Patent Publication Number: US-6908279-B2

Title: Method of installing stationary blades of a turbine and turbine structure having a radial loading pin

Description:
BACKGROUND OF INVENTION 
     The present invention relates to loading pins for reaction nozzles and, more particularly, to improved loading pin configurations for securing the reaction nozzles against the retaining surfaces of the carrier with sufficient force to maintain the design amount of twisting in the airfoil section. 
     A conventional turbine structure includes a rotor having a plurality of rotating blades (buckets) mounted thereto. The blades are mounted in rows to extend radially outward from an outer surface of the rotor. Typically, the blades in a given row are identical to each other but the rotating blades of one row will differ in length and/or shape from those of the other rows spaced therefrom. Each rotating blade has a foil portion that extends radially outwardly from the rotor and a base portion for mounting the blade to the rotor. To that end, the base portion includes a root received in a correspondingly shaped groove. 
     A stationary casing is coaxially supported around the rotor and has a plurality of stationary blades (nozzles) arranged in rows to alternate with the rows of rotating blades. All stationary blades include a foil portion extending from the inner surface of the stationary casing and a base portion including a root for being received in a corresponding groove of the stationary casing. 
     The root of the stationary blade and/or the groove of the stationary housing will be provided with a notch or recess to define a space between the root of the stationary blade and the groove. It is conventional to provide a caulking material or loading pin in the space defined by the notch and/or recess to interconnect the casing and root. Conventionally, the loading pin is formed from brass and is made by machining a surface onto a piece of round stock along its axis so that the pin has a constant cross-section that is generally “D” shaped along its entire length. Thus, conventional loading pins are straight with a machined surface parallel to the longitudinal axis of the pin. 
     SUMMARY OF INVENTION 
     Integral cover reaction nozzles have been designed to maintain an assembled pre-twist that we have recognized cannot be achieved with the previous, conventional nozzle radial loading pin designs. Thus, the invention provides a wedge-like nozzle radial loading pin, preferably formed from steel, that contacts the bottom of a reaction nozzle along a graduated, that is inclined or stepped, surface. This contact will secure the reaction nozzle radially inward against the retaining surface of the carrier dovetail with sufficient force to maintain the designed airfoil pre-twist. Two embodiments of the improved radial loading pin of the invention are described and illustrated by way of example herein below. 
     In a first embodiment, the graduated surface is defined as a continuous taper made by machining a substantially continuously inclined surface into a piece of round stock, along its axis, such that the cross-section through any point of the pin is shaped by the letter “D”. The machined surface is made at an angle to the axis of the pin to create a substantially continuously tapered face that mates with a generally correspondingly tapered surface on the bottom of the reaction nozzle. 
     In an alternative embodiment, rather than a substantially continuous inclined surface, the loading pin comprises one or more discrete steps. More specifically, in an exemplary alternate embodiment, each end of the pin is machined generally parallel to the pin center line but at a different height from the pin centerline, creating two distinct surfaces and a length of machined surface at a slight angle is provided to interconnect the two flat machined surfaces. 
     Thus, the invention may be embodied in a method of installing stationary blades of a turbine comprising: arranging a plurality of stationary blades in a plurality of rows with each stationary blade of a row having a root and an airfoil portion, the stationary blades of a row being mounted by the roots in an annular groove provided in a turbine casing, each annular mounting groove having two opposite sidewalls and a bottom wall at least one of the root of the stationary blades and a wall of said mounting groove defining a recess; inserting a loading pin in the recess, between each said root and the groove, thereby keying the stationary blade root to the casing, said loading pin comprising a part-circumferential wall portion, generally corresponding in cross-sectional shape to cross-sectional shape of said recess, and a graduated wall portion, so that said pin is generally wedge-shaped. 
     The invention may also be embodied in a turbine structure comprising: a rotor having a plurality of rotating blades or buckets mounted thereto, the blades being mounted in rows to extend radially outward from an outer surface of the rotor; a stationary casing is coaxially supported around the rotor and having a plurality of stationary blades or nozzles arranged in rows to alternate with the rows of rotating blades, at least some of said stationary blades including a foil portion extending from an inner surface of the stationary casing and a base portion including a root for being received in a corresponding groove of the stationary casing; at least one of the root of the stationary blade and the groove of the stationary housing including a recess defining a space between the root of the stationary blade and the groove; a loading pin disposed in the space defined by the recess to interconnect the casing and root, said loading pin comprising a part-circumferential wall portion, generally corresponding in cross-sectional shape to cross-sectional shape of said recess, and a graduated wall portion, so that said pin is generally wedge-shaped. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other features 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 longitudinal section of stationary and moving blades of a turbine; 
         FIG. 2  is an elevational view of a loading pin provided in accordance with an exemplary embodiment of the invention; 
         FIG. 3  is an end view taken from the right of  FIG. 2 ; 
         FIG. 4  is an elevational view of a loading pin provided in accordance with an alternate embodiment of the invention; 
         FIG. 5  is an end view taken from the right of  FIG. 4 ; 
         FIG. 6  is a cross-sectional view, looking down the turbine centerline, showing the pin of  FIG. 2  installed between a nozzle and casing; and 
         FIG. 7  is a cross-section view looking down the turbine centerline, the pine of  FIG. 4  installed between a nozzle and casing. 
     
    
    
     DETAILED DESCRIPTION 
     Elastically pre-stressed blades installed under controlled conditions display excellent damping characteristic and are in the position of absorbing dynamic stresses under all operating conditions without endangering their long-term reliable life span. For blades with a sufficient amount of pre-stress, there is no frictional wear and no blade loosening. Therefore, it is important to maintain the prescribed pre-stress. 
     Thus, a design objective is that all installed blades are twisted inside the corresponding groove by a specific twist. The configuration of the nozzle airfoil and the dimensions of the root are selected so that the blade can assume the position inside the groove which is defined by the design criteria. 
     A loading pin provided in accordance with the present invention provides a wedge contact for radially loading the nozzle to secure the nozzle radially inward against the retaining surface of the carrier dovetail with sufficient force to maintain the designed airfoil pre-twist. 
       FIG. 1  schematically illustrates in longitudinal section two stages of a turbine structure. In the illustrated structure, a generally part cylindrical or U-shaped recess  10  is defined at the base of each nozzle root  12  receiving groove  14 . A loading pin  16  is inserted into this recess between the casing  18  and the nozzle  20  for locking these parts with the nozzle in its pre-twisted disposition. To assuredly lock each nozzle and maintain its pre-twist, in an embodiment of the invention, the loading pin  16 , 116 , 216  is generally wedge shaped having a part cylindrical wall portion  22 , 122 , 222  and a graduated, i.e., inclined or stepped, wall portion  24 , 124 , 224 . 
     In a first embodiment, illustrated in  FIG. 2 , the loading pin  116  has a wall portion  124  that is substantially continuously inclined from a first, insert end  126  to a second, proximal end  128  to define a generally tapered or wedge shaped pin  116 . As will be understood from  FIG. 3 , the cross sectional area of the loading pin adjacent the distal, insert end is less than the cross sectional area of the loading pin adjacent said proximal end. Although wall portion  124  is illustrated as a continuously tapered surface, a wall portion comprising a plurality of steps so as to define an effectively continuously inclined surface would be functionally equivalent thereto. 
     A groove  130  is optionally defined longitudinally of the loading pin defining a part circular recess extending from the proximal to the distal ends of the pin. The groove allows the pin material to swage or upset from its original surfaces, thereby increasing the contact area between the pin and the nozzle. The groove also allows, e.g., insertion of a pin removal tool (not shown) so that the pin may be engaged and displaced proximally even if fully inserted below a respective nozzle  120 . Although a part circular groove  130  is illustrated it is to be understood that the cross-sectional shape of this groove is non-critical and a V-shaped, rectangular or other groove configuration could be provided without departing from this invention. 
     As will be appreciated, the insertion of the tapered loading pin  116  illustrated in  FIG. 2  into recess  110 , between the nozzle root  112  and the root groove (carrier dovetail)  114  of the casing, lifts the nozzle slightly from the groove base. This will secure the reaction nozzle radially inward against the retaining surface of the carrier dovetail with sufficient force to maintain the designed airfoil pre-twist. To maximize the surface-to-surface contact between the loading pin and its respective nozzle(s), in an exemplary embodiment, a corresponding portion of the nozzle root  112  is machined to define an inclined surface  132  generally corresponding to the incline of the wall portion  124  of the loading pin  116  so that insertion of the loading pin yields an inclining surface to inclined surface wedge displacement. To ensure that the loading pin will maintain its shape and the corresponding lock of the nozzle with respect to the casing, in an exemplary embodiment, the loading pin is formed from steel. 
     An alternate embodiment of the invention is illustrated in  FIGS. 4–5  and  7 , rather than a tapered or substantially continuously inclined surface, wall portion  224  is comprised of discrete stepped portions. In the illustrated embodiment, a single step is defined along the length of the pin  216 . More specifically, in this end, the pin is machined to define a flat nozzle engaging surface  234 , 236  adjacent each end  226 , 228 , said surfaces being generally parallel to the longitudinal axis of the loading pin, and the loading pin  216  is machined to define an inclined transition or step  238  between the parallel surfaces  234 , 236 . As illustrated by dashed line  240 , the offset between flat surfaces  234 , 236  is limited. As also illustrated, a cutout  242  at the nozzle root may be provided to facilitate pin insertion. 
     A groove  230  is optionally defined longitudinally of the loading pin defining a part circular recess extending from the proximal end  228  to the distal end  226  of the pin  216 . As in the first described embodiment, the groove  230  is provided to allow the pin material to swage or upset from its original surfaces, thereby increasing the contact area between the pin and the nozzle. The groove also allows, e.g., insertion of a pin removal tool (not shown) so that the pin may be engaged and displaced proximally even if fully inserted below a respective nozzle  212 . As mentioned above, although a part circular groove  230  is illustrated for pin retrieval it is to be understood that the cross-sectional shape of this groove is non-critical and a V-shaped, rectangular or other groove configuration could be provided without departing from this invention. 
     As will be appreciated, the insertion of the tapered loading pin  216  illustrated in  FIG. 4  into recess  210 , between the nozzle root  212  and the root groove  214  of the casing, lifts the nozzle slightly from the base base of the dovetail groove  214  to effectively lock the nozzle in its prescribed pre-twisted configuration. Once again, to ensure that the loading pin will maintain its shape and the corresponding lock of the nozzle with respect to the casing, in an exemplary embodiment, the loading pin is formed from steel. 
     As noted above, it is to be understood that while a continuously inclined surface and a single stepped surface are illustrated as embodiments of the invention, the inclined surface need not be continuously inclined but can be provided as a series of discrete steps. Furthermore, the discrete flats  234 , 236  of the steps may themselves be provided as surfaces that are generally parallel to the longitudinal axis of the pin as illustrated in  FIG. 4 , or may be inclined themselves. Moreover, while in the illustrated embodiment, the transition  238  between discrete steps is provided as an inclined surface, it is to be understood that in the alternative, a plurality of discrete, generally perpendicular radial steps may be provided, whereby the cross-sectional area of the pin is increased from the distal end to the proximal continuously or in steps. 
     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.