Abstract:
A bucket spreading tool is disclosed for separating covers of adjacent turbine buckets, the tool includes: an arm for extending a head of the tool between adjacent turbine wheels and for positioning the head between the adjacent buckets of a wheel, wherein the head includes an attachment to an end of the arm and a forward portion having a front side surface shaped to engage a first bucket of said adjacent buckets, and a rear side surface shaped to engage a second bucket of said adjacent buckets.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to the field of steam turbine buckets and, particularly, to machining of buckets mounted in the turbine.  
           [0002]    Steam turbines generally have annular rows of turbine buckets that are mounted on a rotor. Each row of turbine buckets is arranged around a disc wheel mounted on the rotor. Typically each turbine bucket has a blade section, and a upper and lower shroud sections. The upper shroud is generally referred to as the “cover” of the bucket. The buckets are arranged annularly around the outer periphery of the wheel. The wheel is mounted on the turbine shaft. Several rows of turbine wheels are arranged on the turbine shaft. Each wheel is separated by some predefined distance, e.g. approximately five inches (15 cm), to allow for turbine vanes that are arranged between the rows turbine buckets.  
           [0003]    The covers of the buckets are at the outer perimeter of the turbine wheel and bucket assembly. The covers are adjacent the stationary turbine casing. To prevent steam passing over the buckets from leaking over the casing and into the casing, a seal is formed between the casing and covers of the buckets. As part of this seal, sealing teeth are machined onto the upper surface of the covers after the buckets have been assembled onto the wheel. The sealing teeth on the covers are aligned with similarly configured teeth on spill strips of the turbine casing. The non-contact engagement of the sealing teeth on the covers and the teeth on the spill strips prevent steam from leaking past the buckets and into the casing, thereby improving the efficiency of the turbine unit.  
           [0004]    The machining of the bucket covers can create metal burs on the covers, including burs that extend into the gaps between adjacent bucket covers. The standard past practice for machining away burs has involved removal of the buckets from the turbine wheel, which requires disassembly of the turbine. After the burs are ground down on the removed bucket, the buckets and turbine are reassembled. This prior bur removal process is extremely time consuming and expensive.  
           [0005]    There are occasions when the turbine buckets are machined and repaired after they have been assembled on a disc, and the disc has been mounted on the shaft of the turbine. For example, veneer-sealing teeth are often machined into the ICVs, after their buckets have been mounted on a wheel and the wheel mount on a shaft. Machining the veneer-sealing teeth into the ICVs after the buckets have been assembled on a wheel ensures that the teeth on each cover line up and are aligned with their opposite teeth on the stationary spill strips mounted on the turbine housing.  
           [0006]    Machining veneer-sealing teeth often leaves metal burs on ICVs. Some of these metal burs are on the upper surface of the covers and will extend into the gaps between the covers and the spill strips, and other burs may protrude from the sides of the covers and interfere with the interlocking of a cover with its adjacent covers. Shims have been inserted between bucket covers to reduce burs from rolling into the gap between covers.  
           [0007]    However, it is difficult to access the gap between turbine covers to insert and remove shims after the buckets have been mounted on a wheel and the wheel mounted on the turbine shaft. It is especially difficult to access the steep-angle interval cover buckets (ICVs) that have been developed to improve steam turbine efficiency. Nevertheless, ICVs and other types of bucket covers do require additional machining after their buckets have been assembled on the wheel and shims are useful for reducing burs.  
           [0008]    If the burs on ICVs are substantial they can affect the response characteristics of the turbine bucket to vibration. In particular, large burs on ICVs have been shown to produce substantial resonance frequency shifts in the axial and torsion vibration modes of a bucket. Burs may shift the resonance vibration frequencies of a bucket by more than 10 percent (10%) from the expected resonance frequency for the bucket. Accordingly, the resonance frequency shifts caused by the burs on the ICVs can render inaccurate the expected resonance frequencies for buckets.  
           [0009]    Turbine designers rely on the expected resonance frequencies of a bucket to, for example, select the number of upstream nozzles to be adjacent the row of buckets. If a designer properly understands the resonance frequency of the turbine buckets, then the number of upstream nozzles may be selected to minimize the resident frequencies in the buckets. If burs offset the expected resonance frequency of the bucket, the actual vibration resonance of the bucket with burs may unintentionally coincide with vibrations induced by the upstream nozzles and rotating buckets as steam flows from the nozzles to the buckets. If vibrations induced by the steam have frequencies at or near the resonance frequencies of the bucket, then excessive vibration may be induced in the bucket that will cause the bucket to prematurely fail.  
           [0010]    There is a long felt need for better tools to machine burs from buckets and use shims to reduce bur formation, especially from the cover of buckets. Such tools, should make the insertion and removal of shims expeditious and inexpensive.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0011]    A tool has been developed to spread apart the covers of adjacent buckets mount on a turbine wheel. The tool may be applied when the wheels are assembled on a turbine shaft. By separating the covers, shims can be inserted into and removed from the covers.  
           [0012]    In one embodiment the invention is a bucket spreading tool for separating covers of adjacent turbine buckets, wherein the tool includes: an arm for extending a head of the tool between adjacent turbine wheels and for positioning the head between the adjacent buckets of a wheel, and the head has an attachment to an end of the arm and a forward portion having a front side surface shaped to engage a first bucket of said adjacent buckets, and a rear side surface shaped to engage a second bucket of said adjacent buckets.  
           [0013]    The spreading tool may further include a front surface of the head that is cupped to abut a convex surface of the first bucket, and a rear surface of the head is rounded to pivot against the second bucket. Further, the spreading tool head may include a slot between its front and rear side surfaces, wherein said slot is alignable with gap between the covers of the adjacent turbine buckets.  
           [0014]    The bucket spreading tool of the first embodiment may have a tool head that is separable from the arm, and tool head height that is no greater than three inches which is less than the distance between the adjacent turbine wheels. Further the tool head may be formed of a soft metallic material.  
           [0015]    In a second embodiment the invention is a bucket spreading tool for separating covers of adjacent turbine buckets having: an arm for extending a head of the tool between adjacent turbine wheels and for positioning the head between the adjacent buckets of a wheel; the head including an attachment to an end of the arm and a forward portion having a front side surface shaped to engage a first bucket of said adjacent buckets, and a rear side surface shaped to engage a second bucket of said adjacent buckets, wherein the front side surface is concave to mate with a convex airfoil surface of the first bucket, and the rear side surface of the head is convex to pivot against a concave airfoil surface of the second bucket.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a perspective view of a portion of a turbine wheel having buckets;  
         [0017]    [0017]FIG. 2 is a plan view of a bucket spreading tool;  
         [0018]    [0018]FIG. 3 is an enlarged perspective view of a head of the bucket spreading tool; and  
         [0019]    [0019]FIGS. 4 and 5 are enlarged perspective views of the back and front, respectively, the spreading tool between adjacent turbine buckets. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    With reference to FIG. 1, a plurality of turbine buckets  10  are secured to a turbine rotor wheel  12 . The wheel is mounted on a turbine shaft with a series of other wheels. Each bucket may include a dovetail connector  14  formed in a lower portion of bucket  10 . This connector interlocks with a dovetail shaped slot formed on the rim of rotor wheel  12 . Buckets  10 , only three of which are shown here, extend a full 360° about the rotor wheel  12 .  
         [0021]    Each bucket has a blades  16  that extends radially upwardly from the dovetail  14  to a tip  18  of the blade. Covers  20  are formed on the blade tips. The covers are preferably of unitary, or one-piece, construction with the remainder of bucket  10 . The covers interconnect with adjacent covers to couple the row of buckets together about the rotor wheel  12 . Each cover  20  of a bucket has a pair of sides  22 . The sides of covers interlock with the sides of covers of adjacent blades. The covers, especially ICVs, may have a steep slope, as is shown in FIG. 1. This slope renders machining of the covers when mounted on a wheel difficult.  
         [0022]    After installation on the wheel  12 , the covers (ICVs)  20  are machined on their upper surface to include sealing teeth  24  that will line up in sealing engagement with similar teeth on spill strips of the turbine casing. During the machining of the covers  22 , shims  26  are inserted between the sides of adjacent covers. The shims reduce the formation of burs on the covers, especially burs that extending into the gap  32  between the sides of covers. The shims  26  are removed after the machining of the covers is completed.  
         [0023]    Burs may be formed while machining the teeth  24  on the covers. In some instances, the burs are small and produce a negligible frequency shift in the resonance modes of the buckets. In other instances, the burs are sufficiently large that they bridge the gap  32  and produce substantial resonance modes shifts and need to be removed. The burs are substantially prevented by inserting shims  26  (see FIG. 5) between the covers. The bucket covers need to be separated slightly to allow the shims to be inserted.  
         [0024]    [0024]FIG. 2 shows a plan view of a bucket spreading tool  40  which includes an extended slender arm  42 , and a head  44 . The bucket spreading tool separates the buckets to allow for the insertion and later removal of the shims  26 . The head  44  of the tool is inserted between individual turbine buckets and is pivoted to spread apart the buckets and their covers. With the adjacent bucket covers separated, the shims  26  may be inserted. The buckets are spread apart and can be machined while the turbine buckets are assembled on the wheel and the wheels are mounted on the turbine shafts. In addition, the buckets are spread after the turbine casing has been removed to expose the individual rows of turbine buckets, and the turbine nozzles are removed from between each row of turbine buckets.  
         [0025]    After removal of the turbine casing and nozzles, there is a gap  34  (see FIG. 1) between the adjacent rows of turbine buckets is typically 5 inches (15 cm) or larger in large steam turbines. Into this gap  34  is inserted the bucket spreading tool  40 . The bucket spreading tool is inserted tangentially to the array of buckets so that the tool head  44  can be extended between adjacent wheels and inserted perpendicularly between two selected adjacent buckets on the same wheel. The slender arm  42  of the spreading tool  40  is sufficiently long to allow a technician to reach the tool head  44  between any of the adjacent buckets in any of the rows of buckets in a turbine. The arm has a handle  43  at an end opposite to the head  44 . A technician grasps the handle  43  to operate the tool.  
         [0026]    The tool head  44  is secured to the arm  44  by bolts  46 . A notch  48  in the head may fit with a opposite notch  50  on the arm, as shown in FIG. 3. The bolts  46  and notch connection between the head and arm ensure that the head is securely attached to the arm. The arm  42  may be used to apply a slight torque to the head  44  to spread apart adjacent buckets.  
         [0027]    The bolt and notch connection between the tool head  44  and arm  42  allows for relatively easy changing of heads  44  on the arm. Each row of turbine buckets may have a different blade profile. The side surfaces of the front portion  52  of the head  44  are tailored to fit the surfaces of adjacent blades of a particular row of blades. Accordingly, there may be a different heads  44  for each row of blades. By allowing for easy substitution of heads, the arm  42  may be attached to the proper vane head  44  for the particular row of turbine blades desired to be separated and machined.  
         [0028]    The height  54  of the tool head  44  is sufficiently short to allow the head to easily fit between adjacent turbine wheels. For example, the height  54  of the head may be three inches (8 cm) or less.  
         [0029]    The front portion  52  of the head has sides which are shaped to seat on the surfaces of adjacent blades. A forward side surface  56  of the head is cupped to seat on the front surface  58  (see FIG. 4) of a blade. A rear surface  60  of the front portion of the head  52  is rounded to pivot against a back surface  62  of an adjacent blade. The rear surface may have a volute shape so that as the head pivots the width of the head increases to force the buckets apart. As the forward side  56  of the head  44  presses against one blade  58  and the rear side  60  presses against the adjacent blade  62 , the pivoting of the head separates the adjacent blades and creates a separation of the gap  32  (see FIG. 5) between the covers of the two blades.  
         [0030]    The head  44  is pivoted by the technician who pivots the arm  42  along an arc  64 . Once the bucket has been separated and the gap  32  has been widened, a shim  26  remaining in the gap may be removed. To facilitate insertion and removal of shims  26 , a slot  66  on the head of the spreading tool allows easy access to the gap and for removal of the shims. Once the shim has been inserted or removed, the technician pivots the arm and head to allow the adjacent buckets to come together. As the tool head  44  unseats from the blades, the head can be removed from between the blades and the head and arm pulled out from between the turbine wheels.  
         [0031]    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.