Abstract:
A steam turbine having at least a HP blade cascade, an IP blade cascade and a plurality of dummy members that are attached to a common rotor shaft, is provided with, but not limited to, a detection unit that detects a steam flow into an IP chamber, a pressure reducing unit that reduces a pressure difference between both sides of a target dummy member of said plurality of the dummy members when the steam flow into the IP chamber stops, the target dummy member having one side communicating with a part of the IP chamber, and a control unit that controls the pressure reducing unit based on a detection result obtained by the detection unit.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0001]    The present invention relates to a steam turbine and a method of adjusting a thrust force of the steam turbine, particularly regarding a steam turbine and a method of adjusting a thrust force of the steam turbine which are capable of balancing of a thrust force acting on a rotor shaft of the steam turbine which includes at least a high-pressure (HP) blade cascade, an intermediate-pressure (IP) blade cascade and a plurality of dummy members that are attached to a common rotor shaft. 
         [0002]    Since the rotor shaft is subjected to the thrust force acting thereon, the steam turbine is provided with a thrust bearing. With a limited load capacity of the bearing, it is necessary to design the steam turbine in consideration of a thrust balance so that the thrust force acting on the rotor shaft does not exceed the load capacity of the bearing under any operating condition. 
         [0003]    Hence, the dummy members (dummy pistons) and the blade cascades are attached to the same rotor shaft, so as thrust forces in a counter-thrust direction are generated by the dummy members to balance the forces acting in an axial direction of the entire rotor shaft. In this manner, the thrust force acting on the rotor shaft is kept within the scope of the load capacity of the bearing under any operation condition. 
         [0004]      FIG. 13  shows an outline view regarding a conventional steam turbine under a normal operating condition, the conventional steam turbine being provided with dummy members for adjusting the thrust forces. 
         [0005]    In a conventional steam turbine  1  depicted in  FIG. 13 , a turbine casing (not shown) is formed around a rotor shaft  10 . The turbine casing includes an inlet part (not shown) for introducing high-pressure (HP) main steam  22 , an inlet part (not shown) for introducing reheat steam  24  and an inlet part (not shown) for introducing low-pressure (LP) main steam  26 . 
         [0006]    Further, a HP blade cascade  2  to which the HP main steam is supplied, an IP blade cascade  4  to which the reheat steam  24  is supplied and a low-pressure (LP) blade cascade  6  to which the LP main steam  26  is supplied are attached to the rotor shaft  10  in this order. The IP blade cascade  4  and the LP blade cascade  6  have steam inlets that are open to one side, whereas the HP blade cascade  2  has a steam inlet is open to other side being opposite to the one side. Between the steam inlet of the HP blade cascade  2  and the steam inlet of the IP blade cascade  4 , a high-pressure (HP) dummy member  12  is provided. On a steam outlet side of the HP blade cascade  2 , an intermediate-pressure (IP) dummy member  14  and a low-pressure (LP) dummy member  16  are provided in this order. Further, a thrust balance conduit  30  is provided so as to communicate the outlet side of the IP dummy member  14  to a latter half of the IP blade cascade  4 . 
         [0007]    In the steam turbine  1  as described above, the HP main steam  22  from a boiler and the like (not shown) enters the HP blade cascade  2 . And, the HP main steam  22  gives a rotary force to the rotor shaft  10  while the steam passes through the HP blade cascade  2 . The steam that has done the work through the HP blade cascade  2  drops the pressure and the temperature gradually and is discharged out of the steam turbine  1  as a low-temperature reheat steam  28 . The low-temperature reheat steam  28  discharged out of the steam turbine  1  is reheated by a reheat boiler (not shown) to be the reheat steam  24 . 
         [0008]    The IP reheat steam  24  that is reheated by the reheat boiler gives the rotary force to the rotor shaft  10  and gradually reduces the pressure and the temperature while the reheat steam  24  passes through the IP blade cascade  4 . Further, the LP main steam  26  gives the rotary force to the rotor shaft  10  and gradually reduces the pressure and the temperature while the LP main steam  26  passes through the LP blade cascade  6 . 
         [0009]    Further, a part of the high-pressure (HP) main steam  22  passes by the high-pressure (HP) dummy member  12  and a part of the low-temperature reheat steam  28  that has passed through the HP blade cascade and has reduced the temperature and the pressure, passes by the intermediate-pressure (IP) dummy member  14  and the low-pressure (LP) dummy member  16 . 
         [0010]    Further, in  FIG. 13 , the thrust forces acting on the rotor shaft  10  at the cascades and the dummy members on the rotor shaft are represented by encircled numbers,  1  to  6  and an example regarding a set of the pressure values between adjacent pair of each blade cascade (dummy parts) are shown in  FIG. 13 . In addition, the thrust forces indicated by encircled numbers  1 , 2 , 3 , 4 , 5  and  6  denote the thrust forces acting on the LP dummy member  16 , the IP dummy member  14 , the HP blade cascade  2 , the HP dummy member  12 , the IP blade cascade  4  and the LP blade cascade  6 , respectively. The thrust force acing on each of the blade cascades can be computed based on the gas pressure force working on each blade cascade and the thrust force acting on each of the dummy members can be computed based on a pressure difference between both sides of each dummy member and a cross-sectional area of each dummy member. 
         [0011]    As shown in  FIG. 13 , the dummy members  12 ,  14  and  16 , and the thrust balance conduit  30  are provided so as to balance the thrust forces by the steam pressure. In other words, the thrust force acting on the HP dummy member  12  roughly serves as a counterbalance to the thrust force acting on the HP blade cascade  2 , the thrust acting on the IP dummy member  14  roughly serving as a counterbalance to the thrust force acting on the IP blade cascade  4 , the thrust force acting on the LP dummy member  16  roughly serving as a counterbalance to the thrust force acting on the LP blade cascade  6 . Thus, the resultant thrust force acting on the whole steam turbine  1  is balanced. 
         [0012]    Further, in the steam turbine, in order to prevent the thrust bearing from being damaged, the resultant thrust force needs to be brought into balance not only in a case where the steam turbine is operated under a normal operating condition but also in a case where either the HP main steam supply or the reheat steam supply is stopped. 
         [0013]    First, attention is paid to a case where the flow of the HP main steam  22  through the steam turbine  1  as shown in  FIG. 13  is stopped due to a trouble, a tuning operation or the like.  FIG. 14  shows the outline of the state of the steam turbine provided with conventional dummy parts for adjusting thrust balance, when the supply of the HP main steam  22  is stopped. 
         [0014]    As shown in  FIG. 14 , when the supply of the HP main steam  22  is stopped, the flow of the steam streaming through the HP blade cascade  2  stops, causing the pressure difference of the HP blade cascade to be 0. Accordingly, the thrust force represented by the encircled numeral  3  as depicted in  FIG. 14  also becomes 0. Further, the pressure difference of the HP dummy member  12  becomes infinitesimal and, the thrust force represented by the encircled numeral  4  becomes approximately 0. Therefore, as shown in  FIG. 14 , the resultant thrust force developed in the whole steam turbine  1  is substantially balanced, even in a case where the supply of the HP main steam  22  is stopped. 
         [0015]    In the next place, attention is paid to a case in which the flow of the reheat steam  24  and the LP main steam  26  through the steam turbine  1  as shown  FIG. 13  is stopped due to a trouble, a tuning operation or the like.  FIG. 15  shows the outline of the state of the steam turbine provided with conventional dummy parts for adjusting thrust balance, when the supply of the reheat steam and the LP main steam is stopped. 
         [0016]    As shown in  FIG. 15 , when the supply of the reheat steam  24  and the LP main steam  26  is stopped, the flow of the steam streaming through the IP blade cascade  4  and the LP blade cascade  6  ends. Each of the pressures on both sides of the IP blade cascade  4  and the pressures on both sides of the LP blade cascade  6  becomes approximately a level of vacuum pressure. Further, due to the thrust balance conduit  30  that communicates the IP dummy member  14  to the latter half of the IP blade cascade  4 , the pressure between the IP dummy member  14  and the LP dummy member  16  also becomes a level of vacuum pressure. 
         [0017]    In such case, in an LP system (a low-pressure part of the steam turbine), the pressure difference between both sides of the LP blade cascade  6  and the pressure difference between both sides of the LP dummy member  16  become approximately 0, resulting in the thrust force acting on the rotor shaft being 0. 
         [0018]    Further, in relation to the IP system (the intermediate-pressure part of the steam turbine), the pressure at the outlet of the IP dummy member  14  becomes a level of vacuum pressure and in response to the vacuum pressure level, the thrust force represented by the encircled numeral  2  as shown in  FIG. 15  increases. In addition, the pressure difference between both sides of the IP blade cascade  4  becomes approximately 0. In this manner, the thrust force represented by the encircled numeral  5  becomes approximately 0. As a result, the resultant thrust force acting toward the direction of the IP dummy member side (leftward in  FIG. 15 ) increases. 
         [0019]    Further, in relation to a HP system (a high-pressure part of the steam turbine), the thrust force generated in the HP blade cascade  2  represented by the encircled numeral  3  is approximately the same as that of a normal operation condition, whereas the thrust force represented by the encircled numeral  2  generated in the HP dummy member  12  increases by an amount corresponding to the vacuum pressure level at the outlet of the HP dummy member  12 . Thus, the thrust force acting in the direction of the HP dummy member (rightward in  FIG. 15 ) increase. 
         [0020]    Hereby, the increase of the thrust force generated in the IP system is greater than the increase of the thrust force generated in the HP system. Accordingly, the resultant thrust force generated in the whole steam turbine  1  increases in the leftward direction in  FIG. 15 . Thus, the resultant thrust force acting on the whole steam turbine is not balanced. 
         [0021]    In a case where the flow of the reheat steam  24  is stopped, it may be considered that the HP dummy member  12  is upsized so that thrust force in the rightward direction increases and the resultant thrust force is balanced. However, the upsizing of the HP dummy member  12  spoils the balancing in the normal operation and thus, this approach is not appropriate. 
         [0022]    Hence, in relation to each of  FIG. 13  to  FIG. 15 , the IP dummy member  14  is downsized and the LP dummy member  16  is upsized. By this, the balance of the thrust force can be maintained under the normal operating condition, even in a case where the supply of any one of the HP main steam and the reheat steam is stopped. 
         [0023]    In addition, Patent Reference 1 discloses another technology; according to this technology, thrust forces acting on the steam turbine are evaluated based on the measured data such as bearing temperatures. Based on the results of the measurements, the thrust forces acting on the dummy members can be adjusted in an electronic control approach, and the resultant thrust force developed in the whole steam turbine is brought into balance. 
       REFERENCES   
     Patent References  
       [0024]    Patent Reference 1: JP1996-189302 
       SUMMARY OF THE INVENTION 
     Subjects to be Solved 
       [0025]    In the conventional technology as explained above in reference to  FIG. 13  through  FIG. 15 , it is necessary to downsize the IP dummy member  14  and upsize the LP-dummy member  16  so as to balance the resultant thrust force even when the supply of the reheat steam in addition to the HP main steam is stopped in the normal operation. Upsizing of the LP dummy member  16  accompanies upsizing of the casing that is located at the outer periphery of the LP dummy member  16 . Accordingly, the whole steam turbine  1  is inevitably upsized and the manufacturing cost increases. Moreover, when the diameter of the LP dummy member  16  is increased, the steam leakage from the LP dummy member  16  toward the gland increases. Inevitably, there arises a possibility that the performance of the steam turbine  1  deteriorated. In recent years, the LP blade cascade is becoming larger, accompanying upsizing of the LP dummy member. However, it is not desirable to upsize the LP dummy member to balance the thrust forces. 
         [0026]    Further, in the technology as disclosed by Patent Reference 1 where the balancing of the thrust forces is performed by use of an electric control, there is a possibility that the reliability of the electric system may cause a problem. 
         [0027]    In view of the above problems of the related art, it is an object of the present invention to provide a steam turbine and a method of adjusting a thrust force of the steam turbine acting on a rotor shaft of the turbine in an entire operating range of the steam turbine without upsizing a LP dummy member or without using an electric control of a complicated system. 
       Means to Solve the Subjects 
       [0028]    To solve the above issues, the present invention provides a steam turbine having at least a high-pressure (HP) blade cascade, an intermediate-pressure (IP) blade cascade and a plurality of dummy members that are attached to a common rotor shaft. The steam turbine may include, but is not limited to: 
         [0029]    a detection unit that detects a steam flow into an intermediate-pressure (IP) chamber; 
         [0030]    a pressure reducing unit that reduces a pressure difference between both sides of a target dummy member of said plurality of the dummy members when the steam flow into the IP chamber stops, the target dummy member having one side communicating with a part of the IP chamber; and 
         [0031]    a control unit that controls the pressure reducing unit based on a detection result obtained by the detection unit. 
         [0032]    In this manner, the thrust force generated at the IP dummy member when the steam flow into the IP chamber stops can be eliminated. Thus, it is no longer necessary to increase the diameter of the LP dummy member which was conventionally needed to balance the thrust force generated at the IP dummy member. As a result, the diameter of the LP dummy member can be reduced and the thrust forces acting on the rotor of the steam turbine can be balanced in the entire operation range of the steam turbine without using the electric control of the complicated system. 
         [0033]    The above pressure reducing unit may include, but is not limited to, a first conduit that connects the both sides of the target dummy member and a first valve that is provided in the first conduit to adjust the pressure difference between the both sides of the target dummy member. 
         [0034]    In this way, the thrust forces acting on the rotor shaft of the steam turbine can be balanced, with a simple configuration. 
         [0035]    The above steam turbine may further include: 
         [0036]    a third conduit that connects the one side of the pressure reducing unit to an outlet of the IP chamber; and 
         [0037]    a third valve that is provided in the third conduit. 
         [0038]    When the first valve opens while the steam flow into the IP chamber is not stopped, the control unit may control the third valve to open so as to generate the pressure difference between the both sides of the target dummy member. 
         [0039]    In this way, even when the first valve is out of order, the thrust forces generated in the steam turbine can be balanced and, the reliability of the steam turbine can be enhanced. 
         [0040]    The above pressure reducing unit may include, but is not limited to: 
         [0041]    a second conduit that connects the part of the IP chamber and the one side of the target dummy member; and 
         [0042]    a second valve that is provided in the second conduit to adjust the difference between the both sides of the target dummy member. 
         [0043]    The second valve may be closed when the steam flow into the IP chamber stops. 
         [0044]    In relation to the above, the second conduit is often provided even in the conventional steam turbines. Thus, in remodeling or modernizing the conventional existing steam turbine, the pressure reducing unit can be provided by simply fitting the second valve to the existing second conduit without newly installing a conduit to the steam turbine. Thus, the remodeling can be easily accomplished. 
         [0045]    The above steam turbine may further include: 
         [0046]    a bypass conduit that is provided to bypass the second valve; and 
         [0047]    an orifice that is provided in the bypass conduit. 
         [0048]    In this way, the thrust forces generated in the steam turbine can be easily balanced. 
         [0049]    The above steam turbine may also include: 
         [0050]    a third conduit that connects the one side of the pressure reducing unit to an outlet of the IP chamber; and 
         [0051]    a third valve that is provided in the third conduit. 
         [0052]    When the second valve closes while the steam flow into the IP chamber is not stopped, the control unit may control the third valve to open so as to generate the pressure difference between the both sides of the target dummy member. 
         [0053]    To achieve the object of the present invention, the present invention provides a method of adjusting a thrust force of a steam turbine having at least a HP blade cascade, an IP blade cascade and a plurality of dummy members that are attached to a common rotor shaft. The method may include, but is not limited to, the step of reducing a pressure difference between both sides of a target dummy member of said plurality of the dummy members when the steam flow into the IP chamber stops, the target dummy member having one side communicating with a part of the IP chamber. 
         [0054]    Further, in the above method, the pressure difference between the both sides of the target dummy member may be reducible by use of a first valve provided in a first conduit that connects the both sides of the target dummy member. 
         [0055]    Further, in the above method, when the first valve opens while the steam flow into the IP chamber is not stopped, the pressure difference may be generated between the both sides of the target dummy member by opening a third valve which is provided in a third conduit that connects the one side of the target dummy member to an outlet of the IP chamber. 
         [0056]    In the above method of adjusting the thrust force of the steam turbine, the pressure difference between the both sides of the target dummy member may be reducible by use of a second valve provided in a second conduit that connects the part of the IP chamber and the one side of the target dummy member. 
         [0057]    In the above method of adjusting the thrust force of the steam turbine, when the second valve closes while the steam flow into the IP chamber is stopped, the pressure difference may be generated between the both sides of the target dummy member by opening a third valve which is provided in a third conduit that connects the one side of the target dummy member to an outlet of the IP chamber. 
       EFFECTS OF THE INVENTION  
       [0058]    According to the present invention, the steam turbine and the method of adjusting the thrust force of the steam turbine can be provided which are operable to balance the thrust forces in the entire operation range of the steam turbine without upsizing the LP dummy member, as well as, without using the electric control of a complicated system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0059]      FIG. 1  shows a configuration of a single-casing reheat steam turbine provided with a plurality of dummy members for adjusting thrust forces, according to a first preferred embodiment of the present invention. 
           [0060]      FIG. 2  shows an outline of a normal operating state of the steam turbine provided with the dummy members for adjusting thrust forces, according to the first preferred embodiment of the present invention. 
           [0061]      FIG. 3  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces when the supply of the HP main steam is stopped, according to the first preferred embodiment of the present invention. 
           [0062]      FIG. 4  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces when the supply of the reheat steam and the LP main steam is stopped, according to the first preferred embodiment of the present invention. 
           [0063]      FIG. 5  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces in a case where a valve is in an abnormal condition in the normal operating state of the steam turbine, according to the first preferred embodiment of the present invention. 
           [0064]      FIG. 6  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces, after taking a countermeasure against the malfunction of the valve in the normal operating state of the steam turbine, according to the first preferred embodiment of the present invention. 
           [0065]      FIG. 7  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces in a case where another valve is in an abnormal condition in the normal operating state of the steam turbine, according to the first preferred embodiment of the present invention. 
           [0066]      FIG. 8  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces, after taking a countermeasure against the malfunction of said another valve in the normal operating state of the steam turbine, according to the first preferred embodiment of the present invention. 
           [0067]      FIG. 9  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces, according to the first preferred embodiment of the present invention, in a case where the function of a valve is out of order while the supply of the reheat steam and the LP main steam is stopped. 
           [0068]      FIG. 10  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces, according to the first preferred embodiment of the present invention, after taking a countermeasure against the malfunction of the valve while the supply of the reheat steam and the LP main steam is stopped. 
           [0069]      FIG. 11  shows an outline of a HP-IP steam turbine provided with the dummy members for adjusting thrust forces, according to a second preferred embodiment of the present invention. 
           [0070]      FIG. 12  shows an outline of a HP-IP steam turbine provided with the dummy members for adjusting thrust forces, according to a third preferred embodiment of the present invention. 
           [0071]      FIG. 13  shows an outline of a normal operating state of the steam turbine provided with conventional dummy members. 
           [0072]      FIG. 14  shows an outline of a state of the steam turbine provided with conventional dummy parts for adjusting thrust balance, when the supply of the HP main steam is stopped. 
           [0073]      FIG. 15  shows an outline of a state of the steam turbine provided with conventional dummy parts for adjusting thrust balance, when the supply of the reheat steam and the LP main steam is stopped. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0074]    A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shape, its relative positions and the like shall be interpreted as illustrative only and not limitative of the scope of the present invention. 
       PREFERRED EMBODIMENTS 
     First Preferred Embodiment 
       [0075]      FIG. 1  shows a configuration of a single-casing reheat steam turbine provided with a plurality of dummy members for adjusting thrust forces, according to a first preferred embodiment of the present invention. In a steam turbine shown in  FIG. 1 , a low-pressure (LP) casing  32  and a HP-IP casing  34  (a high-intermediate-pressure casing) are formed around a rotor shaft  10 . The HP-IP casing  34  is provided with a high-pressure (HP) steam inlet  23  through which HP steam  22  is supplied to the steam turbine and a reheat steam inlet  25  through which reheat steam  24  is supplied to the steam turbine. Further, the LP casing  32  is provided with a low-pressure (LP) steam inlet  27  through which LP steam  26  is supplied to the steam turbine. 
         [0076]    To the rotor shaft  10 , attached are a HP blade cascade  2  to which the HP main steam is supplied, an intermediate-pressure (IP) blade cascade  4  to which the reheat steam  24  is supplied and a low-pressure (LP) blade cascade  6  to which the LP main steam  26  is supplied in this order. 
         [0077]    In the steam turbine, steam inlet sides of the IP blade cascade  4  and the LP blade cascade  6  are arranged such that the steam streams through the IP blade cascade  4  and the LP blade cascade  6  in the same direction, whereas a steam inlet side of the HP blade cascade  2  is arranged such that the steam streams through the HP blade cascade  2  in the opposite direction. Further, a HP dummy member  12  is provided between the steam inlet side of the HP blade cascade  2  and the steam inlet side of the IP blade cascade  4 . On the steam outlet side of the HP blade cascade  2 , an IP dummy member  14  and a LP dummy member  16  are provided in this order. Furthermore, a thrust balance conduit  30  is provided to communicate the steam outlet side of the IP dummy member  14  to a part of the IP blade cascade  4 . 
         [0078]      FIG. 2  shows an outline of a normal operating state of the steam turbine provided with the dummy members for adjusting thrust forces. Hereby, the same components in  FIG. 2  as in  FIG. 1 ,  FIG. 13  through  FIG. 15  are given common numerals and are not explained further. Herein, a normal operating state means an operating state of the steam turbine in which all of the HP steam  22 , the reheat steam  24  and the LP steam  26  are supplied to the steam turbine. 
         [0079]    Differently from the conventional technology shown in  FIG. 13 , in the first preferred embodiment of the present invention as shown in  FIG. 2 , the diameter of the IP dummy member  14  is upsized in comparison with the conventional dummy member  14 , whereas the diameter of the LP dummy member  16  is downsized in comparison with the conventional dummy member  16 . With the LP dummy member  16  having larger diameter, the thrust forces of the steam turbine as a whole are prevented from being unbalanced. 
         [0080]    Further, a conduit  42  is provided to communicate the steam inlet side of the IP dummy member  14  to the steam outlet side thereof and a valve  43  is provided on the conduit  42 . A conduit  44  is connected to the conduit  42  on a side closer to the steam outlet side of the IP dummy part than the valve  43  and in communication to the steam outlet side of the IP blade cascade  4 . A valve  45  is provided on the conduit  44 . A valve  41  is provided on the thrust balance conduit  30 . 
         [0081]    Further, a control unit  52  is provided. The control unit  52  reads a detected value detected by a pressure sensor  54  which is provided at the reheat steam inlet  25  and controls opening and closing of the valves  41 ,  43  and  45  based on the detected value. In the normal operating state where the reheat steam  24  is supplied to the steam turbine  1  and the pressure detected by the pressure sensor  54  is within a normal pressure range of the reheat steam  24 , the control unit  52  controls the valve  41  to open and the valves  43  and  45  to close as shown in  FIG. 2 . As for the open-close state of the valves in the attached drawings, the valve mark filled in with black indicates an opened state, whereas the valve mark filled in with white indicates a closed state. 
         [0082]    In  FIG. 2  through  FIG. 10  and  FIG. 13  through  FIG. 15 , the unit k denotes a pressure value in kgf/cm 2  to show pressure values as only example values at indicated places. 
         [0083]    As shown in  FIG. 2 , the steam turbine is provided with the dummy members  12 ,  14  and  16 , and the thrust balance conduit  30 . In the normal operating state, the resultant thrust force generated by the steam pressures is balanced. 
         [0084]    Next, a case where the supply of the HP main steam  22  is stopped in the steam turbine  1  shown in  FIG. 2  is explained.  FIG. 3  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces of the present invention when the supply of the HP main steam is stopped. In  FIG. 3  through  FIG. 15 , the control unit  52  is omitted. 
         [0085]    In  FIG. 3 , when the supply of the HP main steam  22  is stopped, no steam is supplied to the HP blade cascade  2 , and the pressure difference at the HP blade cascade  2  becomes 0. Thus, the thrust force represented by the encircled numeral  3  as depicted in  FIG. 14  also becomes 0. Accordingly, the pressure difference at the HP dummy member  12  becomes significantly small and the thrust force represented by the encircled numeral  4  becomes close to 0. Therefore, as shown in  FIG. 3 , even when the supply of the HP main steam  22  is stopped, the resultant thrust force acting on the whole steam turbine  1  is balanced. 
         [0086]    Next, a case where the supply of the reheat steam  24  and the LP main steam  26  is stopped in the steam turbine  1  shown in  FIG. 2  is explained.  FIG. 4  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces of the present invention when the supply of the reheat steam and the LP main steam is stopped. 
         [0087]    In  FIG. 4 , when the supply of the reheat steam  24  and the LP main steam  26  is stopped, no steam is supplied to the IP blade cascade  4  and the LP blade cascade  6 . This causes the pressures at both sides of the IP blade cascade  4  and the LP blade cascade  6  to be approximately at vacuum level. In the HP system (the high-pressure part of the steam turbine), the thrust force represented by the encircled numeral  3  generated at the HP blade cascade  2  is almost the same as the thrust force in the normal operating state of the steam turbine. However, the thrust force represented by the encircled numeral  2  generated at the HP dummy member  12  increases in response to the increase regarding the level of the vacuum pressure at the outlet of the HP dummy member  12 . By this, the thrust force acting on the HP dummy member  12  increases in the direction of the steam flow along the HP dummy member (in the rightward direction in  FIG. 4 ). 
         [0088]    When the control unit  52  (not shown in  FIG. 4 ) determines that the reheat steam  24  is not supplied based on the pressure value detected by the pressure sensor  54  (not shown in  FIG. 4 ), the control unit  52  opens the valve  43 . By this, the pressure difference between both sides of the IP dummy member  14  becomes approximately 0. Specifically, in a case where the reheat steam  24  is not supplied to the steam turbine in the conventional technology, an excessive thrust force is generated at the IP dummy member  14  in the leftward direction. On the other hand, in this preferred embodiment of the present invention, the thrust force can be prevented from being generated at the IP dummy member  14 . 
         [0089]    Further, in the case of  FIG. 4 , the diameter of the LP dummy member  16  is designed so as to generate a counter thrust force (leftward in  FIG. 4 ) approximately by an amount corresponding to the above-described increased thrust force generated in the HP system. Thus, the thrust force generated in the whole steam turbine  1  is balanced. 
         [0090]    In addition, the diameter of the LP dummy member  16  is designed in advance so as to balance the thrust forces in a case where the valves  41  and  43  are opened in the state where the supply of the reheat steam and the LP main steam is stopped and, the diameter of the IP dummy member  14  is designed in advance so as to balance the thrust forces in the normal operating state and the state where the supply of the HP main steam is stopped. In this way, the thrust force is prevented from being generated at the IP dummy member  14  when the supply of the reheat steam and the LP main steam is stopped, and it becomes unnecessary to upsize the diameter of the LP dummy member  16 , apart from the conventional technology in which the diameter upsizing was inevitable. Hence, the diameter of the LP dummy member  16  can be small and the steam leakage to the gland can be reduced. As a result, the performance of the steam turbine can be enhanced. 
         [0091]    Next, the countermeasures against the possible abnormal-conditions that may be caused by providing the valves  41 ,  43  and  45  are explained. 
         [0092]    First, abnormal conditions of the valve  43  are now explained. 
         [0093]      FIG. 5  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces of the present invention in a case where the valve  43  is in an abnormal condition in the normal operating state of the steam turbine. 
         [0094]    In  FIG. 5 , when the valve  43  becomes out of order due to a fault and so on, the valve  43  is opened and then both sides of the IP dummy member  14  are in communication with each other, and the pressure at the steam outlet side of the IP dummy member  14  increases. And, the pressure difference between both sides of the IP dummy member  14  becomes almost 0. Thus, the thrust force generated at the IP dummy member  14  becomes almost 0. As a result, the resultant thrust force of the whole steam turbine  1  becomes unbalanced. 
         [0095]    In the event as described above, the pressure detected by a pressure sensor  56  provided in the thrust balance conduit  30  increases. When the detected pressure value exceeds a prescribed value, then the control unit  52  (not shown in  FIG. 5 ) determines that the valve  43  or  41  is not working properly. 
         [0096]    Once it is determined that the valve  43  or  41  is not working properly, the control unit  52  opens the valve  45 . 
         [0097]      FIG. 6  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces of the present invention, after taking a countermeasure against the malfunction of the valve  43  in the normal operating state of the steam turbine. 
         [0098]    When the control unit  52  opens the valve  45 , the steam outlet side of the IP dummy member  14  communicates with the steam outlet side of the IP blade cascade  4  via the conduit  44 . A part of the steam at the steam outlet side of the IP dummy member  14  streams to the steam outlet side of the IP blade cascade  4 . This causes the pressure at the steam outlet side of the IP dummy member  14  to drop so that the pressure difference between both sides of the IP dummy member  14  is generated, thereby generating the thrust force at the IP dummy member  14 . As a result, the resultant thrust force generated in the whole steam turbine  1  is balanced. In addition, it is necessary to design the conduits  44  and the valve  45  in advance so that the steam flow rate through the conduit  44  is almost the same as the steam flow rate through the valve  43  when the valve  45  is opened in a case when the valve  43  is abnormally opened. 
         [0099]    As described above, even when the valve  43  is in the abnormal condition, the resultant thrust force can be kept balanced; thus, the reliability of the steam turbine  1  can be enhanced with additionally provided simple-components. 
         [0100]    Next, the abnormal-conditions of the valve  41  are explained. 
         [0101]      FIG. 7  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces of the present invention in a case where the valve  41  is in an abnormal condition in the normal operating state of the steam turbine. 
         [0102]    In  FIG. 7 , when the valve  41  becomes out of order because of a fault and so on and the valve  41  is closed, then the steam at the outlet side of the IP dummy member  14  is no longer able to stream toward the IP blade cascade  4  through the thrust balance conduit  30 . On the other hand, when there is a pressure difference between both sides of the IP dummy member  14 , the steam in a labyrinth seal provided at an outer periphery of the IP dummy member  14  leaks toward the steam outlet side thereof. Thus, the pressure difference between both sides of the IP dummy member  14  becomes approximately 0. Accordingly, the thrust force acting on the IP dummy member  14  becomes approximately 0. As a result, the resultant force is unbalanced. 
         [0103]    In the event as described above, the pressure detected by the pressure sensor  56  provided on the thrust balance conduit  30  increases. When the detected pressure exceeds a prescribed value, then the control unit  52  (not shown in  FIG. 5 ) determines that the valve  43  or  41  is not working properly. 
         [0104]    Once it is determined that the valve  43  or  41  is not working properly, the control unit  52  opens the valve  45 . 
         [0105]      FIG. 8  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces of the present invention, after taking a countermeasure against the malfunction of the valve  41  in the normal operating state of the steam turbine. 
         [0106]    When the control unit  52  opens the valve  45 , the steam outlet side of the IP dummy member  14  communicates with the steam outlet side of the IP blade cascade  4  via the conduit  44 . Then, a part of the steam at the steam outlet side of the IP dummy member  14  streams into the steam outlet side of the IP blade cascade  4 . Thus, the pressure at the steam outlet side of the IP dummy member  14  drops so that the pressure difference between both sides of the IP dummy member  14  is generated. Accordingly, the thrust force is generated at the IP dummy member  14  so that the resultant thrust force generated in the whole steam turbine  1  is balanced. 
         [0107]    As described above, even when the abnormal condition of the valve  41  happens, the resultant thrust force is kept balanced. Thus, the reliability of the steam turbine can be enhanced with additional simple-components. 
         [0108]    Next, abnormal-conditions that may occur on the valve  41  in a case where the supply of the reheat steam and the LP main steam is stopped are explained. 
         [0109]      FIG. 9  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces of the present invention, in a case where the function of the valve  43  is out of order while the supply of the reheat steam and the LP main steam is stopped 
         [0110]    As described already based on  FIG. 3 , it is necessary to open the valve  43  in the case where the supply of the reheat steam and the LP main steam is stopped. However,  FIG. 9  shows the case where the valve  43  stays closed. 
         [0111]    In  FIG. 9 , with the valve  43  being closed, both sides of the IP dummy member  14  are not in communication with each other. Thus, the pressure difference between both sides of the IP dummy member  14  is generated so that the thrust force is generated at the dummy member  14 . The thrust force generated at the dummy member  14  causes the resultant thrust force generated in the whole steam turbine  1  to be unbalanced. In the present invention, the diameter of the IP dummy member  14  is greater than that of the conventional IP dummy member. For a corresponding amount, the unbalance (i.e. being out of balance) regarding the developed resultant thrust force increases. 
         [0112]    In the event as described above, the pressure detected by the pressure sensor  56  provided on the thrust balance conduit  30  drops. When the detected pressure value is processed below a prescribed value, then the control unit  52  (not shown in  FIG. 9 ) determines that the valve  43  is not working properly. 
         [0113]    Once it is determined by the control unit  52  that the valve  43  is not working properly, the control unit  52  closes the valve  41 . 
         [0114]      FIG. 10  shows an outline of a state of the steam turbine provided with the dummy members for adjusting thrust forces of the present invention, after taking a countermeasure against the malfunction of the valve  43  while the supply of the reheat steam and the LP main steam is stopped. 
         [0115]    With the valve  41  being closed, the pressure difference between both sides of the IP dummy member  14  becomes approximately 0 due to the steam leakage from the IP dummy member  14 . Accordingly, the thrust force acting on the IP dummy member  14  becomes almost 0. 
         [0116]    In this manner, the resultant thrust force is balanced as is the case with  FIG. 4  in which there is no abnormal condition regarding the valve  43 . 
         [0117]    Specifically, the resultant force is kept balanced, even when the abnormal condition regarding the valve  43  takes place. 
       Second Preferred Embodiment 
       [0118]    The disclosed technology of the present invention is also applicable to HP-IP steam turbines. 
         [0119]      FIG. 11  shows an outline of a HP-IP steam turbine provided with the dummy members for adjusting thrust forces according to a second preferred embodiment of the present invention. 
         [0120]    The HP-IP steam turbine  101  depicted in  FIG. 11  is provided with a turbine casing (not shown) is formed around a rotor shaft (not shown). The turbine casing encloses the inlet parts (not shown) for introducing HP steam and IP steam. 
         [0121]    Further, a high-pressure (HP) chamber blade cascade  102  to which the HP steam is supplied and an intermediate-pressure (IP) chamber blade cascade  104  to which the IP steam is supplied are attached to the rotor shaft such that steam inlets of the HP chamber blade cascade  102  and the IP chamber blade cascade  104  are disposed facing each other. Further, between the steam inlet of the HP chamber blade cascade  2  and the steam inlet the IP chamber blade cascade  104 , a first dummy member  111  and a second dummy member  112  are provided. Further, a third dummy member  113  is provided at a steam outlet of the HP chamber blade cascade  102 . Further, a balance conduit  121  is provided to communicate a location between the first dummy member  111  and the second dummy member  112  to both sides of the third dummy member  113 . Furthermore, a balance conduit  122  is provided to communicate the steam outlet of the third dummy member  113  to the steam outlet of the IP chamber blade cascade  104 . In addition, a valve  141  is provided on the balance conduit  121  between both sides of the third dummy member  113  and the downstream side of the third dummy member  113  and a valve  142  is provided on the balance conduit  122 . 
         [0122]    In relation to the HP-IP steam turbine as described above, the table in  FIG. 11  summarizes a balance of the thrust forces of the cases, when the turbine is operated normally, the supply of the HP steam is stopped (the HP system is closed), and the IP steam is stopped (the IP system is closed). The figures of the thrust forces in the table of  FIG. 7  show not the absolute values but the relative ratios among thrust forces appearing in design calculations. 
         [0123]    As shown in  FIG. 7 , in the normal operation state, the resultant thrust force is substantially balanced when the HP system is closed. In contrast, when the IP system is closed, the resultant thrust force becomes unbalanced because of the thrust force acting on the third dummy member  113  and the resultant thrust force increases rightward. In this event, when the valve  41  (CV 1 ) is opened, the pressure difference between both sides of the third dummy member  113  is reduced and thus, the resultant thrust force generated in the whole steam turbine can be balanced. In addition, when the IP system is closed, the pressure difference between both sides of the third dummy member  113  can be also reduced via the steam leakage through the dummy member  113 , by appropriately closing the valve  142  instead of opening the valve  141 . As a result, the resultant thrust force generated in the whole steam turbine can be balanced. 
       Third Preferred Embodiment 
       [0124]      FIG. 12  shows an outline of the HP-IP steam turbine provided with the dummy members for adjusting thrust forces according to a third preferred embodiment of the present invention. 
         [0125]    The same components in  FIG. 12  as in  FIG. 11  are given common numerals and are not explained further. 
         [0126]    In  FIG. 12 , a first dummy member  111 ′ is provided. The first dummy member  111 ′ is formed by integrating the first dummy member  111  and the second dummy member  112  (shown in  FIG. 11 ), whose diameter is as same as the diameter of the first dummy member  111 . Hereby, the steam turbine in  FIG. 12  is not provided with the balance conduit  121 . Instead, the balance conduit  122  is provided with a bypass conduit  123  that bypasses the valve  142 . Further, an orifice  124  is provided on the bypass conduit  123 . 
         [0127]    In a manner similar to the second preferred embodiment, the resultant thrust force can be balanced, except when the IP system is closed. When the IP system is closed, the resultant thrust force can be balanced by adjusting the opening of the valve  142 . 
         [0128]    In the above event, when it is difficult to adjust the opening of the valve such as setting the opening of the valve  142  at a minimal level, it is recommendable to close the valve  42  and use the orifice  123 . In relation to this event, it is necessary to set the size of the orifice in advance so that with the valve  142  being full-closed, the steam pressure at a back side of the third dummy member  113  is appropriate. 
         [0129]    In other words, in a case where the IP system is closed, the valve  142  is closed and the steam streams through the orifice  124 . Thus, the steam pressure at the back side of the third dummy member  113  is appropriately maintained. Hence, the resultant thrust force can be balanced. 
       INDUSTRIAL APPLICABILITY  
       [0130]    According to the present invention, it is possible to provide a steam turbine and a method of adjusting a thrust force of the steam turbine acting on a rotor shaft of the turbine in an entire operating range of the steam turbine without upsizing a LP dummy member or without using an electric control of a complicated system.