis was used to calculate the frequency response and natural frequency of each rotor under different bearing support stiffness,and the influence of bearing support stiffness on the vibration characteristics of continuous three-braced shafting structure was studied.The results show that with the decrease of bearing support stiffness,the natural frequency of the low-pressure rotor remains unchanged,and the natural frequency of the medium-pressure rotor and the high-pressure rotor decreases,and the excited amplitude increases when the running speed reaches the critical point.In the actual vibration control of the unit,it can be considered to adjust the bearing support stiffness to avoid the working speed and critical interval.