Abstract:
The invention relates a method for improving the hot-starting behavior of a turbomachine. By virtue of the setting of a radial gap formed between a brushing edge of a blade profile and a guide face lying opposite this, in which setting a guide ring forming the guide face can be acted upon with a coolant, the hot-starting behavior can be improved, in that the guide ring is cooled before the start of the machine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to and benefit of European Patent application No. 05009380.6 filed Apr. 28, 2005 and is incorporated by reference herein in its entirety. 
   FIELD OF THE INVENTION 
   The invention relates to a method for setting a radial gap, formed between a brushing edge of a blade profile and a guide face lying opposite this brushing edge, of an axial-throughflow turbomachine in which a guide ring forming the guide face can be acted upon with a coolant. The invention relates, furthermore, to a compressor. 
   BACKGROUND OF THE INVENTION 
   The laid-open publication DE 199 38 274 A1 discloses, in this respect, a method and a device for the controlled setting of the radial gap between stator and rotor arrangements in a gas turbine. The design-related radial gaps are formed between the rotatable moving blades of the rotor of the turbomachine and the guide faces lying opposite them fixedly in terms of rotation on the stator. The guide faces serve for guiding the working medium and are formed by annular segments which are subdivided in the circumferential direction and extend coaxially as a guide ring about the axis of rotation of the rotor in the axial direction. When the gas turbine is in operation, the moving blades of the rotor move at a distance from the guide faces. Conversely, free-standing guide blades can also in each case form a radial gap with respect to a rotating conical or cylindrical guide face arranged on the rotor. In order further to optimize the efficiency of the gas turbine, the radial gaps are to be designed so as to be as small as possible. It is known from the abovementioned laid-open publication to fasten guide rings to a stator by means of holding partners arranged obliquely with respect to the radial direction and, during the operation of the gas turbine, to displace this stator in the direction of the moving blade ends by virtue of the thermal expansion of the material of the guide ring, in order to make the radial gap smaller. 
   Something similar is disclosed in EP 1 163 430 B1. During the operation of a gas turbine, a guide element lying opposite a tip of a turbine moving blade flexes in the direction of the moving blade tip on account of the thermal expansions, thereby making the radial gaps smaller. At the same time, the guide element can be acted upon with cooling air from the rear side, so that it can withstand the temperatures prevailing in the flow duct. 
   Moreover, it is known from GB 2 397 102 A to insulate the guide ring of a turbine with respect to the carrying structure. 
   It is known, furthermore, that the design parameters determining the gap dimension are designed for the hot starting of a gas turbine, in order to satisfy the requirements for the smallest possible operating gap, that is to say radial gap. After the gas turbine has been run down, the casing cools comparatively quickly, as compared with the rotor of the gas turbine. The casing or the guide rings, on account of their cooling, shrink back to their original design size, the still hot rotor initially remaining expanded due to the heat stored in it and cooling and shrinking with a delay. This gives rise to what is known as the contraction effect. This situation may result in the radial gap decreasing as the blades of the rotor touching or even brushing against the casing or the guide ring, thus permanently enlarging the radial gap or possibly even damaging the blade. An enlarged radial gap leads to increased fuel consumption, and damaged blades may make it necessary to carry out premature maintenance with the corresponding extra costs. 
   During the starting, that is to say run-up, of the gas turbine, the centrifugal forces acting on the moving blades bring about a further expansion of these, which may close the radial gap still present before the starting of the gas turbine and cause a harmful and unwanted brushing against of the blades. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to specify a method of the type initially mentioned which improves the hot-starting behavior of the turbomachine in order to increase availability, and at the same time to increase the efficiency. Moreover, the object of the invention is to specify a compressor for this purpose. 
   The object aimed at the method is achieved by means of the features of the claims and the object aimed at the compressor is achieved by means of the features of the claims. 
   The solution proposes that the guide ring be acted upon by coolant before the starting of the turbomachine. The invention in this case proceeds from the knowledge that the hot-starting conditions of the turbomachine are improved by the radial gap being influenced, in that, in the case of a still hot or heated-up turbomachine which, however, is not in operation, the gap dimension of the radial gaps is enlarged by means of the proposed method, as compared with the gap dimension of the radial gap of a gas turbine known from the prior art, in the identical state. The guide ring, having a hammer-shaped cross section, is formed over the circumference by annular segments lying against one another. Since the guide ring lying opposite the moving blades (or the guide blades) is fixed radially further outward (or inward), action upon it by coolant leads to a displacement of the guide faces which is directed away from the opposite brushing edges of the blades. The enlargement of the radial gaps which is thus achieved results in the reduction in the contraction effect described and in the risk of brushing, thus significantly improving the hot-starting behavior of the turbomachine, that is to say the turbomachine could be started earlier with respect to its previous rundown time point. Moreover, the radial gaps no longer need to be dimensioned according to the hot start as an unfavorable operating start. 
   The cooling of the hot guide rings enlarges the radial gaps of the inoperative turbomachine. The radial gap enlargement obtained for this state may also be utilized partially, instead for improving the hot start, in order to design the radial gaps of the turbomachine which is in the inoperative and cold state, say at ambient temperature, so as to be smaller, with respect to a turbomachine known from the prior art. 
   This has an advantageous effect on the operation, in particular on the stationary operation of the turbomachine, particularly in the case of a compressor and in the case of a turbine unit. In this operating state, the method according to the invention is no longer applied, so that the radial gaps become smaller again. During operation, the reduction in size of the design-related radial gaps mitigates, so as to increase efficiency, the losses in the working medium which arise due to the unused leakage flow through the radial gap, particularly in the case of increasing pressure conditions of the working medium in the flow duct. 
   Advantageous embodiments are specified in the dependent patent claims. 
   After the (hot) start of the turbomachine, both the rotor and the casing of the turbomachine heat up, as the operating period continues, to a maximum operating temperature. In this case, both the casing and the rotor expand, so that there is no longer the risk of contraction. Accordingly, the method is particularly advantageous when action upon the guide ring by coolant is stopped during the starting of the turbomachine. After the maximum operating temperature is reached, the thermally induced expansions of the turbomachine, that is to say the stator and the rotor, are concluded. The guide ring consequently also heats up, so that the latter expands and displaces its guide face in the direction of the brushing edges of the blades, thus leading to an efficiency-increasing reduction in size of the radial gaps. This may be employed advantageously particularly when the turbomachine is designed as a compressor of a gas turbine, in which the guide rings are normally uncooled during operation. 
   It is particularly advantageous if the coolant is extracted from an external coolant source. Conventionally, in a turbomachine designed as a gas turbine, coolant in the form of cooling air is extracted from the compressor. Since the method is employed even before the starting of the gas turbine, this is not possible. An external coolant source, for example a separately driven auxiliary compressor or external blower, therefore has to be used for providing the coolant for cooling the guide rings before the hot start of the gas turbine. 
   Preferably, after the start of the turbomachine, the guide ring can be acted upon by heating medium. This is advantageous particularly when the turbomachine is, for example, a compressor or a turbine of a gas turbine, and when the methods known from the prior art, in which material expansions of the guide ring are used to set the radial gap, are applied to the guide ring of a compressor. Air or steam may preferably be used as heating medium. By the guide ring being heated, its guide face grows toward the brushing edge of the blades and thus reduces the size of the radial gap enclosed by them. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained by means of a drawing in which: 
       FIG. 1  shows a longitudinal part section through a turbomachine designed as a gas turbine, with a compressor and a turbine unit, and 
       FIG. 2  shows the detail II of  FIG. 1 , a guide ring in cross section with an opposite blade tip. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows, as an example of a turbomachine, a longitudinal part section through a gas turbine  1 . It has, inside it, a rotor  3  which is rotationally mounted about an axis of rotation  2  and which is also designated at the turbine rotor. A suction-intake casing  4 , a compressor  5 , a toroidal annular combustion chamber  6  with a plurality of coaxially arranged burners  7 , a turbine unit  8  and the exhaust gas casing  9  succeed one another along the rotor  3 . The annular combustion chamber  6  forms a combustion space  17  which communicates with an annular flow duct  18 . There, four turbine stages  10  connected in series form the turbine unit  8 . Each turbine stage  10  and each compressor stage is formed from two blade rings. 
   In the turbine unit  8 , as seen in the flow direction of a hot gas  11 , a guide blade row  13  is followed in the flow duct  18  by a row  14  formed from moving blades  15 . The guide blades  12  are in this case fastened to the stator, whereas the moving blades  15  of a row  14  are mounted on the rotor  3  by means of a turbine disk. A generator or a working machine is coupled (not illustrated) to the rotor  3 . 
   By contrast, in the compressor  5 , a compressor stage is formed by a moving blade row  13  with a ring of guide blades  12  which follows in the flow direction of the air to be compressed. 
   A guide ring  21  lies radially opposite the moving blade  15  on the outside and a guide ring  23  lies radially opposite the guide blade  12  on the inside. The guide rings  21 ,  23  delimit in the radial direction the flow duct  18  extending in the axial direction of the rotor  3 . The guide rings  21 ,  23  may be formed from annular segments lying one against the other over the circumference. 
   After the starting of the gas turbine  1  and as a result of the working medium flowing in the flow duct  18 , all the components of the gas turbine  1  heat up. On account of the temperature rise, these components, that is to say also the rotor  3 , the moving blade  15 , the guide blades  12  and the inner casing  27 , expand with respect to their cold state. 
   When the gas turbine  1  is heated up completely and a no longer varying temperature distribution is established, all the thermally induced expansions are also concluded. The gas turbine  1  is then in a stationary state. 
     FIG. 2  shows the detail II from  FIG. 1 , a cross section through a guide ring  21  with an opposite blade, after all the thermally induced expansions are concluded. In this case, the device shown in  FIG. 2  may be provided both in the turbine unit  8  and/or in the compressor  5  of the gas turbine  1 . 
   The blades each have a blade profile  19  of drop-shaped cross section which has a leading edge  20  capable of having a working medium flowing onto it and a trailing edge  22 . 
   A wall  25  extending cylindrically or conically with respect to the axis of rotation  2  of the gas turbine rotor  3  forms part of a rotationally fixed inner casing  27 . The wall  25  surrounds the annular flow duct  18 . The inner casing  27  or the wall  25  has incorporated in it a groove  29  of hammer-shaped cross section which runs in the circumferential direction and in which the guide ring  21  is arranged. The guide ring  21  thus also surrounds the flow duct  18  coaxially with respect to the axis of rotation  2  of the rotor  3 . 
   Between the wall  25  and the guide ring  21 , an insulating layer  26  may be formed, which shields and insulates the guide ring  21  thermally with respect to the wall  25 , so that the wall  25  or the inner casing  27  does not likewise shrink in the direction of the blade. 
   The guide ring  21  is in this case manufactured from a material which expands under the action of heat, that is to say a temperature rise, preferably in this case expands to a greater extent than the wall  25  or the inner casing  27 , that is to say the guide ring  21  has a higher coefficient of thermal expansion than the wall  25  or the inner casing  27 . 
   The guide ring  21  is designed so as to match essentially with the hammer-shaped groove  29  and bears on the rear side directly, or, as illustrated via the insulating layer  26 , against the groove bottom of the groove  29  and on the front side against a bearing face  50  of the undercut  31 , so that the guide ring  21  is fixed. The bearing face  50  determines the radial position of the guide ring  21  and is in this case arranged radially further outward (or inward) than the guide face  33  lying opposite the tips of the moving blade  15  (or guide blades  12 ). 
   The moving blade  15 , in particular its brushing edge  35 , lies opposite the guide face  33 , facing the flow duct  18 , of the guide ring  21 . A radial gap  36  is formed between the brushing edge  35  of each moving blade  15  and the guide face  33 . When the gas turbine is in operation, the moving blade  15  rotates under and below the face  33 , this being indicated for clarity by the axis of rotation  2  shown in a position which is not true to scale. 
   That face  37  of the guide ring  21  which is on the rear side with respect to the guide face  33  has incorporated in it a groove  39  which forms with the wall  25  or, if present, with the insulating layer  26  a circumferentially running, that is to say annular supply duct  41 . 
   Furthermore, a plurality of, preferably three cooling ducts  43  extend in the circumferential direction, that is to say coaxially with respect to the axis of rotation  2 , and communicate with the supply duct  41  via radial connecting ducts  45 . 
   A feed duct  49 , which opens into the supply duct  41 , extends through the wall  25  from that side  47  of the latter which faces away from the flow duct  18 . 
   After the gas turbine  1  has been shut down, the casing cools more quickly than the rotor  3 , so that the expansions of the casing decrease or diminish more quickly and contract the still hot rotor  3  which is therefore expanded to a greater extent. The gap dimension of the radial gap  36  is thereby reduced. 
   In the event of an early start of the still hot gas turbine  1 , that is to say during a hot start, the centrifugal forces acting on the rotor  3  and the moving blades  15  cause an additional radial growth, which may reduce the size of the radial gap  36  in such a way that the brushing edges  35  may brush harmfully against the guide face  33 . 
   This is where the invention comes in. Before the operation of the still hot gas turbine is resumed, the supply duct  41  is fed by the feed duct  49  with coolant  51  which passes from there via the connecting ducts  45  into the cooling ducts  43  and cools the guide ring  21 . The coolant  51  absorbs the heat still stored in the guide ring  21  and subsequently, via orifices, not shown, is either blown out into the flow duct  18  or recirculated outward from the machine interior via recirculation ducts, likewise not illustrated. By heat which is, in particular, is near the guide face being transported away from the guide ring  21 , the thermal induced material expansions of the guide ring  21  diminish. In conjunction with its local position defined radially on the outside in the groove  29 , the guide face  33  delimiting the flow duct  16  is displaced radially outward into the position  33 ′. As a result of this, the radial gap  36  is enlarged by the amount of the distance X to  36 ′, with the result that the risk of the moving blades  15  brushing against the guide face  33  or  33 ′ in the event of the hot start decreases. This effect may be utilized in order to shorten the duration between the rundown or shutdown and the hot start of the gas turbine. 
   The method is particularly effective when the guide ring  21  is insulated with respect to the wall  25 . In this embodiment, only the guide ring  21  is cooled, not also the wall  25 . This leads to a particularly efficient cooling of the guide ring  21  and prevents the wall  25  from likewise being co-moved in an identical way. This ensures that only the guide ring  21  reduces its thermally induced expansions. 
   After or during the start, that is to say during the process of starting up the gas turbine  1 , the casing heats up and expands. The casing and also the inner casing  27  are displaced radially outward. The risk of the moving blades  15  brushing with their brushing edge  35  against the guide face  33  of the guide rings  21  is reduced, so that, after a predetermined operating period, the cooling of the guide rings  21  can be stopped. 
   At the same time, the gas turbine  1  heats up further, until a no longer varying temperature distribution is established in it. 
   Insofar as the material of the guide ring  21  allows a further temperature rise, the heating medium may even be conducted through the ducts  49 ,  41 ,  45 , instead of the coolant  51 , during the operation of the gas turbine  1 . A further temperature rise in the guide ring  21  causes an additional expansion in the radial direction, as a result of which the radial gap  36  is further reduced. This leads to an increase in efficiency, since less working medium—in the compressor  5 , the gas to be compressed and, in the turbine unit  8 , the expanding hot gas  11 —can escape, unused, through the reduced radial gap  36 . 
   The radial gap  36  may not only be formed between a radially outer guide face  33  and a moving blade  15 , but it may also lie between the rotationally fixed guide blade  12  and the guide face  23  arranged on the rotor  3 . Accordingly, the wall  25  can be part of the rotor  3 , so that a guide blade  12  lies opposite the guide ring  23 . In this case, the displacement directions also change from the outside inward. 
   The method according to the invention for varying the radial gaps  36  is suitable particularly for compressors  5 . However, it may also be used in the turbine unit  8 .