An Investigation Of The Flexural Vibration Behavior Of Slender Rotors In Drum-Type Condensing Turbines

LecturePg. 71-86.With turbine rotors of low shaft elasticity (large diameter and small bearing span) the increase in amplitude at the first point of shaft resonance in the speed range is generally slight. Consequently no particular attention is paid to this point of resonance either at the rotor design stage or during operation of the turbine. However, if a two-cylinder condensing turbine has to be replaced by a single-cylinder machine of similar high efficiency, it is necessary to have a drum-type rotor of large bearing span and small diameter in the region of the first drum stages, and large diameter in the region of the low-pressure stages. This type of rotor has a markedly higher shaft elasticity compared with the rotors of the two-cylinder machine. A rotor of this type was recently built and put into service. This paper describes the rotor and gives its calculated dynamic characteristics. Since operation in the vicinity of the first resonant speed is of greatest interest the paper describes the test results for the properly balanced condition, and for the artificially heavily unbalanced condition. The shaft vibration values measured during the test-run are compared with the assessment criteria for rotor dynamic performance used at present. In order to obtain valid theoretical statements for even more slender rotors, the shaft elasticity was systematically increased in theoretical calculations (by increasing the bearing span). The effects of the shaft elasticity on the magnitude of the resonant speeds, the maximum vibration amplitudes and the stability limit (oil whip) are described. In its original form, the drum-type rotor studied here has only one output shaft coupling. For even higher turbine powers, however, heavy couplings on both shaft ends are necessary. In order to examine the dynamic behavior of these rotors, the original rotor was fitted with an extra mass at the usually free shaft-end to simulate a second coupling. For this rotor, the same calculations and measurements were carried out in the overspeed testing pit as were for the original rotor. The results are given and discussed. A further point examined with this rotor is whether there is a linear relationship between the dynamic bearing force and the magnitude of the unbalance