Patent Publication Number: US-10774665-B2

Title: Vertically oriented seal system for gas turbine vanes

Description:
STATEMENT REGARDING GOVERNMENT FUNDING 
     The subject matter of this disclosure was made with support from the United States government, under Contract Number DE-FE0024006, which was awarded by the U.S. Department of Energy. The government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to the field of gas turbines and, more particularly, to seals for gas turbine vanes that include a shell of a ceramic matrix composite (CMC) material disposed between metal side walls. The seals, which are vertically oriented, are disposed between a respective CMC shell and a metallic side wall. 
     BACKGROUND 
     Some conventional turbo machines, such as gas turbine systems, are utilized to generate electrical power. In general, gas turbine systems include a compressor, one or more combustors, and a turbine. Air may be drawn into a compressor, via its inlet, where the air is compressed by passing through multiple stages of rotating blades and stationary nozzles. The compressed air is directed to the one or more combustors, where fuel is introduced, and a fuel/air mixture is ignited and burned to form combustion products. The combustion products function as the operational fluid of the turbine. 
     The operational fluid then flows through a fluid flow path in a turbine, the flow path being defined between a plurality of rotating blades and a plurality of stationary nozzles disposed between the rotating blades, such that each set of rotating blades and each corresponding set of stationary nozzles defines a turbine stage. As the plurality of rotating blades rotate the rotor of the gas turbine system, a generator, coupled to the rotor, may generate power from the rotation of the rotor. The rotation of the turbine blades also causes rotation of the compressor blades, which are coupled to the rotor. 
     Gas turbine vanes are static components of the turbine section, which are configured to direct hot gases (at temperatures above 2,200° F.) in a hot gas path to the rotating portions of the turbine to achieve rotational motion of the rotor. Though significant advances in high temperature capabilities have been achieved, superalloy components must often be air-cooled and/or protected with a coating to exhibit a suitable service life in certain sections of the gas turbine engine, such as the airfoils. In order to withstand the high temperatures produced by combustion, the airfoils in the turbine section are cooled. Cooling the airfoils represents a parasitic loss to the power plant, since the air that is used to cool the parts has to be compressed but the amount of useful work that be extracted is comparatively small. As such, it is desirable to cool these parts with as low flow of air as possible to allow for efficient operation of the turbine. 
     The volume of cooling air required may be reduced by using more advanced materials, which can withstand the high temperature conditions in the hot gas flowpath. These materials, such as ceramic matrix composites (CMCs), can increase gas turbine efficiency because their properties reduce the cooling requirements for the respective parts. 
     One challenge in developing CMC vanes that efficiently use the available cooling air is sealing between the CMC vane shell and the respective inner and outer side walls that define therebetween the hot gas path. Often, such side walls are made of a metal alloy with different thermal properties than the CMC vane shell, making sealing between the disparate materials difficult. Leakage of the cooling fluids from the vanes can increase the parasitic losses to the turbine (thereby reducing its efficiency) and can increase the temperature of the vanes (thereby leading to increased stress and/or wear). 
     An effective sealing system is needed to address this problem. 
     SUMMARY 
     A seal system for a gas turbine vane includes a first seal layer and an optional second seal layer. Each first seal layer includes multiple seal segments, each of which includes a first leg, a second leg, and an intermediate portion between the first leg and the second leg. Each adjacent pair of seal segments of the first seal is separated by a gap. The seal segments define a substantially complete perimeter of a seal slot in which the first seal is installed. The second seal, which is also segmented, may or may not define the substantially complete perimeter of the seal slot. If present, the second seal segments are circumferentially offset from the first seal segments to block the gaps between the first seal segments. A turbine vane including the present seal system is also disclosed. 
     Specifically, according to one aspect provided herein, the seal system includes a first seal layer and a second seal layer. The first seal layer has one or more first gaps, and the second seal layer includes one or more second seal segments. The one or more second seal segments are positioned adjacent the first seal layer to block the one or more first gaps. The first seal layer and the second seal layer define a substantially complete perimeter of a seal slot in which the first seal layer and the second seal layer are vertically installed. 
     Specifically, according to another aspect provided herein, the seal system includes a first seal having a first plurality of seal segments. Each adjacent pair of seal segments of the first plurality of seal segments is separated by a first gap. The plurality of seal segments defines a substantially complete perimeter of a seal slot in which the first seal is vertically installed. 
     According to another aspect of the present disclosure, a turbine vane includes: a metal spar comprising an airfoil-shaped body; an outer side wall defining an opening through which the metal spar is installed; a ceramic matrix composite vane shell disposed over the airfoil-shaped body of the metal spar, the CMC vane shell having a radially outer vane platform and a radially inner vane platform; and an inner side wall joined to the ceramic matrix composite vane shell. A first seal system is disposed between the outer side wall and the radially outer vane platform, and a second seal system is disposed between the inner side wall and the radially inner vane platform. Each of the first seal system and the second seal system includes a plurality of seal segments vertically oriented within a respective platform seal slot and a respective side wall seal slot. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The specification, directed to one of ordinary skill in the art, sets forth a full and enabling disclosure of the present system and method, including the best mode of using the same. The specification refers to the appended figures, in which: 
         FIG. 1  is a functional block diagram of an exemplary gas turbine that may incorporate various embodiments of the present disclosure; 
         FIG. 2  is an exploded view of a turbine vane, according to one aspect of the present disclosure; 
         FIG. 3  is a cross-sectional view of a portion of the turbine vane of  FIG. 2 , as assembled with an outer side wall, illustrating the present seal system between the outer side wall and a ceramic matrix composite shell; 
         FIG. 4  is an assembled turbine vane including a seal system, according to one aspect of the present disclosure; 
         FIG. 5  is a cross-sectional view of a portion of an assembled turbine vane of  FIG. 3 , illustrating a seal system, according to the present disclosure; 
         FIG. 6  is an exploded view of an alternate seal system as may be used with a turbine vane, according to an aspect of the present disclosure; 
         FIG. 7  is a perspective view of an alternate seal system as may be used with a turbine vane, according to another aspect of the present disclosure; 
         FIG. 8  is a perspective view of the alternate seal system of  FIG. 7 ; 
         FIG. 9  is a perspective view of the seal system of  FIG. 7 ; 
         FIG. 10  is a perspective view of the seal system of  FIG. 7 ; 
         FIG. 11  is an overhead view of the seal system of  FIG. 7 , showing attachment locations between an inner seal layer and an outer seal layer; 
         FIG. 12  is an overhead view of an alternate view of the seal system of  FIG. 7 ; 
         FIG. 13  is an overhead view of a seal system, according to another embodiment of the present disclosure; 
         FIG. 14  is an overhead view of a seal system, according to yet another embodiment of the present disclosure; 
         FIG. 15  is an overhead view of an alternate seal system, according to one aspect of the present disclosure; 
         FIG. 16  is an overhead view of yet another seal system, according to another aspect of the present disclosure; and 
         FIG. 17  is an overhead view of another version of a seal system, according to another aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments of the present disclosure, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the disclosure. 
     To clearly describe the current seal system, certain terminology will be used to refer to and describe relevant machine components within the scope of this disclosure. To the extent possible, common industry terminology will be used and employed in a manner consistent with the accepted meaning of the terms. Unless otherwise stated, such terminology should be given a broad interpretation consistent with the context of the present application and the scope of the appended claims. Those of ordinary skill in the art will appreciate that often a particular component may be referred to using several different or overlapping terms. What may be described herein as being a single part may include and be referenced in another context as consisting of multiple components. Alternatively, what may be described herein as including multiple components may be referred to elsewhere as a single integrated part. 
     In addition, several descriptive terms may be used regularly herein, as described below. The terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. 
     As used herein, “downstream” and “upstream” are terms that indicate a direction relative to the flow of a fluid, such as the working fluid through the turbine engine. The term “downstream” corresponds to the direction of flow of the fluid, and the term “upstream” refers to the direction opposite to the flow (i.e., the direction from which the fluid flows). The terms “forward” and “aft,” without any further specificity, refer to relative position, with “forward” being used to describe components or surfaces located toward the front (or compressor) end of the engine or toward the inlet end of the combustor, and “aft” being used to describe components located toward the rearward (or turbine) end of the engine or toward the outlet end of the combustor. The term “inner” is used to describe components in proximity to the turbine shaft, while the term “outer” is used to describe components distal to the turbine shaft. 
     It is often required to describe parts that are at differing radial, axial and/or circumferential positions. As shown in  FIG. 1 , the “A” axis represents an axial orientation. As used herein, the terms “axial” and/or “axially” refer to the relative position/direction of objects along axis A, which is substantially parallel with the axis of rotation of the gas turbine system. As further used herein, the terms “radial” and/or “radially” refer to the relative position or direction of objects along an axis “R”, which intersects axis A at only one location. In some embodiments, axis R is substantially perpendicular to axis A. Finally, the term “circumferential” refers to movement or position around axis A (e.g., axis “C”). The term “circumferential” may refer to a dimension extending around a center of a respective object (e.g., a rotor). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Each example is provided by way of explanation, not limitation. In fact, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     Although exemplary embodiments of the present disclosure will be described generally in the context of sealing turbine nozzles for a land-based power-generating gas turbine for purposes of illustration, one of ordinary skill in the art will readily appreciate that embodiments of the present disclosure may be applied to other locations within a turbomachine and are not limited to turbine components for land-based power-generating gas turbines, unless specifically recited in the claims. 
     Referring now to the drawings,  FIG. 1  schematically illustrates an exemplary gas turbine  10 . The gas turbine  10  generally includes an inlet section  12 , a compressor  14  disposed downstream of the inlet section  12 , a combustion section  16  disposed downstream of the compressor  14 , a turbine  18  disposed downstream of the combustion section  16 , and an exhaust section  20  disposed downstream of the turbine  18 . Additionally, the gas turbine  10  may include one or more shafts  22  (also known as “rotors”) that couple the compressor  14  to the turbine  18 . 
     During operation, air  24  flows through the inlet section  12  and into the compressor  14 , where the air  24  is progressively compressed, thus providing compressed air  26  to the combustion section  16 . At least a portion of the compressed air  26  is mixed with a fuel  28  within one or more combustors in the combustion section  16  and burned to produce combustion gases  30 . The combustion gases  30  flow from the combustion section  16  to into the turbine  18 , where thermal and/or kinetic energy are transferred from the combustion gases  30  to rotor blades (not shown) attached to the shaft  22 , thereby causing the shaft  22  to rotate. The mechanical rotational energy may then be used for various purposes, such as to power the compressor  14  and/or to generate electricity, via a generator  21  coupled to the shaft  22 . The combustion gases  30  exiting the turbine  18  may then be exhausted from the gas turbine  10 , via the exhaust section  20 . 
     Within the turbine  18 , each row of rotor blades has a corresponding row of stationary vanes  40  that are positioned between and that are attached to an outer side wall  60  and an inner side wall  80 . Collectively, the rotor blades and the adjacent stationary vanes define a turbine stage. Generally, the length of the rotor blades and stationary vanes increases with each stage, and many heavy-duty gas turbines  10  used for power generation have three or four turbine stages. 
     Gas turbines  10  are routinely operated at very high temperatures (e.g., with combustion gas temperatures in excess of 2,200° F., as the gases enter the turbine section  18 ). Such high temperatures require turbine blades and vanes to be cooled to prevent component stress or failure. The amount of air diverted to the turbine section  18  for cooling the blades and vanes  40  negatively impacts the efficiency of the gas turbine  10 . Thus, to address the competing demands for power generation and high efficiency, some gas turbine manufacturers have contemplated using ceramic matrix composite (CMC) materials to create the blades and/or vanes of one or more turbine stages. 
     Two such turbine vanes  40  are shown in an exploded view in  FIG. 2 . Each turbine vane  40  includes a metal (e.g., superalloy) spar  50  that serves as the foundation of the vane  40 . The metal spar  50  includes a mounting flange  52  and a hollow airfoil-shaped body  54  extending from the platform. The metal spar  50  is installed through an opening  64  in the outer side wall  60 , which corresponds in size and shape to the airfoil-shaped body  54  of the metal spar  50 . The opening  64  is surrounded by a mounting ledge  62  that projects radially outward from a platform  66  of the outer side wall  60 . When the metal spar  50  is installed, the mounting flange  52  of the metal spar  50  is in contact with the mounting ledge  62 . 
     One or more seal systems  100 , according to various aspects of the present disclosure, are installed over the airfoil-shaped body  54  of the metal spar  50  and are positioned into a vertically oriented side wall seal slot  67  (see  FIG. 3 ) on the radially inward surface  68  of the outer side wall  60 . A CMC vane shell  70  is positioned over the airfoil-shaped body  54  of the metal spar  50 . The CMC vane shell  70  includes an outer vane platform  72 , an inner vane platform  76 , and an airfoil-shaped body  74  extending radially between the inner vane platform  76  and the outer vane platform  72 . The airfoil-shaped body  74  is hollow or substantially hollow to receive a flow of cooling air. A cavity  75 , which corresponds in size and shape to the airfoil-shaped body  54  of the metal spar  50 , extends through the airfoil-shaped body  74  from the outer vane platform  72  to the inner vane platform  76 . The seal system  100  may be positioned within a vertically oriented platform seal slot  77  on a radially outward surface  78  of the outer vane platform  72 . The platform seal slot  77  is aligned with the side wall seal slot  67 , so that the seal  100  is oriented vertically, or upright, within the seal slots  67 ,  77 . 
     The seal system  100  has one or more layers, and each layer has one or more seal segments. According to one aspect of the present disclosure, the seal system  100  is installed within a corresponding vertically oriented platform seal slot (not shown) on a radially inward surface  79  of the inner vane platform  76  before the CMC vane shell  70  is brought into contact with the inner side wall  80 . The inner side wall  80  includes a platform  82  and an airfoil-shaped extension  84  that projects radially outward from the platform  82 . The airfoil-shaped extension  84  is sized and shaped to fit within the cavity  75  in the CMC vane shell  70 . A vertically oriented side wall seal slot  87  is formed around the airfoil-shaped extension  84  in the platform  82  to engage the one or more seal layers and/or segments of the seal system  100 . Bolts  90 , threaded from the radially inward surface  88  of the platform  82  through bolt holes  85 , secure the inner side wall  80  to the CMC vane shell  70 . 
       FIG. 3  shows a cross-sectional view of a portion of the vane  40 , in which only the CMC vane shell  70  and the outer side wall  60  are illustrated. (That is, the metal spar  50  and the inner side wall  80  are not shown.) One or more seal layers and/or segments of the seal system  100  are disposed within the respective vertically oriented seal slots  67 ,  77  on the radially inward surface  68  of the outer side wall  60  and the radially outward surface  78  of the outer vane platform  72 . The seal system  100  is disposed in a generally vertical orientation (i.e., parallel to a centerline axis of the vane  40 ). 
       FIG. 4  illustrates a perspective view of the CMC vane shell  70  and the seal system  100 , in which the outer side wall  60  is removed to provide a view of the entire seal system  100 . The seal system  100 , which is oriented vertically within the side wall seal slot  77 , includes a first seal segment  102 , a second seal segment  104 , and a third seal segment  106 . A small gap  105  is formed between each pair of adjacent seal segments  102 / 104 ,  104 / 106 ,  106 / 102 . Although the seal segments  102 ,  104 , and  106  are the same height, the seal segments  102 ,  104 , and  106  may be of different lengths. In the exemplary embodiment shown in  FIGS. 2 through 5 , the seal system  100  has a generally airfoil-shaped profile, and the seal segments  102 ,  104 ,  106  are curved to define the substantial perimeter of this profile. 
     As shown herein, the platform seal slot  77  and the seal segments  102 ,  104 ,  106  are perpendicular to the radially outward surface  78  of the outer vane platform  72 . However, the seal slot  77  may be angled, relative to a longitudinal axis of the vane  40 , thereby causing the seal segments  102 ,  104 ,  106  to be angled as well. In this instance (not shown), the side wall seal slot  67  of the outer side wall  60  is offset radially to accommodate the angle of the platform seal slot  77  and the height of the seal segments  102 ,  104 ,  106 . 
       FIG. 5  provides an enlarged cross-sectional view of the CMC vane shell  70  and the seal system  100 , as shown in  FIG. 4 . The airfoil-shaped cavity  75  has a curved leading edge (visible in  FIG. 2 ) and an opposite trailing edge  71 . The platform seal slot  77  is disposed radially outward of the perimeter of the cavity  75  and defines a shape conforming generally to the cross-sectional shape of the cavity  75 . Rather than positioning the gap  105  between seal segments  102 ,  104  at the area of the platform seal slot  77  with the smallest radius, the present seal system  100  disposes the gap  105  at a circumferentially offset position from the trailing edge  71 . Likewise, as shown in  FIG. 4 , the third seal segment  106  has an arcuate profile that spans the leading edge, such that the gaps  105  between the third seal segment  106  and the adjacent seal segments  102 ,  104  are offset from the leading edge. This configuration minimizes leakage that may otherwise occur at areas with a small radius. 
     While  FIGS. 2 through 5  illustrate the seal segments  102 ,  104 ,  106  as being installed between the radially outward surface  78  of the outer vane platform  72  and the outer side wall  60 , it should be appreciated that a corresponding seal system  100  having seal segments  102 ,  104 ,  106  is used to seal between the radially inward surface  79  of the inner vane platform  76  and the inner side wall  80 . 
     The seal segments  102 ,  104 ,  106 , as illustrated, are thin strips or ribbons of sheet metal or metal cloth, which may or may not be coated or sealed. The seal segments  102 ,  104 ,  106  are flexible enough (in the longitudinal direction) to permit insertion into the respective seal slots  67 ,  77 ,  87 , but are rigid enough (in the transverse direction) to retain their upright position in the seal slots  67 ,  77 ,  87 . The seal segments  102 ,  104 ,  106 , as positioned between respective seal slots  67 ,  87  in the side walls  60 ,  80  and the seal slots  77  in the CMC vane shell  70 , are secured in position, minimizing the likelihood of creep during operation of the turbine. The seal segments  102 ,  104 ,  106  may be a single layer, as shown, or may be multiple layers as shown in  FIGS. 6 through 10 . 
     In  FIG. 6 , a seal system  200  is illustrated, which includes a first seal layer  201  and a second seal layer  211 . For purposes of illustration only, the seal layers  201  and  211  are shown as having a generally rectangular shape, although clearly the principles described apply equally to seal systems of other shapes (including the airfoil shape of  FIGS. 2 through 5  and the trapezoidal shape of  FIGS. 7 through 17 ). 
     The first seal layer  201  includes a first seal segment  202 , a second seal segment  204 , a third seal segment  206 , and a fourth seal segment  208 , each of the first seal segments being separated by a gap  205 . The second seal layer  211  includes a fifth seal segment  212 , a sixth seal segment  214 , a seventh seal segment  216 , and an eighth seal segment  218 , each of the second seal segments being separated by a gap  215 . Each of the seal segments  202 - 218  is configured with a generally J- or L-shaped profile, having one long leg (e.g.,  202   a ) and one short leg (e.g.,  202   c ) and defining an intermediate portion (e.g.,  202   b ) between the long leg and the short leg. As illustrated, the intermediate portion (e.g.,  202   b ) may be a curved portion having a radius of curvature that creates an angle of approximately 90-degrees between the long leg and the short leg. 
     In one embodiment, each seal segment  202 - 218  is of the same size (height, length, and thickness) and, therefore, is interchangeable with any other seal segment  202 - 218 . Alternately, all the seal segments  202 - 218  may have the same height, but at least some of the seal segments  202 - 218  be made in different lengths. In another embodiment, the seal segments  202 ,  204 ,  206 ,  208  may have a first height, and the seal segments  212 ,  214 ,  216 ,  218  may have a second height different from the first height. 
     The second seal layer  211  is sized to nest within the first seal layer  201  (or vice versa). Further, it should be noted that the seal segments  212 ,  214 ,  216 ,  218  of the second seal layer  211  are circumferentially offset from the seal segments  202 ,  204 ,  206 ,  208  of the first seal layer  201 , such that the gaps  205 ,  215  between the seal segments  202 - 208  and  212 - 218  are blocked by a respective seal segment of the radially adjacent seal layer  211 ,  201 . This configuration helps to minimize leakage by creating more blockages for air trying to escape the cavity  75  of the vane  40 . 
     The seal layers  201 ,  211  of the seal system  200  are installed in an upright position in a platform seal slot (e.g., slot  77 ) in the radially outward surface  78  of the outer vane platform  72 . The same seal system  200  may be used in a respective platform seal slot (not shown) in the radially inward surface  79  of the inner vane platform  76 . 
       FIGS. 7 through 10  refer to a seal system  300 , according to another aspect of the present disclosure. For the sake of illustration, the seal system  300  is shown installed on the radially outward surface  78  of the outer vane platform  72 , although the seal system  300  is equally applicable for use on the respective platform seal slot (not shown) in the radially inward surface  79  of the inner vane platform  76 . In this exemplary embodiment, the seal system  300  defines the general shape of a trapezoid, although the seal system  300  is applicable to other shapes. 
     The seal system  300  includes a first layer of seal segments  302 ,  304 ,  306 ,  308  that fill substantially all the perimeter of the platform seal slot  77  (excluding gaps  305 ) in the radially outward surface  78  of the outer vane platform  72 . As used herein the phrase “substantially complete” means that the perimeter of the respective seal slot (e.g.,  77 ) is filled except for the small area defining the gaps (e.g.,  305 ). Gaps  305  are defined between adjacent seal segments  302 / 304 ,  304 / 306 ,  306 / 308 ,  308 / 302 . The gaps  305  may or may not be of uniform width. Each seal segment  302 ,  304 ,  306 ,  308  has a first leg (e.g.,  302   a ) and a second leg (e.g.,  302   c ) and define an intermediate portion (e.g.,  302   b ) between the first leg and the second leg. In the exemplary embodiment, the intermediate portion (e.g.,  302   b ) has a radius of curvature that is greater than or less than 90 degrees. 
     In an exemplary embodiment, the first seal segment  302  and the second seal segment  304  have the same dimensions and are mirror images of one another, while the third seal segment  306  and the fourth seal segment  308  have the same dimensions and are mirror images of one another. There is no requirement that seal segments  302 ,  304 ,  306 ,  308  have any particular size relative to one another, provided that the seal segments  302 ,  304 ,  306 ,  308  have a uniform height and include an intermediate portion connecting the respective legs of the seal segment. 
     A second layer of seal segments  312 ,  314 ,  316 ,  318  is disposed radially inward of the first layer of seal segments  302 ,  304 ,  306 ,  308 . Each seal segment  312 ,  314 ,  316 ,  318  is a straight seal segment having a height matching the height of the first layer of seal segments  302 ,  304 ,  306 ,  308 . Each straight seal segment  312 ,  314 ,  316 ,  318  is positioned to block a respective gap  305  between adjacent seal segments  302 / 304 ,  304 / 306 ,  306 / 308 ,  308 / 302 . 
     Specifically, the fifth seal segment  312  is positioned radially inward of the first seal segment  302  and the second seal segment  304  to block the gap  305  between the first and second seal segments  302 ,  304 . The sixth seal segment  314  is positioned radially inward of the second seal segment  304  and the third seal segment  306  to block the gap  305  between the second and third seal segments  304 ,  306 . The seventh seal segment  316  is positioned radially inward of the third seal segment  306  and the fourth seal segment  308  to block the gap  305  between the third and fourth seal segments  306 ,  308 . The eighth seal segment  318  is positioned radially inward of the fourth seal segment  308  and the first seal segment  302  to block the gap  305  between the fourth and first seal segments  308 ,  302 . 
     In the exemplary embodiment in which the seal system  300  defines a trapezoidal shape, the sixth seal segment  314  and the eighth seal segment  318  have the same length, and the seventh seal segment  316  is shorter than the fifth seal segment  312 . Thus, the sixth and eighth seal segments  314 ,  316  are interchangeable with one another, and the lengths of the fifth and seventh seal segments  312 ,  316  ensure their proper placement within the platform seal slot  77 . 
       FIGS. 11 through 16  illustrate various embodiments of the seal system  300  having a first seal layer  301  of one or more seal segments (represented by black lines) and a second seal layer  311  of one or more seal segments (represented by gray lines). The seal segments are vertically oriented in a seal slot  77  (represented by dashed lines). Gaps in the first seal layer  301  are blocked by a respective seal segment of the second seal layer  311 . The second seal layer  311  may be radially inward of the first seal layer  301  (as shown in  FIGS. 11 and 16 ) or may be radially outward of the first seal layer  301  (as shown in  FIGS. 12 through 15 ). 
     In  FIG. 11 , the first seal layer  301  is disposed radially outward of the second seal layer  311 . The first seal layer  301  defines a substantially complete perimeter of the seal slot  77 , which is complete except for the first gaps  305  between the first seal segments  302 ,  304 ,  306 ,  308 . 
     Each of the first seal segments  302 ,  304 ,  306 ,  308  has a generally J- or L-shape with a long leg, a short leg, and an intermediate portion between the long leg and the short leg. Each of the second seal segments  312 ,  314 ,  316 ,  318  is a straight seal. The straight seal segments  312 ,  314 ,  316 ,  318  are disposed in contact with the first seal segments  302 ,  304 ,  306 ,  308 , in such a location as to block the gaps  305  between each pair of adjacent first seal segments  302 / 304 ,  304 / 306 ,  306 / 308 ,  308 / 302 . 
     The number of first seal segments  302 ,  304 ,  306 ,  308  in the first seal layer  301  is equal to the number of seal segments  312 ,  314 ,  316 ,  318  in the second seal layer  311 . Although four seal segments are used in each seal layer  301 ,  311  in the embodiment of  FIG. 11 , other numbers of seal segments may instead be used. 
     To simplify installation of the seal layers  301 ,  311 , each first seal segment (e.g.,  302 ) may be permanently coupled to a radially adjacent second seal segment (e.g.,  318 ). The coupling  335 , which is represented by a rectangle, may be one of a spot weld, a rosette weld, a braze joint, or a flat-headed rivet. By permanently coupling each first seal segment to a radially adjacent second seal segment, the number of units to be installed in the slot  77  is divided in half. 
     Optionally, each first seal segment (e.g.,  302 ) may be temporarily coupled to a second seal segment (e.g.,  312 ) that is circumferentially adjacent to the second seal segment (e.g.,  318 ) to which the permanent coupling  335  is made. The temporary coupling  345 , which is represented by an oval, may be one of an adhesive that is non-durable under the operating temperatures of the gas turbine  10  or a removable fastener, such as tape or a binder clip. The temporary coupling  345  is used to hold all the seal segments  302 - 318  as a single unit to facilitate installation into the seal slot  77 . If a removable fastener is used as the temporary coupling  345 , the removable fastener may be removed after the seal system  300  is positioned within the seal slot  77 . 
     In the seal system  300   a  of  FIG. 12 , the first seal layer  301  is disposed radially inward of the second seal layer  311 . As in  FIG. 11 , the first seal layer  301  defines a substantially complete perimeter of the seal slot  77 , which is complete except for the first gaps  305  between the first seal segments  302 ,  304 ,  306 ,  308 . The number of first seal segments  302 ,  304 ,  306 ,  308  in the first seal layer  301  is equal to the number of seal segments  312 ,  314 ,  316 ,  318  in the second seal layer  311 . Although four seal segments are used in each seal layer  301 ,  311 , other numbers of seal segments may instead be used. 
     As described above, the seal segments  302 ,  304 ,  306 ,  308  may be permanently coupled to radially adjacent seal segments  318 ,  312 ,  314 ,  316 , respectively, as represented by permanent coupling joints  335 . Optionally, the seal segments  302 ,  304 ,  306 ,  308  may be temporarily coupled, via dissolvable or removable fasteners  345 , to seal segments  312 ,  314 ,  316 ,  318  that are circumferentially adjacent to the seal segments  318 ,  312 ,  314 ,  316  with the permanent coupling. 
       FIG. 13  illustrates an exemplary seal system  300   b , in which the first seal layer  301  includes a single seal segment  302 , and the second seal layer  311  includes a single seal segment  312 . The first seal segment  302  fills a substantially complete perimeter of the seal slot  77 , excluding the gap  305 . The straight second seal segment  312  is positioned radially outward of the first seal segment  302  and is located to block the gap  305 . The first seal layer  301  and the second seal layer  311  may be permanently coupled, via coupling  335 . 
     In the illustrated embodiment, the gap  305  is located along the portion of the seal segment  302  with the longest length, although the gap  305  may be located instead along any other portion of the seal segment  302 . It should be understood that the second seal segment  312  is positioned along any portion of the seal segment  302  where the gap  305  is located. 
       FIG. 14  illustrates an exemplary seal system  300   c , in which the number of seal segments in the first seal layer  301  is different from the number of seal segments in the second seal layer  311 . The first seal layer  301  includes three seal segments  302 ,  304 ,  306 , and the second seal layer  311  includes two seal segments  312 ,  314  that are positioned radially outward of the first seal layer  301 . The seal segments  302 ,  304 ,  306  of the first seal layer  301  are spaced to define gaps  305  between the seal segments  302 ,  304 ,  306 , and the seal segments  312 ,  314  are positioned to block these gaps  305 . 
     In the illustrated embodiment, the second seal segment  314  blocks the gaps  305  between seal segments  304 / 306  and  306 / 302 , while the seal segment  312  blocks the gap  305  between first seal segments  302 ,  304 . The seal segments  312 ,  314  of the second seal segment  311  may be permanently coupled to the seal segments  302 ,  304 ,  306 , via permanent couplings  335 . 
       FIG. 15  illustrates an exemplary seal system  300   d , in which the first seal layer  301  is a single seal segment  302 , and the second seal layer  311  is a single seal segment  312  disposed radially outward of the first seal layer  301 . The seal segment  302  defines the gap  305 , and the seal segment  312  defines the gap  315 , which is circumferentially offset from the gap  305 . The gaps  305 ,  315  may be disposed in any location along the respective seal segment  302 ,  312 , as long as the gaps  305 ,  315  do not align circumferentially with one another. 
       FIG. 16  illustrates an exemplary seal system  300   e , in which the first seal layer  301  includes seal segments  302 ,  304 ,  306 ,  308  that each include a first leg (e.g.,  302   a ), a second leg (e.g.,  302   c ), and an intermediate portion  302   b , in the form of an angular corner. In the illustrated embodiment, the second seal layer  311  (including seal segments  312 ,  314 ,  316 ,  318 ) is disposed radially inward of the first seal layer  301 , although the opposite configuration may instead be used. As before, the seal segments  312 ,  314 ,  316 ,  318  are positioned to block the gaps  305  between circumferentially adjacent seal segments  302 / 304 ,  304 / 306 ,  306 / 308 ,  308 / 302 . The segments  302 - 308  of the first seal layer  301  may be coupled to the radially adjacent segments  312 - 318  of the second seal layer  311 , via permanent couplings  335  and/or temporary couplings  345  (neither of which are shown in this illustration). 
       FIG. 17  illustrates a seal system  400  having a first seal layer  401 , a second seal layer  411 , and a third seal layer  421 . The first seal layer  401  includes seal segments  402 ,  404 ,  406 ,  408  with gaps  405  being defined between circumferentially adjacent pairs of seal segments. Each seal segment  402 ,  404 ,  406 ,  408  generally has a dog-leg shape having a first leg (e.g.,  402   a ), a second leg (e.g.,  402   c ), and an intermediate portion (e.g.,  402   b ) connecting the first leg to the second leg. The intermediate portion may be curved (as shown) or may define an angular corner (as in  FIG. 16 ). 
     The second seal layer  411  includes seal segments  412 ,  414 ,  416 ,  418 , which are straight seal segments positioned to block the gaps  405  defined between the respective pairs of seal segments  402 / 404 ,  404 / 406 ,  406 / 408 ,  408 / 402 . 
     The third seal layer  421  includes seal segments  422 ,  424 ,  426 ,  418 , which may or may not have the same shape as the seal segments  402 ,  404 ,  406 ,  408  of the first seal layer  401 . As shown, each of the seal segments  422 ,  424 ,  426 ,  428  generally have a dog-leg shape, and gaps  425  are defined between each pair of seal segments. It is not necessary that the gaps  425  align with the gaps  405 . Rather, the second seal segments  412 ,  414 ,  416 ,  418  are positioned to simultaneously block the gaps  405  in the first seal layer  401  and the gaps  425  in the third seal layer  421 . 
     The seal layers  401 ,  411 ,  421  may be coupled together by permanent couplings  435 , as shown, to simplify installation. 
     Exemplary embodiments of the seal system and methods of installing the same are described above in detail. The methods and seals described herein are not limited to the specific embodiments described herein, but rather, components of the methods and seals may be utilized independently and separately from other components described herein. For example, the methods and seals described herein may have other applications not limited to practice with turbine nozzles for power-generating gas turbines, as described herein. Rather, the methods and seals described herein can be implemented and utilized in various other industries. 
     While the technical advancements have been described in terms of various specific embodiments, those skilled in the art will recognize that the technical advancements can be practiced with modification within the spirit and scope of the claims.