Preliminary Investigations for a Pressure Balanced Back Plate at Low Inclined Brush Seals

Within this paper a continuation in brush seal testing for flexible load regimes in a steam turbine is given. Besides the well-known main design parameters of brush seals, e.g. the bristle pack thickness, the bristle diameter or the lay angle of the bristle pack, this paper focuses on the axial inclination of the bristle pack and particularly the affinity of bristle pack oscillations at low inclined bristle packs and small pressure differences. As it was presented in GT2014-26330, the axial inclination of the bristle pack is an important design parameter for brush seals. Along with a clearly increased blow-down capability and a reduced stiffness the seals tend to exhibit an enhanced axial bristle pack width during pressurization. It was previously shown that a low axial inclination of the bristle pack results in a loose package and in bristle pack oscillations until pressure differences of 10 bar. Above pressure drops of 10 bar the resulting higher abrasive behavior stops and a well sealing brush seal with a loose bristle pack is given. Regarding the renewable energy sources for necessary changes in steam turbine operations, a flexible sealing system with an enhanced wide operating range is requested. To capture all positive behaviors of low inclined brush seals for pressure differences until 10 bar, a design to safely avoid bristle pack oscillations is required. With this background low inclined brush seals with a new back plate design were tested at the Institute’s cold air test facility in Braunschweig up to a pressure difference of 4 bar. The facility allows detailed sealing performance investigations including real time bristle pack observations. The present paper shows and discusses overall experimental results of brush seals with different axial inclinations mounted with an adjustable back plate to determine the influence of the back plate design on the bristle pack oscillations. Furthermore, these new results together with older measurements from 2012 were used to develop a theory regarding the changes that result from contact between the bristle pack and the adjusted back plate. Finally, the design for a pressure balanced back plate will be shown.