Patent Publication Number: US-6669443-B2

Title: Rotor platform modification and methods using brush seals in diaphragm packing area of steam turbines to eliminate rotor bowing

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a steam turbine having brush seals between non-rotatable and rotatable components arranged and located to eliminate thermal bowing resulting from non-uniform distribution of heat about the rotatable component due to heat generated by frictional contact between the brush seal and the rotatable component and particularly relates to apparatus and methods for eliminating thermal bowing as well as axial thrust loads in the event of failure of the brush seal in such turbine. 
     In U.S. Pat. No. 6,168,377, of common assignee herewith, there is disclosed a steam turbine having a brush seal located between a non-rotatable component and a rotatable component of the rotor shaft. Particularly, axial flanges are provided on the dovetails of the buckets, the bucket dovetails being secured in complementary fashion to the dovetail of a rotor wheel. A brush seal comprised of an arcuate array of metal bristles projecting from the non-rotatable component toward the rotatable component, i.e., the flanges on the bucket dovetails, has bristle tips engaging with and bearing against the flange surfaces. As will be appreciated from a review of that patent, the contact between the bristles of the brush seal and the opposing sealing surface, i.e., the flanges, generates heat. 
     As disclosed in that patent, it is recognized that the contact between the brush seal and the sealing surface should be located radially outwardly of the rotor shaft in order to isolate the generated heat from the outer diameter of the rotor. Otherwise, the friction-generated heat may cause a non-uniform temperature distribution about the circumference of the shaft, resulting in non-uniform axial expansion of the rotor and, hence, a bow in the rotor. While various methods and apparatus are disclosed in that patent for eliminating that problem, one such solution locates the friction-generating surface on the bucket dovetail flanges radially outboard of the outer shaft diameter. In that manner, the generated heat is isolated from the rotor, eliminating any tendency of the rotor to bow. 
     That patented design and other designs utilize conventional labyrinth-type packing seals on the inside of the diaphragm web as a backup to the brush seal. These labyrinth seals are located directly adjacent the outer diameter of the shaft. Brush seals are, however, susceptible to wear and failure. Should a brush seal spaced outwardly from the shaft fail, e.g., the brush seal of that patented design, the sealing diameter changes from the bucket dovetail platform to the rotor shaft. This, in turn, adversely changes the pressure distribution on the shaft and the thrust on the rotor in an axial direction. Thus, rotor dynamic constraints limit the number of stages in which brush seals may be used and where labyrinth-type sealing teeth are used in lieu of such seals, there is a decrease in section efficiency due to increased secondary losses. Accordingly, there is a need to provide a sealing system for a steam turbine in which not only is the problem of thermal bowing of the steam turbine rotor due to non-uniform heat distribution resulting from contact between brush seals and complementary sealing surfaces eliminated, but also the axial thrust loads on the rotor bearings are eliminated or minimized in the event of brush seal failure. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, there is provided a brush seal located radially outwardly of the outer diameter or surface of the shaft of the rotatable component to eliminate thermal bowing of the rotor due to non-uniform heat distribution. The brush seal may be applied in combination with a labyrinth seal at substantially the same radial location to eliminate thrust loads in the event of failure of the brush seal. Particularly, a platform is formed about the rotor between adjacent axially spaced wheels carrying the turbine buckets and which platform projects radially outwardly from the surface of the rotor. The platform, in a preferred embodiment, is in the form of an annular pedestal having an axially reduced neck and at least one and preferably a pair of axially extending flanges at the radial outer extremes of the platform. The one or more flanges are in effect cantilevered in an axial direction from the neck of the pedestal and serve as one or more fins enabling heat generated by frictional contact of the brush seal on the platform surface to be dissipated before affecting rotor dynamics. Thus, the platform configuration enables a sufficient area and provide flanges or fins to dissipate the heat locally, mitigating the effect on rotor vibration, thus allowing similar brush seal application to all steam turbine section stages. It will be appreciated that the cantilevered flange or fin provides a void radially between the flange or fin and the rotor surface, i.e., in the wheelspace, whereby the frictional heat generated by brush seal contact with the sealing surface of the platform is dissipated first in an axial direction and then in a radial direction before having any effect on the thermal dynamics of the rotor. The heat dissipation is sufficient to minimize or eliminate a thermal response of the rotor to the frictionally generated heat. 
     The diaphragm between the adjacent wheels has a web extending radially inwardly into the wheel space and, not only carries the brush seal, but also one and preferably a plurality of labyrinth seal teeth. The labyrinth seal teeth terminate in tapered edges spaced from a surface of the platform and preferably serve as backup seals to the brush seal. The labyrinth teeth thus are preferably located on the downstream side of the brush seal. Should the brush seal fail, the labyrinth teeth limit performance degradation. The brush seal, however, may be located downstream of the labyrinth seal teeth or intermediate the labyrinth seal teeth. Also, since the areas of the upstream and downstream sides of the pedestal exposed in the cavity integrally of the diaphragm, i.e., the wheelspace, are substantially equal, no net axial thrust from leakage flows past labyrinth seals occurs. 
     In a preferred embodiment according to the present invention, there is provided a steam turbine comprising a rotatable component including a rotor shaft having a rotor shaft surface and a non-rotatable component about the rotatable component, a brush seal carried by the non-rotatable component for sealing engagement with the rotatable component, at least a pair of wheels on the rotatable component spaced axially from one another, the rotatable component including a plurality of buckets spaced circumferentially from one another on each of the wheels, means for inhibiting non-uniform circumferential heat transfer to the rotor shaft surface due to heat generated by frictional contact between the brush seal and the rotatable component thereby to eliminate or minimize bow of the rotatable component, the inhibiting means including an annular platform projecting radially outwardly of the rotor shaft surface at an axial location between the wheels, the brush seal being disposed between the buckets on the wheels and engaging a sealing surface on the platform radially outwardly of the rotor shaft surface. 
     In a further preferred embodiment according to the present invention, there is provided in a steam turbine having a rotatable component including a rotor shaft having a rotor shaft surface and a non-rotatable component about the rotatable component carrying a brush seal for sealing engagement with the rotatable component along a steam leakage flow path, a method of substantially eliminating bowing of the rotor shaft resulting from circumferential non-uniform distribution of heat about the rotatable component due to heat generated by frictional contact between the brush seal and the rotatable component comprising inhibiting circumferential non-uniform heat transfer to the rotatable component due to heat generated by frictional contact between the rotatable component and the brush seal by locating the area of frictional contact between the rotatable component and the brush seal along a sealing surface spaced radially outwardly of the rotor shaft surface and in radial registration with the rotor shaft surface and a wheelspace portion between the sealing surface and the rotor shaft surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view of a portion of a steam turbine illustrating turbine buckets and diaphragms along the turbine shaft and the location of the combined brush/labyrinth seals; and 
     FIG. 2 is an enlarged fragmentary cross-sectional view illustrating a combined brush and labyrinth seal hereof engaging a radially projecting platform in the wheelspace between adjacent buckets. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, there is illustrated a steam turbine, generally designated  10 , having a rotational component  11 , e.g., a rotor or shaft  12  mounting a plurality of axially spaced wheels  14  for mounting buckets  16 . A series of nozzle partitions  18  are interspersed between the buckets and form with the buckets  16  a steam flow path indicated by the arrow  20 . The partitions are attached to a diaphragm inner web  22  extending between the wheels  14  of the stages of the turbine, the web and partitions collectively defining a stationary component  17 . It will be appreciated that the rotor  12  is a continuous solid elongated piece of metal. 
     As previously noted, brush seals have been employed at various locations along a rotor, i.e., between the stationary and rotational components in a steam turbine to form seals. In accordance with a preferred embodiment of the present invention, a brush seal is provided between the stationary and rotatable components  17  and  11 , respectively, at a location radially outwardly of the outer surface  24  of rotor  12  in such manner as to prevent non-uniform distribution of heat about the rotor due to frictional contact between the tips of the bristles of the brush seal and the rotor. The brush seal seals along a leakage flow path, indicated by the arrow  19  in FIG. 2, from the steam flow path  20 . The brush seal per se may be of conventional construction. For example, as best illustrated in FIG. 2, a brush seal  26  comprises a plurality of preferably metal bristles  28  disposed between a pair of plates  30  and  32  extending circumferentially about the rotor. Brush seal  26  in a preferred embodiment hereof is located and retained in an annular groove  34  formed in the web  22  along a forward portion thereof, i.e., an upstream portion in relation to the direction of leakage steam flow  19 . It will be appreciated that the bristles  28  of the brush seal extend at a cant angle relative to radii of the rotor about its axis of rotation and have tips  38  which engage the rotatable component forming a seal therewith. 
     To prevent a non-uniform distribution of heat about the rotor due to frictional contact between the tips  38  of bristles  28  and a sealing surface of the rotational component, the rotor  12  mounts a platform  40  which projects radially outwardly of the rotor surface  24  and between adjacent wheels  14  of the various rotor stages. Particularly, the platform  40  may comprise an annular, radially extending, pedestal  42  having a neck  44  and at least one and preferably a pair of annular axially extending flanges or fins  46 . As specifically illustrated in FIG. 2, the flanges or fins  46  are cantilevered in an axial direction from the reduced neck  44  and hence register radially with a portion of the wheelspace  48  between the wheels  14 . The outer surface of the platform  40  and particularly the upstream outer annular surface  50  in radial registration with the tips of the bristles  28  serves as a contact sealing surface with the bristle tips  38 . Accordingly, the contact surface between the tips  38  of bristles  28  and the rotational component at which heat is generated by such frictional contact is located both axially and radially spaced from the rotor surface  24 . As a consequence, heat generated by such frictional contact is dissipated first in an axial direction toward a central portion of the platform and then radially inwardly along the neck  44  of platform  40 . The frictional heat generated is thus dissipated along this path. That is, the platform is configured and has sufficient area to dissipate the heat locally, thus minimizing or eliminating any thermal response of the rotor to thermal effects resulting from the brush seal contact with the sealing surface of the platform  40 . 
     One or more labyrinth seal teeth  60  are also carried by the web  22  in one or more annular arrays thereof about the platform  40 . The labyrinth teeth  60  are tapered along their radial inner edges and are spaced a minimal distance from the surface of platform  40  to effect labyrinth-type seals, i.e., afford a tortuous path for any further steam leakage flow escaping past the brush seal. The labyrinth teeth are preferably located downstream of any leakage flow past the brush seal and thus serve as backup seals for the brush seal. Because the brush seal and the labyrinth seal are located substantially on the same diameter, axial rotor thrust resulting from failure of the brush seal is substantially eliminated. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.