Patent Publication Number: US-8534675-B2

Title: Strip seal and method for designing a strip seal

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to European Patent Application No. 09151505.6 filed in Europe on Jan. 28, 2009, the entire content of which is hereby incorporated by reference in its entirety. 
     FIELD 
     The present disclosure relates to metal strip seals that fit into grooved recesses formed in two components and to methods of designing strip seals. 
     Throughout this specification:
         a strip seals refer to strip seals made of metal;   a strip seal pressure drop is the pressure drop across the strip seal during normal operation of the strip seal and excludes the influence of pressure pulsation; and   a gas turbine hot gas component is a component that is as least in part exposed to hot combustion gases as they flow through a gas turbine.       

     BACKGROUND INFORMATION 
     Strip seals, which are also known as feather seals, can be used to eliminate leakage flow between two components arranged adjacently to one another. This is achieved by the two components having groove recesses in edge faces that lie substantially opposite and adjacent to one another. The strip seal seals the gap between the two components by being at least partially received into the groove recesses of the adjacently fitted components so as to span the gap between the components. U.S. Pat. No. 5,531,457 discloses an example of such a strip seal used to reduce leakage flow through the gap between two platforms of a blade. 
     The grooved recesses of fitted components often do not perfectly align due to, for example, manufacturing tolerances or as a result of thermal expansion. If the strip seal is manufactured so as to tightly fit into the groove recesses, a less than perfect groove recess alignment would result in high stress loading of the strip seal, which can result in premature failure. 
     In recognition of this drawback, strip seals can be made thinner than the height of the grooved recesses and flexible orthogonal to the strip seal length. In operation, the pressure differential across the seal, due to the flexibility of the strip seal, forces the strip seal against one surface of the grooved recess to effect a seal. When the pressure differential is low, strip seals are made thinner so as to increase their flexibility strip. To hold thin seals in place, for example during installation, the strip seal may be provided with biasing means which can be dispersed along the strip seal length. An example of biasing means is described in U.S. Pat. No. 3,836,279. 
     During operation, the strip seals can be exposed to periodic pressure pulsations caused by the passing of rotating blades as they pass the non-rotating regions within which the strip seals are contained. Depending on the strength and frequencies of the pressure pulsations, parts of the strip seal that are not biased against faces of the groove recess or otherwise retained can be induced into periodic resonance leading to premature fatigue failure of the strip seal. An application where this drawback is particularly relevant is in the sealing of components in gas turbines where rotating blades of the gas turbine induce pressure pulsation at sealing faces. 
     By reducing the seal length, it is possible to avoid fatigue failure. However, when strip seal length is shorter than the length of the recess groove, sealing is made more complicated. There is therefore a desire for a strip seal which is resilient to fatigue failure induced by pressure pulsation independent of strip seal length. 
     SUMMARY 
     An exemplary embodiment provides a strip seal for sealing two adjacent non-rotating gas turbine components. The components include complimentary grooved recesses configured and arranged to receive the strip seal so that the strip seal, when received into the grooved recesses, extends between the components to provide a seal between a higher pressure medium and a lower pressure medium acting on the components. The strip seal is constituted by a material having a dynamic modulus of at least one of approximately 232 GPa at a temperature of 20° C., approximately 217 GPa at a temperature of 200° C., approximately 201 GPa at a temperature of 400° C., approximately 184 GPa at a temperature of 600° C., approximately 176 at a temperature of 700° C., approximately 169 GPa at a temperature of 800° C., approximately 161 GPa at a temperature of 900° C., and approximately 153 GPa at a temperature of 1,000° C. The exemplary strip seal comprises a pressure face configured to be acted upon by the higher pressure medium, a sealing face configured to be acted upon by the lower pressure medium, a first end, a second end, a length extending between the first end and the second end, a width extending substantially normal to the length, at least two clamping projections distributed along discrete points of the length, where the at least two clamping projections extend from the pressure face and are configured to prevent localized movement of the strip seal when fitted, and a thickness defined as a distance free of projections between the pressure face and the sealing face. In addition, the strip seal has (i) a first ratio of the length of the strip seal extending free of clamping projections from any one of the ends to a clamping projection, to the thickness, of less than 25, and (ii) a second ratio of the length of the strip seal extending free of clamping projections between any two of projections, to the thickness, of less than 200. 
     An exemplary embodiment provides a method for configuring a strip seal for sealing two adjacent components with clamping projections to ensure resilience to induced resonance. The exemplary method includes the steps of: a) determining a resonance frequency to which the strip seal will be exposed during operation; b) clamping the strip seal at the clamping projections; c) applying the clamped strip seal to the frequency determined in step a); d) measuring the response of the clamped strip seal to the applied frequency; e) assessing acceptability of the response measured in step d); and f) if the response assessed in step e) is not acceptable, reconfiguring at least one of a location and number of the clamping projections, and repeating from step b). 
     Another exemplary embodiment provides a method for configuring a strip seal for sealing two adjacent components so that the strip seal ensures resilience to induced resonance, the method including the steps of: a) determining an operational excitation frequency of each component; and b) arranging one or more clamping projections on the strip seal as a function of the determination of step a) and properties of the strip seal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional refinements, advantages and features of the present disclosure are described in more detail below with reference to exemplary embodiments illustrated in the drawings, in which: 
         FIG. 1  is a schematic view of a portion of a gas turbine that has components with strip seals according to an exemplary embodiment of the present disclosure; 
         FIG. 2  is an expanded view of portions of  FIG. 1  showing two adjacently fitted components with strip seals according to an exemplary embodiment of the present disclosure; 
         FIG. 3  is an expanded side view of a component of  FIG. 2  showing a strip seal according to an exemplary embodiment of the present disclosure; 
         FIG. 4  is an expanded side view of the component of  FIG. 2  showing another strip seal according to an exemplary embodiment of the present disclosure; 
         FIG. 5  is a flow chart of a method of configuring a strip seal according to an exemplary embodiment of the present disclosure; and 
         FIG. 6  is a flow chart of a method of configuring a strip seal according to another exemplary embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present disclosure provide a metal gas turbine strip seal which is configured to be resilient to induced resonance independent of strip seal length. Exemplary embodiments of the present disclosure provide a metal strip seals that fit into grooved recesses formed in two components and methods of designing strip seals for use in the above-mentioned way to seal gas turbine components in the hot gas section of a gas turbine. According to an exemplary embodiment, the grooved recesses are formed to be substantially adjacent, when the components are fitted, in such a manner that enables a strip seal to be received into the grooved recesses and span between the components. Under the action of a force caused by differential pressure across the strip seal, the strip seal forms a substantially gastight seal. 
     An exemplary embodiment of the present disclosure is based on the concept of changing the natural frequency of the strip seal so that it is different from the pressure pulsation frequency of the gas turbine. This can be achieved, for example, by providing discrete points along the strip seal length, based on certain criteria, that prevent localized orthogonal movement of the strip seal so that the natural frequency of strip seal lengths between clamped regions are either out of phase with or overtones of the pressure pulsation frequency to which regions of the seal are exposed. 
     An exemplary embodiment provides a strip seal for sealing two adjacent non-rotating gas turbine hot gas components exposed to a pressure pulsation frequency of between about 3000-6000 Hz. The components include complimentary grooved recesses that are configured and arranged to receive the strip seal so that the strip seal, when received into the grooved recesses, extends between the components so as to provide a seal between a higher pressure medium and a lower pressure medium acting on the components. The strip seal can be made of a material having the same or similar dynamic modulus of elasticity shown in Table 1. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Temperature (° C.) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 20 
                 200 
                 400 
                 600 
                 700 
                 800 
                 900 
                 1000 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Dynamic 
                 232 
                 217 
                 201 
                 184 
                 176 
                 169 
                 161 
                 153 
               
               
                 Modulus of 
               
               
                 Elasticity 
               
               
                 (GPa) 
               
               
                   
               
            
           
         
       
     
     The exemplary strip seal can also include a pressure face onto which, in use, the higher pressure medium acts, a sealing face onto which, in use, the lower pressure medium acts, a first end, a second end, a length extending between the first end and the second end, a width extending normal to the length, at least two clamping projections distributed along discrete points of the length and extending out from the pressure face configured to prevent localized movement of the strip seal, and a thickness defined as the distance free of projections between the pressure face and the sealing face. 
     The strip seal can be characterized by two ratios. The first ratio, of less than 25, is the ratio of the strip seal length extending free of clamping projections from, any one of the ends of the strip seal to a clamping projection, to the strip seal thickness. It was found that this ratio equally applies to strip seals without projections and so is a limit currently faced by known strip seals. By providing that the strip seal conforms to a second ratio, of less than 200, that includes the ratio of, the strip seal length extending free of clamping projections between any two projections, to the strip seal thickness, the length of the seal is not limited by induced resonance concerns and so can be made suitable thin for operation at differential pressure conditions below 2 bar. 
     The second ratio value limit is based on the observation that a strip seal with a thickness of between 0.2 mm and 0.8 mm+/−0.1 mm, at points of the strip seal free of clamping projections, is resilient to induced resonance when the second ratio is kept either between 72 and 92 or between 150 and 170. 
     In a further exemplary embodiment, the clamp projections extend only part way across the width of the strip seal so by reducing leak potential around the clamped projections. 
     According to an exemplary embodiment, the clamping projections are configured to prevent localized movement of the strip seal in the traverse direction by being configured to extend from the pressure face so as to bias the sealing face against a wall of the grooved recess. 
     According to an exemplary embodiment, the strip seal has a first layer that forms the pressure face, and a second layer that forms the sealing face. 
     According to an exemplary embodiment, the clamping projections can include stamped projections having an indentation on the sealing face opposite projections on the pressure face, formed projections, and/or a combination of stamped projections and formed projections. 
     An exemplary embodiment of the present disclosure provides a method for configuring a strip seal, for sealing two adjacent components, with clamping projections to ensure resilience, in use, to induced resonance. The method can include the steps of: a) determining the frequency the strip seal will be exposed to during operation; b) clamping the strip seal at the clamping projections; c) subjecting the clamped strip seal to the frequency determined in step a); d) measuring the response of the clamped strip seal to the determined frequency; e) assessing the acceptability of the response of step d), if acceptable the method steps are complete otherwise proceed to step f); and f) reconfiguring the location and/or number of clamping projections then repeat from step b). 
     The exemplary method provides a means of modifying an existing strip seal in a way that does not require reconfiguration of gas turbine components in order to reuse the strip seals. 
     Another exemplary embodiment of the present disclosure provides a method for configuring a strip seal to ensure resilience, in use, to induced resonance. The exemplary method can include the steps of: determining the operational excitation frequency of each component; and arranging one or more clamping projections on the strip seal as a function of the determination of step a) and properties of the strip seal. 
     The properties may include at least one of length, thickness and a material property of the strip seal. 
     In another exemplary embodiment, in accordance with either of these two methods, the determination of step a) is by calculation or by measurement. 
     Other objectives and advantages of the present disclosure will become apparent from the following description, taken in connection with the accompanying drawings wherein by way of illustration and example, embodiments of the disclosure are disclosed. 
     Exemplary embodiments of the present disclosure are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. It may be evident, however, that the disclosure may be practiced without these specific details. 
       FIG. 1  shows a portion of a gas turbine  2  with multiple blades  7  and vanes  10  each of which comprise components  13  which need to be sealed against each other to prevent the loss of a high pressure medium contained in plenums from the lower pressure hot gas of the gas turbine. In order to achieve such sealing according to an exemplary embodiment, strips seals  20  (shown in  FIGS. 2-4 , for example) seal circumferentially distributed non-rotating components  13 . The passing of rotating blades  7  past non-rotating components  13  subjects the components  13  to pressure pulsation. Consequently, seals of these components are exposed to cyclical pressure pulsation. Where the pressure differential across the strip seals  20  (shown in  FIGS. 2-4 , for example) is low, for example below 2 bar, seals are made thin, the resulting flexibility of the strip seals  20  (shown in  FIGS. 2-4 , for example) make then susceptible to failure due to induced resonance. 
     Regions I, II and III, shown in  FIG. 1  are exemplary regions of a gas turbine  2  that include components  13  which may be exposed to pressure pulsation and are subject to low pressure differential. Therefore, these regions are regions were embodiments of the present disclosure may be suitably applied. 
     Region I shown in  FIG. 1  is a heat shield of the first blade  7  of the gas turbine  2 , which in an exemplary gas turbine  2  has a seal pressure differential of less than about 2 bar and as a result has a seal strip thickness of about 0.5 mm. As the blade  7  in this region is unshrouded according to an exemplary embodiment, the component  13  passed over by the tip of the rotating blade  7  is subject to particularly severe pressure pulsation. 
     Region II, shown in  FIG. 1 , is a platform of a vane  10  which in an exemplary gas turbine  2  has a seal pressure drop of less than about 0.5 bar and thus can involve very thin strip seals  20 . Therefore, despite not being exposed to the same degree of pressure pulsation as Region I, seals in this region II may still be prone to premature fatigue failure caused by pressure pulsation due to their thin and therefore flexible nature. 
     Region III, shown in  FIG. 1 , is a heat shield component  13  near the outlet of the exemplary gas turbine  2 . Although the region III is opposite a shrouded blade  7  according to an exemplary embodiment, the seal pressure drop is as that in region II and so it can also be prone to premature fatigue failure caused by pressure pulsation for similar reasons. 
       FIG. 2  is an expanded schematic view of a generic component  13  having features relating to strip seals  20  common to the components  13  in regions I, II and III shown in  FIG. 1 . The components  13 , in use, are adjacent to non-rotating gas turbine hot gas components  13  circumferentially fitted in a gas turbine  2 . Two adjoining components  13  are shown in the exemplary configuration illustrated in  FIG. 2 . A strip seal  20  extending between the components  13  provides a means of sealing the components  13 . To accommodate the strip seals  20 , each of the components  13  has an edge face  16  which defines the joining face between adjoining components  13 . Each edge face  16  of each component  13  has a grooved recess  17  complimentary to the grooved recess  17  of adjacently fitted components  13 . The grooved recess  17  is alignable so as to enable the receiving of a strip seal  20  in the grooved recesses  17  of each adjacent component  13  at the same time so that the received strip seal  20  extends between the adjacent components  13 . In this way, the strip seal  20  provides a seal between the higher pressure medium and the lower pressure medium on either side of the component  13 . The ability of the grooved recesses  17  to receive a strip seal  20  is further enabled by the width  22  of the strip seal  20  relative to the depth of each of the grooved recesses  17 . 
       FIG. 3  is an expanded side view of the components  13  of  FIG. 2  showing a received strip seal  20 . The strip seal  20  has a length  21 , extending between the distal ends  24  of the strip seal  20 , that enables the strip seal  20  to provide a seal along a length of the grooved recess  17 . According to an exemplary embodiment, the strip seal  20  provides a seal between higher and lower pressure mediums acting on the strip seal  20 . The higher pressure medium, acting on a pressure face  26  of the strip seal  20 , presses the sealing face  25  of the strip seal  20  onto a sealing surface  19  of the grooved recess  17 . According to the exemplary embodiment illustrated in  FIG. 3 , the sealing face  25  is the surface substantially parallel to but on the opposite side than the pressure face  26  of the strip seal  20 . In this way, the pressure difference across the strip seal  20  enables the strip seal  20  to seal. 
     The thickness  23  of the strip seal  20 , defined as the dimension between the pressure face  26  and the sealing face  25  of the strip seal  20  free of protrusions or projections  27 , is less than the groove height  18  so that the inserted strip seal  20  is not stressed by any misalignment of adjacently fitted components  13 . 
     To ensure that the strip seal  20  is held firmly in the grooved recess  17 , despite the thickness  23  being less than the groove height  18 , the pressure face  26  of the strip seal  20  is provided with discrete clamping projections  27  along its length  21  that bias the sealing face  25  against the sealing surface  19  of the grooved recess  17 . In this way, the strip seal  20  is held firmly at discrete points in the grooved recess  17  so as to prevent localized movement independent of the pressure difference across the strip seal  20  or pressure pulsations to which the strip seal  20  may be exposed. 
       FIG. 3  further shows exemplary embodiments with clamping projections  27  including formed projections  31  which may be formed by being bonded onto or machining projections  27  on to the pressure face  26  of the strip seal  20 , and stamped projections  30  which may be formed by stamping the sealing face  25  of the strip seal  20  to result in an indentation on the sealing face  25  that corresponds to the stamped projection  30  on the pressure face  26 . To eliminate the potential leak path created by stamping at the sealing face  25 , the strip seal  20 , in another exemplary embodiment, can be configured to comprise a second layer  29 , as shown in  FIG. 4 , that forms the sealing face  25  of the strip seal  20 . This second layer  29 , which is bonded to a first layer  28 , forms the pressure face  26 , and does not have any indentions, thereby ensuring a continuous sealing face  25  absent of any leakage path. 
     In another exemplary embodiment, the clamping projections  30  can extend only part way across the width  22  (shown in  FIG. 2 ) of the strip seal  20  to eliminate seal leakage at the projections. 
     The frequency at which a strip seal  20 , shown generally in the drawings, will be induced to resonant pressure is influenced by the length  21 , thickness  23  and material properties of the strip seal  20 . The material property of particular importance is the dynamic modulus of elasticity at the operating temperature. It has generally be found that a strip seal  20  of a gas turbine  2  component  13  made of material with a dynamic modulus of elasticity which is the same or similar to Table 1 above can be made resilient to induced resonance if the strip seal  20  is made to conform to general length to thickness ratios. 
     An exemplary embodiment provides a strip seal  20  for a gas turbine  2 . The strip seal  20  is resilient to induced resonance when exposed to a pressure pulsation frequency of between about 3000-6000 Hz. The strip seal  20  has a first ratio of the length  21   a  of the strip seal  20  extending free of clamping projections  27  from any one of the ends  24  of the strip seal  20  to a clamping projection  27 , to the thickness  23 , of less than twenty five, and a second ratio of the length  21   b  of the strip seal  20  extending free of clamping projections  27  between any two clamping projections  27 , to the thickness  23 , of less than 200. 
     Very thin seals lack durability and potentially do not have sufficient rigidity for projections  27  to provide an adequate localised claiming function. For gas turbine service, therefore, strip seals can be at least 0.4 mm thick although they can be as thin as 0.2 mm thick. An exemplary embodiment provides a strip seal  20  with a thickness  23  of between 0.2 mm to 0.8 mm, within a tolerance of approximately +/−0.1 mm. Another exemplary embodiment provides a strip seal  20  with a thickness of between 0.3 mm to 0.5 mm, within a tolerance of approximately +/−0.1 mm. A thicker seal is more ridged, and so the advantages that the projections  27  impart is reduced. Therefore projections in another exemplary embodiment are applied to seals thicker than 0.8 mm, within a tolerance of approximately +/−0.1 mm, however with reducing benefit. 
     In a more specific example of this exemplary embodiment, the strip seal  20  has any of these stated preferred thicknesses  23  at points of the strip seal  20  free of clamping projections  27  and a second ratio of between 72 and 92. 
     A yet further specific example of this exemplary embodiment provides a strip seal  20  with any of the stated preferred thicknesses  23  at points of the strip seal  20  free of clamping projections  27  and a second ratio between 150 and 170. 
     As shown in  FIG. 5 , another exemplary embodiment provides a method for arranging clamping projections  27  on a strip seal  20  so as to ensure the induced resonance resilience of the strip seal  20 . In a first step the operational pressure frequency caused by rotating blades  7  is calculated or measured using known calculation methods and techniques. The calculation can be based on rotor frequency, typically 50 Hz or 60 Hz, and the number of blades, which can be about 100 per row. Multiplying the two values together can yield an estimated periodic frequency of between 3000 and 6000 Hz, for example. 
     The next step involves clamping a strip seal  20  with clamping projections  27  at the clamping projections  27 . The strip seal  20  is then subjected to the frequency estimated in the first step. Its excitation response is then measured by means of an accelerometer or the like. The measurement of the excitation response is then used to assess the acceptability of the response assessed by the degree of induced resonance in the strip seal  20 . If the strip seal  20  is not excited by the induced frequency, the performance of the strip seal  20  is considered acceptable and the method is complete. Otherwise, further method steps are performed. 
     If the method is not completed following step  5 , the next step is to reconfigure the strip seal  20  so as to ensure acceptable performance of the strip seal  20 . This can be achieved by forming additional clamping projections  27  along the length  21  in the regions of the strip seal  20  in locations based on the findings of the previous step. 
     In another exemplary embodiment, reconfiguration can be achieved by reducing the number of clamping projections  27  by manufacturing a new strip seal  20  or else removing existing clamping projections  27 . Subsequently new clamping projections  27  may be formed in different locations. The end result may be a strip seal  20  with more, the same or less clamping projections  27 . 
     As the skilled person would appreciate, the preferred reconfiguration for a given application is dependent on many factors, not limited to seal resonance performance, wherein different circumstances can be optimally served by different strip seal  20  reconfiguration methods, and the various exemplary embodiments described herein can provide useful alternatives. 
     Another exemplary method provides a method that can be used in conjunction with strip seal  20  manufacture that ensures the strip seal  20  is resilient to induced resonance during exposure to operational pressure pulsing. The exemplary method comprises the steps shown in  FIG. 6 . First, the operational excitation frequency of each blade  7  is calculated or measured by known techniques. Next, using known properties of the strip seal  20 , one or more clamping projections  27  are arranged on the strip seal  20  in an arrangement, which can be confirmed by calculation, that minimizes induced resonance of the strip seal  20  when exposed to the estimated excitation frequency of the first step. 
     In a further exemplary embodiment, the known properties of the strip seal  20  used in the calculation can include, for example, the length  21 , the thickness  23 , the width  22 , and a material property of the strip seal  20  such as the dynamic modulus of elasticity. 
     While exemplary embodiments have been described with reference to gas turbines  2 , embodiments of the disclosure can be used in other applications where there is potential for premature failure due to induced resonance. 
     Further, although the present disclosure has been herein shown and described in what is conceived to be exemplary embodiments, it will be recognized by those skilled in the art that departures can be made within the scope of the disclosure, which is not to be limited to details described herein but is to be accorded the full scope of the appended claims so as to embrace any and all equivalent devices and apparatus. 
     It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein. 
     
       
         
           
               
             
               
                   
               
               
                 Reference Numbers 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                  2 
                 gas turbine 
               
               
                   
                  7 
                 blade 
               
               
                   
                 10 
                 vane 
               
               
                   
                 13 
                 component 
               
               
                   
                 16 
                 edge face 
               
               
                   
                 17 
                 grooved recess 
               
               
                   
                 18 
                 groove height 
               
               
                   
                 19 
                 sealing surface 
               
               
                   
                 20 
                 strip seal 
               
               
                   
                 21 
                 length 
               
               
                   
                 21a 
                 length from end to a projection 
               
               
                   
                 21b 
                 length from a projection to a projection 
               
               
                   
                 22 
                 width 
               
               
                   
                 23 
                 thickness 
               
               
                   
                 25 
                 sealing face 
               
               
                   
                 24 
                 end 
               
               
                   
                 26 
                 pressure face 
               
               
                   
                 27 
                 clamping projections 
               
               
                   
                 28 
                 first layer 
               
               
                   
                 29 
                 second layer 
               
               
                   
                 30 
                 stamped projections 
               
               
                   
                 31 
                 formed projections 
               
               
                   
                 I, II, III 
                 gas turbine regions