Abstract:
A bucket for use in the low-pressure section of a steam turbine engine is provided. The bucket has a vane length of at least about 45 inches. The bucket is comprised of a dovetail section disposed near an inner radial position of the bucket, a tip shroud disposed near an outer radial position of the bucket and a part span shroud disposed at an intermediate radial position. The intermediate radial position is located between the inner and outer radial positions. The bucket is comprised of a titanium-based alloy having between about 2% and about 6.25% by weight aluminum, up to about 3.5% vanadium, up to about 2.25% tin, up to about 2.25% zirconium, between about 1.75% and about 5.0% molybdenum, up to about 2.25% chromium, up to about 0.7% silicon and up to about 2.3% iron, with the balance being titanium.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to high strength buckets for use in the last stage of steam turbine engines. Specifically, the invention relates to the application of certain titanium-based alloys for use in making high strength, last stage turbine buckets having vane lengths of about 45 inches or greater. 
         [0002]    It is generally recognized that the performance of a steam turbine is greatly influenced by the design and performance of later stage buckets operating at reduced steam pressures. Ideally, the last stage bucket should efficiently use the expansion of steam down to the turbine exhaust pressure, while minimizing the kinetic energy of the steam flow leaving the last stage. 
         [0003]    The service requirements of steam turbine buckets can be complex and demanding. Last stage buckets, in particular, are routinely exposed to a variety of severe operating conditions, including the corrosive environments caused by high moisture and the carry-over from the boiler. Such conditions can lead to serious corrosion and pitting problems with the bucket material, particularly in longer, last stage turbine buckets having vane lengths of 40 inches or greater. Thus, for some time, last stage buckets for turbines have been the subject of repeated investigations and development work in an effort to improve their efficiency under harsh operating conditions since even small increases in bucket efficiency and life span can result in significant economic benefits over the life of a steam turbine engine. 
         [0004]    Last stage turbine buckets are exposed to a wide range of flows, loads and strong dynamic forces. Thus, from the standpoint of mechanical strength and durability, the primary factors that affect the final bucket profile design include the active length of the bucket, the pitch diameter and the operating speed in the operative flow regions. Damping, bucket fatigue and corrosion resistance of the materials of constriction at the maximum anticipated operating conditions also play an important role in the final bucket design and method of manufacture. 
         [0005]    The development of larger last stage turbine buckets, e.g., those with vane lengths of about 40 inches or more, poses additional design problems due to the inertial loads that often exceed the strength capability of conventional bucket materials such as Ti 6 Al 4 V and iron-based alloys. Steam turbine buckets, particularly last stage buckets with longer vanes, experience higher tensile loadings and thus are subject to cyclic stresses which, when combined with a corrosive environment, can be very damaging to the bucket over long periods of use. In addition, the steam in the last stages normally is “wet,” i.e., containing a higher amount of saturated steam. As a result, water droplet impact erosion of the bucket material often occurs in the last stage. Such erosion reduces the useable service life of the bucket and the efficiency of the steam turbine as a whole. 
         [0006]    In the past, it has been difficult to find bucket materials capable of meeting all of the mechanical requirements for different end use applications, particularly mechanical designs in which longer vane buckets, i.e., those having vane lengths about 40 inches or more, have been employed. Invariably, the longer buckets have increased strength requirements and, as noted above, suffer from even greater erosion and pitting potential. The higher stresses inherent in longer vane designs also increase the potential for stress corrosion cracking at elevated operating temperatures because the higher strength required in the bucket material tends to increase the susceptibility to stress cracking at operating temperatures at or near 400 degrees Fahrenheit (F). The effects of pitting corrosion and corrosion fatigue also increase with the higher applied stresses in last stage buckets having longer vane lengths. Many times, an alloy selected to satisfy the basic mechanical design requirements of other turbine stages simply will not meet the minimum mechanical strength and erosion resistance requirements of last stage buckets. 
         [0007]    Previous approaches to solving the problems with longer vane lengths in last stage turbine buckets vary widely, depending on the end use requirements. In some cases, where the service demands are less severe, a single bucket material may be acceptable. However, in order to increase erosion resistance, the bucket normally requires hardening through localized heat treatment (e.g., flame or induction hardening) at the leading edge to provide additional erosion resistance. Alternatively, an erosion resistant shielding material (such as stellite) can be attached to the bucket by brazing, gas tungsten arc or electron beam welding. These prior art physical attachment methods almost invariably lead to some degree of degradation in the weld heat affect zone over time, and thus the bucket may ultimately fail at those points. In addition, the risk of defective welds always exists and can result in costly scrapping of entire bucket assemblies in some cases. 
         [0008]    Another known method of manufacturing or repairing steam turbine buckets involves welding an insert to a leading edge of the bucket blade and then hardening a part of the insert to provide a leading outer edge with a hardened surface. Again, however, the hardening step normally does not extend as far as the junction between the insert and the blade itself. Thus, a portion of the insert can remain unhardened and vulnerable to premature failure or erosion under harsh operating conditions. 
         [0009]    Commonly-owned U.S. Pat. No. 5,351,395 to Crawmer et al., discloses a method for providing a bi-metallic structure that satisfies many of the requirements for highly stressed last stage buckets used in steam turbines. The method described in the &#39;395 patent includes attaching an erosion resistant insert material, with both the bucket and insert material being in a near optimum weldability condition. The bi-metallic assembly is heat treated after welding to optimize bucket properties without significant distortion. The insert material can also be hardened after machining (by flame hardening or other conventional methods) to provide improved erosion resistance. It has been found, however, that the bi-metallic structure disclosed in the &#39;395 patent does not solve all of the pitting and corrosion problems inherent in last stage turbine buckets, particularly those with vane lengths of about 40 inches or greater. 
         [0010]    Accordingly, a need exists in the art for a last stage bucket having longer vane length, improved stiffness, improved dampening characteristics and low vibratory stresses. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0011]    In one aspect of the present invention a bucket for use in the low pressure section of a steam turbine is provided. The bucket is formed with a vane length of at least about 45 inches. The bucket includes a dovetail section disposed near an inner radial position of the bucket, a tip shroud disposed near an outer radial position of the bucket, and a part span shroud disposed at an intermediate radial position. The intermediate radial position is located between the inner and outer radial positions. The bucket is comprised of a titanium-based alloy having between about 2% and about 6.25% by weight aluminum, up to about 3.5% vanadium, up to about 2.25% tin, up to about 2.25% zirconium, between about 1.75% and about 5.0% molybdenum, up to about 2.25% chromium, up to about 0.7% silicon and up to about 2.3% iron, with the balance being titanium. 
         [0012]    In another aspect, a steam turbine is provided comprising a low pressure turbine section having a plurality of last stage buckets arranged about a turbine wheel. The last stage buckets have a vane length of about 45 inches or greater. At least one last stage bucket comprises a dovetail section disposed near an inner radial position of the bucket, a tip shroud disposed near an outer radial position of the bucket, and a part span shroud disposed at an intermediate radial position. The intermediate radial position is located between the inner and outer radial positions. The last stage buckets are comprised of a titanium-based alloy having between about 2% and about 6.25%) by weight aluminum, up to about 3.5% vanadium, up to about 2.25% tin, up to about 2.25% zirconium, between about 1.75% and about 5.0% molybdenum, tip to about 2.25% chromium, up to about 0.7% silicon and tip to about 2.3% iron, with the balance being titanium. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a perspective partial cut away illustration of a steam turbine; 
           [0014]      FIG. 2  is a perspective illustration of a bucket according to one embodiment of the present invention: 
           [0015]      FIG. 3  is an enlarged, perspective illustration of the curved, axial entry dovetail according to one embodiment of the present invention; 
           [0016]      FIG. 4  is a perspective illustration of one embodiment of a tip shroud that can be used with the bucket of  FIG. 2 ; 
           [0017]      FIG. 5  is a perspective illustration showing the interrelation of adjacent tip shrouds; 
           [0018]      FIG. 6  is a perspective illustration of the part span shrouds that can be used with the bucket of  FIG. 2 ; 
           [0019]      FIG. 7  is a perspective illustration showing the interrelation of adjacent part span shrouds. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      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 . 
         [0021]    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. 
         [0022]    In  FIG. 1 , and as one example embodiment, the low pressure turbine can be seen to have five stages. The five stages can be referred to as L 0 , L 1 , L 2 , L 3  and L 4 . L 4  is the first stage and is the smallest (in a radial direction) of the five stages. L 3  is the second stage and is the next stage in an axial direction. L 2  is the third stage and is shown in the middle of the five stages. L 1  is the fourth and next-to-last stage. L 0  is the last stage and is the largest (in a radial direction). It is to be understood that five stages are shown as one example only, and a low pressure turbine can have more or less than five stages. 
         [0023]      FIG. 2  is a perspective view of a turbine bucket  20  that may be used with turbine  10 . Bucket  20  includes a blade portion  102  that includes a trailing edge  104  and a leading edge  106 , wherein steam flows generally from leading edge  106  to trailing edge  104 . Bucket  20  also includes a first concave sidewall  108  and a second convex sidewall  110 . First sidewall  108  and second sidewall  10  are connected axially at trailing edge  104  and leading edge  106 , and extend radially between a rotor blade root  112  and a rotor blade tip  114 . A blade chord distance is a distance measured from trailing edge  104  to leading edge  106  at any point along a radial length  118  of blade  102 . In the exemplary embodiment, radial length  118  or vane length is approximately forty five inches. In other embodiment, length  118  is about 40 to about 50 inches or more. Although radial length  118  is described herein as being equal to approximately 45 inches, it will be understood that radial length  118  may be any suitable length for radial length  118  depending on the specific application. Root  112  includes a dovetail  121  used for coupling bucket  20  to a rotor disk along shaft  14 . 
         [0024]      FIG. 3  illustrates an enlarged view of dovetail  121 . In the exemplary embodiment, dovetail  121  is a curved axial entry dovetail that engages a mating slot defined in the rotor disk. In one embodiment, the dovetail  121  has four convex projections  302 . In other embodiments, dovetail  121  could have more or less than four convex projections. The curved axial entry dovetail is preferred in order to obtain a distribution of average and local stress, protection during over-speed conditions and adequate low cycle fatigue (LCF) margins. 
         [0025]      FIG. 4  illustrates an enlarged view of one embodiment of a bucket tip  4  having an integral tip shroud  410 . The tip shroud  410  improves the stiffness and damping characteristics of bucket  20 . A sealing rib  420  can be placed on the outer surface of the tip shroud. The rib  420  functions as a sealing means to limit steam flow past the outer portion of bucket  20 . Rib  420  can be a single rib or formed of multiple ribs, a plurality of straight or angled teeth, or one or more teeth of different dimensions (e.g., a labyrinth type seal). 
         [0026]      FIG. 5  illustrates an initially assembled view of the tip shrouds  410 . The tip shrouds  410  are designed to have a gap  510  between adjacent tip shrouds, during initial assembly and/or at zero speed conditions. As can be seen, the ribs  420  are also slightly misaligned in the zero-rotation condition. As the turbine wheel is rotated the buckets  20  begin to untwist. As the RPMs approach the operating level (e.g., about 1800 to about 3600 RPM), the buckets untwist due to centrifugal force, the gaps  510  close and the ribs  420  become aligned with each other. The interlocking shrouds provide improved bucket stiffness, improved bucket damping, and improved sealing at the outer radial positions of buckets  20 . 
         [0027]      FIG. 6  and  FIG. 7  illustrate the part span shroud  610  located between the tip shroud  410  and root section  112 . The part span shrouds  610  are located on the convex and concave sidewalls of bucket  20 . During zero-speed conditions, a gap exists between adjacent part span shrouds of neighboring buckets. This gap is closed as the turbine wheel begins to rotate and approach operating speed, and as the buckets untwist. The part span shrouds are aerodynamically shaped to reduce windage losses and improve overall efficiency. The bucket stiffness and damping characteristics are also improved as the part span shrouds contact each other during bucket untwist. As the buckets untwist, the tip shrouds  410  and part span shrouds  610  contact their respective neighboring shrouds. The plurality of buckets  20  behave as a single, continuously coupled structure that exhibits improved stiffness and dampening characteristics when compared to a discrete and uncoupled design. An additional advantage is a rotor exhibiting reduced vibratory stresses. 
         [0028]    The bucket herein described can be comprised of a titanium alloy having the exemplary weight percentages shown below in Table 1: 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Al 
                 V 
                 Sn 
                 Zr 
                 Mo 
                 Cr 
                 Si 
                 Fe 
                 Ti 
               
               
                   
               
             
             
               
                 2% 
                 0 
                 0 
                 0 
                 1.75% 
                 0 
                 0 
                 0 
                 Balance 
               
               
                 to 6.25% 
                 to 3.5% 
                 to 2.25% 
                 to 2.25% 
                 to 5.0% 
                 to 2.25% 
                 to 0.7% 
                 to 2.3% 
               
               
                   
               
             
          
         
       
     
         [0029]    The titanium-based alloys used to form buckets according to the invention, i.e., alloys used for buckets having vane lengths of at least about 45 inches, exhibit a minimum ultimate tensile strength at room temperature of 145 ksi; 0.2 percent yield strength of 130 ksi at room temperature; minimum ultimate tensile strength at 400 degrees F. of  125  ksi; and a minimum 0.2 percent yield strength of 110 ksi at 400 degrees F. The preferred alloys display either a beta or alpha beta structure and achieve a minimum fracture toughness of about 50 ksi root square inches. 
         [0030]    Various steam turbine buckets having vane lengths of about 45 inches were formed in accordance with the invention using the above titanium alloy composition ranges. As noted above, a number of design factors can affect the final bucket profile and specific alloy employed, such as the active length of the bucket, the pitch diameter and the operating speed of the bucket in the operative flow regions. Damping, bucket fatigue and corrosion resistance of the alloy at the maximum anticipated operating conditions also play a role in the final bucket design using titanium-based alloys falling within the above preferred composition ranges. Exemplary profiles for longer vane last stage buckets capable of being formed with titanium alloys according to the invention are described in commonly-owned U.S. Pat. No. 5,393,200, entitled “Bucket for the Last Stage of Turbine.” 
         [0031]    After formation, each bucket according to aspects of the invention is stress relieved and the bucket surfaces machined to the finished profile using conventional finishing and heat treatment steps. Various exemplary buckets having vane lengths of about 45 inches or greater have been subjected to conventional mechanical strength and corrosion resistance tests within the nominal and maximum anticipated operating temperatures for last stage steam turbines. The titanium-based alloy materials used in buckets according to the invention exhibited improved corrosion resistance and better-than-average strength characteristics. 
         [0032]    An exemplary process for manufacturing a titanium-based steam turbine bucket according to the invention includes the following steps. Initially, a titanium billet comprised of an alloy composition as described above is formed and forged into a bucket using a conventional screw press, hammer forging and/or hydraulic press. Optionally, the forged bucket can be heat treated and quenched to provide stress relief and to develop the mechanical strength properties. Depending on the specific end use application, the bucket can also be aged using conventional means and then machined into a final operational configuration (typically with the machining being performed on all sides, i.e., 360 degrees). 
         [0033]    While the above process has been developed for longer buckets, e.g., last stage steam turbine buckets having vane lengths of about 45 inches or more, the process can be adjusted using varying titanium alloy compositions within the above-noted composition ranges, depending on the specific bucket design and end use requirements. 
         [0034]    The bucket according to aspects of the present invention is preferably used in the last stage of a low pressure section of a steam turbine. However, the bucket could also be used in other stages or other sections (e.g., high or intermediate) as well. One preferred span length for bucket  20  is about 45 inches and this radial length can provide a last stage exit annulus area of about 112 ft 2  (or about 10.4 m 2 ). This enlarged and improved exit annulus area can decrease the loss of kinetic energy the steam experiences as it leaves the last stage buckets. This lower loss provides increased turbine efficiency. 
         [0035]    As embodied by aspects of the present invention, an improved bucket for a steam turbine has been provided. The bucket is preferably used in the last stage of a low pressure section of a steam turbine. The bucket&#39;s integral tip shrouds and part span dampers provides improved stiffness and damping characteristics. The curved axial entry dovetail also improves the distribution of average and local stresses at the dovetail interface. 
         [0036]    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.