Abstract:
An impingement insert sleeve is provided that is adapted to be disposed in a coolant cavity defined through a stator vane. The insert has a generally open inlet end and first and second diametrically opposed, perforated side walls. A metering plate having at least one opening defined therethrough for coolant flow is mounted to the side walls to generally transverse a longitudinal axis of the insert, and is disposed downstream from said inlet end. The metering plate improves flow distribution while reducing ballooning stresses within the insert and allowing for a more flexible insert attachment.

Description:
FEDERAL RESEARCH STATEMENT 
     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 INVENTION 
     The present invention relates generally to cooling gas turbines, for example, for electrical power generation and, more particularly, to the provision of a metering plate in an impingement insert for metering flow into that impingement insert. 
     The traditional approach for cooling turbine blades and nozzles is to use high pressure cooling air extracted from a source, such as from the intermediate and last stages of the turbine compressor. A series of internal flow passages are typically used to achieve the desired mass flow objectives for cooling the turbine blades. External piping is generally used to supply air to the nozzles, with the air typically exiting into the hot gas stream of the turbine to provide air film cooling of the nozzle surface. 
     In advanced gas turbine designs, it was recognized that the temperature of the hot gas flowing past the turbine components could be higher than the melting temperature of the metal. It was therefore necessary to develop a cooling scheme that more assuredly protects the hot gas path components during operation. Steam has been demonstrated to be a preferred cooling media for cooling gas turbine nozzles (stator vanes), particularly for combined-cycle plants. See, for example, U.S. Pat. No. 5,253,976, the disclosure of which is incorporated herein by this reference. However, because steam has a higher heat capacity than the combustion gas, it is inefficient to allow the coolant steam to mix with the hot gas stream. Consequently, it is desirable to maintain cooling steam inside the hot gas path components in a closed circuit. Certain areas of the components of the hot gas path, however, cannot practically be cooled with steam in a closed circuit. For example, the relatively thin structure of the trailing edges of the nozzle vanes effectively precludes steam cooling of those edges. Therefore, air cooling may be provided in the trailing edges of nozzle vanes. For a complete description of the steam cooled nozzles with air cooling along the trailing edge, reference is made to U.S. Pat. No. 5,634,766, the disclosure of which is incorporated herein by reference. 
     In turbine nozzles there are typically impingement inserts disposed inside the nozzle cavities to augment heat transfer coefficients and, therefore, increase cooling of the airfoil walls. Metering plates may be used in an impingement cooled multiple cavity nozzle to balance the total cooling system overall flow distribution. The use of a metering plate in an impingement insert creates a flow disruption that causes reduced total pressure in the area just below the metering plate and therefore meters the flow into that particular impingement insert. A typical metering plate has one or more orifice holes to control the flow into the insert. Conventionally, the metering plate is placed on top of the insert inlet prior to or during assembly into the nozzle. Because of the reduced total pressure in the area below the metering plate, the metering plate reduces the pre-impingement pressure in the insert thereby reducing ballooning stresses in the insert. 
     SUMMARY OF INVENTION 
     The present invention provides a cooling system for cooling the hot gas components of a nozzle stage of a gas turbine, in which closed circuit steam or air cooling and/or open circuit air cooling systems may be employed. In the closed circuit system, a plurality of nozzle vane segments are provided, each of which comprises one or more nozzle vanes extending between inner and outer walls. The vanes have a plurality of cavities in communication with compartments in the outer and inner walls for flowing cooling media in a closed circuit for cooling the outer and inner walls and the vanes per se. This closed circuit cooling system is substantially structurally similar to the steam cooling system described and illustrated in the prior referenced U.S. Pat. No. 5,634,766, with certain exceptions as noted below. Thus, cooling media is provided to a plenum in the outer wall of the segment for distribution therein and passage through impingement openings in a plate for impingement cooling of the outer wall surface of the segment. The spent impingement cooling media flows into leading edge and aft cavities extending radially through the vane. Return intermediate cooling cavities extend radially and lie between the leading edge and aft cavities. A separate trailing edge cavity may also be provided. 
     The cooling media that flows through the leading edge and aft cavities flows into a plenum in the inner wall and through impingement openings in an impingement plate for impingement cooling of the inner wall of the segment. The spent impingement cooling media then flows through the intermediate return cavities for further cooling of the vane. 
     Impingement cooling is also provided in the leading and aft cavities of the nozzle vane, as well as in the intermediate, return cavities of the vane. More specifically, impingement inserts are disposed inside the nozzle cavities to augment heat transfer coefficients and, therefore, increase cooling of the airfoil walls. The inserts in the leading and aft cavities comprise sleeves having a collar at their inlet ends for connection with integrally cast flanges in the outer wall of the cavities and extend through the cavities spaced from the walls thereof. These inserts have impingement holes in opposition to the walls of the cavity whereby cooling media, e.g. steam, flowing into the inserts flows outwardly through the impingement holes for impingement cooling of the vane walls. Return or exit channels may be provided along the inserts for channeling the spent impingement cooling media. Similarly, inserts in the return intermediate cavities have impingement openings for flowing impingement cooling medium against the side walls of the vane. These inserts also may have return or exit channels for collecting the spent impingement cooling media and conducting it to the cooling media outlet. 
     Typically, nozzles do not have metering plates as a part of the impingement insert design. Of the known designs using inserted metering plates, the metering plate is placed on and connected to the top of the insert. As used herein, ‘top of the insert’ refers to the entrance end or inlet end of the insert with respect to the direction of coolant flow therethrough. Thus, intermediate inserts through which coolant flow flows radially outwardly would have a metering plate, if provided, disposed at a radially inner end thereof. 
     While metering plates are considered generally effective to balance cooling flow to different cavities of the nozzle as required and to reduce ballooning stresses on the insert, that is not to say that improvement thereof cannot be made. Indeed, in an embodiment of the invention, by relocating the metering plate from its conventional top end placement, significant improvements to the flow distribution and insert assembly can be achieved. 
     Thus, in an embodiment of the invention, an impingement insert metering plate is provided that improves the mechanical robustness of the insert assembly, improves the assembly of the insert to the nozzle and improves the manufacturing assembly of the insert. 
     Accordingly, the invention is embodied in an impingement insert sleeve for being disposed in a coolant cavity defined through a stator vane, the insert having a generally open inlet end and first and second diametrically opposed, perforated side walls. A metering plate having at least one opening defined therethrough for coolant flow is mounted to the side walls to generally transverse a longitudinal axis of the insert, downstream from said inlet end. 
     The invention is further embodied in a turbine vane segment, comprising inner and outer walls spaced from one another; a vane extending between the inner and outer walls and having leading and trailing edges, the vane including a plurality of discrete cavities between the leading and trailing edges and extending lengthwise of the vane for flowing a cooling medium; an insert sleeve within one the cavity and spaced from interior wall surfaces thereof, the insert sleeve having an inlet end through which cooling medium flows into the insert sleeve, the insert sleeve having a plurality of openings therethrough for flowing the cooling medium through the openings into the space between the sleeve and the interior wall surfaces for impingement against the interior wall surface of the vane; and a metering plate having at least one opening for cooling medium flow defined therethrough, the metering plate being mounted to the insert sleeve so as to substantially traverse a flow path defined therethrough, the metering plate being spaced from the inlet end of the insert sleeve. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic, cross-sectional view of an exemplary first stage nozzle vane; 
     FIG. 2 is a schematic cross sectional view showing an impingement insert disposed within a nozzle cavity and having a metering plate connected in a conventional manner to an inlet end thereof; 
     FIG. 3 is a schematic cross-sectional view illustrating a conventional end connection of the metering plate; 
     FIG. 4 is a schematic perspective view of the assembly of a recessed metering plate to an insert as an embodiment of the invention; 
     FIG. 5 is a schematic cross sectional view showing an impingement insert disposed within a nozzle cavity and having a recessed metering plate assembly embodying the invention connected thereto; and 
     FIG. 6 is a schematic cross-sectional view illustrating the insert/nozzle connection in an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     As discussed previously, the present invention relates in particular to cooling circuits for the first stage nozzles of a turbine, reference being made to the previously identified patents for disclosures of various other aspects of the turbine, its construction and methods of operation. 
     Referring now to FIG. 1, there is schematically illustrated in cross-section a vane  10  comprising one of the plurality of circumferentially arranged segments of the first stage nozzle. It will be appreciated that the segments are connected one to the other to form an annular array of segments defining the hot gas path through the first stage nozzle of the turbine. Each segment includes radially spaced outer and inner walls  12  and  14 , respectively, with one or more nozzle vanes  10  extending between the outer and inner walls. The segments are supported about the inner shell of the turbine (not shown) with adjoining segments being sealed one to the other. It will therefore be appreciated that the outer and inner walls and the vanes extending therebetween are wholly supported by the inner shell of the turbine and are removable with the inner shell halves of the turbine upon removal of the outer shell as set forth in U.S. Pat. No. 5,685,693. For purposes of this description, the vane  10  will be described as forming the sole vane of a segment. 
     As shown in a schematic illustration of FIG. 1, the vane has a leading edge  18 , a trailing edge  20 , an outer wall  12  and an inner wall  14 . The outer wall includes outer side rails  26 , a leading rail  28  and a trailing rail  30  that define a plenum  32  with outer cover plate  34 . An impingement plate  36  is disposed generally in parallel to the outer wall for impingement cooling of the outer wall. It is to be noted that the terms outwardly and inwardly or outer and inner as used herein refer to the generally radial direction. 
     In this example, the nozzle vane has a plurality of cavities, for example, a leading edge cavity  42 , aft cavities  52 ,  54  and a plurality of intermediate return cavities  44 ,  46 ,  48 ,  50 . Thus, the cooling medium, such as steam flows in through a steam inlet  22 , through impingement plate  36  to impingement cool the outer wall  12  and then flows radially inwardly through, e.g., the leading edge cavity  42  and aft cavities  52 ,  54 . The post impingement cooling media flows into a plenum  73  defined by the inner wall  14  and a lower cover plate  76 . Radially inwardly of the inner wall is an impingement plate  74  (FIGS.  2 - 3 ). As a consequence, it will be appreciated that spent impingement cooling steam flows through the impingement openings of the impingement plate  74  for impingement cooling of the inner wall  14 . The spent cooling medium then flows towards the openings of the intermediate cavities for return flow to a steam outlet  24 . 
     In FIGS. 2 and 3, a single insert disposed in a single cavity is illustrated. By way of non-limiting example, insert  64  disposed in cavity  44  is schematically shown. In a conventional manner, the insert sleeve is disposed in the cavity in spaced relation from the side walls  38 ,  40  and partition wall(s)  72  defining the respective cavity. The impingement openings  78  lie on opposite sides of the sleeves for flowing the cooling medium, e.g., steam, from within the insert sleeve through the impingement openings for impingement cooling of the side walls of the vane, generally as discussed above. The spent cooling steam then flows through the gaps between the insert sleeve and the walls of the cavity to the outlet  24  for return to the coolant supply. 
     As illustrated in FIGS. 2 and 3, to secure the impingement insert in the nozzle cavity, an insert collar  80  is conventionally provided peripherally of the opening at the inlet end  82  of the impingement insert at the interface of the impingement insert and the flash rib  84  of the nozzle airfoil wall. The insert collar  80  is secured to the flash rib  84  by a braised or welded connection. As mentioned above, typically nozzles do not have a metering plate as a part of the impingement insert design. Of the known designs using insert metering plates, the metering plate  86  is provided on the inlet end  82  of the insert. This is either done when the insert  64  is assembled, or as a part of the nozzle to insert assembly. Thus, the metering plate  86  is conventionally secured to the insert collar  80  at the inlet end  82  of the impingement insert  64 . 
     An embodiment of the invention is illustrated in FIGS. 4,  5  and  6 . To facilitate an understanding of the illustrated assembly, reference numbers generally corresponding to those used above and in FIGS. 1-3 are used in FIGS. 4-6, but incremented by 100. In the embodiment illustrated in FIGS. 4-6, the metering plate  186  is placed below, that is downstream of the impingement insert inlet end  182 . More specifically, lower than or downstream from the first row  188  of impingement insert cooling holes  178 . Thus, the metering plate  186  is recessed at least to below the first row of impingement insert cooling holes  188 . At least one, and generally a plurality of openings  190  for cooling media flow are defined through the metering plate  186 . 
     The use of a recessed metering plate  186  as illustrated by way of example in FIGS. 4-6 reduces the possibility of restricting flow to the metering plate due to interference with additional nozzle parts such as the inner wall impingement plate  174 . Furthermore, flow distribution is improved because the variability in flow between the impingement plate  174  above the insert and the insert itself would be significantly reduced by the elimination of the metering plate placement on top of the insert  164 . 
     Placing the metering plate as illustrated in FIGS. 4-6, spaced from the insert inlet  182 , has the advantage of reducing ballooning stresses (internal pressure) within the insert. Indeed, due to the mechanical strength of the metering plate  186  and also due to the reduced pre-impingement pressure caused by the total pressure loss across the metering plate, the ballooning of the insert walls is reduced. 
     In addition, the connection at assembly of the insert  164  to the metering plate  186  forms a more significant structure to hold the form of the insert  164  during subsequent handling and assembly. This improves the profile of the insert thereby improving the cooling flow to the nozzle. 
     The placement of the metering plate further away from the impingement insert inlet  182  also allows for a more flexible insert attachment at that inlet. This is a very significant improvement to the manufacturing assembly. The conventional relatively rigid inlet end of the insert, due to the presence of the insert collar  80  requires very precise machining to enable a good connection joint (braise or weld) to the flash rib  84 . The more flexible inlet end  182  enabled by the invention, with the spacing of the metering plate from the inlet end, and the consequent elimination of the insert collar  82  at the inlet end is an important factor in improving the connection as the insert can, to a varying degree, be formed to the opening. 
     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.