The present invention relates to brush seals between static and rotary components, sealing between high and low pressure regions and particularly relates to a balanced pressure brush seal for producing either a desired nonuniform or a substantially uniform pressure distribution on opposite sides of the bristle pack achieving an anti-hysterisis effect.
Typically, brush seals disposed between static and rotary components, for example between a packing and a rotary shaft or between a shroud and a bucket tip in a turbine include a bristle pack disposed between a fence on the high pressure side and a pressure plate on the low pressure side of the seal. The bristle pack normally includes a plurality of densely packed bristles fixed at one end adjacent the static component and cantilevered to extend substantially freely toward the rotor where the tips of the bristles engage the rotor. Typically, the bristles extend normally in the direction of rotation i.e. in the circumferential direction of rotation of the rotor forming an angle with the radius. The fence as well as the pressure plate have free ends which terminate short of the surface of the rotary component. Because of the pressure drop across the bristles, including the frictional engagements between the bristles and the pressure plate, and between layers of bristles, temporary excursions of the rotor, for example during turbine start up may permanently deflect or bend the bristles such that the tips no longer engage the rotor. Alternately, temporary rotor excursions may displace bristles radially away from the rotor with the seal under pressure whereby the bristle to bristle and bristle to pressure plate friction prohibits the bristles from returning to their position very close to the rotary component. This of course degrades the seal and has other deleterious effects on the wear life of the seal.
This phenomenon known as an anti-hysterisis effect, previously has been addressed. For example, in U.S. Pat. No. 5,799,952, a brush seal is provided having radially extending circumferentially spaced grooves formed in the pressure plate in an effort to obtain a balancing of the pressure on opposite sides of the bristle pack. In another known form of brush seal, a plurality of circumferentially extending grooves of varying width at discrete radial locations about the brush seal segment have been proposed and constructed in order to substantially balance the pressure on the opposite sides of the brush seals. It is also noted that suppliers of brush seal backing plate material, such as 409 stainless steel, have material availability problems as well as manufacturing difficulties in the formation of the brush seals. Accordingly, it has been found desirable to provide an improved pressure balanced brush seal which is readily and easily manufactured, provides substantially balanced pressure and which may be formed of more readily available materials.