Abstract:
Method and apparatus for assembling steam turbines are provided. The method of assembling a steam turbine includes providing an annular outer member, providing an annular inner member, coupling a plurality of airfoils to the inner member with a plurality of generally radial fastener assemblies such that the plurality of airfoils extend substantially radially outward from the inner member, and coupling each of the plurality of airfoils to the outer member.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to steam turbines, and more particularly, to methods and apparatus for assembling steam turbines. 
     At least some known steam turbines include a turbine configuration that includes a plurality of stages of diaphragms. Within at least some known turbines, the last few stages of diaphragms are called fillet fabrications that are constructed of an annular outer ring, an annular inner ring, and a plurality of circumferentially-spaced airfoils, partitions, and/or nozzles, extending there-between. To facilitate enhancing the structural integrity of such diaphragms, the airfoils are welded to the inner and outer rings. More specifically, to facilitate achieving a pre-determined weld strength, known fillet fabrications include a large weld fillet at the interface defined between the airfoil and the ring. 
     During the fabrication of at least some known fillet fabrications, a flowpath surface of the inner ring and outer ring are first scribed with lines facilitate positioning the airfoils prior to the individual airfoils being welded in position. However, because known airfoils are typically heavy and are difficult to maneuver, the welding process may be a time-consuming and laborious task. In other known fabrication methods, a complex fixture is used to facilitate aligning and holding the airfoils during welding. However, known fixtures are expensive. Moreover, within each method of fabrication, weld distortion may occur due to local heating and shrinkage of the weld material during fabrication of the diaphragm. As a result, often extensive labor-adjustments and/or machining of the assembled diaphragm is necessary to ensure that pre-determined tolerances and throat limitations defined between circumferentially-adjacent airfoils are satisfied. Additionally, distorted airfoils or rings generally can not fully obtain desired tolerances, such that stage performance may be compromised. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a method for assembling a steam turbine is provided. The method comprises providing an annular outer member, providing an annular inner member, coupling a plurality of airfoils to the inner member with a plurality of fastener assemblies such that the plurality of airfoils extend substantially radially outward from the inner member, and coupling each of the plurality of airfoils to the outer member. 
     In another aspect, a diaphragm for a steam turbine is provided. The diaphragm includes a radially outer and radially inner member that are configured to extend substantially circumferentially within the steam turbine, and at least one airfoil that extends substantially radially between the outer and inner members. The at least one airfoil is coupled to one of the radially outer and radially inner members with a fastener assembly. 
     In a further aspect, a steam turbine is provided. The steam turbine includes at least one diaphragm including a radially outer and radially inner member that are configured to extend substantially circumferentially within the steam turbine, and a plurality of airfoils that extend between the outer and inner members. The plurality of airfoils are circumferentially spaced from each other and are coupled to one of the outer and inner members by a fastener assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary known opposed flow, or double flow, steam turbine; 
         FIG. 2  is an enlarged schematic view of an exemplary diaphragm that may be used with the steam turbine shown in  FIG. 1 ; 
         FIG. 3  is an enlarged schematic view of a portion of the diaphragm shown in  FIG. 2 ; and 
         FIG. 4  is an enlarged view of a portion of the diaphragm shown in  FIG. 3  and taken along area  4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic illustration of an exemplary known opposed-flow steam turbine  10 . Turbine  10  includes first and second low pressure (LP) sections  12  and  14 . As is known in the art, each turbine section  12  and  14  includes a plurality of stages of diaphragms (not shown in  FIG. 1 ). A rotor shaft  16  extends through sections  12  and  14 . Each LP section  12  and  14  includes a nozzle  18  and  20 . A single outer shell or casing  22  is divided along a horizontal plane and axially into upper and lower half sections  24  and  26 , respectively, and spans both LP sections  12  and  14 . A central section  28  of shell  22  includes a low pressure steam inlet  30 . Within outer shell or casing  22 , LP sections  12  and  14  are arranged in a single bearing span supported by journal bearings  32  and  34 . A flow splitter  40  extends between first and second turbine sections  12  and  14 . 
     It should be noted that although  FIG. 1  illustrates a double flow low pressure turbine, as will be appreciated by one of ordinary skill in the art, the present invention is not limited to being used with low pressure turbines and can be used with any double flow turbine including, but not limited to intermediate pressure (IP) turbines or high pressure (HP) turbines. In addition, the present invention is not limited to being used with double flow turbines, but rather may be used with single flow steam turbines as well, for example. 
     During operation, low pressure steam inlet  30  receives low pressure/intermediate temperature steam  50  from a source, for example, an HP turbine or IP turbine through a cross-over pipe (not shown). The steam  50  is channeled through inlet  30  wherein flow splitter  40  splits the steam flow into two opposite flow paths  52  and  54 . More specifically, the steam  50  is routed through LP sections  12  and  14  wherein work is extracted from the steam to rotate rotor shaft  16 . The steam exits LP sections  12  and  14  and is routed, for example, to an intermediate pressure turbine (not shown). 
       FIG. 2  is an enlarged schematic view of an exemplary diaphragm  100  that may be used with steam turbine  10  (shown in  FIG. 1 ). In one embodiment, diaphragm  100  is a last stage diaphragm  100  of turbine  10 . Diaphragm  100  includes an annular inner web or ring  102 , an annular outer ring  104 , and a plurality of nozzles or airfoils  106  extending therebetween. Outer ring  104  is radially outward of, and substantially concentrically aligned with, inner ring  102 . Nozzles  106  are spaced circumferentially between rings  102  and  104  and each extends substantially radially between inner and outer rings  102  and  104 , respectively. 
     A radially outer surface  110  of inner ring  102  and a radially inner surface  112  of outer ring  104  define radially inner and radially outer boundaries of a flowpath defined through diaphragm  100 . 
       FIG. 3  is an enlarged schematic view of a portion of diaphragm  100 .  FIG. 4  is an enlarged view of a portion of diaphragm  100  taken along area  4 . In the exemplary embodiment, diaphragm inner ring  102  is fabricated from a rolled or forged ring of material. Alternatively, diaphragm inner ring  102  may be fabricated in any means that enables ring  102  to function as described herein. Ring  102  includes a plurality of alignment openings  111  and a plurality of coupling openings  112 . In the exemplary embodiment, openings  111  are pin openings and openings  112  are bolt openings. Openings  111  and  112  each extend generally radially through inner ring  102  between flowpath surface  110  and a radially inner surface  114  of inner ring  102 . 
     Openings  111  and  112  are each spaced circumferentially about inner ring  102 . More specifically, in the exemplary embodiment, openings  111  are spaced a distance D downstream from openings  112 . Alternatively, openings  111  may be formed at any location with respect to openings  112  that facilitates assembly of diaphragm  100  as described herein. Moreover, in the exemplary embodiment, openings  111  have a diameter d a  that is smaller than a diameter d o  of each opening  112 . Alternatively, opening diameter d a  may be approximately the same size, or larger than coupling opening diameter d o . More specifically, in the exemplary embodiment, each opening diameter d a  is approximately the same size as a diameter d p  of each alignment pin  130  inserted therein. 
     In the exemplary embodiment, alignment openings  111  are drilled using a precision machining process. Alternatively, openings  111  may be formed using any process that enables openings  111  to function as described herein. Specifically, the location of openings  111  facilitates determining circumferential spacing between circumferentially adjacent airfoils  106  along the inner flowpath. Moreover, the location of openings  111  also facilitates aligning each airfoil  106  axially relative to inner ring  102  and more specifically, relative to flowpath surface  110 . For example, in an alternative embodiment, openings  111  are forward of openings  112 . 
     Openings  112  are spaced circumferentially about inner ring  102  and each includes a recessed or countersunk portion  140  that extends inward from radially inner surface  114  towards flowpath surface  110 . Between countersunk portion  140  and flowpath surface  110 , openings  112  have a diameter d o  that is smaller than a diameter d cs  of countersunk portion  140 . In the exemplary embodiment, countersunk portion diameter d cs  is larger than a diameter d bh  of each coupling bolt  150  received therein, and opening diameter d o  is larger than a corresponding diameter d bb  of each coupling bolt shank  152 . 
     In the exemplary embodiment, coupling openings  112  are drilled using a precision machining process. Alternatively, openings  112  may be formed using any process that enables openings  112  to function as described herein. Specifically, the location of openings  112  facilitates determining a throat area defined between circumferentially adjacent airfoils  106 . In the exemplary embodiment, openings  112  are slightly oversized to facilitate accommodating slight alignment modifications while setting individual throat areas. 
     During fabrication of diaphragm  100 , initially openings  111  and  112  are formed generally radially within inner ring  102 . A first airfoil  106  is then positioned relative to inner ring flowpath surface  110 , and an alignment pin  130  is slidably received within a respective alignment opening  111 . More specifically, alignment pin  130  is inserted generally radially from inner surface  114 , through inner ring  102 , and into the airfoil  106  positioned against flowpath surface  110 . Each pin  130  is received in a friction fit within a respective opening  111 . Pins  130  facilitate positioning airfoils  106  both circumferentially with respect to each other, as well as axially with respect to inner ring flow path surface  110 . Alternatively, a plurality of pins  130  may be used to facilitate aligning each airfoil  106  with respect to every other airfoil. 
     Airfoils  106  are then oriented with respect to diaphragm  100  and coupling openings  112  are then formed within inner ring  102  and within airfoils  106 . In the exemplary embodiment, the portion of openings  112  defined within airfoils  106  is threaded. Each coupling bolt  150  is then inserted within each opening  112  to facilitate securing each airfoil  106  to inner ring  102 . More specifically, even as bolts  150  are threadably coupled within each airfoil  106 , an orientation of airfoils  106  may still be rotated slightly to adjust individual nozzle throat areas. In an alternative embodiment, a plurality of bolts  150  are used to facilitate securing each airfoil  106  to inner ring  102 . 
     After each respective throat area has been defined, each airfoil  106  is tack-welded to outer ring  104  to facilitate maintaining an orientation of each airfoil  106  as other airfoils  106  are coupled within diaphragm  100 . After each throat area has been set, coupling bolts  150  are securely fastened within openings  112  such that a head portion  170  of each bolt  150  is received within each respective opening countersunk portion  140 . As such, bolts  150  do not create any additional rings, ledges, or protrusions that could adversely affect fluid flow through diaphragm  100 . In an alternative embodiment, openings  112  receive only a portion of bolts  150 . 
     After airfoils  106  are spaced circumferentially around inner ring  102  and outer ring  104  has been tack-welded to each airfoil  106  included within diaphragm  100 , airfoils  106  are then securely welded to outer ring  104 . In one embodiment, a plurality of additional alignment openings (not shown) is formed to facilitate securing each airfoil  106  in its final orientation. More specifically, airfoils  106  are not welded sequentially in order circumferentially about diaphragm  100 , but rather are welded in patterns that facilitate even welding and reducing welding distortion and deformation. 
     Accordingly, a diaphragm is formed in a manner that is more cost-effective and less time-consuming than known diaphragms. Specifically, because diaphragm  100  includes a bolted inner ring  102 , during fabrication, less welding is performed on diaphragm  100 , such that the cycle time required for fabrication of diaphragm  100  is reduced in comparison to known diaphragms. Moreover, because inner ring coupling openings  112  are slightly oversized, openings  112  facilitate more accurate throat area definitions to be formed in a more cost-effective manner than is possible with known diaphragms. As a result, turbine performance and efficiency is facilitated to be enhanced. In addition, because diaphragm  100  requires much less welding than known diaphragms, weld distortion is reduced within diaphragm  100 , such that turbine performance is facilitated to be improved. 
     Exemplary embodiments of diaphragms and steam turbines are described above in detail. Although the diaphragms are herein described and illustrated in association with the above-described steam turbine, it should be understood that the present invention may be used with any steam turbine configuration. More specifically, the diaphragms are not limited to the specific embodiments described herein, but rather, aspects of each diaphragms may be utilized independently and separately from other turbines or diaphragms described herein. 
     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.