Assessment of Rotordynamic Characteristics for Two Types of Damper Seals Operating in a Steam Turbine

Damper seals (such as pocket damper seal (PDS), hole-pattern/honeycomb 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 a comprehensive assessment and comparison on the rotordynamic characteristics of a staggered labyrinth seal (LABY), a fully partitioned pocket damper seal (FPDS), and a hole-pattern damper seal (HPS), used as the balance piston seal in a 150 MW 3000 rpm multistage single casing steam turbine. The staggered labyrinth seal is an original seal scheme where the steam turbine experienced strong subsynchronous rotordynamic instability, preventing full load operation of the machine. The fully partitioned pocket damper seal and the hole-pattern damper seal were designed (the same sealing clearance, inner diameter and axial length) to increase the rotor net damping and eliminate the subsynchronous vibration. The straight swirl brake device also was implemented at seal entrance as a solution scheme to reduce the swirl velocity and enhance the seal net damping capability at high inlet preswirl condition (inlet preswirl ratio is 0.5). Numerical results of seal rotordynamic force coefficients, cavity pressure, and swirl velocity developments were presented and discussed, using a proposed transient computational fluid dynamics (CFD)-based perturbation method based on the multiple-frequency elliptical-orbit rotor whirling model. Results show that although the HPS seal and the FPDS seal both produce the obviously larger effective damping and the similar lower crossover frequency than the LABY seal, from a rotordynamic view, the FPDS seal scheme with entrance swirl brake is better for the present steam turbine due to the relatively larger effective damping and smaller direct stiffness, as a replacement scheme of the original staggered labyrinth seal, to resolve the subsynchronous vibration instability and allow full load operation of the steam turbine.