Abstract:
A packing ring holder assembly for packing seal segments about a steam turbine rotor includes a nozzle box and a holder cooperable to form a dovetail for securing packing ring segments to the nozzle box. A spring is interposed between the nozzle box and packing ring segment to bias the latter radially inwardly for sealing with the rotor. The holder includes a shoulder for preventing axial movement of the spring in one direction.

Description:
[0001]     The present invention relates to turbines having a packing ring seal about a rotor and particularly relates to steam turbines having a nozzle box and packing ring holder secured thereto for securing the packing ring seal about the turbine rotor.  
       BACKGROUND OF THE INVENTION  
       [0002]     In steam turbines, a nozzle box is used to change the direction of steam flowing into the turbine from a generally radial direction to an axial direction for flow through the various stages of the turbine. The nozzle box surrounds the turbine rotor and packing ring seals are typically used to seal between the nozzle box and the rotor. Because of reduced clearance with the rotor, certain nozzle boxes have required a split packing ring holder assembly to retain the shaft seal packings, i.e. the sealing segments about the rotor. These prior packing ring holder assemblies each included a relatively thin annular piece terminating at axially opposite ends in radially extending flanges. A second piece of the holder assembly bolted and clamped one of the flanges of the first piece between the second piece and the nozzle box. The first and second pieces had axially extending flanges which cooperated with a generally dovetail shaped sealing segment to secure the sealing segment in sealing relation to the rotor. Each sealing segment carried labyrinth seal teeth for sealing relative to the rotor.  
         [0003]     Because of the limitations of space between the nozzle box and the rotor, the packing ring tended to become unstable. The packing ring segments were generally spring biased toward the rotor and the springs tended to be off center. The thin annular ring also had a tendency to bend. As a consequence, the seal packing was unstable, tended to vibrate, increased the difficulty of installation in the field and had longer outage times during repair and replacement. Consequently, there has developed a need for a packing ring sealing assembly for a steam turbine having, inter alia, increased sealing segment stability, easier installation and shortened outage time.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0004]     In a preferred embodiment of the present invention, there is provided a packing assembly for a steam turbine comprising: a nozzle box for facilitating flow of steam into a first stage of the turbine and including an arcuate groove; an arcuate packing ring segment about an axis of the turbine and including a seal face for sealing against a rotor of the turbine; and a packing ring segment holder secured to the nozzle box, the nozzle box and the holder having surfaces cooperable with the segment to secure the segment to the nozzle box and holder substantially against relative axial movement and with portions of said segment in said groove.  
         [0005]     In a further preferred aspect of the present invention, there is provided a packing assembly for sealing about a rotor of a steam turbine comprising: a nozzle box for facilitating flow of steam into a first stage of the turbine; an arcuate packing ring segment about an axis of the turbine and including a seal face for sealing about the rotor, a pair of axially extending flanges and a neck between the seal face and the flanges; the nozzle box including a first flange and extending toward the packing ring segment in a first axial direction and lying on one axial side of the segment; and a packing ring segment holder secured to the nozzle box and extending along an opposite side of the segment, the holder having a second flange extending axially toward the packing ring segment in a second axial direction opposite the first axial direction; the segment being secured to the nozzle box with the first and second flanges of the nozzle box and the holder respectively located radially inwardly of the packing ring segment flanges to secure the segment and the nozzle box to one another. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a fragmentary cross-sectional view illustrating a prior art packing ring seal holder assembly for securing a packing ring segment to a nozzle box for sealing about a steam turbine rotor;  
         [0007]      FIG. 2  is a fragmentary cross-sectional view of a packing seal ring holder assembly for steam turbines in accordance with a preferred embodiment of the present invention; and  
         [0008]      FIG. 3  is a partial axial view of the assembly of  FIG. 2 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]     Referring now to  FIG. 1 , there is illustrated the shaft  10  of a steam turbine having a nozzle box  12  surrounding the shaft  10 . It will be appreciated that the nozzle box  12  enables the steam flowing into the turbine to change direction from a generally radial inward direction to an axial direction such that the steam flows generally axially into the initial stage of the steam turbine. In the prior art illustration of  FIG. 1 , a packing seal assembly is illustrated sealing between the nozzle box and the rotor  10 . The seal assembly generally designated  14  includes a two part holder assembly secured to the nozzle box  12  for securing an arcuate sealing segment  18  in sealing relation about rotor  10 . The sealing segment  18  preferably comprises a plurality of segments for example six in number, each extending approximately 60° and having labyrinth seal teeth  16  along its sealing face for sealing about the rotor  10 . The seal ring segments  18  have a generally dovetail connection with the two part seal holder assembly  14 .  
         [0010]     The seal holder assembly  14  included a first arcuate piece  20  having generally radially directed flanges  22  and  24  adjacent opposite ends. The first piece  20  is secured to the nozzle box  12  by bolts  25  passing through a holder  26  and through the flange  22  of the first piece  20 . The second flange  24  and the holder  26  included axially extending flanges for cooperation with a dovetail connection on the sealing segment  18  to secure the seal segment against axial movement while enabling limited radial movement under the bias of a spring, not shown. As explained previously, the two part assembly became unstable and tended to become distorted. Additionally the spring tended to move off center and generally the two part holder assembly tended to bend resulting in steam leakage paths about the seal ring segment.  
         [0011]     Referring now to  FIGS. 2 and 3 , there is illustrated a packing ring holder assembly generally designated  30  in accordance with a preferred embodiment of the present invention. The packing ring holder assembly  30  is comprised of the nozzle box  32  and a holder  34  for securing a sealing segment  36  in sealing relation to the rotor  38 . The sealing segment  36  comprises one of a plurality of arcuate sealing segments  36 , for example six in number, each 60° in circumferential extent about the rotor and secured to the nozzle box. The sealing segment  36  includes a plurality of labyrinth teeth  40  on the seal face  42  of the segment  36  for sealing about rotor  38 . The sealing segment  36  includes a dovetail along its radial outer side including a pair of axially extending flanges  44  defining a neck portion  46  between the flanges and the seal face  42 . The surface of the seal segment  36  opposite the seal face  42  includes an axial extending slot  50  adjacent each of the opposite ends of the segment for reasons which will become clear.  
         [0012]     A groove  52  is formed along the inner radial face of the nozzle box  32 . The groove is defined in part by a flange  54  extending axially toward the seal segment  36 . The groove  52  also includes a circumferentially extending slot  56  of greater diameter than the groove  52  and opening in an axial direction.  
         [0013]     The seal holder  34  comprises arcuate segments which are secured by bolts  57  to an axial face of the nozzle box. The segments of the holder  34  may have different lengths than the seal segments  36  for example two segments of about 90° each in the upper half of the turbine and one segment of about 180° in the lower half of the turbine. The seal holder  34  includes an axially extending flange  60  and a radially inner axially extending flange  62 . As illustrated, the flange  54  of the nozzle box and the flange  62  of holder  34  extend axially toward one another and constitute surfaces received in the dovetail of the sealing segments to retain the seal segments on the nozzle box.  
         [0014]     A pair of springs preferably coil springs  70  are provided in the groove  52  of the nozzle box  32  adjacent opposite ends of each seal segment  36  to bias the seal segments  36  in a radial inward direction. Particularly, the radial outer ends of the coil springs  70  are captured against axial movement in one direction in the slot  56  by the flange  60  of the holder  34  and against axial movement in an opposite direction by a margin of the slot. The radial inner end of each spring  70  is captured against circumferential movement by the slot  50  formed in the radial outer surface of the segment  36 .  
         [0015]     In a preferred embodiment, the holder  34  and nozzle box  32  are secured to one another by the bolts  57  during the manufacture of the original equipment. The groove  52  is preferably formed concurrently in the holder and nozzle box after the holder is initially secured to the nozzle box. In this manner the holder  34  can be removed from the nozzle box after formation of the groove and reinstalled in the field with the groove  52  being precisely and accurately reestablished for fit about the seal segment  36 .  
         [0016]     A packing anti-rotation pin  80  is disposed between the seal ring segment  36  and the holder  34  to prevent relative circumferential movement of the segment and holder as well as between the segment and the nozzle box. As illustrated in  FIG. 2 , lock washers  82  are applied between the bolt heads and the holder  34  with edges being peened to preclude rotation of the bolts during usage.  
         [0017]     It will be appreciated that the nozzle box  32  is configured as part of the direct securement of packing ring segment to the nozzle box. That is, the nozzle box forms part of the attachment mechanism necessary to secure the packing ring segment to the nozzle box. This in turn enables the use of a more robust holder  34  and avoids use of an intermediate piece between the nozzle box and packing ring segment as in the prior art. The present holder assembly thus provides additional stability, and a reduced tendency for vibration. The increased thickness of the bolted holder  34  and the overall configuration of the mounting structure substantially eliminates any tendency toward the establishment of leakage paths as in the prior art assembly.  
         [0018]     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.