Patent Publication Number: US-2007120329-A1

Title: Apparatus and method for steam turbine variable clearance packing

Description:
BACKGROUND OF THE INVENTION  
      This application relates generally to steam turbines, and more specifically, to seals between rotating and stationary components of a steam turbine.  
      In rotary machines such as turbines, seals are provided between rotating and stationary components. For example, in steam turbines it is customary to provide a plurality of arcuate packing ring segments (sometimes referred to as seal ring segments) to form a labyrinth seal between the stationary and rotating components. Typically, the arcuate packing ring segments are disposed in an annular groove in the stationary component concentric to the axis of rotation of the machine and hence concentric to the sealing surface of the rotating component. Each arcuate seal segment carries an arcuate seal face in opposition to the sealing surface of the rotating component. In labyrinth type seals, the seal faces carry a radially directed array of axially spaced teeth, and which teeth are radially spaced from an array of axially spaced annular teeth forming the sealing surface of the rotating component. The sealing function is achieved by creating turbulent flow of a working media, for example, steam, as it passes through the relatively tight clearances within the labyrinth defined by the seal face teeth and the opposing surface of the rotating component.  
      The ability to maintain proper clearances without physical contact between the rotating equipment and stationary components allows for the formation of an effective seal. If this radial clearance between the seal faces of the segments and the opposing seal surfaces of the rotating component becomes too large, less turbulence is produced and the sealing action is compromised. Conversely, if the clearance is too tight, the sealing teeth may contact the rotating element, with the result that the teeth lose their sharp profile and tight clearance and thereafter create less turbulence, likewise compromising the sealing action. Therefore, because greater clearance is required at transient operating conditions of the turbine than at design point/steady state operating conditions, variable clearance packing rings are desirable.  
     BRIEF SUMMARY OF THE INVENTION  
      Disclosed herein is a packing ring assembly having a first packing ring segment, a second packing ring segment, a third packing ring segment, a fourth packing ring segment, a first half of a stationary component, a second half of a stationary component, and one or more seal keys. The first packing ring segment and the second packing ring segment are located within the first half of a stationary component. The third packing ring segment and the fourth packing ring segment are located within the second half of a stationary component. The seal keys are located at a midline between the first half and the second half of the stationary component such that each of the seal keys supports the first or second packing ring segment.  
      Further disclosed herein is a steam turbine having a shaft, a stationary component, a packing ring assembly, and one or more seal keys. The packing ring assembly has a first packing ring segment, a second packing ring segment, a third packing ring segment, and a fourth packing ring segment. The first and second packing ring segments are located within a first half of the stationary component. The third and fourth packing ring segments are located within a second half of the stationary component. The seal keys are located at a midline between the first half and the second half of the stationary component such that each of the seal keys supports the first or second packing ring segment.  
      Yet further disclosed herein is a method for maintaining proper operation of a labyrinth seal between a rotor shaft and a stationary component of a rotary machine A first packing ring segment and a second packing ring segment are attached to a first half of a stationary component. A third packing ring segment and a fourth packing ring segment are attached to a second half of a stationary component. One or more seal keys are attached to the first half of the stationary component such that each of the seal keys supports and contains the first or second packing ring segment within the first half of the stationary component and permits the third or fourth packing ring segments to travel radially from the rotor shaft within the second half of the stationary component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures:  
       FIG. 1  is a section view of a portion of a steam turbine for use in accordance with an embodiment of the invention;  
       FIG. 2  is an enlarged view of the exemplary steam turbine of  FIG. 1  for use in accordance with an embodiment of the invention;  
       FIG. 3  is a cross-section view of the exemplary steam turbine of  FIG. 2  for use in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to  FIG. 1 , a portion of a steam turbine  10  is illustrated having a turbine shaft  12  disposed in a stationary component, for example a turbine diaphragm  14  comprising first and second diaphragm halves  16  and  18 , respectively. A labyrinth seal is provided at the turbine shaft-to-diaphragm interface to prevent leakage. The labyrinth seal includes a packing ring assembly  20 , sometimes referred to as a seal ring assembly, as shown in  FIG. 1  having four arcuate segments extending around the turbine shaft  12 . The packing ring segments are arranged such that the first packing ring segment  22  and the second packing ring segment  24  are disposed on a first side of a midline between the first diaphragm half  16  and the second diaphragm half  18 , while the third packing ring segment  26  and the fourth packing ring segment  28  are disposed on a second side of the midline between the first diaphragm half  16  and the second diaphragm half  18 . It will be appreciated that in the assembly of  FIG. 1 , and the remaining figures, the segments comprise positive pressure, variable clearance packing ring segments movable between open outermost large clearance and closed innermost small clearance positions about the turbine shaft  12  at startup and at speed operations, respectively. The packing ring segments may be moved to their open, outermost largest diameter positions by springs  30  disposed within the packing ring segments. To displace the packing ring segments into their closed smaller diameter position, a plurality of passages (not shown) are provided in the diaphragm or the packing ring segment to introduce the flowing medium, for example, steam, along the outer face of the packing ring segments whereby the packing ring segments may be displaced inwardly toward the turbine shaft  12  against the bias of the springs  30 . This type of positive pressure variable clearance packing ring segment is further disclosed, and hereby incorporated by reference, in GE Docket No. 193439, Cantor Colbum LLP Docket No. GS1-0202, titled Variable Clearance Packing Ring Arrangement.  
      As further illustrated in  FIGS. 2 and 3 , each packing ring segment has a sealing face  38  and radially projecting teeth  40 , each sealing face  38  is formed by a pair of segment sealing flanges  42  extending axially away from one another. The radial outer portions of the packing ring segments include segment locating flanges  44  which similarly extend from the packing ring segment in axially opposite directions away from one another. An axially reduced neck  45  extends between the segment sealing flanges  42  and the segment locating flanges  44 . The packing ring segments are disposed in a generally dovetail-shaped annular groove  32  within the diaphragm  14 . The annular groove  32  is defined along the radially innermost portions of the diaphragm  14  by a pair of diaphragm locating flanges  34  (illustrated in  FIG. 3 ) which extend axially toward one another defining a slot  36  therebetween. The packing ring segments are positioned such that the axially reduced neck  45  of the packing ring segments is fitted within the diaphragm slot  36 .  
      Additionally, a pair of seal keys  46  are installed at the outermost radial positions of the packing ring segments at the midline between the first diaphragm half  16  and the second diaphragm half  18 . The seal keys  46  are secured, for example, by bolts  49 , to the first diaphragm half  16  in diaphragm slot  48  and projects radially inwardly to extend into packing ring segment slot  50  formed along an end, adjacent to the midline between the first diaphragm half  16  and the second diaphragm half  18 , of each of the packing ring segments. In other words, each of the packing ring segments define one half of the packing ring segment slot  50 , more specifically, the first  22  and fourth  28  packing ring segments define one segment slot  50  and the second  24  and third  26  packing ring segments define the other diametrically opposed segment slot  50 .  
      The seal keys  46  support the first  22  and second  24  packing ring segments against circumferential displacement under gravity forces. The seal keys  46  further prevent the first  22  and second  24  packing ring segments from applying forces, resulting from circumferential displacement, on the third  26  and fourth  28  packing ring segments. The seal keys  46  contain the first  22  and second  24  packing ring segments within the first diaphragm half  16  and therefore the seal keys  46  also minimize the gap between the first  22  and second  24  packing ring segments. Additionally the seal keys  46  allow for horizontal displacement (the horizontal direction is defined as the horizontal plane of  FIGS. 1 and 2  formed by the midline between the first and second diaphragm halves and the central axis of the turbine shaft  12 ) of the first  22  and second  24  segments while allowing radial displacement for the third  26  and fourth  28  segments. Radial displacement in the third  26  and fourth  28  packing ring segments allows for greater radial clearance at the lower half vertical centerline of the turbine shaft  12 , where turbine shaft  12  bowing is the greatest.  
      The seal keys  46  further comprise a threaded hole  52  for engagement of a setscrew  54 . The threaded hole  52  is oriented such that adjustment of the setscrew  54  results in displacement of the first  22  or second  24  packing ring segment. As the setscrew  54  is adjusted to extend beyond the seal key  46 , the exposed setscrew  54  end comes into contact with the packing ring segment and drives the packing ring segment circumferentially away from the seal key  46 . Adjustment of the first  22  and/or second  24  packing ring segment allows for proper alignment of all packing ring segments into true center positions.  
      The above mentioned characteristics allow for the packing ring assembly  20  and seal keys  46  to be entirely enclosed in their respective diaphragm halves thus preventing assembly problems. Further, the packing ring assembly  20  and seal keys  46  may be installed and utilized with any existing dovetail size and any existing diaphragm assembly. Therefore, the packing ring assembly  20  and seal keys  46  may be installed in existing industrial steam turbines with little or no modification to diaphragm components. Additionally, the packing ring assembly  20  and seal keys  46  may be retrofitted into existing steam turbines. The packing ring assembly  20  and seal keys  46  may be provided as a kit having the desired parts and hardware to easily remove the existing packing ring assembly from the diaphragm and replace it with the packing ring assembly  20  and seal keys  46 , with little or no modifications to the diaphragm, at a maintenance interval or overhaul of the steam turbine.  
      While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims.