Patent Publication Number: US-7713024-B2

Title: Bling nozzle/carrier interface design for a steam turbine

Description:
BACKGROUND OF THE INVENTION 
   Steam turbine designs consist of static nozzle segments that direct the steam flow into rotating buckets that are connected to a rotor. In steam turbines, the nozzle construction is typically called a diaphragm stage. Typical diaphragm stages are constructed using one of two methods. The first method is a “band/ring” method that uses an assembly comprised of a plurality of airfoils contained in inner and outer bands and then that banded airfoil assembly is welded into inner (web) and outer rings. The second method involves welding airfoils directly to inner and outer rings using a fillet weld at the interface. The second method is typically used for larger airfoils, where access for creating the weld is possible. 
   However, there are drawbacks to using these methods. One drawback is the inherent weld distortion of both the flow path and the steam path sidewalls. In this regard, current methods of steam turbine nozzle construction consist of high heat input welds using significant amounts of metal filler or deep electron beam welds. This material and heat input causes the flow path to distort and the airfoils often need to be adjusted after welding and stress relief. The result of the distortion is turbine efficiency losses in the steam turbine flow path. 
   Other methods using single nozzle construction into rings still have welds and mechanical interfaces that are difficult to model and analyze. They also are not as robust to stress level due to the weld interface and interfaces between the nozzles. Another method is to put “hooks” on the nozzle and slide each nozzle into a circumferential groove in the carrier. This method is also difficult and time consuming to analyze using finite element methods for stresses. Additionally, the frequency analysis is not as robust due to in determinant and changing boundary conditions between the nozzles and the carrier. 
   Thus, in general, current methods of constructing nozzle diaphragms are costly and time consuming in both engineering and manufacturing and all of the current methods consist of some type of weld or mechanical interface between nozzle and rings. 
   “Bling” design nozzles are currently used very little in steam turbine design. A bling is basically an entire nozzle flow path that is machined out of two half rings with no welding or assembly features. The bling has many valuable design qualities. First, blings have much lower stress levels because there are no weld joints or mechanical discontinuities in the load path. Second, the airfoil tolerances can be greatly improved over welded techniques. Third, they are easier to design and have more determinant frequency characteristics. In this regard, the 3D modeling and finite element analysis of the stress and frequencies is simpler, quicker and more robust due to the simplicity of the design. 
   An issue with current bling constructions is the interface between the carrier and the bling. In most diaphragm designs there are “crush pins” or small spacers to keep a tight tolerance between the diaphragm and the casing in the axial direction. The spacers act to keep the diaphragm loaded in the aft direction against the steam face. This helps assembly and also aids in the removal of the diaphragm after years of operation. In this regard, after years of operation, corrosion occurs on the surfaces and if the diaphragm to casing interface is tight on both axial faces, then it would be very difficult to get the diaphragm out as it would tend to lock into place due to the corrosion. Blings also “roll” or deflect downstream more than the slid in nozzle design. Many diaphragms use only the crush pins on the lower half (usually 3) and the upper half has a larger gap to the front face. This at times allows the diaphragm upper half to unseat off the back face and allow debris to get behind the face and cause a leakage path. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention relates to an improved interface between the bling and carrier to improve the loading, sealing, and disassembly of the blings, while reducing downstream deflection. 
   Thus, the invention may be embodied in a turbine comprising: a turbine nozzle assembly having at least one stator airfoil and including an inner sidewall at a radially inner end of the stator airfoil and an outer sidewall structure at the radially outer end of said stator airfoil; and an outer ring carrier having a radially inwardly open groove; wherein said outer sidewall structure is configured to slideably engage said groove in a radial direction while being restricted from moving in an axial direction with respect thereto, a forward contact area between said outer sidewall structure and said outer ring carrier comprising a forward circumferential land defined on one of an aft facing surface of said groove and an upstream axial face of the outer sidewall structure, said forward circumferential land having a radial dimension substantially less than a radial dimension of said groove, and an aft contact area between said outer sidewall structure and said outer ring carrier comprising a aft circumferential land defined on one of a forward facing surface of said groove and a downstream axial face of the outer sidewall structure, said aft circumferential land having a radial dimension substantially less than the radial dimension of said groove. 
   The invention may also be embodied in a turbine comprising: a turbine nozzle assembly having at least one stator airfoil and including an inner sidewall at a radially inner end of the stator airfoil and an outer sidewall structure at the radially outer end of said stator airfoil; and an outer ring carrier having a radially inwardly open groove; wherein said outer sidewall structure is configured to slideably engage said groove in a radial direction while being restricted from moving in an axial direction with respect thereto, and a forward interface between said groove and said outer sidewall structure being constructed and arranged for contact therebetween solely adjacent a radially outer end wall of said groove and the aft interface between said groove and said outer sidewall structure being constructed and arranged for contact solely adjacent a radially inner portion of said groove, remote from said radially outer end wall of said groove, whereby loading, sealing, and disassembly of the nozzle assembly with to the outer ring carrier, while reducing downstream deflection of the nozzle assembly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a schematic elevational view of a conventional stage having a nozzle diaphragm formed using the band/ring method; 
       FIG. 2  is a schematic elevational view of a conventional stage having a nozzle diaphragm formed using the bling type construction; 
       FIG. 3  is a schematic elevational view of a bling nozzle/carrier interface design according to a first example embodiment of the invention; 
       FIG. 4  is an enlarged, schematic elevational view of the bling/carrier interface face of  FIG. 3 ; 
       FIG. 5  is an enlarged, schematic elevational view of the bling/carrier interface with a forward seal according to another example embodiment of the invention; and 
       FIG. 6  is a schematic elevational view of a singlet construction according to yet another example embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As noted above, with the bling nozzle design, the airfoil shapes are cut into two 180° rings to form the bling stage. This removes the mechanical fits and welded part of the fabrication, solving the issues mentioned above. The invention relates in particular to an improved interface design between the bling outer ring and the carrier. The design serves several purposes. It improves the assembly by allowing relief in appropriate areas to aid placement of the bling into the casing. Moreover, it improves the future disassembly after years of corrosion occurs between the bling and casing interface. It also reduces the ability of the bling to deflect downstream due to strategically placed contact areas. Further, it improves the contact pressure between the bling and the casing, thus improving steam face sealing. 
     FIG. 1  shows the traditional construction of an impulse type turbine stage that uses partition, bands and rings welded into an assembly. More specifically, this traditional construction uses a diaphragm assembly  10  comprised of a plurality of airfoils  12  contained in inner and outer bands  14 ,  16  that are welded as at  18 ,  20 ,  22 ,  24  into an inner ring (web)  26  and an outer ring  28 . 
     FIG. 2  shows the traditional bling construction. More specifically, this traditional bling construction uses a crush pin or small spacer  130  to keep a tight tolerance between the diaphragm  110  and casing  134  in the axial direction as shown at the steam face or axial contact face  136 . As noted above, this helps assembly and also aids in the removal of the diaphragm after years of operation. Also illustrated are a horizontal joint bolt hole  138  and the interpacking seal  140  disposed at the interface between the bling  110  and the rotor  142 . 
   As noted above, in general this invention relates to the features of the mechanical interface between a bling-type nozzle and the carrier (shell or casing) it is assembled into. The key to the design is the strategic placement of interfacing features with the shell and the removal of “crush pins” from the design. The first interface feature is a forward (or upstream) outer contact area defined, e.g., by an undercut (recess) on the casing side. This relief on the casing side allows for a small contact area that only engages once the bling is almost all the way into the casing groove. This allows for ease of assembly. Also, the disassembly is improved because the contact is very small and disengages as soon as the bling is slightly lifted. 
   The aft face, or steam contact face also has a very small contact area. Placing the recess on the bling in this situation allows for quick disengagement from the casing groove when disassembling the hardware. This also reduces the corrosion issue to only a small circumferential land to reduce the possibility of “sticking” or frozen joint when disassembling the hardware several years later. One other major benefit of this small circumferential aft land is that it helps concentrate the axial load of the bling in a small area. This in turn reduces the leakage across the steam face. 
   Another major benefit, or improvement, of this design over typical straight walls, crush pins and larger forward face gap, is the ability of this configuration to limit the deflection of the bling. When the bling is under steam path loading it wants to default downstream at the inner ring area due to the aerodynamic loads (pressure) on the airfoil. Designs that use hooks on the nozzles, when slid into a carrier groove, have very limited deflection down steam due to the hook interface. When putting a bling in the groove, part of the beneficial load path is gone to reduce deflection. The configuration of this design, in the illustrated example embodiments, has the forward land outboard and inner land inboard. The couple, having a relatively tight gap at the forward interface, limits the ability of the part to roll within the groove. The separation of these two faces allow for the load couple to react out part of the bling downstream deflection. 
   If the entire face was straight with a tight gap then there may be a significant issue with corrosion causing the parts to get “frozen” in place making disassembly difficult, if not impossible without destroying the hardware. If the entire faces were straight and the forward had a large gap and crush pin, as in the structure illustrated in  FIG. 2 , then there is no ability to limit the deflection (rolling) of the bling. 
     FIGS. 3 and 4  illustrate an example embodiment of the invention wherein, rather than providing a crush pin disposed between the bling  210  and the casing or shell  234 , a small circumferential land  244  is defined in the carrier. More specifically, the bling is comprised of airfoil(s)  212  and inner and outer side walls  214 , 216 . In the embodiment of  FIGS. 3 and 4 , the outer side wall is received in a circumferentially extending groove  246  of the casing or shell  234 . A small circumferential land  244  is defined as an axial contact face along the upstream side of the groove  246  adjacent the end wall  248  of the groove, thus providing for limited contact between the carrier  234  (casing, shell) to both reduce downstream deflection (roll), and for ease of assembly because it is provided in lieu of a local standoff or crush pin  130 . 
   The steam face  236  also provides for limited contact as compared, for example, to the bling construction of  FIG. 2 . More specifically, as illustrated, a small circumferential land  250  is provided on the downstream axial face of the outer side wall  216  for engaging the casing or shell  234 . In this example embodiment, the small circumferential land  244  of the casing groove  246  and the small circumferential steam face land  250  are defined by forming a circumferential recess  252  in the casing or shell  234  and by machining the bling  210  to create a circumferential recess  254  radially outside the steam face land  250 . 
     FIG. 5  is a schematic illustration similar to  FIG. 4  but illustrating the incorporation of a V or W seal  256  between the nozzle carrier  234  and bling outer ring  216 . A “C” seal may be also used. The seal is not possible in most cases in the traditional designs as the forward part of the diaphragm, where it met with the casing, had a large gap and was of larger tolerance because of the provision of the crush pin. Now that the gap is held to tighter tolerances, and is a continuous surface, it is possible for a seal to be incorporated that is pressure activated or spring loaded while using the bling design. 
   The ring/carrier interface proposed in the embodiments of  FIGS. 3-5 , can also be applied to a “singlet” construction as illustrated in  FIG. 6 . This is a single nozzle  310  with its inner and outer side wall  314 ,  316  machined along with the airfoil  312 . The singlet nozzle outer wall  316  is then welded to a solid outer ring  358  using small axial welds forward  360  and aft  362  as low heat welds to resemble a bling configuration. The assembly of the singlet nozzle and outer ring is then seated into a corresponding groove  346  of the nozzle carrier  334 . The nozzle assembly (outer ring  358  and nozzles welded thereto) are not welded to the carrier. The nozzle assembly can move radially in the carrier groove. As in the embodiment of  FIGS. 3 and 4 , a small circumferential land  344  is defined by recessing the casing groove  346  circumferentially as at  352  on the upstream side of the nozzle assembly and by defining a small circumferential steam face land  350  by recess machining  354  of the solid ring  358  on the downstream side of the nozzle assembly. 
   The mechanical features of the interface between the singlet and the outer ring are used as an assembly and alignment feature and allow for improved reliability and risk abatement. In this regard, the mechanical lock between the ring  358  and nozzles  310  means that in the event of failure of an airfoil, the rings and nozzles cannot go downstream as there is a mechanical interface preventing the assembly from failing due to the pressure. Additionally, the mechanical lock serves the purpose of a pre-determined and repeatable weld stop. In this regard, the weld beam (assuming an EB weld) would stop when it hits the radial interlock interface. A further advantage of the  FIG. 6  embodiment is that the radially outerface of the nozzle outer side wall  316  is configured as a flat end instead of a more costly circumferential cut end. The example embodiment of  FIG. 6  has an inner ring  364  that is mechanically locked and braised or welded to the nozzle inner side wall  314  or simply mechanically locked to the nozzle. 
   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.