Abstract:
A steam turbine including a rotor rotatable about an axis and a casing concentrically surrounding the rotor. A reduction in the starting times and reduced production costs may be achieved in that the casing includes a high-mass hollow-cylindrical basic carrier and a plurality of shells concentrically surrounding the basic carrier. The shells are each produced from a bent metal sheet and interspaces capable of being filled with steam are defined between adjacent ones of the plurality of shells.

Description:
Priority is claimed to German Patent Application No. DE 103 53 451.2, filed on Nov. 15, 2003, the entire disclosure of which is incorporated by reference herein. 
   The present invention relates generally to steam turbines and particularly to a steam turbine with a rotor rotatable about an axis and surrounded concentrically by a casing. 
   BACKGROUND 
   Steam turbines of the modern type of construction for high efficiencies and high steam inlet temperatures comprise a rotor which is rotatable about an axis of rotation and which is surrounded by a casing. The casing is subdivided into an inner casing, which surrounds the rotor concentrically, and an outer casing, which surrounds the inner casing together with the rotor. Between the rotor and the inner casing is formed a steam duct which is in the form of an annular gap and through which the steam is conducted for the performance of work. The blading of the steam turbine is arranged in the steam duct and consists of alternately arranged rings of stationary guide vanes and of moving blades fastened on the rotor. The guide vanes are arranged on the inner wall of the inner casing, said inner wall delimiting the steam duct (see, for example, EP-A1-0 952 311 or U.S. Pat. No. 5,695,317 or U.S. Pat. No. B1-6,315,520). 
   Both the outer casing and the inner casing are conventionally horizontally divided thick-walled castings composed of a comparatively costly high-temperature alloy. For example, a steel casting is used for the outer casing. The inner casing, exposed to especially high pressures and temperatures, mostly consists of a special nickel-based alloy. Steam turbines with steam inlet temperatures of about 700° C. or above are currently in the planning stage. Pressures of several 100 bar, for example 350 bar, occur in this case. 
   According to current estimates, the starting time of steam turbines of the type described amounts to several hours, since, because of the large dimensions and wall thicknesses, the rotors and the casing require a long time before they can be heated to the operating temperature, without excessively high thermal stresses being generated. Further disadvantages of a high-mass inner casing are the high costs, since the nickel-based alloy is a very costly material. Also, for such large castings, there are long delivery times of several months. Since the inner casings are divided and the parts are screwed to one another via flanged connections, high-mass parting line flanges are present which make up a considerable proportion of the entire casing weight. Large and costly parting line screws for the flanged screw connection also have to be employed correspondingly. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a steam turbine that avoids the described disadvantages of known steam turbines and is distinguished, particularly by virtue of a novel casing design, by a reduced starting time during operation and by more cost-effective and quicker production. A further or alternate object of the present invention is to specify a method for the production of such a steam turbine. 
   The present invention provides a steam turbine ( 10 , 40 , 50 ) with a rotor ( 18 ) rotatable about an axis ( 47 ) and surrounded concentrically by a casing ( 11 ), characterized in that the casing ( 11 ) comprises a high-mass hollow-cylindrical basic carrier ( 21 ) and a plurality of shells ( 21 , 12 , 22 , 13 , 23 , . . . , 17 , 27 ) which surround the basic carrier ( 21 ) concentrically and are produced from a bent metal sheet and between which interspaces ( 48 ) capable of being filled with steam are provided. 
   The casing is constructed from a high-mass hollow-cylindrical basic carrier and a plurality of shells which surround the basic carrier concentrically and are produced from a bent metal sheet, interspaces capable of being filled with steam being provided between these shells. Instead of a high-mass cast inner casing which absorbs both the internal pressure and the shearing force, therefore, sheets, preferably in standard dimensions, are used in a plurality of layers. These absorb essentially only the internal pressure. The guide vanes are mounted in the basic carrier when the casing is an inner casing. Said basic carrier absorbs essentially only the shearing force and the load moment and transmits the shearing force to the axial guide and the load moment to the supports. The casing is mounted and guided via the basic carrier. The basic carrier is subjected to little internal pressure stress and can therefore be constructed with a small wall thickness. 
   A preferred embodiment of the present invention is distinguished in that the basic carrier is composed of a plurality of annular carrier segments arranged one behind the other in the axial direction and connected to one another, in that the shells have a barrel-shaped design, the next outer shell in each case surrounding all the further inward-lying shells both in the radial and in the axial direction, and in that the shells are connected on the end faces in each case to one of the carrier segments of the basic carrier in a steamtight manner. By the basic carrier being divided axially into segments connected to one another, the production of the casing is simplified considerably. 
   Good accessibility for assembly and maintenance is achieved in that the basic carrier and the shells are subdivided in a horizontal midplane into an upper and a lower part or into upper and lower segment halves and into upper and lower shells which are in each case screwed to one another in pairs via flanged screw connections. Preferably, to form the flanged screw connections, in each case horizontal flanges are attached, in particular welded, to the upper and lower shells. 
   Assembly may be further simplified in that the upper shells are in each case screwed to the upper segment halves, preferably via a semiannular flanged connection, and in that the lower shells are in each case welded to the lower segment halves. 
   So that the pressure drop from the inside outward can be apportioned correctly to the individual shells, it is advantageous that the carrier segments of the basic carrier are connected to one another in such a way that steam can flow out of the interior of the basic carrier into the interspaces between the shells. 
   Another embodiment of the present invention is characterized in that the steam for operating the steam turbine is conducted to the rotor from outside through all the shells of the casing by means of at least one inlet pipe, and in that the at least one inlet pipe, in its passage through a shell, is in each case sealed off by means of a piston ring seal. 
   In particular, the casing may be an inner casing or a combined inner and outer casing, there being formed, between the basic carrier and the rotor, a steam duct, in which is arranged a blading comprising guide vanes and moving blades, and the guide vanes of the blading being fastened to the inner wall of the basic carrier. The casing may in this case be of single-flow or double-flow design. 
   The casing may, however, also be an outer casing. The basic carrier then carries no blading on the inside, but seals instead. 
   Preferably, the shells are produced in each case from a standard metal sheet consisting of a high-temperature nickel-based alloy, in particular of Alloy 617, with a sheet thickness, dependent on the position of the shell in the casing, of several millimeters, in particular of between 3 and 11 millimeters. 
   A less costly material, preferably rolled sheet steel, may also be used in the outer shells, in which the steam temperature is markedly lower than in the inner shell. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be explained in more detail below by means of exemplary embodiments, with reference to the drawings in which: 
       FIG. 1  shows a cross section through the set-up of a multishell inner casing of a steam turbine according to a first preferred exemplary embodiment of the present invention; 
       FIG. 2  shows, in the form of an enlarged detail, the flanged connection of an upper and a lower shell of the exemplary inner casing from  FIG. 1 ; 
       FIG. 3  shows a longitudinal section through a double-flow inner casing of the type illustrated in  FIG. 1 , with the central inlet pipes for the steam inlet; 
       FIG. 4  shows an illustration, comparable to  FIG. 3 , of a single-flow inner casing of the type illustrated in  FIG. 1 ; 
       FIG. 5  shows, in two views from the side and in the axial direction (part  FIGS. 5A  and B), two upper segment halves of a basic carrier according to the present invention which are welded to one another via short round bars; and 
       FIG. 6  shows, in an illustration comparable to  FIG. 5 , the upper half, cast or forged in one piece, of a basic carrier according to the present invention, in which the radial passage of steam between the segments is ensured by means of bores. 
   

   DETAILED DESCRIPTION 
   The present invention is based on arranging a large number of bent sheets one behind the other instead of a high-mass cast casing or inner casing. Since the sheets are comparatively thin and are effectively insulated from one another thermally by means of the gap lying between them, the thermal stresses are low. The casing is therefore suitable for starting in a very short time. Further advantages are: 
   The delivery time is markedly shorter than where cast casings are concerned, since the sheets are commercially available. With standard dimensions, the sheets are already available in commercial depots. Alternatively, a specific depot may be set up. 
   A smaller quantity of costly nickel-based material is required, specifically for three reasons: 
   On account of the insulation of the sheets from one another, the temperature decreases sharply from the inner shell to the outer shell. A more cost-effective material can therefore be used in the colder outer shells. 
   Owing to the sharper temperature drop from the inner shell to the outer shell, as compared with the conventional cast casing, the temperatures in the outer shells are lower. Accordingly, the material strength, which increases with a decrease in temperature, is higher there, so that a smaller wall thickness of the shell is sufficient. 
   The high-mass parting line flange of a conventional cast casing forms a considerable proportion of the entire casing weight. In the casings according to the present invention, because of the low pressure difference from shell to shell, only very small flanges, which are very light as compared with the casing shell, are required. 
   The parting line screws of the flanged screw connections in the case of the shells divided in a horizontal midplane can have a very small design, as compared with conventional parting line screws. As a result, on the one hand, they can be delivered more quickly, since commercially available thin raw material can be used for manufacture. On the other hand, they can be produced more cost-effectively, since the commercially available raw material is more cost-effective and since they can be manufactured on smaller machines. 
   Instead of a high-mass cast inner casing which absorbs both the internal pressure and the shearing force, sheets in standard dimensions are used in a plurality of shells. These absorb essentially only the internal pressure. The guide vanes are mounted in the basic carrier. The latter absorbs essentially only the shearing force and the load moment and transmits the shearing force to the axial guide and the load moment to the supports. The casing is mounted and guided via the basic carrier. The basic carrier is subjected to almost no internal pressure stress and can therefore be constructed with a small wall thickness. 
     FIG. 1  shows the basic principle in a cross-sectional illustration by means of an exemplary embodiment: the inner casing  11  of a steam turbine  10 , said inner casing surrounding a rotor  18  concentrically, is illustrated. Between the rotor  18  and the inner casing  11 , a steam duct  20  in the form of an annular gap is left free, in which is arranged a blading  19  comprising guide vanes and moving blades. 
   Metal sheets bent into a barrel shape and having a thickness of a few millimeters, preferably of between 2 and 11 millimeters, which consist of the here six upper shells (upper halves)  12 ,  13 ,  14 ,  15 ,  16  and  17  and of the six lower shells (lower halves)  22 ,  23 ,  24 ,  25 ,  26  and  27 , are laid in the manner of onion skins around the steam duct  20  and the rotor  18 . The upper and lower shells are fixed to one another in each case via a welded-on small horizontal flange  28 ,  29  (see also  FIG. 2 ) and a flanged screw connection  30 . The guide vanes in the steam duct  20  are mounted on a basic carrier  21  which likewise consists of an upper part  21   a  and of a lower part  21   b  which are both fixed to one another by means of a small flange (horizontal flange halves  52 ,  53  in  FIGS. 5 and 6 ) and a flanged screw connection  30 . Between the upper and lower shells  12 , . . . ,  17  and  22 , . . . ,  27 , interspaces  48  are left free which are filled with steam via orifices in the basic carrier  21  during operation. The steam pressure decreases from the inside outward from interspace to interspace of the upper and lower shells  12 , . . . , 17  and  22 , . . . , 27 . 
     FIG. 2  shows, in the form of an enlarged detail, an exemplary horizontal flanged connection of the upper and lower shells from  FIG. 1  (flanged screw connection  30  in  FIG. 1 ). In  FIG. 2 , an upper shell  17  is welded to a flange upper part  28 . The lower shell  27  is welded to the flange lower part  29 . The associated weld seams are given the reference symbol  36 . A screw bolt  32  is inserted through in bores  35  in the flange upper part  28  and lower part  29 , said screw being braced by means of nuts  33  and  34  and sealing off the parting line  31  between the flange upper part  28  and the flange lower part  29 . 
     FIG. 3  shows a longitudinal section through a double-flow inner casing  11  of a steam turbine  40 . The inner casing  11  again comprises a basic carrier  21  with an upper part  21   a  and lower part  21   b  and also upper shells  12 , . . . , 17  and lower shells  22 , . . . , 27 . The rotor  18  rotates about the axis  47 . A flow  44  and  45  together with the corresponding blading  19  is arranged on each of the two sides of central inlet pipes  37 ,  38 . The steam flows through the inlet pipes  37  (top) and  38  (bottom) to the rotor  18  and is then apportioned to a left flow  44  with the blading  19  and to a right flow  45  with the blading  19 . The inlet pipes  37  and  38  are led via piston ring seals  39  (top) and  41  (bottom) through the upper shells  12 ,  13 ,  14 ,  15 ,  16 ,  17  and the lower shells  22 ,  23 ,  24 ,  25 ,  26 ,  27  and the innermost ring (carrier segment  46 ′) of the basic carrier  21 . 
   The basic carrier  21 , on the one hand, is divided horizontally into an upper part  21   a  and a lower part  21   b  and, on the other hand, is subdivided axially into carrier segments  46  (more precisely, segment halves  46   a, b ) which carry the guide vanes of the blading  19  on the insides and to which the upper and lower shells  12 , . . . , 17  and  22 , . . . , 27  are fastened. In the example of  FIG. 3 , six carrier segments  46  (12 segment halves  46   a,b ) are provided on each of the two sides of the innermost carrier segment  46 ′ through which the inlet pipes  37 ,  38  are led. The individual carrier segments  46  are connected to one another in such a way that steam can flow out of the steam duct into the interspaces  48  of the upper and lower shells. For mounting the upper shells  12 ,  13 ,  14 ,  15 ,  16 ,  17 , it is necessary to screw these to the upper segment halves  46   a  of the basic carrier  21  by means of a semiannular flanged connection  42 . The lower shells  22 ,  23 ,  24 ,  25 ,  26 ,  27  can be connected to the lower segment halves  46   b  by means of weld seams  43 . 
   In  FIG. 3 , the connection of the upper and lower shells  12 ,  13 ,  14 ,  15 ,  16 ,  17  and  22 ,  23 ,  24 ,  25 ,  26 ,  27  to the basic carrier  21  takes place axially, in each case approximately at the segment center. If, however, this screw or welded connection is formed at that end of the carrier segment  46  which is directed downstream with respect to the steam flow, the carrier segment  46  of the basic carrier  21  is acted upon by an external pressure. The horizontal flange screws which hold together the upper and lower part  21   a, b  of the basic carrier  21  may then have a very small design or they may even be dispensed with completely. 
     FIG. 4  shows a longitudinal section through a single-flow inner casing  11  of a steam turbine  50 . The steam flows through the inlet pipes  37  (top) and  38  (bottom) to the rotor  18  and then to the left through the blading  19 . A residual steam flows through the casing seal  49  (here, labyrinth seal) arranged on the right side. The inlet pipes  37  and  38  are led via piston ring seals  39  (top) and  41  (bottom) through the upper shells  12 ,  13 ,  14 ,  15 ,  16 ,  17  and the lower shells  22 ,  23 ,  24 ,  25 ,  26 ,  27  and the inner carrier segment  46 ′ of the basic carrier  21 . The inner carrier segment  46 ′ carries the first guide vanes of the blading  19  on the left side and the casing seals  49  on the right side. The carrier segments  46  on the right of this inner carrier segment  46 ′ carry the further casing seals, and the carrier segments  46  on the left of this inner carrier segment  46 ′ carry the further guide vanes. Between the carrier segments  46 ,  46 ′ are orifices which allow the steam to flow into the interspaces  48  of the upper and lower shells. Between the carrier segments  46 ,  46 ′ which carry the casing seals, these orifices may be dispensed with. This rules out the situation where hot steam flows out of the seals into the interspaces  48  of the upper and lower shells. The seals should, however, be designed in such a way that the pressure difference between the interspaces  48  and the seals  49  is low. Preferably, the pressure in the interspaces should be slightly higher, so that, in the event of a leak, colder steam flows out of the interspace into the seal, not vice versa. 
   For mounting the upper shells  12 ,  13 ,  14 ,  15 ,  16 ,  17 , it is necessary to screw these to the upper segment halves  46   a  of the basic carrier  21  by means of a semiannular flanged connection  42 . Should the turbine not have to be dismantled again, the upper shells  12 ,  13 ,  14 ,  15 ,  16 ,  17  may also be connected to the upper segment halves  46   a  of the basic carrier  21  by means of a welded connection. 
   The lower shells  22 ,  23 ,  24 ,  25 ,  26 ,  27  may again be connected to the lower segment halves  46   b  by means of weld seams  43 . 
     FIG. 5  shows two upper segment halves  46   a  of a basic carrier  21 . In the example illustrated, the segment halves  46   a  are connected to short round bars  51 , for example by welding. The segment halves  46   a  themselves may consist, for example, of bent sheet metal or of forged half rings. The round bars  51  for the adjacent segment halves not yet welded on are also illustrated. The voids between the round bars  51  allow the steam to flow into the interspaces  48  of the upper and lower shells (see  FIGS. 3 and 4 ). Round flanges  54  with bores  55  distributed over the circumference are attached to the segment halves  46   a , the upper shells ( 12 , . . . , 17  in  FIG. 1 to 4 ) being screwed to said round flanges. Furthermore, horizontal flange halves  52  and  53  are attached, by means of which the upper segment halves  46   a  illustrated can be screwed to the associated lower segment halves (not illustrated). 
     FIG. 6  shows the upper half or the upper part  21   a  of a basic carrier  21 . In contrast to  FIG. 5 , the segment halves  46   a  are connected in that the entire upper part  21   a  of the basic carrier  21  is cast or forged in one piece. The permeability of the steam into the interspaces  48  of the upper and lower shells (see  FIGS. 3 and 4 ) is ensured by means of bores  56 . The round flanges  54  having the bores  55  are attached to the segment halves  46   a , the upper shells being screwed to said round flanges. Furthermore, horizontal flange halves  52  and  53  are again provided, by means of which the illustrated upper part  21   a  of the basic carrier  21  can be screwed to the lower part  21   b  (not illustrated). 
   The claws and webs for the supports and guides of the casings  11  are not shown in the figures. Claws and webs are attached, for example, to the outermost carrier segments  46  of the basic carrier  21 . 
   In the figures, the casing  11  is designed as an inner casing. However, an outer casing, too, may be produced in the multishell design according to the present invention with a stepped pressure reduction. Instead of the basic carrier segments with blading (left side in  FIG. 4 ), basic carrier segments with casing seals  49  (right side in  FIG. 4 ) are used on both sides. 
   The inner and outer casings configured according to the present invention may also be combined. For example, in  FIG. 4 , one or more shells are added on the outside, in which only seals are attached to the associated additional carrier segments of the basic carrier. Then, in a similar way to the inlet pipes  37  and  38 , an outlet pipe with piston seals is led through these added shells, the steam being capable of flowing outward through said outlet pipe. 
   The bent shells (upper and lower shells) can be produced in a simple and cost-effective way by means of the method of end-controlled bending, as disclosed in German patent specification DE-C2-43 10 773. 
   For an exemplary 400-MW steam turbine with an HP and MP part, it is necessary in the HP part to have 5 shells consisting of Alloy 617 which have stepped wall thicknesses of 9 to 10.5 mm. The MP part has 3 shells consisting of Alloy 617 with stepped wall thicknesses of 3.8 to 5.8 mm. In each case 3 stages of the turbine (3 guide vane rings and 3 moving blade rings) are assigned to a carrier segment of the basic carrier. 
   Overall, the present invention affords the following advantages:
         Starting of a 700° C./720° C. turbine possible within a few minutes (instead of 5 hours at the present time).   Reduced delivery time for the casing.   Cost saving with regard to the casing and to the casing screws.   Casings: standard metal sheets, if appropriate standardized, are used instead of cast iron (conventional design). The standardization results in a cost benefit. There is additionally also a cost benefit because less nickel-based material is required, since a change to the material of the next lower quality is possible directly in the next “onion skin”.   Parting line screws: small screws are mass products or in any event can be manufactured everywhere and are therefore inexpensive.   As a result of standardization, the sheets and the individual segments of the basic carrier can be kept in stock. Standard sheets may also alternatively be procured from the sheet manufacturer&#39;s depot. The delivery time is thereby drastically reduced, since there is no dependence on the long delivery time of a casting foundry.