Patent Publication Number: US-2011070072-A1

Title: Rotary machine tip clearance control mechanism

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to turbine engines and more specifically to a tip clearance control mechanism for turbine blades of the turbine engines. 
     A turbine stage consists of a row of stationary vanes followed by a row of rotating blades in an annulus. The flow is partially expanded in the vanes and directs it to the rotating blades, where it is further expanded to generate required power output. For the safe mechanical operation, there exists a minimum physical clearance requirement between the tip of the rotating blade and outer annulus wall of the casing of the turbine engine. This clearance varies based on the rotor dynamic and thermal behaviors of the rotor and casing. Reduction in physical clearance will lead to reliability issues. 
     Turbine blades are rotating airfoil-shaped components in series of stages designed to convert thermal energy from a working fluid, such as gas or steam, into mechanical work of turning a rotor. The high-energy flow leaving through clearance area will account for 20% loss in the stage performance and thus reduction in power output. Performance of a turbine can be enhanced by sealing the outer edge of the blade tip to prevent the working fluid from escaping the working flowpath into the gaps between a blade tip and an outer casing of turbine. A common manner for sealing the gap between the turbine blade tips and the turbine casing is through blade tip shrouds. Not only do shrouds enhance turbine performance, but also serve as a vibration damper, especially for large, radial-length turbine blades. The shroud acts as a mechanism to raise the blade natural frequency and in turn minimizes failures due to extended resonance time of the blade at a natural frequency. 
     A portion of a typical turbine blade with a shroud (also referred to as turbine bucket cover or tip cover) is shown in  FIG. 1 . The turbine blade  10  includes an airfoil section  11  and shroud  12 . The shroud  12  may be manufactured integral to the airfoil  11 . The airfoil further contains a leading edge  15  and trailing edge  16  that run generally perpendicular to shroud  12 . The shroud  12  has a thickness and has sidewalk  17 , which may be cut to create an interlocking configuration when adjacent turbine blades are present. The interlocking mechanism occurs along two bearing faces  13 , where adjacent turbine blades (not shown) contact at shroud  12 . It is the interlocking of the turbine blade shrouds  12  at bearing faces  13 , that creates means for damping out vibrations, as well as for sealing the working fluid within the turbine gas-path. An additional feature of a typical turbine blade shroud may be a knife-edge tip seal (also known as rotor tooth or tooth seal)  14 . Depending upon the size of the blade shroud, one or more tip seals may be utilized. These seals run parallel to each other, typically perpendicular to the engine axis  18 , and extend outward from shroud  12 . The purpose of these seals is to engage the shroud blocks of the turbine casing (not shown) to further minimize leakage around the blade tip and reduce mechanical impact in case the bucket tip rubs the casing. Tip leakage diverts working fluid that would otherwise flow in a main flow path and perform work on the turbine blades. Tip leakage may further result in elevated tip vortex size and intensity, which may penetrate the main steam flow path downstream from the blade, raising backpressure and thereby lowering the efficiency of the stage. However, a clearance between the tip seal and the casing shroud needs to be provided to account for thermal expansion and asymmetry of rotation. A fully covered turbine blade tip has better aerodynamics performance over uncovered bucket tip because of reduced tip vortex size, intensity, and tip leakage. 
     While the purpose of the shroud is to seal the working fluid within the flow path as well as to provide a means to dampen vibrations, the shroud has its disadvantages as well. A drawback to the shroud concept is the weight the shroud adds to the turbine blade. During operation, the turbine blades spin on a disk, about the engine axis. A typical industrial application includes disk speeds up to 3600 revolutions per minute. The blades are held in the disk by an interlocking cut-out between the blade root and the disk. As the turbine blade spins, the centrifugal forces cause the blade to load outward on the turbine disk at this attachment point. The amount of loading on the disk and hence the blade root, which holds the blade in the disk, is a function of the blade weight. That is, the heavier the blade, the more load and stresses are found on the interface between the blade root and disk, for a given revolutions per minute. Excessive loading on the blade root and disk can reduce the overall life of each component. Another drawback to shrouds is creep curling of the blade shrouds. Depending on the thickness of the shroud, the shroud edges can “curl” up at their ends and introduce severe bending stresses in the fillets between the shroud and blade tip. Shrouds curl due to the bending load on the edges of the shroud from gas pressure loads as well as centrifugal loads. The curling of a shroud is analogous to the bending of a cantilevered beam due to a load at the free end of the beam. An industry known fix to this curling phenomenon is to increase the section thickness of the shroud uniformly which will result in a stiffer shroud and more resistance to curling. The downside to simply increasing the shroud thickness uniformly is the additional weight that is added to the shroud by this additional material. 
     As described above, to prevent the tip cover from rubbing turbine casing wall and to further reduce tip leakage, one or several seal teeth can be placed on the top of a tip cover. In some applications, rotor teeth can be accompanied by stator teeth suspended from a casing shroud and integrated with the rotor teeth. 
     Accordingly, it would be desirable to limit tip leakage for turbine blades, while at the same time providing enhanced stage efficiency. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to a first aspect of the present invention, a rotary machine tip clearance control mechanism for a stage of a low pressure steam turbine is provided. The clearance mechanism provides a rotor blade including an airfoil with a rotor blade shroud disposed on an outer radial tip of the airfoil. A rotor tooth projects radially outward on the rotor blade shroud, with the rotor tooth being generally centered with respect to an axial chord of the airfoil. A casing shroud may be disposed radially outward from the rotor blade shroud. At least one stator tooth is provided on the casing shroud. The stator tooth may project inward radially and be disposed axially relative to the rotor tooth. A clearance area may be formed between the rotor blade shroud and the casing shroud, the clearance area being adapted to limit leakage flow and increase stage efficiency. 
     A further aspect of the present invention provides a method for providing a tip clearance control mechanism for rotor blade of a stage of a low pressure steam turbine between a rotor blade shroud of a rotor blade and a casing shroud. The method includes selecting a shape for a tip shroud; selecting a rotor tooth shape; selecting a number of rotor teeth; selecting a stator tooth shape; and selecting a number of stator teeth. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  illustrates a prior art shroud for a turbine blade; 
         FIG. 2A  illustrates a plurality of arrangements of rotor tip shroud configurations with variations in the number and arrangement of stator teeth and number and arrangement of rotor teeth; 
         FIG. 2B  illustrates a plurality of stator tooth structures; 
         FIG. 2C  illustrates a plurality of rotor tooth structures; 
         FIG. 2D  illustrates various exemplary tip clearance control configurations wherein stator teeth may be formed as an integral part of the casing. 
         FIG. 3  illustrates a turbine blade incorporating an inventive clearance mechanism on a rotor blade shroud; 
         FIG. 4A  illustrates geometric parameters for components of a rotor shroud and casing shroud for an inventive tip clearance control mechanism; 
         FIG. 4B  illustrates a top view of a top surface of a rotor blade shroud for a large blade of a low pressure steam turbine; 
         FIG. 5A  illustrates leakage flow past a single conventional rotor tooth disposed at a typical location on an aft end of a rotor blade shroud; 
         FIG. 5B  illustrates leakage flow control for a preferred embodiment of the clearance mechanism on a last stage blade for a low pressure turbine; 
         FIG. 6A  illustrates leakage flow past a single conventional rotor tooth disposed at a typical location on an aft end of a rotor blade shroud for a next-to-last stage of a low pressure turbine; 
         FIG. 6B  illustrates leakage flow for the preferred embodiment of the clearance mechanism for a next-to-last stage of a low pressure turbine; and 
         FIG. 7  illustrates a flowchart for the method  500  for forming a clearance mechanism for the rotor blade of low pressure steam turbine. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following embodiments of the present invention have many advantages, including reducing bucket tip leakage flow, increasing rotor torque work, minimizing mixing losses caused by tip vortex, improving turbine performance and reducing tip cover weight. 
     The present invention relates to a rotary machine with a tip clearance control mechanism for reducing leakage flows, minimizing tip vortex size and penetration into main flow that will improve turbine efficiency. The tip clearance control mechanism includes inventive arrangements of a rotating shroud and a shape of the shroud, teeth of various shapes and locations on the rotating shroud, and one or more teeth of various shapes and locations on a stationary shroud or casing wall configurations providing comparable tip clearance control. The reduction in leakage flow is a function of how these components are assembled together, which defines a clearance passage between the rotating shroud and the stationary shroud. 
       FIGS. 2A ,  2 B  2 C and  2 D illustrate a variety of exemplary inventive arrangements that may be considered for establishing an inventive tip clearance control mechanism for a rotary machine. 
       FIG. 2A  illustrates a plurality of exemplary inventive arrangements  30 ,  35  of rotor tip shroud configurations with variations in the number and arrangement of stator teeth and number and arrangement of rotor teeth. The rotor tip shroud arrangements may include a top surface with a slope on the upstream side mating with a generally level downstream side. The rotor tip shroud may include a top surface with a slope on the upstream side with a generally level downstream side separated by a step. 
       FIG. 2B  illustrates a plurality of exemplary inventive stator tooth structures. The stator tooth  50  includes extends into the clearance passage in a generally upstream direction with respect to leakage flow. The upsteam surface  51  of the stator tooth  50  may be contoured to smoothly blend into the upsteam surface  52  of the stationary shroud  45 . The downsteam surface  53  may be flat. Stator tooth  55  may include a first section  56  of roughly constant thickness extending into the clearance passage normal to the stationary shroud  45  and a generally tapered section  57  extending further into the clearance passage in a downstream direction with respect to the leakage flow. Stator tooth  60  may include a first section  61  of roughly constant thickness extending into the clearance passage normal to the stationary shroud  45  and a generally tapered section  62  extending further into the into the clearance passage in an upstream direction with respect to the leakage flow. Stator tooth  65  may include a first section  66  of roughly constant thickness extending into the clearance passage normal to the stationary shroud  45  and a tapered section  67  extending further normal into the clearance passage, the taper provided on the downstream side with respect to leakage flow. A top surface  68  may be provided on stator tooth  67 . Stator tooth  70  may include a first section  71  of constant thickness extending into the clearance passage with a second section  72  extending from the first section and tapering on both sides to an edge  73 . 
       FIG. 2C  illustrates a plurality of exemplary inventive rotor tooth structures. Rotor tooth  80  may include a generally curved upstream surface  81  and a generally flat downstream surface  82  with respect to leakage flow. The rotor tooth  80  may extend into the clearance passage in a generally upstream direction with respect to the leakage flow. Rotor tooth  80  may extend from a first surface  84  on the upsteam side of the rotor shroud  83  and from a downstream surface  86  of a different elevation on the rotor shroud  83 . Rotor tooth  85  may extend normal to the rotor shroud  83  into the clearance passage. Rotor tooth  85  may include a first section  87  with constant thickness and an outer section  88  with tapered thickness, wherein the taper is provided on the downstream side  89  with respect to the leakage flow, terminating at top surface  89 . 
       FIG. 2D  illustrates various exemplary tip clearance control configurations of casing options  90  wherein stator teeth may be formed as an integral part of a casing for a rotary machine. A rotor blade  10  with a rotor tip shroud  85  may include one or more rotor teeth  80  within a trenched area  91  of a casing  92  for a rotary machine. An inner wall  75  of the casing  92  may define an outer boundary for leakage flow  94  past the rotor tip. Depending on the physical configuration of the casing, either stator tooth may be physically formed or the step in the casing may provide an obstruction to leakage flow in lieu of the discrete stator tooth. As an example, a relative height of the steps of the trench  91  of the casing  92  may itself provide the clearance control in conjunction with the rotor teeth (tooth)  80 . In  FIG. 2D , the first step  93  of the trench  91  may provide a first resistance to leakage flow  94 , functionally replicating a first stator tooth  95  forward relative to the rotor tooth  80  within casing wall layout  96 . The second step  97  of the trench  91  may act as a resistance replicating a second stator tooth  98  aft relative to the rotor tooth  80 , wherein the discrete second stator tooth would be formed with a recessed casing wall  99 . 
     In the present invention, a rotating blade may include a shroud along with teeth, which is placed in between a set of stationary teeth located on a shroud attached at the casing wall of the turbine. When the flow passes through the clearance passage, vortices are formed, which reduce the effective clearance and lead to reduced clearance flow. 
       FIG. 3  illustrates an embodiment of a rotor blade component of the inventive tip clearance control mechanism for a stage of a low pressure turbine. The rotor blade  110  includes a sloped rotor blade shroud  112  that provides a sloped pressure side top surface  120  of the blade shroud and a relatively flat suction side top surface  121 . The blade shroud  112  may be mounted to an airfoil  111  of a blade  110  for a turbine engine. The airfoil  110  may include a root  130 , a tip  131 , a leading edge  190 , a trailing edge  191 , a pressure side  133  and a suction side  134 . A dovetail arrangement  135  in the root section  130  engages the airfoil  111  to the rotor wheel of the turbine engine (not shown). The blade shroud  112  may be formed integral with the airfoil  111  at the tip end  131 . The rotor blade shroud  112  includes a root tooth  145  positioned with respect to an axial chord of the blade ( FIG. 4A ,  4 B). 
       FIG. 4A  illustrates geometric parameters for components of a rotor shroud and casing shroud for an inventive tip clearance control mechanism. Inlet cavity  115  provides a steam inlet path  113  at the tip end of the rotor blade  110 . The tip clearance control mechanism  100  includes a rotor tooth  145 , a front stator tooth  140  and a rear stator tooth  150 . The rotor tooth  145  may have a thickness  146  and a height  147  and a location  148  with respect to an axial chord line of the blade airfoil as shown in  FIG. 4B . The front stator tooth  140  may have a thickness  141  and a height  142  and an axial position  143  relative to the relative to the rotor tooth  145 . The rear stator tooth  150  may have a thickness  151  and a height  152  and an axial position  153  relative to the relative to the rotor tooth  145 . Teeth on the rotor shroud  112  and the casing shroud  116  may include an angular forward/aft orientation α  149  relative to a radial direction. The arrangement may further include a step  155  disposed at a distance  156  from the pressure side of the rotor shroud sidewall with a step height  157  below the remaining top surface of the rotor shroud. 
       FIG. 4B  illustrates a top view of a top surface  180  of a rotor shroud  112  for a large rotor blade  110  of a low pressure steam turbine. A top view of the airfoil  111  for the rotor blade and an axial chord  185  of the airfoil are shown in phantom. The axial chord  185  traverses between the leading edge  190  and trailing edge  191  of the rotor blade. Location of the rotor tooth  145  may be designated with respect to the percentage distance between the leading edge and trailing edge (e.g. 50% chord length  195 , 40% chord length  196 ). 
     According to an embodiment of the present invention, a rotary machine tip clearance control mechanism  200  is provided for a stage of a low pressure turbine. The stage may be a final stage of the low pressure turbine. Further, the stage may be a stage at either end of a double-flow low pressure steam turbine. The rotary machine tip clearance control mechanism may provide a rotor blade including an airfoil with a rotor blade shroud disposed on an outer radial tip of the airfoil. A rotor tooth may project radially outward on the rotor blade shroud. The rotor tooth may be generally centered with respect to an axis chord of the airfoil. Further, a casing shroud may be disposed radially outward from the rotor blade shroud. A front stator tooth on the casing shroud, the front stator tooth projecting inward radially and being disposed axially forward relative to the rotor tooth. An aft stator tooth may be provided on the casing shroud, the aft stator tooth projecting inward radially and being disposed aft relative to the rotor tooth. A clearance area is formed between the rotor blade shroud and the casing shroud. The clearance area is adapted to limit leakage flow and increase stage efficiency. 
     The rotor tooth may be arranged at about 40% to 50% of axial chord length for the airfoil of the blade. Preferentially, the rotor tooth may be set at about 50% of the axial chord length. The rotor tooth height may range from about 0.19 inch to about 0.35 inch. Preferentially, the rotor tooth height may be set at about 0.35 inch. The rotor tooth thickness may be about 0.13 inch. The front stator tooth may be disposed forward from the from the rotor tooth axis by about 0.6 inch to about 0.825 inch. Preferentially the front stator tooth may be located at about 0.8 inch forward from the rotor tooth axis. The front stator tooth height may be sized at about 0.3 inch to about 0.8 inch. The front stator tooth height may be preferentially sized at about 0.6 inch. The aft stator tooth may be disposed about 0.3 inch to about 1.2 inch aft of the rotor tooth axis. The aft stator tooth may preferentially be located about 0.8 inch aft of the rotor tooth. The aft stator tooth may include a height of about 0.2 inch. 
     The top surface  180  of the shroud may be sloped radially outward between the pressure side of the sidewall for the rotor blade shroud and the rotor tooth. The top surface  180  on the suction side may be essentially flat. In the first embodiment a step rising about 0.16 inch in the top surface of the shroud aft of the pressure side of the shroud sidewall by about 1.02 inch may be provided. In a preferred arrangement, no step in the rotor blade shroud top surface is provided. 
     Further, the tooth for the front stator tooth, the rotor tooth and the aft stator tooth may include a saw-tooth outer edge wherein the height of the pressure side of the tooth extends further into the clearance area than the height of the suction side of the tooth. 
     In a further embodiment, the rotor teeth may include a forward or reverse angle with respect to an outward radius from the rotor wheel, however the preferred embodiment provides for rotor teeth projecting radially outward from the rotor blade shroud. 
       FIG. 5A  illustrates leakage flow past a single conventional rotor tooth disposed at a typical location on an aft end of a rotor blade shroud. Tip leakage flow  170  from inlet cavity  115  enters the broad clearance space  172  between a top surface  173  of a baseline rotor blade shroud  174  and casing shroud  175 . Leakage flow  170  is restricted only by the one clearance space  176  between the tip  177  of the rotor tooth  178  and the casing shroud  175 . 
       FIG. 5B  illustrates leakage flow for the preferred embodiment of the tip clearance control mechanism  200 . The clearance mechanism  200  includes a front stator tooth  210  positioned on the casing shroud  175  close to a forward end  241  of the rotor blade shroud  240 , a rotor tooth  220  generally centered with respect to the axial chord  185  ( FIG. 4B ) on the rotor blade shroud, and an aft stator tooth  230  generally disposed proximate to an aft end  242  of the rotor blade shroud. Specific positioning for a preferred embodiment has been described above with respect to the parameters of  FIGS. 4A and 4B . 
     As the flow comes into rotating blade region, a portion of flow enters the cavity  115  and tries to pass through clearance region between the rotor blade shroud  240  and the casing shroud  175 . With the presence of front stator tooth  210  there is formation of a first vortex  261  in the inlet cavity  115 , diverting some of the leakage flow  270 . The forward location of the front stator tooth  210  also encourages the leakage flow  270  downward onto the top surface  243  of the rotor blade shroud  240 . This leakage flow  270  then passes to the forward face  221  of the rotor tooth  220 , where it moves upwards and forms the second vortex  262  between the rotor blade shroud  240  and the casing shroud  175 . This second vortex  262  makes the leakage flow to be “hard-pressed” on to the forward face  221  of the rotor tooth  220  and forces the high-speed flow to take a sharp turn over the rotor tooth edge  223 . Since flow is taking a sharp turn over the controlling gap, the effective flow area reduces resulting in less leakage over the rotor tooth  220 . Similarly, the leakage flow  170  passing over the rotor tooth edge is urged against the forward edge  231  of the aft stator tooth  230 , thereby being forced inward radially and creating a third vortex  263 . The third vortex  263  forces the tip leakage flow against the forward face of the aft stator tooth, making passage over the sharp edge  232  of the aft stator tooth more difficult and further limiting leakage flow. 
       FIG. 6A  illustrates leakage flow past a single conventional rotor tooth disposed at a typical location on an aft end of a rotor blade shroud for a next-to-last low pressure turbine blade. A baseline clearance mechanism provides a sloped rotor blade shroud  374  without teeth. Multiple knife-edges  376  project from a casing shroud  375 . Leakage flow  370  from inlet cavity  115  enters the broad clearance space  372  between a top surface  373  of a baseline rotor blade shroud  374  and casing shroud  375 . Leakage flow  370  is restricted only by the clearance space  372  between the top surface  373  of the rotor blade shroud  375  and the knife edges  376 . 
       FIG. 6B  illustrates leakage flow for a preferred embodiment of the clearance mechanism for a next-to-last stage of a low pressure turbine. The clearance mechanism  400  includes a rotor tooth  420  generally centered with respect to the axial chord ( FIGS. 4A and 4B ) on the rotor blade shroud  440 , and an aft stator tooth  440  generally disposed proximate to an aft end  442  of the rotor blade shroud. Specific positioning for a preferred embodiment has been described above with respect to  FIGS. 4A and 4B . 
     As the flow comes into rotating blade region, a portion of flow enters the cavity and tries to pass through region  472  between the top surface  473  of rotor blade shroud  440  and casing shroud  475 . The leakage flow  470  then flow past the forward face  421  of the rotor tooth  420  and through restricted clearance  480  between tip  422  of the rotor tooth and the casing shroud  475 . The leakage flow  470  passing over the rotor tooth edge  422  is urged against the forward face  431  of the aft stator tooth  430 , thereby being forced inward radially and creating a vortex  460 . The vortex  460  further forces the leakage flow  470  against the forward face  431  of the aft stator tooth  430 , making passage over the sharp edge  432  of the aft stator tooth  430  more difficult and further limiting the tip leakage flow. 
     Tip leakage flow is one of the major loss sources in a low pressure turbine. A reduction of tip leakage flow reduction may lead directly proportional to turbine performance gain. The inventive clearance mechanism reduces the tip clearance flow by 50% over prior art base case. The inventive tip clearance mechanism leads to an improvement of about 0.5% in final stage efficiency and 0.6% in penultimate stage efficiency, resulting in an improvement of about 0.25% overall efficiency improvement for the low pressure turbine. 
     In a further aspect of the present invention a method is provided for a tip clearance control mechanism for a rotor blade of a low pressure steam turbine between a tip shroud of a rotor blade and a casing shroud.  FIG. 7  illustrates a flowchart for the method  500  for forming a clearance mechanism for the rotor blade of low pressure steam turbine. The method includes selecting a shape for a tip shroud in step  505 ; selecting a rotor tooth shape in step  510 ; selecting a number of rotor teeth in step  515 ; selecting a stator tooth shape in step  530 ; and selecting a number of stator teeth in  540 . 
     The method may further include step  520  for establishing the rotor teeth height step  525  locating the rotor teeth on the rotor tip shroud. The method may also further include the step  545  for sizing the height of the stator teeth and step  550  for locating the stator teeth on the stator shroud. The method may further include, in step  560 , forming at least one stator tooth as an integral part of an inner wall for the casing for the rotary machine. 
     The method for selecting a shape for a tip shroud may further include selecting a shape from one of a sloped upstream top surface and a level downstream top surface, a sloped upsteam top surface and a level downsteam top surface with a step therebetween; and a sloped upsteam top surface and a sloped downsteam top surface. 
     The step of selecting a rotor tooth shape may include: selecting a rotor tooth shape from one of an angled rotor tooth and a normal rotor tooth relative to the top surface of the rotor shroud. The step of selecting a rotor tooth shape may include selecting an arrangement with one rotor tooth or two rotor teeth. 
     The step of selecting a stator tooth shape may include selecting a stator tooth shape from one of an angled stator tooth, a normal stator tooth, a compound stator tooth with one of a tapered outer end extending in one of an upstream and a downsteam orientation, and a normal stator tooth with an outer end tapering to one of an edge and one of a top surface. 
     While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention.