Assessment of Rotordynamic Characteristics for the Hole-Pattern Damper Seal in a 150MW Steam Turbine

Damper seals and swirl brakes have been commonly used for many years as a solution for the rotor vibration instability in multiple-stage centrifugal compressor. However, there are few literatures focusing on the application assessment of damper seals and swirl brakes in high-pressure steam turbines. This paper presents two attempt schemes (swirl brake, damper seal) to resolve the strong subsynchronous vibration instability for a 150MW 3000rpm multistage single casing steam turbine. The rotor vibration instability has been demonstrated to be caused by the steam whirl forces generated at the balance piston seal location in combination with a long flexible rotor. The original balance piston seal scheme is a staggered labyrinth seal with both teeth on stator and rotor, yielding high cross-coupling stiffness and large negative effective damping at high inlet preswirl condition. To resolve the subsynchronous vibration instability and allow full load operation of the steam turbine, the hole-pattern damper seal schemes with/without inlet swirl brake were designed as the replacement of the original staggered labyrinth seal. A comprehensive assessment and comparison were conducted on the rotordynamic characteristics for the original staggered labyrinth seal and the new hole-pattern damper seals, using a proposed transient CFD-based perturbation method based on the multiple-frequency elliptical-orbit rotor whirling model. Results show that the present hole-pattern damper seal with inlet swirl brake produces the largest effective damping and the lowest crossover frequency, is a good balance piston seal scheme for the present steam turbine to increase the rotor net damping and eliminate the subsynchronous vibration.