Fatigue Strength of the Low-Pressure Rotors of Steam Turbines

Resumo

The effect of different factors on the fatigue state and fatigue strength of the low-pressure rotors of stationary steam power turbines is analyzed. Levels of variable stresses acting in the annular grooves beneath the thrust rings of the adapter disks of the low-pressure rotors are determined for the case of T-175/210-130 turbine units. Amplitudes of variable bending stresses with 50 Hz frequency in the grooves as functions of the displacements of the supports of the low-pressure rotors relative to the adjacent supports of the generator and an average-pressure rotor are obtained. It is shown that the position of the supports on the side of the generators when they turn out to be below the adjacent supports is the most unfavorable position from the standpoint of the resulting cyclic stress. With an unfavorable misalignment of the supports, the stresses produced by the intrinsic weight of the rotors together with the stresses from transverse oscillations of the shaft may exceed 20 MPa. The level of dynamic stresses in the grooves falls with a higher position of the supports of the low-pressure rotors relative to adjacent supports. Data are obtained that confirm and substantiate previously expressed theories of the influence of variable bending stresses acting in a corrosive-active medium on the formation of the fatigue cracks in the low-pressure rotors of certain types of central heating steam turbines. To reduce the formation of cracks, it necessary to achieve optimum alignment of the low-pressure rotors with adjacent rotors besides maintaining a required water and chemical regime. In selecting an optimum alignment, the influence of all the factors should be kept in mind and the variation of the support responses and operating time of the turbine unit in each of the characteristic operating modes should be taken into account.