Abstract:
A bucket for use on a steam turbine rotor wheel, the bucket comprising a shank portion and an airfoil portion, the airfoil portion having a radially outer tip with a tip cover adapted to be engaged, in use, by a similar tip cover on an adjacent bucket, wherein a radial step is formed in the tip cover and the airfoil portion along a leading edge of the airfoil portion.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to steam turbines and more specifically, to the design of last-stage steam turbine buckets with integral covers.  
         [0002]     The tip areas of last-stage steam turbine buckets or blades with integral covers operate in a wet steam condition, typically with supersonic relative velocity between the steam flow and the buckets. The action of high speed, wet steam flow on the buckets can produce erosion, and can contribute to corrosion damage of the metal surfaces in the tip areas. The covers between adjacent buckets contact each other during operation by virtue of the bucket&#39;s rotation caused by the untwisting effect of the applied centrifugal forces. Connection or contact of the integrally covered buckets during operating conditions enhances the rigidity of the bucket structure and improves vibration damping. The presence of moisture on these contact areas can contribute to stress corrosion cracking. The design of the last stage bucket, therefore, must be tolerant of wet steam in existing environmental conditions. Moreover, any flow disturbing elements at the bucket tip region must be avoided to minimize aerodynamic losses.  
         [0003]     The tip bucket design for certain last stage turbine buckets results in a pocket area (or simply, pocket) being formed between adjacent bucket tip covers that tends to trap moisture produced by adjacent surfaces of the bucket covers and leading and trailing edges of the adjacent airfoils. The trapped moisture in the pocket area can cause damage to the buckets themselves as well as the damping contact surfaces of the covers.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0004]     The present invention identifies an improved bucket tip and cover shape that avoids erosion and corrosion of the steam turbine bucket and reduce aerodynamic losses, thus improving the reliability and efficiency of the steam turbine. This design change is achieved without impacting other features that are critical to the performance of the turbine and reliability of the bucket.  
         [0005]     In an exemplary embodiment, the last stage turbine buckets have integral covers disposed at the tip of the buckets that are generally similar to the known covers, but with a subtle yet significant shape change as further described below. To solve the problems experienced with the existing cover design, the cover has been modified to the extent that a radial step is formed between the airfoil leading edge tip and the cover top surface that eliminates the above-described pocket area, thus reducing moisture entrapment potential and also reducing aerodynamic drag force or aerodynamic losses. In one variant, the radial surface portion of the step is curved toward the adjacent bucket cover surface. In a second variant, the radial surface portion of the step is curved more severely to substantially smoothly merge with the adjacent bucket cover surface. The precise shape of the step may be optimized to balance the stress level, addition of mass and the impact on the aerodynamic design.  
         [0006]     Accordingly, in one aspect, the invention provides a bucket for use on a steam turbine rotor wheel, the bucket comprising a shank portion and an airfoil portion, the airfoil portion having a radially outer tip with a tip cover adapted to be engaged, in use, by a similar tip cover on an adjacent bucket, wherein a radial step is formed in the tip cover and the airfoil portion along a leading edge of the airfoil portion.  
         [0007]     In another aspect, the invention provides a bucket for use on a steam turbine rotor wheel, the bucket comprising a shank portion and an airfoil portion, the airfoil portion having a radially outer tip with a tip cover adapted to be engaged, in use, by a similar tip cover on an adjacent bucket, wherein a radial step is formed in the tip cover and the airfoil portion along a leading edge of the airfoil portion; wherein the step is formed by a first airfoil surface extending in a flow direction away from the leading edge and a second tip cover surface extending radially away from the first airfoil surface; and wherein the leading edge is radially shortened by forming the radial step; and further wherein the tip cover is integral with the airfoil portion.  
         [0008]     In still another aspect, the invention provides a method of eliminating a moisture-trapping pocket between adjacent top covers at radially outer ends of respective airfoil portions of turbine buckets comprising: a) radially shortening leading edges of the turbine buckets to create radial steps between the leading edges and top surfaces of the tip covers; and b) cutting radial surface portions of the radial steps such that the radial surface portions more smoothly merge with adjacent radial surfaces at trailing edges of adjacent buckets.  
         [0009]     The invention will now be described in detail in connection with the drawings identified below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a partial perspective view of a pair of buckets having integral covers in accordance with a known design;  
         [0011]      FIG. 2  is an enlarged detail taken from  FIG. 1 ;  
         [0012]      FIG. 3  is a partial perspective view illustrating a bucket tip cover design in accordance with a first embodiment of the invention; and  
         [0013]      FIG. 4  is a partial perspective view illustrating a bucket tip cover design in accordance with a second embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     With reference to  FIG. 1 , a plurality (two shown) of like turbine blades or buckets  10 ,  12  are secured to a turbine rotor wheel (not shown) by means of a dovetail or other suitable joint generally indicated at  14 . The buckets  10 ,  12  extend a full 360° about the turbine wheel, thereby forming a “row” of buckets. Each bucket in the row is generally identical, though occasionally the last bucket (or “notch blade”) and two buckets adjacent to the notch blade can have some geometrical differences to facilitate assembly. As is well understood, the dovetail or other joints  14  are designed for mating and sliding engagement with a complementary dovetail or other shape formed on the rim of the rotor wheel. The type of bucket dovetail and the manner of loading the buckets onto the wheel may vary and, in any event, is not significant to this invention.  
         [0015]     Blade portions  16 ,  18  of the buckets  10 ,  12 , respectively, extend upwardly from the dovetail portions  18  to respective tips  20 ,  22 . The tips  20 ,  22  are formed with respective integral covers  24 ,  26  which couple the entire row of buckets together, substantially 360° about the wheel described in detail.  
         [0016]     With reference to  FIG. 2 , the integral cover  26  is set back from the leading edge  28  of the blade in the direction of steam flow, indicated by the flow arrow  30 . Note, however, that the radially outer (or top) surface  32  of the cover is flush with (or lies in the same plane as) the radially outer tip surface  34  of the blade portion  18 . The bucket cover  24  of the adjacent blade  10  has a trailing edge portion  36  defined in part by side surfaces  38 ,  40  and a back face  42 . During operation, centrifugal forces cause the back face  42  to engage a generally parallel front face  44  of the cover  26 , leaving a pocket area or pocket  46  between the leading edge  28  of bucket  12 , front face of the bucket cover  26  and the curved trailing edge side surface  40  of cover  24 . This pocket or pocket area is susceptible to moisture collection as described above.  
         [0017]      FIG. 3  illustrates a first embodiment or variant of a bucket cover re-design that substantially eliminates the pocket  46  shown in  FIG. 2 . In this embodiment, the bucket  48  includes a blade portion  50 , with a radial step or notch  52  cut into the leading edge  54  of the blade portion  50  and associated tip cover  56 , such that a portion of the leading edge  54  is radially shortened. Specifically, the step or notch is defined by a radially shortened surface portion  58  of the leading edge  54  and a curved radial surface  60  cut along the side of the cover  56 . This cut also reduces the surface area of the front face  62  of the tip cover  56 , and thus substantially eliminates the pocket discussed above, while providing a smoother interface for continuity of flow between surface  60  and side surface  64  of adjacent cover  66 .  
         [0018]     In  FIG. 4 , a variation of the radial step is illustrated and, for convenience, reference numerals are the same as used in  FIG. 3  but with the prefix “1” added. Thus, the bucket  148  includes a blade portion  150 , with a radial step or notch  152  cut into the leading edge  154  of the blade portion  150  and associated tip cover  156 . The radial cut is defined by radially shortened surface portion  158  and a curved radial surface  60 . In this instance, however, the curved radial surface portion  160  of the step is curved more severely to remove additional cover material and substantially eliminate that portion of the front face  162  of the tip cover  156  exposed to wet steam flow. There is now a relatively smooth transition between the curved side surface  164  of the cover  166  and the radial surface portion  160  of the cover  156 .  
         [0019]     The radial shortening of the leading edge  54  or  154  of the blade portion  50  or  150  does not significantly impact performance, and the substantial elimination of the moisture-trapping pocket prevents moisture from collecting and causing potential corrosion damage to the blades and their respective covers.  
         [0020]     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.