Patent Document

BACKGROUND OF THE INVENTION 
   This invention relates generally to turbine rotor assemblies and, more particularly, to seal systems for sealing radial leakage in turbine bucket covers. 
   Known steam turbines are classified as action turbines, or “constant-pressure” turbines and reaction turbines, or “excess-pressure” turbines. Each turbine includes a turbine shaft that includes moving blades, or buckets positioned circumferentially around the shaft, and includes an inner casing with guide blades, or diaphragms positioned between axially spaced buckets. 
   In the case of a constant-pressure turbine, the entire energy gradient is converted essentially into kinetic flow energy in the ducts that are narrowed by the guide blades. During the process, the velocity rises and the pressure falls. In the moving blades, the pressure and relative velocity remain essentially constant, being achieved through ducts having a uniform clear width. Because the direction of the relative velocity changes, action forces occur that drive the moving blades and, thus, cause rotation of the turbine shaft. The magnitude of the absolute velocity decreases considerably when the flow passes around the moving blades, resulting in a flow that transfers a large part of its kinetic energy to the moving blades and, therefore, to the turbine shaft. 
   In the case of an excess-pressure turbine, only part of the energy gradient is converted into kinetic energy when the flow passes through the guide blades. The rest of the energy gradient brings about an increase in relative velocity within the moving-blade ducts formed between the moving blades. Where the blade forces are almost exclusively action forces in the constant-pressure turbine, in an excess-pressure turbine, a greater or lesser fraction resulting from the change in the velocity magnitude is added. The term “excess-pressure” turbine is derived from the pressure difference between the downstream and the upstream side of the moving blade. In an excess-pressure turbine, therefore, a change in the velocity magnitude takes place when the pressure varies. 
   A turbine typically includes at least one rotor including a plurality of rotor buckets or blades that extend radially outwardly from a plurality of wheels attached to a common annular shaft. Specifically, the rotor buckets are attached to the wheels with dovetail joints. A radial extrema of the bucket, or tip may support a cover that joins the tips of a plurality of buckets circumferentially around the periphery of the turbine. In some known bucket designs the cover is individual to each bucket and integrally cast with the bucket during manufacture. 
   A gap between adjacent bucket covers may define a radial leakage path that may allow the working fluid to escape the bucket surfaces and adversely affect an operational efficiency of the turbine. 
   To facilitate reducing radial leakage, rotor assemblies may include sealing covers. At least some known rotor assemblies include additional loose fitting seals that are separate from the bucket covers. The seals block the leakage path to reduce radial leakage. 
   During operation, forces induced within the bucket may cause the adjacent bucket covers to flex and to separate from each other increasing the width of the leakage gap. Over time, continued leakage between the adjacent bucket covers may erode the covers where adjacent covers adjoin and increase leakage flow, adversely affecting the turbine operational efficiency. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect, a method of fabricating a rotor assembly for a turbine is provided. The method facilitates minimizing radial leakage of a working fluid, the rotor assembly is rotatable about a longitudinal axis, and includes a radially outer rim and a plurality of buckets that extend radially outward from the radially outer rim, each of the buckets includes a blade including a pair of opposing sidewalls. The method includes forming a cover on each bucket radial tip wherein the cover includes a leading edge side in a direction of rotation, and a trailing edge side in the direction of rotation wherein the leading edge and trailing edges are parallel with respect to each other, and wherein the leading edge and trailing edges are skewed with respect to the axis of rotation. The method includes forming an extension in the cover leading edge side, forming a groove in the cover trailing edge side, and attaching each bucket to the radially outer rim, such that the extension in the cover leading edge side of one cover is in mating engagement with the groove in the cover trailing edge side of an adjoining cover. 
   In another aspect, a rotor assembly for a turbine is provided. The rotor assembly is rotatable about a longitudinal axis of the rotor assembly, and includes a radially outer rim and a plurality of buckets that extend radially outward from the radially outer rim wherein each of the buckets includes a blade that includes a pair of opposing sidewalls. The rotor assembly includes a cover formed on each bucket radial tip wherein the cover includes a leading edge side in a direction of rotor assembly rotation, and a trailing edge in the direction of rotor assembly rotation wherein the leading edge and trailing edges are parallel with respect to each other, and wherein the leading edge and trailing edges are skewed with respect to the axis of rotation. The rotor assembly also includes an extension formed in the cover leading edge side, and a groove formed in the cover trailing edge side, and wherein each bucket is attached to the radially outer rim, such that the extension in the cover leading edge side of one cover is in mating engagement with the groove in the cover trailing edge side of an adjoining cover. 
   A steam turbine is provided that includes a rotor assembly rotatable about a longitudinal axis of the rotor assembly, that includes a radially outer rim and a plurality of buckets that extend radially outward from the radially outer rim wherein each of the buckets includes a blade that includes a pair of opposing sidewalls, a cover that is formed on each bucket radial tip, wherein the cover includes a leading edge side in a direction of rotor assembly rotation, and a trailing edge in the direction of rotor assembly rotation wherein the leading edge and the trailing edge are parallel with respect to each other, and wherein the leading edge and the trailing edge are skewed with respect to the axis of rotation, and the cover is formed integrally with the blade. The steam turbine also includes a semi-circular extension formed in the cover leading edge side, and a semi-circular groove formed in the cover trailing edge side, and wherein each bucket is attached to the radially outer rim, such that the extension in the cover leading edge side of one cover is in mating engagement with the groove in the cover trailing edge side of an adjoining cover such that adjoining sides of the covers are slidingly engaged and in substantially metal-to-metal contact. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective partial cut away view of a steam turbine; 
       FIG. 2  is a partial cross-sectional view of a rotor assembly including a damper system and that may be used with the gas turbine engine shown in  FIG. 1 ; 
       FIG. 3  is an enlarged perspective view of an exemplary portion of the rotor assembly including a plurality of adjacent buckets shown in  FIG. 2 ; 
       FIG. 4  is an enlarged elevation view of the cover that may be used with buckets shown in  FIG. 3 ; and 
       FIG. 5  is a plan view of an exemplary embodiment of the cover that may be used with the buckets shown in FIG.  3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a perspective partial cut away view of a steam turbine  10  including a rotor  12  that includes a shaft  14  and a low-pressure (LP) turbine  16 . LP turbine  16  includes a plurality of axially spaced rotor wheels  18 . A plurality of buckets  20  are mechanically coupled to each rotor wheel  18 . More specifically, buckets  20  are arranged in rows that extend circumferentially around each rotor wheel  18 . A plurality of stationary nozzles  22  extend circumferentially around shaft  14  and are axially positioned between adjacent rows of buckets  20 . Nozzles  22  cooperate with buckets  20  to form a turbine stage and to define a portion of a steam flow path through turbine  10 . 
   In operation, steam  24  enters an inlet  26  of turbine  10  and is channeled through nozzles  22 . Nozzles  22  direct steam  24  downstream against buckets  20 . Steam  24  passes through the remaining stages imparting a force on buckets  20  causing rotor  12  to rotate. At least one end of turbine  10  may extend axially away from rotor  12  and may be attached to a load or machinery (not shown), such as, but not limited to, a generator, and/or another turbine. Accordingly, a large steam turbine unit may actually include several turbines that are all co-axially coupled to the same shaft  14 . Such a unit may, for example, include a high-pressure turbine coupled to an intermediate-pressure turbine, which is coupled to a low-pressure turbine. In one embodiment, steam turbine  10  is commercially available from General Electric Power Systems, Schenectady, N.Y. 
     FIG. 2  is a partial cross-sectional view of a rotor assembly  100  that may be used with steam turbine  10 . Rotor assembly  100  includes a plurality of buckets  102 . Each bucket  102  includes a dovetail  104  attached to a complementary shaped extension of a shaft  106 , a blade  108  extending radially outwardly from its respective dovetail  104 , and a cover  110  formed on a radial extrema, or tip of each blade  108 . In the exemplary embodiment, covers  110  are formed integrally with blades  108 . 
   Blades  108  are configured for cooperating with a motive or working fluid, such as steam. In the exemplary embodiment illustrated in  FIG. 2 , rotor assembly  100  is a section of steam turbine  10 , with blades  108  configured for suitably extracting energy from the motive fluid steam in succeeding stages. Outer surfaces or shoulders  112  of dovetails  104  define a radially inner flowpath surface of the turbine section as steam is directed from stage to stage. 
   Blades  108  rotate about the axial centerline axis up to a specific maximum design rotational speed, and generate centrifugal loads in rotating components. Centrifugal forces generated by rotating blades  108  are carried by dovetails  104  and portions of shaft  106  directly below each blade  108 . Rotation of rotor assembly  100  and blades  108  from steam passing past blades  108  removes energy from the steam. 
   Blades  108  each include a leading edge  114 , a trailing edge  116 , and an airfoil  118  extending therebetween. 
     FIG. 3  is an enlarged perspective view of a portion of rotor assembly  100  including a plurality of adjacent buckets  102 . Airfoil  118  includes a high-pressure side  120  and a circumferentially opposite low-pressure side  122 . High-pressure side  120  and low-pressure side  122 , respectively, extend between axially spaced apart leading and trailing edges  124  and  126 , respectively and extend in radial span between a rotor blade tip  128  and a rotor blade root  130 . A blade chord  132  is measured between airfoil  118  leading and trailing edges  124  and  126 , respectively. 
   Cover  110  includes a leading edge side  134  in a direction  136  of rotation, and a trailing edge side  138  in direction  136  of rotation. Cover  110  also includes an inlet edge side  140  and an outlet edge side  142  wherein inlet and outlet are referenced to a direction  144  of steam flow past airfoil  118 . Leading edge side  134  is parallel with trailing edge side  138 , and inlet edge side  140  and outlet edge side  142  are parallel. Leading edge side includes an extension  146  that extends circumferentially from leading edge side  134  toward trailing edge side  138  of adjacent cover  110 . Trailing edge side  138  of cover  110  each includes a groove  148  that extends inwardly from trailing edge side  138  into cover  110 . Groove  148  is sized and positioned to receive extension  146  in a sliding engagement. 
     FIG. 4  is an enlarged elevation view of cover  110  that may be used with buckets  102  shown in FIG.  3 . Leading edge side  134  and trailing edge side  138  of adjacent covers  110  meet to form a joint  149  when rotor assembly  100  is assembled. Leading edge side  134  includes extension  146 , a inner radial portion  150  and an outer radial portion  152 . Inner radial portion  150  extends radially inwardly from a first extent  154  of extension  146  to an undersurface  156  of cover  110 . Outer radial portion  152  extends radially outwardly from a second extent  158  of extension  146  to an outersurface  160  of cover  110 . Trailing edge side  138  includes groove  148 , a inner radial portion  162  and an outer radial portion  164 . Inner radial portion  162  extends radially inwardly from a first extent  166  of groove  148  to undersurface  156  of cover  110 . Outer radial portion  164  extends radially outwardly from a second extent  168  of groove  148  to an outersurface  160  of cover  110 . 
   In the exemplary embodiment, joint  149  includes metal-to-metal contact between corresponding surfaces of adjacent covers  110 . Specifically, inner radial portion  150  of leading edge side  134  and inner radial portion  162  of trailing edge side  138  are butted together such that no intended gap exists between their respective surfaces. Likewise, outer radial portions  152  and  164  are similarly butted together such that substantially no gap exists between their respective mated surfaces. Additionally, extension  146  is received in groove  148  such that no intended gap exists between their respective surfaces. Further, leading edge side  134  and trailing edge side  138  are able to slide approximately axially, with respect to each other. Leading edge side  134  and trailing edge side  138  are restrained from sliding radially with respect to each other due to the engagement in the radial direction of extension  146  in groove  148 . 
   In operation, during a startup of turbine  10 , steam  24  is admitted into inlet  26 . Steam  24  is directed past airfoil  118  between shoulder  112  and covers  110 . Initially, steam  24  is hotter than airfoil  118 , shoulder  112 , and covers  110 , and transfers heat to airfoil  118 , shoulder  112 , and covers  110 . The heat causes expansion of airfoil  118 , shoulder  112 , and covers  110  which may be uneven and may tend to cause warpage of airfoil  118 , shoulder  112 , and covers  110 . Additionally, steam  24  may tend to leak past joint  149  due to a pressure gradient that may exist across covers  110 . To relieve compressive forces which may build up in airfoil  118  due to a thermal expansion of airfoil  118 , covers  110  of adjacent buckets  102  may slide axially, allowing the compressive forces to dissipate. Because of the metal-to-metal engagement of sides  134  and  138  including extension  146  and groove  148 , respectively, steam  24  is facilitated being blocked during expansion of the components within turbine  10 . 
     FIG. 5  is a plan view of an exemplary embodiment of cover  110  that may be used with bucket  102  as shown in FIG.  3 . Each cover  110  includes leading edge side  134  that is parallel with trailing edge side  138 , and inlet edge side  140  that is parallel with outlet edge side  142 . An angle  170  is formed between sides  138  and  142 . In the exemplary embodiment, angle  170  is an acute angle. Angle  170  being an acute angle facilitates sealing joint  149  during all operational modes of turbine  10 . Sides  134  and  142  form an angle  172 . In the exemplary embodiment, angle  172  is an obtuse angle. Because angles  170  and  172  are not right angles, sides  134  and  138  have a skew with reference to the longitudinal axis of rotor assembly  100 . The skew in sides  134  and  138  facilitates maintaining a seal along joint  149 . Additionally, in the exemplary embodiment, sides  134  and  138  are straight from side  140  to side  142 . Some known bucket covers are S-shaped or Z-shaped when observed from a plan perspective. An S or Z shape to cover  110  would cause adjacent covers  100  to bind as they expanded due to thermal growth if they tried to slide in a first direction, or would cause a gap in joint  149  if they slid in the opposite direction. 
   The above-described rotor assembly is cost-effective and highly reliable. The rotor assembly includes an integral cover for each bucket that interlocks with each adjacent bucket to facilitate sealing radial leakage of working fluid. More specifically, seals include an extension and a groove formed in the cover that cooperate during various operational modes to maintain contact between the covers of adjacent covers. The covers are skewed with respect to a longitudinal axis and are slidably engaged to facilitate maintaining a seal between adjacent covers while facilitating stress relief between adjacent covers. During operation, the differential expansion of turbine bucket airfoils and covers may tend to open the cover seals. Additionally, a force generated by the differential expansion turbine bucket airfoils and covers may induce compressive stress in the turbine bucket airfoils and covers. The groove and extension seal, and skewed cover facilitate relieving the stress while maintaining the seal between adjacent covers. As a result, the bucket covers facilitate sealing a radial leakage area in the rotor assembly. 
   Exemplary embodiments of rotor assembly components are described above in detail. The components are not limited to the specific embodiments described herein, but rather, components of each assembly may be utilized independently and separately from other components described herein. Each rotor assembly component can also be used in combination with other rotor assembly components. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.

Technology Category: 4