Transient vibrations of a bent rotor having residual imbalance during its rundown

Abstract

The model of a single-span rotor having an initial nonremovable sag and residual imbalance is used for estimating the amplitudes of its resonance vibrations at check points (on the supports and in the middle of the span) in the vicinity of critical speeds lying below the operating angular speed passed by the rotor as it rotates with an angular deceleration in the period from the turbine disconnection moment to its full stop (during the rundown). Two cases of installing the rotor on yielding anisotropic supports (sleeve-type bearings with an elliptic bore) are considered: during rig tests when there are two yielding supports, and when the rotor interacts with the remaining part of the assembled turbine set shaft system, which was modeled by using a yielding support at the left-hand end and a fixed hinge at the right-hand end. The analysis procedure is illustrated by calculations carried out for a K-300-23.5 turbine’s high-pressure rotor having an initial sag and residual imbalance. The values of both excitation initiating factors were taken equal to their maximum permissible levels established by the limitations imposed from turbine sets operating experience on the hot-test values for the initial sag of a thermally unstable rotor and for its residual imbalance. The list of considered cases included lumped imbalance of the rotor resulted from separation of a blade or disk section and distributed residual imbalance remaining after preliminary balancing of the rotor on a rig. An analysis of the obtained results is presented.