Patent Publication Number: US-9850781-B2

Title: Steam turbine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-134449, filed on Jun. 27, 2013; the entire contents of all of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a steam turbine. 
     BACKGROUND 
     At a low-pressure part of a nuclear power turbine, a geothermal turbine, or a thermal power turbine, a temperature and a pressure of steam being a working fluid are low. Accordingly, a part of the steam is condensed during expansion work to be water droplets to adhere to an inner wall of a steam passage, a stationary blade, and a rotor blade. The water droplets generated at the steam passage grow into the water droplets whose particle diameters are large. The water droplets whose particle diameters are large collide with a leading edge and so on of the rotor blade, and thereby, the rotor blade is eroded and a collision resistance relative to a rotation of the rotor blade is generated to lower turbine efficiency. 
     Here, a flow of the steam and so on in a vicinity of a turbine stage at a final stage in a general low-pressure turbine is described.  FIG. 12  is a view illustrating a meridian cross section in a vicinity of a final turbine stage in a conventional low-pressure turbine. Note that in  FIG. 12 , a dotted line represents a streamline of the steam, and a solid line represents a trace of the generated water droplets. 
     As illustrated in  FIG. 12 , diaphragm outer rings  310   a ,  310   b , diaphragm inner rings  311   a ,  311   b  are included at an inner side of a casing  300 . Between the diaphragm outer rings  310   a ,  310   b  and the diaphragm inner rings  311   a ,  311   b , plural stationary blades  312   a ,  312   b  are supported in a circumferential direction to make up a stationary blade cascade. 
     At an immediate downstream side of the stationary blade cascade, a rotor blade cascade in which plural rotor blades  322   a ,  322   b  are implanted into rotor disks  320   a ,  320   b  of a turbine rotor in the circumferential direction is made up. A single stage turbine stage is made up by the stationary blade cascade and the rotor blade cascade positioning at the immediate downstream thereof. In  FIG. 12 , a final turbine stage  330  and a turbine stage  331  at upstream side for one stage than the final turbine stage  330  are illustrated. At the rotor blades  322   a ,  322   b , a speed energy of the steam expanded at the stationary blades  312   a ,  312   b  is converted into a rotational energy to generate a motive power. 
     As illustrated in  FIG. 12 , the steam expands along the steam passage which increases in size as it goes downstream. The water droplets are generated at a part where the pressure and the temperature of the steam descend, for example, at the turbine stage  331  which is at upstream for one stage than the final turbine stage  330 . 
     A part of the generated water droplets flows toward the diaphragm outer ring  310   a  of the turbine stage  330  affected by the centrifugal force and the coriolis force. Accordingly, a lot of water droplets adhere to an inner surface of the diaphragm outer ring  310   a  of the turbine stage  330  to form a water film. 
     Besides, remaining water droplets collide with and adhere to a surface of the stationary blade  312   a  to form the water film. A water film reaching a trailing edge of the stationary blade  312   a  is blown and torn off by a steam flow at the trailing edge to be the water droplets. The water droplets collide with the rotor blade  322   a  at the immediate downstream side to erode the rotor blade  322   a , activate a force in a reverse direction with a rotational direction to lower turbine efficiency. 
     Here,  FIG. 13  is a view illustrating a distribution of wetness of the steam in a blade height direction of the stationary blade at the final turbine stage of the conventional low-pressure turbine. Note that a vertical axis represents a blade height ratio in which each blade height position is divided by a blade height. For example, when the blade height ratio is “1”, it indicates a blade tip of the stationary blade, and when the blade height ratio is “0” (zero), it indicates a blade root of the stationary blade. As illustrated in  FIG. 13 , the wetness becomes high at the tip side of the stationary blade, namely, at the diaphragm outer ring side. Accordingly, an adverse effect caused by the generated water droplets becomes remarkable at the diaphragm outer ring side. 
     In the conventional steam turbine, a technology to remove the generated water droplets and water film has been studied to suppress the erosion by the water droplets and the lowering of the turbine efficiency. As the technology to remove the water droplets and the water film, there is a technology providing plural through holes in the circumferential direction of the diaphragm outer ring to remove the water film adhered to an inner surface of the diaphragm outer ring. 
     However, when the plural through holes are provided in the circumferential direction of the diaphragm outer ring, the through holes are formed at a limited area between the stationary blade and the rotor blade, and therefore, it is impossible to enough expand a bore diameter. Accordingly, it is necessary to form a lot of through holes in the circumferential direction to uniformly remove the water film and the water droplets in the circumferential direction. This incurs complication of a manufacturing process, and increase in manufacturing cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a cross section in a vertical direction of a steam turbine according to a first embodiment. 
         FIG. 2  is a view illustrating a cross section in a vertical direction at an upper half side of a final turbine stage at the steam turbine according to the first embodiment. 
         FIG. 3  is a view illustrating an A-A cross section in  FIG. 2 . 
         FIG. 4  is a plan view when a diaphragm outer ring of the steam turbine according to the first embodiment is seen from outside in a radial direction. 
         FIG. 5  is a view illustrating a cross section in a vertical direction of a part of the upper half side of the final turbine stage at the steam turbine according to the first embodiment to explain a constitution of another annular slit. 
         FIG. 6  is a view illustrating a cross section in a vertical direction of a part of the upper half side of the final turbine stage at the steam turbine according to the first embodiment to explain a constitution of still another annular slit. 
         FIG. 7  is a view illustrating a cross section in a vertical direction of a part of the upper half side of the final turbine stage at the steam turbine according to the first embodiment to explain a constitution of yet another annular slit. 
         FIG. 8  is a view illustrating a cross section in a vertical direction of a part of the upper half side of the final turbine stage at the steam turbine according to the first embodiment to explain a constitution of another stationary blade. 
         FIG. 9  is a view illustrating a cross section in a vertical direction at an upper half side of a final turbine stage in a steam turbine according to a second embodiment. 
         FIG. 10  is a view illustrating a B-B cross section in  FIG. 9 . 
         FIG. 11  is a view illustrating a circumferential distribution of a suction velocity in a radial direction at an inlet of the annular slit. 
         FIG. 12  is a view illustrating a meridian cross section in a vicinity of a final turbine stage at a conventional low-pressure turbine. 
         FIG. 13  is a view illustrating a distribution of wetness of steam in a blade height direction of a stationary blade at the final turbine stage of the conventional low-pressure turbine. 
     
    
    
     DETAILED DESCRIPTION 
     In one embodiment, provided is a steam turbine where wet steam flows in a turbine stage in low-pressure. This steam turbine includes: rotor blades implanted in a circumferential direction to a turbine rotor provided to penetrate in a casing; stationary blades provided at an immediate upstream side of the rotor blades in the circumferential direction, and making up a turbine stage together with the rotor blades; diaphragm outer rings provided inside the casing, each including an annular extending part surrounding a periphery of the rotor blades, and supporting the stationary blades from outside in a radial direction; and diaphragm inner rings supporting the stationary blades from inside in the radial direction. The steam turbine further includes: an annular slit formed at an inner surface of the diaphragm outer ring between the stationary blades and the rotor blades along the circumferential direction; and communication holes provided in plural at an outer surface of the diaphragm outer ring along the circumferential direction, communicated to the annular slit from the outer surface side of the diaphragm outer ring, and communicated to a suction part sucking liquid via the annular slit. 
     Hereinafter, embodiments of the present invention are described with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a view illustrating a cross section in a vertical direction of a steam turbine  10  according to a first embodiment. Note that the steam turbine described hereinbelow is a low-pressure turbine. 
     As illustrated in  FIG. 1 , the steam turbine  10  includes a casing  20 . A turbine rotor  21  is provided to penetrate in the casing  20 . Rotor disks  21   a  are formed at the turbine rotor  21 . Plural rotor blades  22  are implanted to the rotor disk  21   a  in a circumferential direction. Rotor blade cascades including the plural rotor blades  22  in the circumferential direction are made up in plural stages in an axial direction of the turbine rotor  21 . Note that the turbine rotor  21  is rotatably supported by a not-illustrated rotor bearing. 
     Diaphragm outer rings  23  are provided at an inner side of the casing  20 . Each diaphragm outer ring  23  extends annularly toward a downstream side, and includes an annular extending part  24  surrounding a periphery of the rotor blade  22 . Diaphragm inner rings  25  are provided at an inner side of the diaphragm outer ring  23 . 
     Besides, plural stationary blades  26  are disposed in the circumferential direction between the diaphragm outer ring  23  and the diaphragm inner ring  25  to make up a stationary blade cascade. The diaphragm outer ring  23  supports the stationary blade  26  from outside in a radial direction. The diaphragm inner ring  25  supports the stationary blade  26  from inside in the radial direction. The stationary blade cascades are included in plural stages alternately with the rotor blade cascades in the axial direction of the turbine rotor  21 . One turbine stage is made up by the stationary blade cascade and the rotor blade cascade positioning at an immediate downstream side. 
     An annular steam passage  27  where main steam flows is formed between the diaphragm outer ring  23  and the diaphragm inner ring  25 . A flow passage cross section of the steam passage  27  gradually expands as it goes, for example, downstream. A gland sealing part  28  is provided between the turbine rotor  21  and the casing  20  to prevent leakage of the steam toward outside. Besides, a sealing part  29  is provided between the turbine rotor  21  and the diaphragm inner ring  25  to prevent that the steam leaks therebetween toward downstream side. 
     Besides, a steam inlet pipe (not-illustrated) to introduce the steam from a crossover pipe  30  into the steam turbine  10  is provided at the steam turbine  10 . This steam inlet pipe is provided to penetrate the casing  20 . An exhaust passage (not-illustrated) to exhaust the steam having performed expansion work at each turbine stage is provided at a downstream side of a final turbine stage. This exhaust passage is communicated to a condenser (not-illustrated). 
     Next, a constitution of the turbine stage which becomes low-pressure and where wet steam flows is described in detail. 
     Here, the final turbine stage is exemplified to be described as the turbine stage where the wet steam flows. Note that the turbine stage where the wet steam flows is not limited to the final turbine stage. Accordingly, when the wet steam flows, the turbine stage includes the similar constitution as the following final turbine stage even if it is the turbine stage at upstream than the final turbine stage. 
       FIG. 2  is a view illustrating a cross section in a vertical direction at an upper half side of the final turbine stage at the steam turbine  10  according to the first embodiment.  FIG. 3  is a view illustrating an A-A cross section in  FIG. 2 .  FIG. 4  is a plan view when the diaphragm outer ring  23  of the steam turbine  10  according to the first embodiment is seen from outside in the radial direction. Note that in  FIG. 2  and  FIG. 3 , water films  80  adhered to the stationary blades  26 , an annular slit  40 , and a communication hole  50  are also illustrated. 
     As illustrated in  FIG. 1 , the annular extending part  24  of the diaphragm outer ring  23  surrounds the periphery of the rotor blade  22  with a minute gap. The annular slit  40  is formed between the stationary blade  26  at an inner surface of the diaphragm outer ring  23  and the rotor blade  22  along the circumferential direction. Namely, the annular slit  40  is made up of an annular groove successive in the circumferential direction. 
     This annular slit  40  is formed so as not to penetrate the diaphragm outer ring  23  from an inner surface  23   a  of the diaphragm outer ring  23  along outside in the radial direction. A groove depth of the annular slit  40  heading from the inner surface  23   a  of the diaphragm outer ring  23  toward outside in the radial direction is, for example, approximately 20% to 50% of a thickness of the diaphragm outer ring  23 . 
     Plural communication holes  50  are formed at an outer surface  23   b  of the diaphragm outer ring  23  with a predetermined interval (pitch) toward the circumferential direction. The communication holes  50  are formed from the outer surface  23   b  of the diaphragm outer ring  23  to a position communicating to the annular slit  40 . The communication holes  50  are, for example, formed along a radial line R extending from a center axis of the turbine rotor  21  in the radial direction at a cross section perpendicular to a turbine rotor axis direction as illustrated in  FIG. 3 . Namely, the communication holes  50  are, for example, radially formed centering on the center axis of the turbine rotor  21 . The communication hole  50  is made up of, for example, a round hole and so on as illustrated in  FIG. 4 . 
     Besides, as illustrated in  FIG. 2 , a protruding ridge part  60  protruding toward outside in the radial direction is formed at an upstream side than the communication hole  50  of the outer surface  23   b  of the diaphragm outer ring  23 . This protruding ridge part  60  is in contact with a protruding ridge part  70  protruding toward inside in the radial direction formed at an inner surface  20   b  of the casing  20 . The protruding ridge part  60  functions as a sealing part. A space surrounded by the diaphragm outer ring  23  at downstream side than the sealing part and the casing  20  is communicated to, for example, an exhaust chamber (not-illustrated) to exhaust the steam. Namely, the communication hole  50  is communicated to, for example, the exhaust chamber (not-illustrated) to exhaust the steam. Note that the communication hole  50  may be constituted to be, for example, directly communicated to the condenser without being intervened by the exhaust chamber. 
     Next, operations of the steam turbine  10  are described with reference to  FIG. 1  to  FIG. 3 . 
     The steam flowing into the steam turbine  10  via the steam inlet tube (not-illustrated) from the crossover tube  30  passes through the steam passage  27  including the stationary blades  26  and the rotor blades  22  of each turbine stage while performing the expansion work, to rotate the turbine rotor  21 . 
     A pressure and a temperature of the steam are lowered as it goes downstream. The pressure and the temperature of the steam are lowered to be wet steam, and water droplets are generated. 
     A part of the generated water droplets is affected by the centrifugal force and the coriolis force, and flows toward the diaphragm outer ring  23  side. Accordingly, a lot of water droplets adhere to the inner surface of the diaphragm outer ring  23  to form the water film  80 . Besides, the remaining water droplets collide with and adhere to a surface of the stationary blade  26  to form the water film  80  as illustrated in  FIG. 2 . The water film  80  adhered to the stationary blade  26  accumulates at an outer periphery side of the steam passage  27 , namely, at the diaphragm outer ring  23  side by the centrifugal force. 
     Here, a pressure of the annular slit  40  at the steam passage  27  side is approximately the same as an outlet pressure of the stationary blade  26 . The outlet pressure of the stationary blade  26  is larger than a pressure at an opening formed at an outer periphery of the diaphragm outer ring  23  of the communication hole  50  communicated to the exhaust chamber (not-illustrated) exhausting the steam. 
     Accordingly, the water droplets flowing at the diaphragm outer ring  23  side and the water film  80  adhered to the inner surface of the diaphragm outer ring  23  and the stationary blade  26  are sucked from the annular slit  40  toward the communication hole  50  side. The water droplets and the water film sucked from the annular slit  40  are introduced into, for example, the exhaust chamber at a low-pressure side via the communication hole  50 . The annular slit  40  is formed along the circumferential direction. Accordingly, the water droplets and the water film dispersed in the circumferential direction are surely collected. Note that the exhaust chamber which is in low-pressure than the opening of the communication hole  50  and sucks the water droplets and the water film functions as a suction part. 
     The steam passing through the final turbine stage passes through the exhaust chamber (not-illustrated), and is introduced to the condenser (not-illustrated). 
     As stated above, according to the steam turbine  10  of the first embodiment, the annular slit  40  is provided along the circumferential direction, and thereby, it is possible to surely collect (remove) the water droplets and the water film dispersed in the circumferential direction. It is thereby possible to suppress erosion caused by collision of the water droplets with the rotor blade  22  at the immediate downstream side of the stationary blade  26 , and lowering of turbine efficiency. 
     Here, a constitution of the steam turbine  10  of the first embodiment is not limited to the above-stated constitution. 
       FIG. 5  to  FIG. 7  are views each illustrating a cross section in a vertical direction of a part of an upper half side of the final turbine stage at the steam turbine  10  according to the first embodiment to explain a constitution of another annular slit  40 . 
     As illustrated in  FIG. 5 , an end part  40   a  at an upstream side of the annular slit  40  which faces the inner surface  23   a  of the diaphragm outer ring  23  may be constituted such that, for example, the end part  40   a  positions on a circumference including an intersection point X between a trailing edge of a blade tip of the stationary blade  26  and the diaphragm outer ring  23 . Namely, at the intersection point X, the end part  40   a  at the upstream side of the annular slit  40 , the trailing edge of the blade tip of the stationary blade  26 , and the inner surface  23   a  of the diaphragm outer ring  23  intersect. 
     The annular slit  40  is constituted as stated above, and thereby, it is possible to collect the water film before the water film remaining at the intersection point X scatters. 
     Besides, as illustrated in  FIG. 6 , the end part  40   a  at the upstream side of the annular slit  40  which faces the inner surface  23   a  of the diaphragm outer ring  23  may be chamfered. Here, an example in which chamfering (C chamfering) processing the end part  40   a  to a corner surface is performed is illustrated, but chamfering (R chamfering) processing the end part  40   a  to a round surface may be performed. 
     In this case, as illustrated in  FIG. 6 , it may be constituted such that an end part at an upstream side of the chamfered surface positions on the circumference including the intersection point X between the trailing edge of the blade tip of the stationary blade  26  and the diaphragm outer ring  23  as same as, for example, the end part  40   a  at the upstream side of the annular slit  40  illustrated in  FIG. 5 . 
     As stated above, the chamfering is performed, and thereby, it is possible to collect the water film remaining at the trailing edge of the blade tip of the stationary blade  26  before the water film scatters. 
     Further, as illustrated in  FIG. 7 , an acute angle made up of the turbine rotor axis direction and the inner surface  23   a  of the diaphragm outer ring  23  where the annular slit  40  is formed is set to be α. An acute angle made up of the turbine rotor axis direction and an inner surface  23   c  of the diaphragm outer ring  23  at the upstream side than the annular slit  40  is set to be β. The inner surface  23   a  may be constituted such that the acute angle α becomes smaller than the acute angle β. 
     Namely, expansion of the inner surface  23   a  of the diaphragm outer ring  23  where the annular slit  40  is formed toward downstream side is smaller than expansion of the inner surface  23   c  of the diaphragm outer ring  23  at upstream side than the annular slit  40  toward downstream side. Note that the acute angle α is illustrated by, for example, an angle made up of the turbine rotor axis direction and the inner surface  23   a  at the end part  40   a  at the upstream side of the annular slit  40  facing the inner surface  23   a  of the diaphragm outer ring  23  as illustrated in  FIG. 7 . 
     It is constituted as stated above, and thereby, the water film and the water droplets reached the annular slit  40  along the inner surface  23   c  of the diaphragm outer ring  23  having the acute angle β collide with an end part  40   b  and an end surface  40   c  at the downstream side of the annular slit  40 . Accordingly, it is possible to surely introduce and suck the water film and the water droplets into the annular slit  40 . 
     Here, the inner surface  23   a  of the diaphragm outer ring  23  where the annular slit  40  is formed may be constituted such that the acute angle α becomes “0” (zero). In this case, the inner surface  23   a  is in parallel to the turbine rotor axis direction, and therefore, a process when the annular slit  40  is formed becomes easy. 
       FIG. 8  is a view illustrating a cross section in a vertical direction of a part of an upper half side of the final turbine stage at the steam turbine  10  according to the first embodiment to explain a constitution of another stationary blade  26 . 
     As illustrated in  FIG. 8 , at a trailing edge  26   a  positioning at 90% or more of a blade height from a blade root of the stationary blade  26 , the trailing edge may be gradually extended as it goes to a blade tip  26   b . Note that an end part  90   a  at a downstream side of an extending part  90  made up by extending the trailing edge may be constituted so as to intersect with the end part  40   a  at the upstream side of the annular slit  40  facing the inner surface  23   a  of the diaphragm outer ring  23 . 
     Here, a reason why a range of the part where the extending part  90  is formed is set to be 90% or more of the blade height from the blade root is that the wetness exceeds 0.1 (10%) at 90% or more of the blade height as illustrated in  FIG. 13 . As a result of an internal observation of an actual operating steam turbine, it turns out that formations of the water film and waterway become remarkable when the wetness exceeds 0.1. 
     As stated above, the extending part  90  is included, and thereby, it is possible to collect the water film before the water film remaining at the end part  90   a  at the downstream side of the extending part  90  scatters. 
     Note that in the first embodiment, an example in which the annular slit  40  and the communications holes  50  are provided for one stage between the stationary blade  26  and the rotor blade  22  is illustrated, but the constitution is not limited thereto. The annular slit  40  and the communication holes  50  may be, for example, included in plural stages in the turbine rotor axis direction between the stationary blade  26  and the rotor blade  22 . 
     Second Embodiment 
       FIG. 9  is a view illustrating a cross section in a vertical direction of an upper half side of a final turbine stage in a steam turbine  11  according to a second embodiment.  FIG. 10  is a view illustrating a B-B cross section in  FIG. 9 . Note that the same reference symbols are added for the same components as the constitution of the steam turbine  10  according to the first embodiment, and redundant descriptions are not given or simplified. 
     In the steam turbine  11  according to the second embodiment, a constitution is the same as the constitution of the steam turbine  10  according to the first embodiment except a constitution of a communication hole. Here, the communication hole is mainly described. 
     As illustrated in  FIG. 9  and  FIG. 10 , a communication hole  51  inclines relative to a radial line R extending from the center axis of the turbine rotor  21  in the radial direction at a cross section perpendicular to the turbine rotor axis direction. This inclination direction is not particularly limited, but it is preferably a reverse direction to a turning direction of the steam passing through the stationary blade  26  so as to suppress the flowing of the steam into the communication hole  51 . The communication hole  51  is preferably formed by, for example, a round hole in which a shape at a cross section perpendicular to a center axis O becomes a circle. 
     An inclination angle θ being an acute angle made up of the center axis O of the communication hole  51  and the radial line R is preferably more than “0” (zero) degree and 75 degrees or less. The inclination angle θ is set to be more than “0” (zero) degree, and thereby, a communication area between the communication hole  51  and the annular slit  40  increases, and an opening area in the circumferential direction directly sucking the water film and the water droplets from the annular slit  40  increases. Accordingly, it is possible to surely collect the water droplets and the water film dispersed in the circumferential direction. When the inclination angle θ exceeds 75 degrees, it becomes difficult to form the communication hole  51  from a point of view of manufacturing. A more preferable range of the inclination angle θ is 30 degrees or more and 75 degrees or less. 
     When the communication hole  51  is formed by the round hole, a pitch of the communication hole  51  in the circumferential direction is P, a diameter of the round hole of the communication hole  51  is D, and the inclination angle of the communication hole  51  is θ, it is preferable that a relationship of the following expression (1) is satisfied.
 
 P /( D ·secθ)≦5  expression (1)
 
     When a value of P/(D·secθ) is “5” or less, an effect sucking the water film and the water droplets is obtained in the circumferential direction without pause even if the inclination angle θ is less than 30 degrees. Note that a lower limit value of the P/(D·secθ) is preferably approximately two to maintain strength of the diaphragm outer ring  23  at a part where the communication hole  51  is not provided, and because a ratio sucking not only the water droplets but also the accompanying main stream increases when a hole area is excessive. 
     The expression (1) is satisfied, and thereby, it is possible to surely collect the water droplets and the water film dispersed in the circumferential direction. Besides, it is preferable to satisfy a relationship of the following expression (2) in addition to the expression (1) to enable a more surely collection of the water droplets and the water film.
 
 L/W≧ 3  expression (2)
 
     Here, “L” is a groove depth (refer to  FIG. 10 ) of the annular slit  40  in the radial direction, “W” is a groove width (refer to  FIG. 9 ) of the annular slit  40  in the turbine rotor axis direction. 
     When a value of L/W is three or more, it is possible to surely collect the water droplets and the water film dispersed in the circumferential direction. A maximum value of the L/W is, for example, approximately 20 from a point of view of reducing an abrasion cost of a lathe cutter which processes the groove. Besides, the grove width W is preferably 10 mm or less in consideration of application for an actual product, a size of the groove depth L, and so on. 
     Here,  FIG. 11  is a view illustrating a circumferential distribution of a suction velocity in the radial direction at an inlet of the annular slit  40 . Note that the inlet of the annular slit  40  positions at the steam passage  27  side. 
     Here,  FIG. 11  illustrates the circumferential distribution within a range of the pitch P in the circumferential direction of the communication hole  51  centering on the communication hole  51 . Besides, results illustrated in  FIG. 11  are results obtained by a computational fluid analysis. 
     In each of analysis models F 1  to F 4 , the communication hole  51  was formed by the round hole, the P/D was set to be 10, and the inclination angle θ of the communication hole  51  was set to be 60 degrees. As for the L/W, it was two in F 1 , it was three in F 2 , it was eight in F 3 , and it was  16  in F 4 . 
     As illustrated in  FIG. 11 , in F 1  whose value of the L/W is two, a spread of the circumferential distribution of the suction velocity in the circumferential direction is small. When the value of the L/W is “3” or more (F 2  to F 4 ), the spread of the circumferential distribution of the suction velocity in the circumferential direction is wide, and it can be seen that the water droplets and the water film are sucked from the annular slit  40  at approximately a whole range of the pitch P. It is thereby turned out that it is possible to uniformly perform the suction of the water film and the water droplets at the annular slit  40  along the circumferential direction when the value of the L/W is three or more (F 2  to F 4 ). 
     According to the steam turbine  11  of the second embodiment, the annular slit  40  is included along the circumferential direction, and the inclination angle θ of the communication hole  51  relative to the radial line R is set to be within the above-stated range, and thereby, it is possible to surely collect the water droplets and the water film dispersed in the circumferential direction. It is thereby possible to suppress the erosion caused by the collision of the water droplets with the rotor blade  22  at the immediate downstream side of the stationary blade  26  and the lowering of the turbine efficiency. Besides, the value of the L/W is set to be within the above-stated range, and thereby, it is possible to more surely collect the water droplets and the water film dispersed in the circumferential direction. 
     Note that in the second embodiment, it is also possible to include each constitution according to  FIG. 5  to  FIG. 8  described in the first embodiment, and it is possible to obtain the similar operation and effect as each constitution. 
     According to the above-described embodiments, it becomes possible to surely remove the generated water droplets and the water film along the circumferential direction. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.