Abstract:
The invention relates to a turbine in which a bypass-passage extends through a base member of a stationary vane to join seal cavities of adjacent rotating blade rows so that seal flow passing between a casing and shrouds of the rotating blades at least partially bypasses the turbine main flow passage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European Patent Application 14189908.8 filed Oct. 22, 2014, the contents of which are hereby incorporated in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to multi stage turbines, including steam turbines and gas turbines and more specifically to means to reduce efficiency loss caused by leakage flow through seals of shrouded rotating blades. 
       BACKGROUND INFORMATION 
       [0003]    An axial flow turbine, for example a steam turbine, comprises a casing and a rotor which is rotatably supported within the casing. The rotor comprises a shaft and a plurality of rotor blade rings which are attached behind one another to the shaft. During operation of the turbine working fluid is expanded progressively by the blade rings to bring about driving the shaft. 
         [0004]    Each rotor blade ring is formed by a plurality of rotor blades being circumferentially arranged, wherein two adjacent rotor blades form a blade passage. The rotor blades are aerodynamically profiled such that, when the working fluid passes the blade passages, the flow is turned and thereby a circumferential force on the rotor blades is generated. The circumferential forces on each blade of the rotor blade ring effect turning the rotor thereby generating shaft power. 
         [0005]    The rotor blades are fixed to the shaft and extend therefrom towards the casing. The lateral ends of the rotor blades at the casing are formed into blade tips, wherein at the blade tips the rotor blade ring is shrouded by a shroud. The shroud is fixed to the blade tips and spaced apart from the casing thereby forming a tip clearance. The height of the tip clearance is dimensioned such that during operation of the turbine it is prevented that the shroud scrubs at the casing. Due to the fact that static pressure of the flow upstream of the rotor blade ring is higher than static pressure of the flow downstream of the rotor blade ring, during operation of the turbine a leakage flow passes the tip clearance. 
         [0006]    The main flow passes the blade passages for shaft power generation, whereas the leakage flow bypasses the rotor blade ring via the tip clearance. Therefore, the leakage flow does not participate to the shaft power generation and does not flow through the blade passage. Further, the leakage flow after being re-entrained into the main flow path interferes with the main flow. Therefore, the main flow is locally inhomogeneous resulting in a mismatched flow. Furthermore, the tip clearance flow mixes with the main flow and generates disadvantageous dissipation. As consequence of this, the presence of the tip clearance flow affects the turbine efficiency. 
         [0007]    In particular in high pressure turbines with low aspect ratio blades, the loss caused by the tip clearance flow is significantly high compared with the total losses of the turbine. 
         [0008]    A remedy to reduce this negative effect of the tip clearance flow on the aerodynamic efficiency of the turbine is to take measurements reducing the tip clearance flow. A measurement, for example, is to provide a labyrinth seal on the outer circumference of the shroud within the tip clearance in order to reduce the mass flow of the tip clearance flow. As an alternative, a sealing element is fixed at the casing in the tip clearance. For fixing the sealing element to the casing, in the casing a circumferential groove is provided into which the sealing element is mortised. 
         [0009]    Each of the solutions reduces tip leakage but does not eliminate the flow. As a result there is a continuing need to address turbine efficiency losses resulting from blade tip leakage. 
       SUMMARY 
       [0010]    A turbine is disclosed that is configured to address the problem of rotating blade leakage flow reducing turbine efficiency by creating turbulence in the main working fluid flow passage. 
         [0011]    It attempts to addresses this problem by means of the subject matter of the independent claim. Advantageous embodiments are given in the dependent claims. 
         [0012]    The disclosure is based on the general idea of providing a bypass around the stationary vanes in order to at least reduce the re-entry flow of the leakage fluid passing between shrouded rotating blade tips and the casing. 
         [0013]    One general aspect includes a turbine comprising a rotor with a rotational axis, a casing enclosing a rotor to form a flow passage therebetween having first and second sealing means. The turbine also includes a first rotating blade row in the flow passage having a plurality of circumferentially distributed first blades each with a first root connected to the rotor and a first shroud adjacent the first sealing means. The turbine additionally has a stationary vane row, each that with vane airfoil that extends into the flow passage. The stationary vane row is axially adjacent and downstream of the first rotating blade row having a plurality of circumferential distributed stationary vanes. Each of the stationary vanes has a base member connected to the casing. A second rotating blade row is located in the flow passage axially adjacent and downstream of the stationary vane row. This second rotating blade row has a plurality of circumferentially distributed second rotating blades each with a second root connected to the rotor and a second shroud adjacent the second sealing means. A first cavity is formed by the first shroud, the first sealing means and the base member while a second cavity is formed by the second shroud, base member and the second sealing means. 
         [0014]    Further aspects may include one or more of the following features. The turbine wherein each stationary vane has a leading edge wherein the first end of the bypass-passage is located at a point of the first cavity circumferentially between the leading edges of two circumferentially adjacent stationary vanes. The turbine wherein the bypass-passage is radially displaced from the rotor rotational axis. The turbine wherein the bypass-passage is parallel to the rotor rotational axis. The turbine wherein the bypass-passage is angled from the rotational axis in a direction to the normal operating rotation of the rotor of between −30 degrees and 30 degrees, preferably between 0 degrees and 10 degrees. The turbine wherein the bypass-passage has a uniform cross sectional area along its length. The turbine configured as a gas turbine, or impulse type steam turbine. The turbine wherein the base member is a steam turbine diaphragm. 
         [0015]    In a general aspect the turbine of claim comprising a plurality of bypass-passages. 
         [0016]    Other aspects and advantages of the present disclosure will become apparent from the following description, taken in connection with the accompanying drawings which by way of example illustrate exemplary embodiments of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    By way of example, an embodiment of the present disclosure is described more fully hereinafter with reference to the accompanying drawings, in which: 
           [0018]      FIG. 1  is a perspective view of a turbine section with a bypass-passage according to an exemplary preferred embodiment of the disclosure; 
           [0019]      FIG. 2  is a top sectional view of the turbine section of  FIG. 1 ; 
           [0020]      FIG. 3  is a sectional view of steam turbine with a diaphragm and bypass-passage of an exemplary embodiment around the diaphragm; and 
           [0021]      FIG. 4  is an expanded view of the base member of  FIG. 1  showing a bypass-passage of non-uniform cross sectional area. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Exemplary embodiments of the present disclosure are now described with references 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 to provide a thorough understanding of the disclosure. However, the present disclosure may be practiced without these specific details, and is not limited to the exemplary embodiments disclosed herein. 
         [0023]    Pitch is the distance in the direction of rotation between corresponding points on adjacent blades. In this description, the points correspond to the leading edge of circumferentially adjacent stationary blades wherein 0% pitch corresponds to the leading edge of the upstream blade as taken from the circumferential direction of rotation of the rotating blades of the turbine and 100% pitch corresponds to the leading edge of the downstream blade as taken from the circumferential direction of rotation of the rotating blades of the turbine. 
         [0024]    An exemplary embodiment of a turbine shown in  FIG. 1  includes a rotor  10  and a casing  15  enclosing the rotor  10  so as to form a flow passage  19  therebetween. A plurality airfoils  20   a , 30   a  of circumferentially distributed rotating blades  20  and stationary vanes  30  are located in the flow passage  19 . The rotating blades  20  and stationary vanes  30  are arranged such that there is an upstream row of rotating blades  20  adjacent a downstream row of stationary vanes  30  which are in turn adjacent a further row of rotating blades  21 . The number of rotating blades  20  and stationary vanes  30  shown in  FIG. 1  is only limited in order to explain an exemplary embodiment and therefore is not a limiting example of a turbine to which exemplary embodiments of this disclosure can be applied. 
         [0025]    The turbine includes sealing means  16 ,  17  that provide a seal between the stationary casing  15  and the shrouds  22 ,  23  of the rotating blades  20 ,  21 . Depending on the configuration of the turbine, the sealing means  16 ,  17  could be mounted on the casing  15 , as shown in  FIG. 2 , or else mounted on an extension ring  18   a,    18   b  such that each of the seal means  16 ,  17  are in a first cavity  40  and a second cavity  42  respectively that are both located outside the flow path  19 . In an exemplary embodiment shown in  FIG. 3  an extension ring  18   a,    18   b  is mounted to a downstream base member  32 . In an exemplary embodiment shown in  FIG. 3  an extension ring  18   a  is mounted to the base member  32  and an extension ring  18   b  is mounted to a downstream base member  32 . In a not shown exemplary embodiment an extension ring  18   a  is mounted to an upstream base member  32 . 
         [0026]    Each of the rotating blades  20 ,  21  includes a blade root  24  that fixes the rotating blade  20 ,  21  to the rotor  10 . At a distal end of each rotating blade  20 ,  21 , that is, at an end nearest the casing  15 , the rotating blades  20 ,  21  have a shroud  22 ,  23 . The shroud  22 ,  23  is configured such that there is a leakage flow of working fluid that passes between the shroud  22 ,  23  and the casing  15 . A sealing means, typically located between the casing  15  and the shroud  22 ,  23 , limits the leakage flow. 
         [0027]    The stationary vanes  30 , located between the rows of rotating blades  20 ,  21 , each have a base member  32  that supports or connects the stationary vane  30  to the casing  15 . The form of the base member  32  is dependent on the configuration of turbine. For example, in an exemplary embodiment applied to an impulse type steam turbine, the base member  32  is a diaphragm  32  configured as a ring to support the stationary vanes  30  of the stationary vane row. In another not shown exemplary embodiment, the base member  32  is a vane root  32  connecting each stationary vane  30  to the casing  15 . In another not shown exemplary embodiment, the base member is a combination of the casing  15  and a vane attachment means. 
         [0028]    The first cavity  40  is formed by the first shroud  22 , the first sealing means  16  and the base member  32  while the second cavity  42  is formed by the second shroud  23 , base member  32  and the second sealing means  17 . 
         [0029]    An exemplary embodiment shown in  FIG. 1  further includes a bypass-passage  44  that extends from a first end at the first cavity  40  through the base member  32  to a second end at the second cavity  42  wherein both the first end and the second are located outside of the flow passage  19 . The purpose of the bypass-passage  44  is to direct leakage flow flowing over the shroud  22  of the upstream rotating blades  20  to the downstream row of rotating blades  21  by bypassing the flow passage  19  all together and thus bypass the airfoil  30   a  of the vane  30 . As little or no leakage fluid from the first cavity  40  returns to the flow passage  19  a source of turbulence in the flow passage  19 , and thus efficiency lost, is thus eliminated or at least reduced. 
         [0030]    In an exemplary embodiment the bypass-passage  44  has a first end located at a point of the first cavity  40  circumferentially between the leading edges  34  of two circumferentially adjacent stationary vanes  30 . In this exemplary embodiment circumferential between includes a point axially and/or radially displaced from a point on a line projected between leading edges  34  of two circumferentially adjacent stationary vanes  30 . That is, the first end of the bypass-passage  44  may be at any point in the first cavity upstream of the projected line. 
         [0031]    The configuration of the bypass-passage  44  is dependent on the type of turbine and whether or not the bypass-passage  44  is retrofitted to the turbine or else configured as part of the original design. As such it may be straight or else include at least one non-linear section, such as a curve or corner. 
         [0032]    In an exemplary embodiment shown in  FIG. 3  the turbine is an impulse type steam turbine with a diaphragm  32  configured as a ring to encircle and support stationary vanes  30  of the stationary blade row. In this exemplary embodiment, the bypass-passage  44  is formed through the diaphragm  32 . 
         [0033]    In an exemplary embodiment shown in  FIG. 4 , the bypass-passage  44  has different cross sectional areas along its length. In a first portion the bypass-passage  44  has a larger cross-sectional area while at an end region the bypass-passage  44  has a reduced cross-sectional area. This exemplary embodiment may be applicable for retrofits where it may be easier to drill long passages with a larger drill bit. This is enabled by the presence of a smaller pilot hole formed by the smaller cross-sectional area of the bypass-passage  44  that defines the flow capacity of the bypass-passage  44 . 
         [0034]    In an exemplary embodiment shown in  FIG. 1  the flow passage  19  is skewed from the rotational axis  12  to preferably follow an expansion of the flow passage  19 . In a not shown exemplary embodiment the flow passage  19  is parallel to the rotational axis  12 . 
         [0035]    In an exemplary embodiment shown in  FIG. 2  the bypass-passage  44  forms an angle  46  with the rotational axis  12  that angles the bypass-passage  44  in the direction of rotational direction  14  of the rotating blades  20 . In an exemplary embodiment shown in  FIG. 2  the first end of the bypass-passage  44  is located along a pitch of the stationary vanes  30 . 
         [0036]    In an exemplary embodiment shown in  FIG. 3  the turbine is an impulse type steam turbine with diaphragm  32  configured as a ring to support stationary vanes  30  of the a stationary blade row. In this exemplary embodiment, the bypass-passage  44  is formed around the diaphragm  32 . When this exemplary embodiment is retrofitted to a steam turbine it may be necessary to ensure that steam does not further bypass the sealing means. Where the sealing means includes extension rings  18   a,    18   b  each of which is itself mounted on the diaphragm  32  of this row of stationary vanes  30  or an adjacent row, additional casing seals  48  spanning between either or both of the extension rings  18   a,    18   b  and the casing  15  may be required. 
         [0037]    Although the disclosure has been herein shown and described in what is conceived to be the most practical exemplary embodiment, it can be embodied in other specific forms. For example, exemplary embodiments may equally be applied to gas turbines and all types of steam turbines including high pressure steam turbines, intermediate pressure steam turbines, reaction bladed steam turbines and impulse bladed steam turbines. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the disclosure is indicated by the appended claims rather that the foregoing description and all changes that come within the meaning and range and equivalences thereof are intended to be embraced therein.