Abstract:
A seal assembly for installation between rotating and stationary components of a machine includes a first plurality of leaf spring segments secured to the stationary component in a circumferential array surrounding the rotating component, the leaf spring segments each having a radial mounting portion and a substantially axial sealing portion, the plurality of leaf spring segments shingled in a circumferential direction.

Description:
This invention relates generally to seals between rotating and stationary machine components and specifically, to a leaf seal assembly between a stator and a turbine rotor. This invention was made with Government support under Contract No. DE-FC21-95MC31176 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Gas turbine operation depends upon the controlled flow of air between rotating and static components. Seals are introduced between these components in order to direct the flow in the desired paths. For example, it may be desirable in some cases to prevent flow in some directions, such as into bearing housings, while in other cases, it may be desirable to direct a controlled amount of flow to actively purge cavities, cool components, and prevent hot flowpath gases from contacting rotor components. Examples of this latter type of seal may be found in the compressor discharge secondary flow circuit and in the paths around the turbine nozzle diaphragms. Establishing a seal that provides a controlled amount of leakage, independent of operating condition, thermal transients, and operating age, has been an ongoing design objective. 
     A number of different seal designs have been used in the gas turbine industry. These include: “pumpkin teeth” seals; labyrinth seals; honeycomb seals; and brush seals. All of these designs are intended to provide a “tortuous” path for the air and thus minimize the leakage across them. Table I describes the prior seals, qualitatively comparing them based on certain key features: 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                 PUMP- 
                   
                 HONEY- 
                   
               
               
                 FEATURE 
                 KIN 
                 LABYRINTH 
                 COMB 
                 BRUSH 
               
               
                   
               
             
             
               
                 rotor stator contact 
                 no 
                 possible 
                 yes 
                 yes 
               
               
                 deterioration w/ usage 
                 no 
                 if contact 
                 yes 
                 slight 
               
               
                 adapts to transients, 
                 no 
                 no 
                 no 
                 yes 
               
               
                 operating points 
               
               
                 relative leakage flow 
                 high 
                 lower (because of 
                 lower still 
                 very 
               
               
                   
                   
                 lower clearances) 
                   
                 low 
               
               
                 “windage” temperature 
                 slight 
                 slight 
                 higher 
                 lower 
               
               
                 rise of leakage air 
               
               
                 adapts to casing non- 
                 no 
                 no 
                 no 
                 yes 
               
               
                 symmetry 
               
               
                   
               
             
          
         
       
     
     Note that in many applications, some minimum level of air leakage is required to ensure cavity purge flow high enough to preclude contact of rotor structural components with hot gaspath gasses. For this reason, holes allowing air to bypass the seal may be included with brush or honeycomb seals. One difficulty with this approach, however, is that to ensure safe operation, the holes must be sized to provide sufficient flow based a new seal configuration. If the seal later deteriorates, and leaks more flow than when new, the total flow past the seal will be greater than design requirements. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention utilizes a circumferential array of leaf seals clamped (or otherwise fixed) into the stator component. In the preferred embodiment, the individual leaf seal components or segments each comprise a primary spring and a backing spring, each attached to the stator. The primary spring has an attachment portion and a seal portion that extends generally axially along the rotor, curving toward and then away from the rotor surface in the flow direction. The backing spring also has an attachment portion and a more sharply curved backing portion that engages substantially tangentially the backside of the primary spring. The backing spring serves two purposes: 
     1. Because of its different curvature and possibly different thickness, the effective stiffness of the two springs together will be non-linear (i.e.: the load-deflection curve will be a curve, not a straight line). This will allow the seal opening, and thus performance, to be optimized over a greater range of operating conditions. 
     2. Since the two springs will rub against each other, they will serve as friction dampers for each other, preventing vibration and fatigue that might result from aerodynamic instabilities or flutter of the seal. 
     The collective array of leaf seal segments are circumferentially overlapped or shingled, requiring the backing springs to be slightly shorter in tangential length than the primary springs. The primary springs are assembled with a slight radial gap relative to the rotor when the machine is not in operation. When the machine is started, a pressure differential develops across the seal, with the higher upstream pressure trying to push the seal open. The force pushing the seal open will be based upon the difference between the total upstream pressure and the total downstream pressure. As soon as fluid flow past the seal begins, however, the force will drop. This is because the air attains a velocity and the force depends upon the difference between the static upstream and downstream pressures. The downstream velocity will be low, so this static pressure will approximate the total pressure, but the upstream static pressure (opening force) will drop by an amount proportional to the flow rate/velocity. 
     Since the force opening the seal decreases as the flow rate/velocity increases, and the spring force closing the seal increases with the seal opening, the seal can be designed to allow a controlled “leakage” for any given design point. 
     Accordingly, in one aspect, the invention relates to a seal assembly for installation between rotating and stationary components of a machine comprising a first plurality of leaf spring segments secured to the stationary component in a circumferential array surrounding the rotating component, the leaf spring segments each having a radial mounting portion and a substantially axial sealing portion, the plurality of leaf spring segments being shingled in a circumferential direction. 
     In another aspect, the invention relates to a turbine rotor and stator arrangement including a leaf spring seal assembly comprising a rotor; a stator surrounding the rotor in radially spaced relation thereto; and a leaf spring seal between the rotor and the stator, the leaf spring seal including a first plurality of leaf spring segments fixed to the stator in a circumferential array about the rotor, the leaf spring segments being circumferentially shingled, and each having a sealing portion defining a predetermined radial gap between the sealing portion and the rotor when the rotor is at rest. 
     In still another aspect, the invention relates to a method of sealing a radial gap between a rotor and a stator comprising: a) mounting a first plurality of leaf spring segments to the stator; and b) arranging the first plurality of leaf spring segments in a circumferentially shingled array about the rotor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side section of leaf seal components between a stator and rotor in accordance with an exemplary embodiment of the invention; and 
     FIG. 2 is a schematic end view illustrating the manner in which the leaf seal components are shingled or overlapped in a circumferential direction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, a rotor  10  and stator  12  are illustrated schematically, with a radial gap  14  therebetween. In a gas turbine environment, air flow in the direction indicated by arrow  16  is controlled by a leaf spring seal assembly  18 . The seal assembly  18  comprises individual leaf seal components or segments that are arranged circumferentially about the rotor  10  in a “shingled” or overlapped arrangement best seen in FIG.  2 . The controlled flow is desired to, for example, actively purge cavities while cooling components and preventing hot flowpath gases from contacting rotor components, e.g., in compressor discharge secondary flow circuits and in paths around the turbine nozzle diaphragms. 
     Each individual leaf seal component or segment includes a primary leaf spring segment  20  with a mounting portion  22  and a sealing portion  24 . The former is attached to the stator  12  by any suitable means (e.g. mechanically clamped or otherwise suitably fixed to the stator), while the sealing portion curves in an axial direction toward then away from the rotor, in a direction of flow, with a minimum radial gap at  26 . The primary spring segment  20  is substantially flat in a tangential direction, i.e., as viewed in FIG. 2, noting that each leaf seal segment is oriented generally tangentially with respect to the surface  28  of the rotor. The extent of the leaf seal segment in the tangential direction is referred to herein as its tangential length. 
     Each leaf seal segment also includes a secondary or backing leaf spring segment  30 , also attached to the stator  12  in a similar fashion. The backing spring segment  30  is located behind or downstream of the primary spring with a more sharply curved portion  32  engaging the backside of the primary spring in a generally tangential fashion. 
     Referring to FIG. 2, it will be apparent that in order to overlap or shingle the individual leaf seal assemblies without interference, the backing spring segments  30  will have a shorter tangential length than the primary springs. Nevertheless, the backing spring will be centered on the primary spring as shown in phantom in FIG.  2 . 
     Both the primary and secondary spring segments  20 ,  30  are preferably constructed of spring steel, the specified alloy composition and the thickness of each dependent on the application. The tangential length of the primary springs will depend on the number of segments employed, which is again, application specific. It will be appreciated that the respective effective stiffnesses of spring segments  20 ,  30  may vary based on different degrees of curvature, different thicknesses and different alloys. This means that the spring stiffness will be non-linear, thus permitting seal opening and thus performance to be optimized over a wide range of operating conditions. 
     A further advantage to the leaf spring seal consisting of chordal segments shingled over each other is that non-symmetry of the casing can be accommodated, as the different segments may self-adjust to different deflections to maintain a consistent clearance. 
     Prior to starting the machine, the primary spring segments  20  should be assembled with a slight gap (at  26 ) from the rotor surface. At this time, P1 (pressure upstream of the seal  18 )=P2 (pressure downstream of the seal  18 )=P3 (pressure at the radial gap  26 ). When the machine is started, P1 will increase more than will P2. In the potential application illustrated (the high pressure packing seal of a particular machine) the pressure across the seal  18  will approach 2:1 at full speed/full load, resulting in choked flow across the seal. At low P1/P2, the flow across the seal assembly  18  will be low, and so the flow velocity will be low, as will the opening force. As P1/P2 increases, the Mach number of flow across the seal will also increase, and the ratio of P1/P3 will decrease. While P3 will still be greater than is P2, it will not be as much above it as is P1, so there will be a reduced force opening the seal. This will be countered by the spring force exerted by both the primary spring segments  20  and backing spring segments  30  closing the gap  26 . Thus, the flow will be controlled by P1/P2 and the designed opening of the seal at operating conditions. Thus, the seal gap  26  and leakage adapt themselves to the operating conditions. Again, the backing spring segments  30  will preclude any instability as the seal adjusts. 
     Typically, the rotor  10  and the stator  12  will have different transient responses to changes in the operating conditions of the machine. At startup, the rotor  10  will rotate in the direction of arrow  34 , and may grow rapidly towards the stator  12 , due to centrifugal loading. Subsequent to that, the stator  12  will typically respond thermally more quickly than will the rotor  10 , and grow away from the rotor, which will subsequently make up some of that gap. As the machine is shut down, the process will be reversed. The pressure ratio across the seal assembly  18  will be largely independent of the thermal transient changes, and so the opening force, and the seal gap  26  will adapt to those changes. As the gap  26  follows the opening forces, however, the spring forces will act to counter those force changes, thus minimizing flow variations. 
     The surfaces of the primary spring segments  20  and backing spring segments  30  are all smooth, so there will be very little windage-induced temperature increase. Since there is no contact between the seal assembly  18  and the surface of the rotor  10 , there will be little if any deterioration of the seal. 
     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.