Abstract: A rotor oscillation preventing structure for a steam turbine includes: a stator vane 3; a moving blade 1; a shroud cover 2 installed on an outer circumferential side distal end of the moving blade 1; and a plurality of seal fins 6 installed, at any interval in the axial direction of a rotor, on an wall surface of a stationary body located on an outer circumferential side of the shroud cover 2, a whirl preventing structure comprised of whirl preventing plates 9 or whirl preventing grooves 11 is provided at a shroud cover inlet return portion 10 of the shroud cover 2 so as to block the whirl flow of leakage flow 8 on an upstream side in an operating steam flow direction of the seal fins to reduce an absolute velocity component of the leakage flow 8 in a rotational direction of the rotor.

Abstract: Whirl flow of leakage flow is reduced to reduce the occurrence potential of steam whirl. A rotor oscillation preventing structure for a steam turbine includes: a stator vane 3; a moving blade 1; a shroud cover 2 installed on an outer circumferential side distal end of the moving blade 1; and a plurality of seal fins 6 installed, at any interval in the axial direction of a rotor, on an wall surface of a stationary body located on an outer circumferential side of the shroud cover 2, a whirl preventing structure comprised of whirl preventing plates 9 or whirl preventing grooves 11 is provided at a shroud cover inlet return portion 10 of the shroud cover 2 so as to block the whirl flow of leakage flow 8 on an upstream side in an operating steam flow direction of the seal fins to reduce an absolute velocity component of the leakage flow 8 in a rotational direction of the rotor.