Seal for a joint or juncture – Process of dynamic sealing – Close proximity seal
Reexamination Certificate
2001-02-26
2002-08-20
Knight, Anthony (Department: 3676)
Seal for a joint or juncture
Process of dynamic sealing
Close proximity seal
C277S305000, C277S350000, C277S355000, C277S412000
Reexamination Certificate
active
06435513
ABSTRACT:
TECHNICAL FIELD
The present invention relates to brush seals for rotary machines such as steam and gas turbines and particularly relates to brush seals and labyrinth-brush seal combinations for use with rotary machines.
BACKGROUND
Rotary machines, such as steam and gas turbines, used for power generation and mechanical drive applications are generally large machines consisting of multiple turbine stages. In turbines, high pressure fluid flowing through the turbine stages must pass through a series of stationary and rotating components, and seals between the stationary and rotating components are used to control leakage. The efficiency of the turbine is directly dependent on the ability of the seals to prevent leakage, e.g., between the rotor and stator. Turbine designs are conventionally classified as either impulse, with the majority of the pressure drop occurring across fixed nozzles, or reaction, with the pressure drop more evenly distributed between the rotating and stationary vanes. Both designs employ rigid tooth, i.e., labyrinth, seals to control leakage. Traditionally, rigid labyrinth seals of either a hi-lo or straight shaft design are used. These types of seals are employed at virtually all turbine locations where leakage between rotating and stationary components must be controlled. This includes interstage shaft seals, rotor end seals, and bucket (or blade) tip seals. Steam turbines of both impulse and reaction designs typically employ rigid, sharp teeth for rotor/stator sealing. While labyrinth seals have proved to be quite reliable, their performance degrades over time as a result of transient events in which the stationary and rotating components interfere, rubbing the labyrinth teeth into a “mushroom” profile and opening the seal clearance.
Another type of seal used in many environments, including rotary machines, is a brush seal. Brush seals are generally less prone to leakage than labyrinth seals. A brush seal can also accommodate relative radial movement between fixed and rotational components, for example, between a rotor and a stator, because of the flexure of the seal bristles. Brush seals also generally conform better to surface non-uniformities. The result of using brush seals is better sustained rotary machine performance than is generally possible with labyrinth seals.
DISCLOSURE OF THE INVENTION
In accordance with a preferred embodiment of the present invention, there is provided brush seal segments in the environment of a rotary machine such as a turbine. Brush seals per se have general applicability to rotary machines and can be used in lieu of labyrinth seals. Brush seals are advantageous in that context and provide improved sealing, while occupying considerably less axial space as compared with conventional labyrinth seals. As a result, more compact rotary machine, e.g., turbine, designs can be realized. Alternatively, by employing brush seals, the span that would normally be occupied by labyrinth teeth can be used to allow additional turbine stages, resulting in increased turbine efficiency. As a further advantage, application of brush seals at end packing locations can reduce leakage to the point that the need for a gland sealing/exhauster system, for example, in a steam turbine, is eliminated. At rotor end seals, it is also possible to use brush seals in conjunction with face seals. Further, in certain steam rotary machine applications, some leakage is desirable for cooling of components such as rotors. At these locations, brush seals can be used in conjunction with orifices or other flow bypass mechanisms to ensure that the proper amount of leakage is obtained.
A typical brush seal for use in the present invention comprises a bristle pack, i.e., bristles sandwiched between two metallic plates. The bristles are generally alloy steel wires, drawn to a diameter of 0.002-0.006 inches, although the exact diameter depends on the specific seal application. Larger wire diameters are used for seals exposed to a high pressure differential between the upstream and downstream sides. The backing (downstream) plate, or in the present invention a labyrinth tooth, prevents the bristles from deflecting axially under pressure load. As a result, fence height (h) is a critical design variable. Fence height is the distance the bristles extend freely from their support, i.e., the distal end of the support plate or contact points between the bristles and the labyrinth tooth, to their free ends, which typically are in engagement with the rotating component. For a steam turbine application, where the expected maximum radial rotor deflection is approximately 0.040 inches, the fence height must therefore be a minimum of 0.040 inches. Fence heights vary significantly, particularly in gas turbines, depending on the seal location, from 0.030 for bearing seals, to 0.120 for high pressure packing seals to 0.300 for turbine interstage seals.
During shaft radial excursions, the bristles must be able to temporarily deflect without buckling. In order to accommodate these excursions, the bristles are not oriented in a perfectly radial direction, but are instead canted at some angle. Typically, this angle is between 30 and 60 degrees. Increased angles are used to allow for increased radial shaft excursions. If the bristles were straight in a radial direction, the rotor would interfere with the bristles and the bristles would act as columns rather than deflect as beams. This would result in increased wear and not accommodate radial excursions of the shaft.
The bristles of brush seals are typically mounted between a pair of plates or arcuate segments, with the bristles and plates being welded to one another at the ends of the bristles remote from the tips engaging with the opposite component of the seal. Conventionally, the brush seal is cut into a number of segments, typically four, with the cuts at the ends of each of the segments oriented at the same angle as the “cant” angle of the bristles. That is, with the angle of the bristles typically being on the order of 45° relative to radii of the arcuate segments, the ends of each of the segments are likewise cut at the same angle and therefore parallel the linear extent of the bristles as they project from the segment at that segment end. As a result, the bristles lie at an angle affording the capability of accommodating radial excursions of the rotating component. By cutting the segments of the seal at the angles of the bristles, the bristles may be secured at that angle in the segments without loss of bristles, resulting in an assembled seal with a full 360° of bristles for maximum sealing effectiveness.
In certain applications, however, for example conventional labyrinth seals, the labyrinth seal segments are generally cut in a radial direction for ease of manufacturing and assembly. Further, when retrofitting brush seals to existing labyrinth seals or supplying brush seals as original equipment in combination with labyrinth seals, it has been found desirable according to the present invention to cut the brush seal segment ends in a radial direction rather than in the direction of the “cant” of the bristles. This results in making surfaces at the segment end interfaces that are perfectly straight along radii of the segments with no interlocking pieces and no projections that can be damaged during assembly. However, cutting brush seal segments at the ends of the segments inconsistent with the bristle orientation angle, i.e., the cant angle, results in the loss of an area of the bristles. For example, where a brush seal is employed on a fixed component for sealing about a rotating shaft, the bristles of the brush seal are attached to the seal segments along the outer diameter and project radially inwardly at the cant angle. With the ends of the seal segments cut along radii, there are areas at the juncture of the seal segments where no “canted” bristles are present. That is, with the bristles canted, for example, at a 45° angle, and the end segments lying along radii, there is a triangular area on one end of one segment in which b
Bagepalli Bharat S.
Cromer Robert Harold
Dinc Osman Saim
Skinner David Robert
Turnquist Norman Arnold
Beres John L.
Knight Anthony
Nixon & Vanderhye
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