Brush Seals Used in Steam Environments—Chronological Wear Development and the Impact of Different Seal Designs

Abstract

During the last decades, turbo machine efficiency was considerably increased by using more efficient seals. Brush seals, as a compliant contacting filament seal, have become an attractive alternative to conventional labyrinth seals in the field of aircraft engines as well as in stationary gas and steam turbines. The aim of today's research related to brush seals is to understand the characteristics and their connections, in order to be able to make performance predictions, and to ensure the reliability over a defined operating period. The wear behavior is essentially influenced by frictional contacts at the seal-to-rotor interface during operation. For realistic investigations with representative circumferential velocities, the TU Braunschweig, Germany, operates a specially developed steam test rig which enables endurance investigations under varying operating steam conditions up to 50 bar and 450 °C. Wear measurements and the determination of seal performance characteristics, such as blow down and bristle stiffness, are enabled by an additional test facility, using pressurized cold air up to 8 bar as a working fluid. This work presents the chronological wear development on both rotor and seal sides in a steam test lasting 25 days or 11 days, respectively. Interruptions after stationary and transient intervals were made in order to investigate the degree of wear. Two different seal arrangements, a single tandem seal, and a two-stage single seal arrangement, using different seal elements were considered. Besides a continuous wear development, the results clearly show that the abrasive wear of the brush seal and rotor is mainly caused by transient test operations, particularly by enforced contacts during shaft excursions. Despite the increasing wear to the brushes, all seals have shown a functioning radial-adaptive behavior over the whole test duration with a sustained seal performance. Thereby, it could be shown that the two-stage arrangement displays a load shift during transients, leading to a balanced loading and unloading status for the two single brush seals. From load sharing, and in comparison with the wear data of the tandem seal arrangement, it can be derived that the two-stage seal is less prone to wear. However, the tandem seal arrangement, bearing the higher pressure difference within one configuration, shows a superior sealing performance under constant load, i.e., under stationary conditions.

Research Status and Development Trend of Cylindrical Gas Film Seals for Aeroengines

“…When the unit is running at high speed, the rotor and brush filament will produce friction, and the constant friction will cause the brush filament to accumulate a large amount of frictional heat; when the rotor undergoes radial offset, the friction and wear between the brush filament and the rotor will be more intense, resulting in the chattering of the brush filament, which will reduce the stability of the seal [33][34][35][36][37][38][39][40]. With the increasing temperature of the brush tip, when the maximum temperature is close to the melting point of the brush, the metal mechanical properties of the material in the brush tip area decrease, resulting in a sharp increase in leakage and shortening of the service life, which directly restricts the application of brush seals in shaft-end seals and the interstage seals of large-scale rotating machinery [41][42][43].…”

Estimation of the rotordynamic characteristics of a single brush seal using least-squares and instrumental variable methods under super-heated steam environment

“…The stiffness of the brush seal was extracted through the circumferential pressure distribution between the bristle packs, and owing to the blow-down effect, the dynamic characteristics of the brush seal depend largely on the eccentricity of the shaft. Raben et al 23 successfully analyzed chronological wear development of the clamped brush seal and investigated blow-down effect and bristle stiffness under super-heated steam environment. The low inclined bristle of the tandem geometry presented a continuously increasing wear to the rotor and bristle tip surface and showed the performance influenced by the bristle pack design.…”

“…As suggested by Nordmann and Massmann, 17,18 the dynamic characteristics of triboelements have been utilized to analyze a given rotordynamic system. 19–23 In particular, additional stiffness or damping could be the source of destabilizing force, inducing the steam whirl. 4 In addition, thermal instability, such as the Newkirk effect, can appear due to unavoidable frictional heating resulting from contact between the bristles and rotor.…”

Application of blade tip shroud brush seal to improve the aerodynamic performance of turbine stage

“…To widen the application of brush seal in steam turbines, Raben et al. 19 established the steam test rig to investigate the leakage flow and wear characteristics of brush seals under varying operating steam conditions up to 50 bar and 450℃.…”