Abstract:
A steam turbine with an operating temperature of more than 650° C. includes a thermally highly-loaded housing (G) which is produced by a casting technique at least partially from a nickel-based alloy. Production of such a steam turbine is simplified by the housing (G) being divided into a multiplicity of smaller housing sections (G 1 , . . . , G 8 ; G 21 , G 22 , G 31 , G 32 ) which are interconnected and together form the housing (G).

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to Swiss application no. 01720/08, filed 31 Oct. 2008, the entirety of which is incorporated by reference herein. 
       BACKGROUND 
       [0002]    1. Field of Endeavor 
         [0003]    The present invention relates to the field of steam turbines, and more particularly to a steam turbine with an operating temperature of more than 650° C. 
         [0004]    2. Brief Description of the Related Art 
         [0005]    Large steam power plants in the power range of about 1000 MW have steam turbine generator units which are divided into high-pressure turbines, medium-pressure turbines, and low-pressure turbines (see, for example, the article by L. Busse et al., “World&#39;s highest capacity steam turbosets for the lignite-fired Lippendorf power station”, ABB Review 6/1997, pages 13-22 (1997)).  FIG. 4  in that article shows the tangential feed of the reheated steam to the medium-pressure steam turbine from two opposite sides via corresponding valve units which each include a stop valve and an intercept valve. 
         [0006]    Currently, a medium-pressure steam turbine of this order of magnitude has a valve unit on each side of the turbine, as is reproduced in  FIG. 1  of the present application. The medium-pressure steam turbine  10  of  FIG. 1 , which is shown in the direction of view parallel to the axis  16  of the machine, has an outer housing  11  and an inner housing  12  which are both formed concentrically to the axis  16 . The inner housing  12  encloses the (non-visible) rotatably mounted rotor with the associated blading. The reheated steam flows from two opposite sides via steam inlets  13 ,  14  tangentially into a spirally-formed intake region into the housing  12 , is deflected there into an axial direction, impinges upon the blading of the rotor, and discharges again through the steam exhaust  15 . The inflow of steam, which is carried out via two steam feed lines  17 ,  18 , is influenced by two valve units VE 1  and VE 2 . Comparable valve units are known, for example, from publication WO-A1-03/093653. 
         [0007]    The valve units VE 1  and VE 2  from  FIG. 1  include a stop-valve section V 11  or V 21  and a control-valve section V 12  or V 22  which can be operated in each case by (hydraulic) drives. The valves V 11 , . . . , V 22  have an approximately spherical shape inside the valve unit, as is also to be seen in FIG. 2 of WO-A1-03/093653. For large steam power plants (approx. 1100 MW), the spherical stop-valve section V 11  or V 21  typically weighs 39 tons (with a diameter of 1100 mm), and the spherical control-valve section V 12  or V 22  typically weighs 28 tons (with a diameter of 815 mm). If the associated housing is produced in a conventional manner as a one-piece cast part, this has a weight of &gt;60 tons. 
         [0008]    For steam turbines with an operating temperature of &gt;650° C., the housing of the valve unit, which is exposed to the full steam pressure, has to be produced from a nickel-based alloy. This, however, has the following disadvantages:
       The costs per kilogram of the material are very high.   It is basically difficult in the case of nickel-based alloys to machine the inner surfaces of the workpieces. This especially applies in the case of very large workpieces.   It is a problem to find manufacturers for the casting of such large workpieces from nickel-based alloys.   Owing to the sizes of the cast pieces, segregation of individual elements occurs during the casting, which leads to flawed areas with uneven composition in the workpiece. The degree of segregation in this case depends upon the size of the workpiece.       
 
       SUMMARY 
       [0013]    One of numerous aspects of the present invention relates to a steam turbine of the aforementioned type, which can avoid the disadvantages of conventional solutions and is characterized especially by a reduction of the size of the components which are to be cast from a nickel-based alloy. 
         [0014]    Another aspect is directed towards dividing large cast pieces formed of one material into a plurality of small cast pieces of the same material which are produced separately and then interconnected in a materially-bonding manner. An important point relates to the division of a required large component of a nickel-based alloy into a plurality of smaller sub-components which are produced as separate cast parts and then interconnected. This procedure is particularly important and advantageous for very large turbines with an output power of for example &gt;500 MW. The proposed procedure (more sub-components, although each smaller in dimensions or in weight) can be used in the case of steam turbines within the scope of the invention not only on the valve units, which are arranged on a steam turbine, and their housings, but also on the inner housing of the steam turbine itself. 
         [0015]    According to another aspect, at least some of the housing sections are welded to each other for forming the housing. For nickel-based alloys, only a slight reduction of the creep limit results on the weld seam compared with steel when loaded by a high internal pressure, so that a large mechanical strength of the welded body is ensured. In this case, the housing sections are preferably welded to each other in planes which are oriented perpendicularly and/or parallel to the direction of flow of the steam. 
         [0016]    Another aspect includes that the steam turbine is a medium-pressure steam turbine, and in that the housing which comprises a multiplicity of smaller housing sections is the inner housing of the medium-pressure steam turbine. In particular, the inner housing is divided into at least two shell sections along a parting plane which lies parallel to the axis of the steam turbine. For mechanical reinforcement of the inner housing, in this case provision can additionally be made for reinforcing elements, especially shrink-rings which are arranged in a distributed manner in the axial direction and tightly encompass the inner housing. 
         [0017]    A further aspect includes that the steam turbine is a medium-pressure steam turbine. For controlling the steam feed to the medium-pressure steam turbine, at least one valve unit is arranged at a steam inlet of the medium-pressure steam turbine, and the housing, which is divided into a multiplicity of smaller housing sections, is the housing of the at least one valve unit. 
         [0018]    Two steam inlets, which are opposite each other, are preferably provided on the medium-pressure steam turbine, wherein a valve unit is associated with each of the steam inlets, and wherein the housings of the two valve units are divided in each case into a multiplicity of smaller housing sections. 
         [0019]    According to a further aspect, each valve unit comprises a first valve and a second valve which are connected in series in the direction of flow and accommodated together in the housing, wherein the first valve is formed as a stop valve and the second valve is formed as a control valve. 
         [0020]    In particular, the housing of each valve unit has in each case an essentially spherical housing section for the first and second valve, wherein the spherical housing sections are divided in each case into a number of shell sections, which are interconnected in each case by at least one weld seam which extends in the direction of flow. 
         [0021]    Furthermore, the housing of each valve unit has a steam feed line which leads to the first valve, and the steam feed line is formed in each case as a one-piece housing section and is connected to the housing section of the first valve by a circumferential weld seam. 
         [0022]    Furthermore, the housing of each valve unit has a connecting port which projects from the second valve, and each connecting port is formed as a one-piece housing section and is connected to the housing section of the second valve by a circumferential weld seam. 
         [0023]    For mechanical reinforcing of the housing, in order to achieve high mechanical strength despite the reduction in material, provision can be made for reinforcing elements, especially in the form of shrink-rings which tightly encompass the housing at different points, especially in the region of the spherical housing sections. 
         [0024]    For reducing the portion of nickel-based alloy it is also conceivable, however, according to another aspect of the invention, to form the housing of each valve unit as an inner section of a double-walled housing construction, wherein the inner section is formed of a nickel-based alloy and the outer section is formed of a steel. 
         [0025]    A further aspect includes that the steam turbine is a medium-pressure steam turbine, for controlling the steam feed to the medium-pressure steam turbine, at least one valve unit is arranged on the medium-pressure steam turbine, and the valve unit comprises at least two parallel-operating valves of the same type. By the division of a valve into two parallel valves, the valves can be of smaller design, which leads to a corresponding reduction in the size of the associated cast sections. The at least two valves are preferably formed as control valves to which the steam is fed separately and downstream of which the steam is merged. 
         [0026]    The at least two valves can also be formed, however, as stop valves, to which the steam is fed separately, wherein a third valve, which is formed as a control valve, is arranged downstream of the two stop valves in the direction of flow and at which the steam from the two stop valves is merged. 
         [0027]    The two stop valves are preferably connected to the one control valve via connecting lines, and the control valve is connected to the steam turbine via a connecting port, and the connecting lines are oriented perpendicularly to the connecting port. 
         [0028]    Alternatively, the connecting lines together with the connecting port can also form, however, a configuration in the shape of a “Y”. 
         [0029]    Another alternative is that the connecting lines together with the connecting port form a fork-shaped configuration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The invention is subsequently to be explained in more detail based on exemplary embodiments in conjunction with the drawing. In the drawings, 
           [0031]      FIG. 1  shows, in a partial sectional view transverse to the machine axis, a large medium-pressure steam turbine with steam feed on two sides and associated valve units for controlling the steam feed, as is suitable for realization of the invention; 
           [0032]      FIG. 2  shows, in a perspective view, the housing, which is divided into sub-elements, of a valve unit from  FIG. 1  according to one exemplary embodiment of the invention; 
           [0033]      FIG. 3  shows, in a perspective view, the housing, which is divided into sub-elements and reinforced with additional shrink-rings, of a valve unit from  FIG. 1  according to another exemplary embodiment of the invention; 
           [0034]      FIG. 4  shows, in a greatly simplified view, the division of the stop valve of a valve unit according to  FIG. 1  into two smaller, parallel-operating stop valves according to a further exemplary embodiment of the invention; 
           [0035]      FIG. 5  shows, in a view which is comparable to that of  FIG. 4 , a further exemplary embodiment of the invention, in which the steam lines or steam pipes are arranged in a “Y” configuration; 
           [0036]      FIG. 6  shows in a view which is comparable to that of  FIG. 4 , a further exemplary embodiment of the invention, in which the steam lines or steam pipes are arranged in a fork-shaped configuration; and 
           [0037]      FIG. 7  shows, in a perspective view, the inner housing, which is divided into sub-elements, of a steam turbine according to  FIG. 1  according to an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0038]    In  FIG. 2 , the housing G, which is divided into sub-elements, of a valve unit (VE 1 ) from  FIG. 1  according to an exemplary embodiment of the invention, is shown in a perspective view. The housing G in the example is divided into four housing sections G 1 , G 2 , G 3  and G 4  in the direction of flow of the steam which enters through a steam feed line  17  and exits through a connecting port  19 . The housing section G 1  is the connecting port  19  with a connecting flange  19 ′. The housing section G 4  is the steam feed line  17 . The two housing sections G 2  and G 3  are essentially spherically formed and are part of the two valves V 11  and V 12  ( FIG. 1 ), specifically a stop valve and a control valve. The two housing sections G 2  and G 3  in their turn include two half-shells, specifically the upper half-shell G 22  or G 32  and the lower half-shell G 21  or G 31 . 
         [0039]    So that the valve unit VE 1  withstands the internal pressures of the through-flowing steam, the half-shells G 21 , G 22  and G 31 , G 32  are interconnected by (longitudinal) weld seams S 2  and S 4  which extend parallel to the direction of flow. The housing sections G 1 , . . . , G 4  in their turn are connected by weld seams S 1 , S 3  and S 5  which extend in the circumferential direction. 
         [0040]    In order to mechanically reinforce the longitudinally welded, spherical housing sections G 2  and G 3 , outer reinforcing elements, especially shrink-rings  20 ,  21  according to FIG.  3 , can be additionally provided or shrunk onto the pipe connectors of these housing sections. Axially screwed flanges, however, may also be used alternatively or additionally to the shrink rings  20 ,  21  for reinforcing. Although in  FIG. 3  only 1 shrink-ring  20  or  21  is shrunk on the pipe connecter for the valve drive in each case, further shrink-rings can be provided between the two spherical housing sections G 2  and G 3  of the valves and between the housing section G 2  and the connecting flange  19 ′. These further shrink-rings have to be moved in the axial direction in order to enable the longitudinal welding (weld seams S 2 , S 4 ), and are then heated and shifted onto their final position. 
         [0041]    It would be advantageous if the shrink-rings  20 ,  21  were to have a thermal expansion coefficient which is larger in comparison to the housing G (for example rings formed of austenitic steel for a housing of a nickel-based alloy). Alternatively to this, the rings could also be assembled from ring segments, however, which can be installed one after the other, as is described and disclosed, for example, in publication DE-A1-197 58 160. 
         [0042]    In order to even further reduce the weight of the individual cast pieces or housing sections of nickel-based alloy, it is advantageous to produce colder parts of these sections from a cheaper steel with expansion coefficients which are similar to the nickel-based alloy, such as from a 1-2% CrMoV cast steel. The similar thermal expansion coefficients ensure that there are fewer stresses in the component during welding and during operation. 
         [0043]    Further improvements can be achieved if the valve units are formed with double walls. In this case, only the inner section is constructed from a nickel-based alloy with longitudinal weld seams and if necessary is additionally reinforced with shrink-rings. The outer section is produced from steel and if necessary is reinforced by flanges and/or longitudinal weld seams and/or shrink-rings. In the gap between the walls a medium would then circulate in order to limit the high temperatures to the inner section and to reduce the thermal conduction to the outer section. The heat which is absorbed by this medium could then be used for improving the efficiency of the steam-cyclic process. 
         [0044]    The gap between the inner and outer sections can especially be filled with cooling steam, wherein the pressure of the cooling steam can be greater than or less than the steam pressure inside the inner section. Another variant empties the gap, for example, by connecting the outer section to the condenser via a pipeline, as a result of which a vacuum is almost created within the gap. This vacuum acts as thermal insulation between the inner and outer sections and leads to a lower temperature in the outer section. This insulating effect can be augmented by the inner wall of the outer section being provided with a highly reflective surface, for example, by a coating, since the heat transfer as a result of radiation is consequently reduced. 
         [0045]    A reduction of the individual sub-components can also be achieved, however, by a “functional” breaking-down by, for example, two or more parallel-operating smaller valves with smaller spherical housing sections being used instead of a larger valve with a large spherical housing section in a valve unit of a steam turbine. The steam for the steam turbine would then be fed separately to the two or more smaller valves and merged downstream of the valves (for example by a “Y”-shaped pipe section) and fed to the steam turbine at one point. Such valve distributions can be undertaken on the two sides of the steam turbine if the feed is carried out on opposite sides. 
         [0046]    Also, in the case of a configuration of the valve units according to  FIGS. 1-3 , such a “functional” distribution or breaking-down can be advantageously used. Since the spherical housing section G 3  of the stop valve V 11  or V 21  is appreciably larger than the spherical housing section G 2  of the control valve V 12  or V 22 , it is advantageous to replace the stop valve in the two valve units VE 1  and VE 2  with two or more smaller, parallel-operating stop valves, the steam of which is then merged in the (single) control valve which is located downstream and from there is fed to the steam turbine. 
         [0047]    In  FIGS. 4 ,  5 , and  6 , three different possibilities of the configuration are represented for this distribution: In  FIG. 4 , the valve unit VE 3  includes a control valve V 1  with the drive A 1 , and two stop valves V 2  and V 3  with the drives A 2  and A 3 . The steam is fed to the stop valves V 2 , V 3  by steam feed lines  22 ,  23  which come from the bottom perpendicularly to the plane of the drawing. The stop valves V 2 , V 3  are connected to the (single) control valve V 1  via two connecting lines  24 ,  25 . The control valve V 1  feeds the merged steam via a connecting port  19  with connecting flange  19 ′ to the (not shown) steam turbine. The connecting lines  24  and  25  in this case are oriented perpendicularly to the connecting port  19 . 
         [0048]    In an alternative configuration ( FIG. 5 ), the connecting lines  24 ′ and  25 ′ and the connecting port  19  of the valve unit VE 4  form a configuration in the shape of a “Y”. In a further alternative configuration ( FIG. 6 ), the connecting lines  24 ″ and  25 ″ and the connecting port  19  of the valve unit VE 5  form a fork-shaped configuration. In all three cases, “Y”-shaped pipe sections can be dispensed with on account of the merging of the steam flows in the control valve V 1 . 
         [0049]    If one of the principles of the invention is applied to the inner housing  12  of the steam turbine  10  ( FIG. 1 ), according to  FIG. 7  the shell sections TS 1 , TS 2  of the inner housing  12 , which is split in a first parting plane T 1  on account of accessibility, can be additionally split (housing sections G 7 , G 8  in  FIG. 7 ) in the axial direction and/or in the circumferential direction, for example along a parting plane T 2  which extends perpendicularly to the axis, in order to reduce size and weight of the individual cast pieces. An increased mechanical strength can also be achieved in this case with additional shrink-rings which encompass the inner housing  12  on the outside. 
       LIST OF DESIGNATIONS 
       [0000]    
       
         
           
               10  Medium-pressure steam turbine 
               11  Outer housing 
               12  Inner housing 
               13 ,  14  Steam inlet 
               15  Steam exhaust 
               16  Axis 
               17 ,  18  Steam feed line 
               19  Connecting port 
               19 ′ Connecting flange 
               20 ,  21  Shrink-ring 
             G Housing (valve unit) 
             G 1 , G 2 , G 3 , G 4  Housing section 
             G 5 , G 6 , G 7 , G 8  Housing section 
             G 21 , G 22  Shell section 
             G 31 , G 32  Shell section 
             S 1 , . . . , S 5  Weld seam 
             T 1 , T 2  Parting plane 
             TS 1 , TS 2  Shell section 
             VE 1 , . . . , VE 5  Valve unit 
             V 1 , V 2 , V 3  Valve 
             V 11 , V 12  Valve 
             V 21 , V 22  Valve 
               22 ,  23  Steam feed line 
               24 ,  24 ′,  24 ″ Connecting line 
               25 ,  25 ′,  25 ″ Connecting line 
           
         
       
     
         [0075]    While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.