Patent Publication Number: US-9416684-B2

Title: Method for a temperature compensation in a steam turbine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2012/065215 filed Aug. 3, 2012, and claims the benefit thereof. The International Application claims the benefit of European Application No. 11180026 filed Sep. 5, 2011. All of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a steam turbine comprising an outer housing, an inner housing arranged inside the outer housing and a rotor rotatably mounted inside the inner housing, wherein an interspace is formed between the inner housing and the outer housing, wherein a sealing steam seal is formed between the outer housing and the rotor. 
     The invention further relates to a method for avoiding distortion of the housing of a steam turbine. 
     BACKGROUND OF INVENTION 
     The turbine housing of a steam turbine generally comprises an inner housing and an outer housing, an interspace being formed between the inner housing and the outer housing. These two housing parts in turn have an upper half and a lower half; in the case of high-pressure turbines the outer housing is also embodied as a barrel-type design. In particular after shutdown of the steam turbine there appear, at and between the housings, temperature differences between the lower half and the comparatively hot upper half which can be several degrees Kelvin. 
     If the steam turbine is shut down, the outer housing cools faster than the inner housing. As a consequence of free or natural convection, this induces an upward flow in the interspace between the inner housing and the outer housing which causes heat to be introduced into the upper half of the outer housing. This, in turn, may lead to distortion of the housing, particularly in the upper half of the outer housing, with the result that, there, undesirable stresses arise in the housing material and clearances are closed. Distortion of the inner housing can lead to undesirable rubbing-induced damage if, in adverse situations, turbine blades rub against the housing. 
     Steam turbines generally have an inner housing and an outer housing which surrounds the inner housing, with this double casing housing construction forming an interspace. The inner housing is at least partly encased, in its axial extent, by a cladding arranged in the interspace. 
     After a steam turbine is shut down or has shed load, there is a certain quantity of steam in the space between the inner and outer housing, depending on the prevailing pressure. Natural convection gives rise to temperature stratification between the upper and lower regions in the housing or, as the case may be, in the interspace. These different temperatures lead to distortion of the outer housing, which is also termed bowing. This “bowing” is to be avoided as the inner housing rests on the outer housing, inside the latter, and the distortion can cause the inner housing to be misaligned with respect to the rotor, which, at worst, results in closure of the radial clearances and possibly in rubbing. 
     Until now, this problem has been solved by providing a sufficiently large radial clearance. However, this causes a deterioration in efficiency. 
     SUMMARY OF INVENTION 
     Aspects of the invention are directed to structures for preventing or at least minimizing distortion of the outer housing of a turbine engine, such as when the turbine is cooling down. Moreover, a method for avoiding distortion of the housing when the turbine is shut down should be indicated. 
     In one non-limiting embodiment, a steam turbine comprising an outer housing, an inner housing arranged inside the outer housing and a rotor rotatably mounted inside the inner housing, wherein an interspace is formed between the inner housing and the outer housing, wherein a sealing steam seal is formed between the outer housing and the rotor, wherein the outer housing has an inflow opening for introducing the sealing steam into the interspace. 
     In another non-limiting embodiment, a method for avoiding distortion of the housing of a steam turbine when the steam turbine is shut down, in which, in an interspace formed between an inner housing and an outer housing surrounding the inner housing, the introduction of sealing steam into the interspace, via an opening in the outer housing, induces turbulence in the medium in the interspace. 
     The invention is based on the knowledge that the formation of temperature strata in the interspace is avoided by injecting sealing steam. This causes turbulence in the strata and thus a reduction in the thermal stresses, which in turn reduces the distortion of the outer housing. 
     The inflow opening is fluidically connected to the sealing steam seal, which is a comparatively cost-effective solution. 
     Advantageously, a sealing steam line is formed which, on one hand, allows sealing steam to be introduced into the sealing steam seal and, on the other hand, has a branch line which is fluidically connected to the inflow opening. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention will be explained in more detail below with reference to a schematic drawing, in which: 
         FIG. 1  shows a cross section view of a steam turbine, 
         FIG. 2  shows a cross section view of a steam turbine, as seen in the flow direction. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  shows, represented in a cross section view, a steam turbine  1  comprising an outer housing  2  and an inner housing  3  arranged inside the outer housing  2 . Inside the inner housing  3 , a rotor  4  is mounted rotatably about an axis of rotation  5 . An interspace  6  is formed between the inner housing  3  and the outer housing  2 . The inner housing  3  and the outer housing  2  can both be split into a first, upper partial region, the upper half, and a second, lower partial region, the lower half. 
     The steam turbine  1  further has a seal region  7  which separates an outer space  8  from an inner space  9  of the steam turbine  1 . No fluidic connection should be made in the seal region  7  between the rotor and the outer housing  2 . To that end, the seal region  7  has a sealing steam opening  10  which is formed such that cold sealing steam, which can be introduced from outside, can flow into a gap between the outer housing  2  and the rotor  4 . Mixed steam is extracted again with the aid of what are termed vapor steam extractors  11 . 
     A sealing steam seal  12  is thus formed between the outer housing  2  and the rotor  4 . The outer housing  2  has an inflow opening  13  for introducing sealing steam into the interspace  6 . Turbulence is thus induced in the steam in the interspace  6  by the sealing steam flowing in via the inflow opening  13 , which increases the natural convection in the interspace  6  and thus avoids bowing of the outer housing  2 . 
     The inflow opening  13  is fluidically connected to the sealing steam opening  10 . 
       FIG. 2  shows a cross section of the steam turbine  1  as seen in the direction of the axis of rotation  5 . In the exemplary embodiment represented in  FIG. 2 , the inflow opening  13  is arranged in a 12 o&#39;clock position in the outer housing  2 . 
       FIG. 1  shows a sealing steam line  14  in which sealing steam is formed in a sealing space between the sealing steam seal  12  and the rotor  4 . 
     The sealing steam line  14  is represented by dashed lines. A representation of the sealing steam line for the further sealing steam openings  10  shown in  FIG. 1  has been omitted for reasons of clarity. 
     The sealing steam line  14  comprises a branch line  16  which is fluidically connected to the inflow opening  13 . A valve  17  is arranged in the branch line  16  to regulate the flow of sealing steam. 
     By virtue of the equalization of the temperature distribution in the outer housing  2 , brought about as a consequence of the turbulence induced in the steam in the interspace  6  via the inflow opening  13 , the natural convection is counteracted such that housing distortions after shutdown and as the turbine cools are reliably prevented.