Patent Publication Number: US-10781712-B2

Title: Steam valve

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of U.S. application Ser. No. 15/287,145, filed on Oct. 6, 2016, which is a continuation of International Application No. PCT/JP2015/001960 filed on Apr. 7, 2015, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-079781 filed on Apr. 8, 2014; the entire contents of all of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a steam valve. 
     BACKGROUND 
     A steam turbine pipe system includes a main steam pipe which leads steam generated in a boiler to a steam turbine. In this main steam pipe, a steam valve for regulating a flow rate of the steam and shutting off the steam is provided. 
       FIG. 18  is a view illustrating a vertical cross section of a conventional steam valve  300 . The conventional steam valve  300  provided in a main steam pipe is what is called a combined steam valve in which a steam control valve and a main stop valve are combined in one casing  310 . 
     As illustrated in  FIG. 18 , the steam valve  300  includes: a steam control valve  320  movable in an up and down direction when driven from above; and a main stop valve  330  provided under the steam control valve  320  coaxially with the steam control valve  320  and movable in the up and down direction when driven from under. The steam valve  300  further includes a cylindrical strainer  340  disposed so as to surround the steam control valve  320  and the main stop valve  330 . The strainer  340  prevents a foreign object in the steam led through a steam inlet  311  from flowing downstream. 
     The steam control valve  320  includes a valve rod  321  and a valve element  323 . The valve rod  321  penetrates through an upper cover  350  and is movable in the up and down direction when driven from above. The valve element  323  is provided annularly on a lower end side of the valve rod  321  and has a dented portion  322  in its lower surface. 
     The main stop valve  330  includes a valve rod  331 , a valve element  332 , and a guide tube  333 . The valve rod  331  penetrates through a bottom portion of the casing  310  and is movable in the up and down direction when driven from under. The valve element  332  is provided on an upper end side of the valve rod  331  and protrudes radially outward from the valve rod  331  all over the circumferential direction. The valve element  332  is housed in the dented portion  322  of the valve element  323  of the steam control valve  320 . The guide tube  333  is a cylinder fixed to the bottom portion of the casing  310  and having the valve rod  331  penetrating therethrough at its center. 
     Under the valve element  323  of the steam control valve  320  and the valve element  332  of the main stop valve  330 , a valve seat  360  which comes into contact with these valve elements is provided. When the valve element  323  of the steam control valve  320  and the valve element  332  of the main stop valve  330  are pressed while in contact with the valve seat  360 , it is possible to shut off the flow of the steam. 
     At the bottom side of the casing  310 , a drain discharge hole  312  for discharging a drain generated during warming for putting the steam turbine into operation is provided. As illustrated in  FIG. 18 , due to arrangement and structural reasons, the drain discharge hole  312  is formed in a sidewall of the casing  310  at the bottom side so as to extend laterally (in the horizontal direction in  FIG. 18 ). Further, a drain pipe  370  is provided on the drain discharge hole  312  to lead the drain to the outside. A shutoff valve  380  is provided in the drain pipe  370 . When the shutoff valve  380  is opened, the drain generated during the warming is led to a condenser. Then, the shutoff valve  380  is closed after the warming. That is, during the normal operation of the steam turbine, the drain pipe  370  constitutes a pipe part whose one end communicates with the inside of the steam valve  300  and whose other end is closed. 
     The steam valve  300  having such a structure is supplied with the steam superheated by a superheater of the boiler disposed upstream of the steam valve  300 , through the steam inlet  311 . The steam led through the steam inlet  311  passes through the strainer  340  to pass between the valve element  323  of the steam control valve  320  and the valve element  332  of the main stop valve  330 , and the valve seat  360 . Flows of the steam having passed downward in a through portion provided at the center of the valve seat  360  are bent along a steam passage downstream of the valve seat  360 . Then, the steam is discharged through a steam outlet  313  to be led to a high-pressure turbine. 
     At this time, the flow rate of the steam is adjusted by a valve opening degree of the steam control valve  320 . Specifically, when a required flow rate of the steam is small, the valve opening degree of the steam control valve  320  is small, and when the required flow rate of the steam is large, the valve opening degree of the steam control valve  320  is large. When the required flow rate of the steam is small, a gap between the valve element  323  of the steam control valve  320  and the valve seat  360  is narrow and thus the flows of the steam are narrowed in this gap. Then, a flow velocity of the steam increases. 
     Here, as illustrated in  FIG. 18 , partial steam F of the steam having passed between the valve element  323  of the steam control valve  320  and the valve seat  360  flows downward along a side surface of the guide tube  333 . Then, the steam F flows along the shape of a flange portion  333   a  which is provided on the guide tube  333  at the bottom side of the casing  310  to protrude outward all over the circumference direction. At this time, a component of velocity directed outward is added, and as illustrated in  FIG. 18 , the steam F flows outward. 
     Part of the steam F spreading in the circumferential direction flows toward an opening  312   a  of the drain discharge hole  312 . The drain pipe  370  is influenced by the steam F flowing toward the drain discharge hole  312 . Then, during the normal operation of the steam turbine, pressure fluctuation occurs in the drain pipe  370  between the opening  312   a  of the drain discharge hole  312  and the shutoff valve  380 . 
     In steam turbines, temperature and pressure of steam have recently been increased for higher efficiency. Further, downsizing and the like of devices are also being considered in order to reduce a manufacturing cost. These increase the flow velocity of the steam flowing between the valve element  323  of the steam control valve  320  and the valve seat  360  and the flow velocity of the steam flowing in the steam passage in the steam valve  300 . This tends to increase the pressure fluctuation occurring in the drain pipe  370  due to the aforesaid steam F. 
     In the above-described conventional steam valve  300 , the operation under a narrowed valve opening degree of the steam control valve  320 , that is, under a low flow rate of the steam passing through the steam valve  300 , sometimes causes an abnormal temperature increase of the drain pipe  370  more on the steam valve  300  side than the shutoff valve  380 . A possible reason for this to occur may be a thermoacoustic effect caused by the pressure fluctuation in the drain pipe  370 . Such an abnormal temperature increase may cause breakage of the drain pipe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a system diagram illustrating an example of a power generation plant including a steam valve of a first embodiment. 
         FIG. 2  is a perspective view of the steam valve of the first embodiment. 
         FIG. 3  is a view illustrating a vertical cross section of the steam valve of the first embodiment. 
         FIG. 4  is a perspective view of a flow direction changing part in the steam valve of the first embodiment. 
         FIG. 5  is a perspective view of the flow direction changing part with another structure, in the steam valve of the first embodiment. 
         FIG. 6  is a view illustrating a vertical cross section of a steam valve of a second embodiment. 
         FIG. 7  is a perspective view of a flow direction changing part in the steam valve of the second embodiment. 
         FIG. 8  is a view illustrating an A-A cross section in  FIG. 6 . 
         FIG. 9  is a vertical cross-sectional view of a steam valve of a third embodiment. 
         FIG. 10  is a perspective view of a flow direction changing part in the steam valve of the third embodiment. 
         FIG. 11  is a view illustrating a B-B cross-section in  FIG. 9 . 
         FIG. 12  is a view illustrating a vertical cross section of a steam valve of a fourth embodiment. 
         FIG. 13  is a view illustrating a vertical cross section of a steam valve of a fifth embodiment. 
         FIG. 14  is a view illustrating a C-C cross section in  FIG. 13 . 
         FIG. 15  is a view illustrating a vertical cross section of a steam valve of a sixth embodiment. 
         FIG. 16  is a view illustrating a vertical cross section of a steam valve of a seventh embodiment. 
         FIG. 17  is a view illustrating a vertical cross section of a steam valve of an eighth embodiment. 
         FIG. 18  is a view illustrating a vertical cross section of a conventional steam valve. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present invention are described with reference to the drawings. 
     A steam valve of an embodiment includes: a first valve element which is provided to be movable in an up and down direction and adjusts a flow rate of steam; a second valve element which is provided under the first valve element coaxially with the first valve element to be movable in the up and down direction and shuts off a flow of the steam; a valve seat with which the first valve element and the second valve element come into and out of contact; and a guide tube slidably supporting a valve rod including the second valve element, and having a flange portion protruding outward all over the circumferential direction, at a bottom side in the steam valve. 
     The steam valve further includes: a casing housing the first valve element, the second valve element, the valve seat, and the guide tube; a drain discharge hole formed in a sidewall of the casing at a bottom side; a drain pipe provided with a shutoff valve and communicating with the drain discharge hole; and a flow direction changing part which changes a direction in which the steam having passed between the first valve element and the valve seat and flowing along the guide tube, flows toward the drain discharge hole. 
     First Embodiment 
       FIG. 1  is a system diagram illustrating an example of a power generation plant  250  including a steam valve  10  of a first embodiment. 
     As illustrated in  FIG. 1 , in the power generation plant  250 , steam heated in a superheater  252  of a boiler  251  is led to a high-pressure turbine  254  via a main steam pipe  253  and the steam valve  10  provided in the main steam pipe  253 . Note that the steam valve  10 , which will be describe later, is a combined steam valve having functions of a main stop valve and a steam control valve. 
     Here, for example, in a combined cycle system in which a gas turbine and a steam turbine are combined, the boiler  251  functions as an exhaust heat recovery boiler. In this case, the boiler  251  functions by using exhaust gas from the gas turbine. In a case where the combined cycle system including the gas turbine is not constituted, the boiler  251  burns, for example, a fossil fuel to use its heat. 
     The steam having worked in the high-pressure turbine  254  passes through a low-temperature reheat pipe  255  to be reheated in a reheater  256  of the boiler  251  and is led to an intermediate-pressure turbine  259  via a high-temperature reheat pipe  257  and a reheat steam valve  258  provided in the high-temperature reheat pipe  257 . 
     The steam having worked in the intermediate-pressure turbine  259  is led to low-pressure turbines  261  via a crossover pipe  260 . The steam having worked in the low-pressure turbines  261  is returned to water in a condenser  262 . Then, the water is supplied again to the superheater  252  of the boiler  251  by a feed pump  264  via a low-pressure feedwater heater  263  and a high-pressure feedwater heater  265 . Further, for example, the high-pressure turbine  254 , the intermediate-pressure turbine  259 , and the low-pressure turbines  261  drive a power generator  266  to make it generate power. 
     Note that the structure of the power generation plant  250  described here is an example, and its structure is not limited to this. 
     Next, the structure of the steam valve  10  of the first embodiment is described. 
       FIG. 2  is a perspective view of the steam valve  10  of the first embodiment.  FIG. 3  is a view illustrating a vertical cross section of the steam valve  10  of the first embodiment.  FIG. 4  is a perspective view of a flow direction changing part  80  in the steam valve  10  of the first embodiment. Note that the steam valve  10  described here is the combined steam valve having the functions of the main stop valve and the steam control valve, which is provided in the main steam pipe. 
     As illustrated in  FIG. 2  and  FIG. 3 , the steam valve  10  of the first embodiment includes a casing  20  forming a steam passage in which the steam led through a steam inlet  21  in, for example, a horizontal direction is led vertically downward, and the steam led vertically downward is led in, for example, the horizontal direction to flow out through a steam outlet  22 . An upper cover  25  covering the casing  20  from above is coupled to the top of the casing  20  with fixing bolts  24 . The upper cover  25  thus coupled to the casing  20  with the fixing bolts  24  prevents the steam flowing in the casing  20  from leaking outside. 
     As illustrated in  FIG. 3 , the casing  20  includes therein a steam control valve  30  movable in an up and down direction when driven from above and a main stop valve  40  provided under the steam control valve  30  coaxially with the steam control valve  30  and movable in the up and down direction when driven from under. The casing  20  further includes therein a cylindrical strainer  50  disposed so as to surround the steam control valve  30  and the main stop valve  40 . The strainer  50  prevents a foreign object in the steam led through the steam inlet  21  from flowing downstream. The strainer  50  is, for example, a porous member or a porous plate. 
     The steam control valve  30  adjusts a flow rate of the steam. In order to shut off the flow of the steam, the steam control valve  30  is closed. The steam control valve  30  includes a valve rod  31  and a valve element  32 . The valve rod  31  penetrates through the upper cover  25  and is supported to be movable in the up and down direction when driven from above. The valve element  32  is annularly provided on a lower end side of the valve rod  31  and has a dented portion  33  in its lower surface. Around the outer periphery of the valve element  32 , a cylindrical guide  34  which guides the up and down movement of the valve element  32  is provided. The valve element  32  functions as a first valve element. 
     The main stop valve  40  shuts off the flow of the steam. The main stop valve  40  includes a valve rod  41 , a valve element  42 , and a guide tube  43 . The valve rod  41  penetrates through a bottom portion of the casing  20  and is supported so as to be movable in the up and down direction when driven from under. The valve element  42  is provided on an upper end side of the valve rod  41  and protrudes radially outward from the valve rod  41  all over the circumferential direction. The valve element  42  is housed in the dented portion  33  of the valve element  32  of the steam control valve  30 . That is, the valve element  42  is capable of entering and exiting from the dented portion  33  of the valve element  32 . The valve element  42  functions as a second valve element. 
     The guide tube  43  is fixed to the bottom potion of the casing  20  and is a cylinder at the center of which the valve rod  41  slidably penetrates therethrough. The guide tube  43  supporting the valve rod  41  enables the main stop valve  40  to stably move in the up and down direction. 
     The guide tube  43  includes a flange portion  43   a  protruding outward all over the circumferential direction, at the bottom portion side of the casing  20 . An upper end portion of the flange portion  43   a  is, for example, a slanting surface  43   b  slanting downward as illustrated in  FIG. 3  and  FIG. 4 . The shape of the upper end portion of the flange portion  43   a  is not limited to this and may be a curved surface protruding downward, for instance. 
     A height I of the flange portion  43   a  is an axial-direction distance between a lower end of the flange portion  43   a  in contact with a bottom surface of the casing  20  and an upper end of the flange portion  43   a . The height I of the flange portion  43   a  is, for example, less than about three times an axial-direction distance J between the bottom surface of the casing  20  and an upper end of a later-described drain discharge hole  23 . 
     Under the valve element  32  of the steam control valve  30  and the valve element  42  of the main stop valve  40 , a valve seat  60  which comes into contact with these valve elements is provided. The valve seat  60  has a hollow annular shape having a steam passage  61  at its center. For example, when the valve element  32  of the steam control valve  30  and the valve element  42  of the main stop valve  40  are pressed while in contact with the valve seat  60 , the flow of the steam can be shut off. 
     The casing  20  has, at its bottom side, the drain discharge hole  23  which discharges a drain generated during warming for putting the steam turbine into operation. As illustrated in  FIG. 3 , the drain discharge hole  23  is formed in a sidewall  20   a  of the casing  20  at the bottom side to extend laterally (in  FIG. 3 , a left horizontal direction) because of arrangement and structural reasons. 
     A drain pipe  70  which leads the drain outside is provided so as to communicate with the drain discharge hole  23 . The drain pipe  70  on the drain discharge hole  23  side is disposed in a substantially horizontal direction, for instance. Note that the substantially horizontal direction includes not only the horizontal direction but also a direction inclined downward by about 0.5 to 2 degrees so as to make the drain flow down. The drain pipe  70  is provided with a shutoff valve  71 . 
     When the shutoff valve  71  is opened, the drain generated during the warming is led to the condenser. Then, the shutoff valve  71  is closed after the warming. That is, while the steam turbine is in operation, the drain pipe  70  constitutes a pipe part whose one end communicates with the inside of the steam valve  10  and whose other end is closed. 
     The steam valve  10  further includes a flow direction changing part  80  which changes a direction in which the steam having passed between the valve element  32  of the steam control valve  30  and the valve seat  60  flows toward the drain discharge hole  23 . Here, an example where the flow direction changing part  80  is provided on a side surface  44  of the guide tube  43  is described. 
     As illustrated in  FIG. 3  and  FIG. 4 , the side surface  44  of the guide tube  43  has, on its drain discharge hole side, a ridge part  81  protruding outward and extending in the axial direction. As illustrated in  FIG. 3 , a downstream end portion of the ridge part  81  faces the drain discharge hole  23 . The ridge part  81  functions as the flow direction changing part  80 . Note that  FIG. 4  is a view of the flow direction changing part  80  seen from the drain discharge hole  23  side. 
     The ridge part  81  has a wider circumferential width as it goes more downstream as illustrated in  FIG. 4 , for instance. In the example here, the ridge part  81  has a rectangular axial-direction cross section as illustrated in  FIG. 3 , but the axial-direction cross sectional shape of the ridge part  81  is not limited to any particular shape. The ridge part  81  only needs to have a ridge shape capable of dividing the steam F flowing along the guide tube  43 , at an upper portion of the guide tube  43 . For example, the ridge part  81  may have a larger protrusion height as it goes more downstream. In this case, the cross sectional shape of the ridge part  81  in the cross section in  FIG. 3  is trapezoidal. 
     Owing to the presence of the ridge part  81 , the steam F flowing along the guide tube  43  can be divided at the ridge part  81  as a boundary. Further, since the ridge part  81  has a larger circumferential width as it goes more downstream, the direction of the steam F flowing toward the drain discharge hole  23  facing the downstream end of the ridge part  81  can be surely changed at the downstream end. In other words, the increase of the circumferential width of the ridge potion  81  as it goes more downstream makes it possible to divide the flow of the steam F in a direction in which the steam F gets more away from the drain discharge hole  23 . 
       FIG. 5  is a perspective view of the flow direction changing part  80  having another structure, in the steam valve  10  of the first embodiment. Note that  FIG. 5  is a view of the flow direction changing part  80  seen from the drain discharge hole  23  side. 
     As illustrated in  FIG. 5 , in an upper end portion  82  of the ridge part  81 , the circumferential width of the end portion  82  may be reduced in a tapered manner, for instance. This structure enables the smooth division of the flow at a branching part of the flow of the steam F, that is, at the end portion  82 . This can reduce a pressure loss in the end portion  82 . 
     Next, the flow of the steam in the steam valve  10  is described. 
     For example, the steam superheated by the superheater  252  of the boiler  251  illustrated in  FIG. 1  is supplied through the steam inlet  21  illustrated in  FIG. 3 . The steam led through the steam inlet  21  passes through the strainer  50  and passes between the valve element  32  of the steam control valve  30  and the valve element  42  of the main stop valve  40 , and the valve seat  60 . 
     At this time, the main stop valve  40  is fully opened, for instance. That is, a gap between the valve element  42  of the main stop valve  40  and the valve seat  60  is set to the maximum. Further, the opening degree of the steam control valve  30  is set according to a required flow rate of the steam. That is, the opening degree is adjusted according to the flow rate of the steam that is to flow in the gap between the valve element  32  of the steam control valve  30  and the valve seat  60 . Here, a description is given, assuming a condition under which the temperature of the drain pipe  70  is likely to be abnormal, that is, a case where the flow rate of the steam that is to flow is small. In this case, the valve opening degree of the steam control valve  30  is small. 
     In the case where the valve opening degree of the steam control valve  30  is small, the gap between the valve element  32  of the steam control valve  30  and the valve seat  60  is narrow and thus the flow of the steam is narrowed in this gap. Then, the velocity of the steam increases. Partial steam F of the steam having passed between the valve element  32  of the steam control valve  30  and the valve seat  60  flows down along the side surface of the guide tube  43  as illustrated in  FIG. 3  and  FIG. 4 . 
     Out of the flows of the steam F along the side surface of the guide tube  43 , the flow of the steam F reaching the ridge part  81  branches off at the ridge part  81  as the boundary. In  FIG. 4 , the flow branches off to left and right of the ridge part  81  as the boundary. The steam F that has branched off flows so as to be more apart from the ridge part  81  as it goes more downstream. 
     Accordingly, at the downstream end of the ridge part  81 , there occurs no flow of the steam F toward the drain discharge hole  23  facing the downstream end. Consequently, since pressure fluctuation of the flow of the steam F is not transmitted to the drain pipe  70  via the drain discharge hole  23 , the abnormal temperature increase of the drain pipe  70  can be prevented. Here, the flow of the steam F toward the drain discharge hole  23  refers to a flow of the steam flowing into an opening  23   a  of the drain discharge hole  23  mainly due to a dynamic pressure of the flow (the same applies to the below). 
     The steam F having branched off then flows toward the steam outlet  22  together with other steam. The steam discharged through the steam outlet  22  is led to the high-pressure turbine  254 . 
     Here, the description has been given assuming the case where the valve opening degree of the steam control valve  30  is small, but the same operation and effect can also be obtained in a case where the valve opening degree of the steam control valve  30  is large. 
     Here, a reason why the abnormal temperature increase of the drain pipe  70  can be prevented by not transmitting the pressure fluctuation of the flow of the steam F to the drain pipe  70  is described. 
     Here, the frequency of pipe pressure fluctuation of a cylinder whose inside diameter is R is represented by f(Hz). In general, a heat flux q (W/m 2 ) generated by a thermoacoustic effect due to pressure fluctuation in a boundary layer near a pipe wall is found by the expression (2), using a relation of the expression (1) in which a pipe pressure fluctuation amplitude P is divided by a pipe average pressure P 0  and the obtained value is made dimensionless (e.g. Arakawa, Kawahashi, Transactions of the Japan Society of Mechanical Engineers, Vol. 62 No. 598, B(1996), p. 2238-2245). 
     
       
         
           
             
               
                 
                   
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     Since the inner perimeter of the cylinder is πR, a heat generation amount Q (W/m) per unit length of the cylinder is found by the expression (3). 
     
       
         
           
             
               
                 
                   
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     Here, υ is a coefficient of kinematic viscosity. 
     As is apparent from the expression (3), the heat generation (thermoacoustic effect) due to the pipe pressure fluctuation is proportional to a square of the dimensional pressure amplitude. This shows that reducing the pipe pressure fluctuation can reduce the heat generation. 
     As described above, according to the steam valve  10  of the first embodiment, the presence of the flow direction changing part  80  (ridge part  81 ) can prevent the pressure fluctuation of the flow of the steam F along the side surface of the guide tube  43  from being transmitted to the drain pipe  70  via the drain discharge hole  23 . This can prevent the abnormal temperature increase of the drain pipe  70 , enabling to provide the highly reliable steam valve  10 . 
     Second Embodiment 
       FIG. 6  is a view illustrating a vertical cross section of a steam valve  11  of a second embodiment.  FIG. 7  is a perspective view of a flow direction changing part  80  in the steam valve  11  of the second embodiment.  FIG. 8  is a view illustrating an A-A cross section in  FIG. 6 . Note that, in the following embodiment, the same components as those in the structure of the steam valve  10  of the first embodiment are denoted by the same reference signs and duplicated description is not given or simplified. 
     The structure of the steam valve  11  of the second embodiment is the same as the structure of the steam valve  10  of the first embodiment except the structure of the flow direction changing part  80 . Therefore, the flow direction changing part  80  is mainly described here. 
     As illustrated in  FIG. 6  to  FIG. 8 , the flow direction changing part  80  includes a tubular body  90  whose one end is coupled to the drain discharge hole  23  and whose other end has an upper half tip  91  protruding in the length direction (longitudinal direction) beyond its lower half tip  92 . The flow direction changing part  80  further includes a communication hole  93  formed in the tubular body  90  in the length direction and communicating with the drain discharge hole  23 . For example, the communication hole  93  can be formed on a lower half side of the tubular body  90  all along the length direction. For example, the tubular body  90  is constituted by forming the communication hole  93  in a column whose other end has the aforesaid shape. 
     Here, assuming, for example, that the cross sectional shape is a circle in the cross section illustrated in  FIG. 8 , the upper half side refers to an upper side of the circle two-divided vertically by a horizontal straight line passing the center of the circle. Assuming that the cross sectional shape in the cross section illustrated in  FIG. 8  is a circle, the lower half side refers to a lower side of the circle two-divided vertically by the horizontal straight line passing the center of the circle. 
     An end portion of the communication hole  93  on a side different from the drain discharge hole  23  side opens in the steam valve  11 . However, since the upper half tip  91  of the tubular body  90  protrudes toward the guide tube  43  beyond the lower half tip  92 , the end portion of the communication hole  93  on, for example, the tip side cannot be seen from above. Incidentally, the drain generated during the warming for putting the steam turbine into operation passes through the communication hole  93  to flow into the drain discharge hole  23 . 
     The tubular body  90  has a cylindrical shape, for instance. As illustrated in  FIG. 6 , a height H of the tubular body  90  is smaller than the height I of the flange portion  43   a  of the guide tube  43 . The tubular body  90  is fixed to the bottom surface of the casing  20  by welding or the like, for instance. Further, the example where the cross section of the communication hole  93  is rectangular is given here, but this cross section may be circular. 
     In the steam valve  11  including such a flow direction changing part  80 , the partial steam F of the steam having passed between the valve element  32  of the steam control valve  30  and the valve seat  60  flows along the side surface of the guide tube  43 . When the steam F flows along the flange portion  43   a  of the guide tube  43 , a component of velocity directed outward is added, and as illustrated in  FIG. 6 , the steam F flows outward. Part of the flow of the steam F flowing outward flows down onto the tubular body  90  to flow along an outer surface of the tubular body  90 . At this time, the flow of the steam F collides with the top of the tubular body  90 , and as illustrated in  FIG. 8 , branches off. In  FIG. 8 , the flow of the steam F branches off to left and right of the tubular body  90 . That is, the flow direction of the steam F is changed by the tubular body  90 . 
     The steam F flowing down onto the tubular body  90  thus once collides with the tubular body  90  to be changed in its flow direction. The height H of the tubular body  90  is smaller than the height I of the flange portion  43   a  of the guide tube  43 , and the tip-side end portion of the communication hole  93  is covered by the protruding upper half tip  91 . Accordingly, the flow of the steam F toward the communication hole  93  does not occur. Consequently, since the pressure fluctuation of the flow of the steam F is not transmitted to the drain pipe  70  via the communication hole  93  and the drain discharge hole  23 , the abnormal temperature increase of the drain pipe  70  can be prevented. 
     As described above, according to the steam valve  11  of the second embodiment, the presence of the flow direction changing part  80  can prevent the pressure fluctuation of the flow of the steam F along the side surface of the guide tube  43  from being transmitted to the drain pipe  70  via the drain discharge hole  23 . This can prevent the abnormal temperature increase of the drain pipe  70 , enabling to provide the highly reliable steam valve  11 . 
     Third Embodiment 
       FIG. 9  is a view illustrating a vertical cross section of a steam valve  12  of a third embodiment.  FIG. 10  is a perspective view of a flow direction changing part  80  in the steam valve  12  of the third embodiment. Note that  FIG. 10  is a view of the flow direction changing part  80  seen from the drain discharge hole  23  side.  FIG. 11  is a view illustrating a B-B cross section in  FIG. 9   
     The structure of the steam valve  12  of the third embodiment is the same as the structure of the steam valve  10  of the first embodiment except the structure of the flow direction changing part  80 . Therefore, the flow direction changing part  80  is mainly described here. 
     As illustrated in  FIG. 9  and  FIG. 10 , the flow direction changing part  80  includes groove portions  100  formed in the side surface of the guide tube  43  on the drain discharge hole  23  side and arranged in the axial direction in a plurality of tiers. For example, these groove portions  100  are each formed along a half circumference of the side surface of the guide tube  43  which is the cylinder, that is, along ½ of the circumference. 
     Here, the side surface of the guide tube  43  on the drain discharge hole  23  side is described. As illustrated in  FIG. 11 , a straight line parallel to the center line of the drain discharge hole  23  and passing the center O of the guide tube  43  is represented by L. Note that the center O of the guide tube  43  is also the center of the valve rod  41  of the main stop valve  40 . 
     In this case, a side surface corresponding to regions each having a center at the center O of the guide tube  43  and each having an angle θ in each direction from the straight line L (region having an angle 2θ) is defined as the side surface of the guide tube  43  on the drain discharge hole  23  side. In the above-described example, this angle θ is 90 degrees, and the range of the side surface of the guide tube  43  on the drain discharge hole  23  side is the half circumference of the side surface of the guide tube  43  which is the cylinder. 
     The angle θ is preferably set to 15 to 90 degrees, for instance. Setting the angle θ within this range makes it possible to reduce the velocity of the flow toward the drain discharge hole  23 , out of the flows of the steam F along the side surface of the guide tube  43 . 
     In the steam valve  12  including such a flow direction changing part  80 , the partial steam F of the steam having passed between the valve element  32  of the steam control valve  30  and the valve seat  60  flows along the side surface of the guide tube  43 . The steam F flowing along the side surface of the guide tube  43  on the drain discharge hole  23  side enters the inside of the groove portions  100  as illustrated in  FIG. 9  and  FIG. 10  and is disturbed on the surfaces of the groove portions  100  to form vortices. Consequently, the flow direction of the steam F is changed, and in addition, the flow of the steam F is damped, so that the velocity of the steam F decreases. Then, the steam F flowing down up to the downstream side of the guide tube  43  has a low velocity. 
     Even if the flow of the steam F having such a low velocity reaches the drain discharge hole  23 , the pressure fluctuation transmitted to the drain pipe  70  via the drain discharge hole  23  is very small. This can prevent the abnormal temperature increase of the drain pipe  70 . 
     As described above, according to the steam valve  12  of the third embodiment, the presence of the flow direction changing part  80  makes it possible to not only change the flow direction in which the steam flows toward the drain discharge hole  23  but also damp the flow of the steam F to decrease the velocity of the flow. Accordingly, the pressure fluctuation transmitted to the drain pipe  70  via the drain discharge hole  23  is small. This can prevent the abnormal temperature increase of the drain pipe  70 , enabling to provide the highly reliable steam valve  12 . 
     Fourth Embodiment 
       FIG. 12  is a view illustrating a vertical cross section of a steam valve  13  of a fourth embodiment. The structure of the steam valve  13  of the fourth embodiment is the same as the structure of the steam valve  10  of the first embodiment except the structure of the flow direction changing part  80 . Therefore, the flow direction changing part  80  is mainly described here. 
     As illustrated in  FIG. 12 , the flow direction changing part  80  is the valve seat  60  whose inner peripheral surface is curved such that a passage cross section of the steam passage  61  at the center of the valve seat  60  becomes larger as it goes more downward from a throat portion S of the steam passage  61 . Here, the throat portion S is a cross section where the passage cross section becomes smallest in the steam passage at the center of the valve seat  60 . 
     For example, with the passage sectional area at the throat portion S being 1, the passage sectional area at an outlet of the steam passage  61  is preferably about 2.2 to 3 times. By setting it within this range, the flow of the steam having passed between the valve element  32  of the steam control valve  30  and the valve seat  60  can spread outward. The curve of the inner peripheral surface of the valve seat  60  in order for the passage sectional area at the outlet of the steam passage  61  to fall within the aforesaid range is preferably gentle enough to prevent the separation of the flow. 
     In the steam valve  12  including such a flow direction changing part  80 , the steam having passed between the valve element  32  of the steam control valve  30  and the valve seat  60  flows along the inner peripheral surface of the valve seat  60 . Then, at the outlet of the valve seat  60 , a flow spreading outward is obtained. 
     In this case, the flow of the steam along the side surface of the guide tube  43  is minimized. Accordingly, the flow of the steam flowing along the side surface of the guide tube  43 , and on the downstream side, flowing toward the drain discharge hole  23  is minimized. Accordingly, the pressure fluctuation of the flow of the steam is not transmitted to the drain pipe  70  via the drain discharge hole  23 . This can prevent the abnormal temperature increase of the drain pipe  70 . 
     The flow having passed through the steam passage  61  of the valve seat  60  to spread outward spreads in the steam passage downstream of the valve seat  60  to flow toward the steam outlet  22 . 
     As described above, according to the steam valve  13  of the fourth embodiment, the presence of the flow direction changing part  80  can minimize the flow of the steam along the side surface of the guide tube  43 . Accordingly, the pressure fluctuation due to the steam flowing toward the drain discharge hole  23  is not transmitted to the drain pipe  70 . This can prevent the abnormal temperature increase of the drain pipe  70 , enabling to provide the highly reliable steam valve  13 . 
     Fifth Embodiment 
       FIG. 13  is a view illustrating a vertical cross section of a steam valve  14  of a fifth embodiment.  FIG. 14  is a view illustrating a C-C section in  FIG. 13 . The structure of the steam valve  14  of the fifth embodiment is the same as the structure of the steam valve  10  of the first embodiment except the structure of the flow direction changing part  80 . Therefore, the flow direction changing part  80  is mainly described here. 
     As illustrated in  FIG. 13  and  FIG. 14 , the flow direction changing part  80  is constituted by narrowing a gap between the sidewall  20   a  of the casing  20  on the side where the drain discharge hole  23  is formed and the guide tube  43 . For example, as illustrated in  FIG. 13  and  FIG. 14 , the sidewall  20   a  of the casing  20  on the side where the drain discharge hole  23  is formed protrudes toward the guide tube  43 , so that the gap between the sidewall  20   a  and the guide tube  43  can be small. 
     Here, as illustrated in  FIG. 13 , in the gap, an interval between the sidewall  20  a of the casing  20  and the flange portion  43   a  of the guide tube  43  is smallest, in the vertical section of the steam valve  14  including the center of the drain discharge hole  23 . A distance D of this smallest gap is set to the minimum distance allowing the drain to be led to the drain discharge hole  23 , for instance. 
     In the steam valve  14  including such a flow direction changing part  80 , the partial steam F of the steam F having passed between the valve element  32  of the steam control valve  30  and the valve seat  60  flows along the side surface of the guide tube  43 . However, the gap between the sidewall  20   a  of the casing  20  where the drain discharge hole  23  is formed and the guide tube  43  is small. Accordingly, the steam F flows along the side surface of the guide tube  43  so as to keep away from this gap. 
     Accordingly, there occurs no flow of the steam F toward the drain discharge hole  23 . Consequently, the pressure fluctuation of the flow of the steam F is not transmitted to the drain pipe  70  via the drain discharge hole  23 . This can prevent the abnormal temperature increase of the drain pipe  70 . 
     As described above, according to the steam valve  14  of the fifth embodiment, owing to the presence of the flow direction changing part  80 , there occurs no flow of the steam F toward the drain discharge hole  23 . Accordingly, there is no transmission of the pressure fluctuation to the drain pipe  70  due to the steam flowing toward the drain discharge hole  23 . This can prevent the abnormal temperature increase of the drain pipe  70 , enabling to provide the highly reliable steam valve  14 . 
     In the example described here, the sidewall  20   a  of the casing  20  on the side where the drain discharge hole  23  is formed protrudes toward the guide tube  43  to narrow the gap between the sidewall  20   a  and the guide tube  43 , but this structure is not restrictive. 
     For example, in a cross section corresponding to the cross section illustrated in  FIG. 13 , a structure composed of the steam control valve  30 , the main stop valve  40 , the valve seat  60 , and the strainer  50  may be put closer to the sidewall  20   a  of the casing  20  on the side where the drain discharge hole  23  is formed. That is, the center axis of the aforesaid structure may be deviated toward the drain discharge hole  23  from the vertical center axis of the steam passage downstream of the valve seat  60 . In this case as well, the same operation and effect as the operation and effect in the steam valve  14  illustrated in  FIG. 13  can be obtained. 
     Sixth Embodiment 
       FIG. 15  is a view illustrating a vertical cross section of a steam valve  15  of a sixth embodiment. The structure of the steam valve  15  of the sixth embodiment is the same as the structure of the steam valve  10  of the first embodiment except the structure of the flow direction changing part  80 . Therefore, the flow direction changing part  80  is mainly described here. 
     As illustrated in  FIG. 15 , the flow direction changing part  80  is constituted by making the outside diameter of the sidewall of the guide tube  43  above the flange portion  43   a  equal to the outside diameter of the flange portion  43   a . Here, in  FIG. 15 , the shape before the outside diameter of the sidewall above the flange portion  43   a  is increased is illustrated by the broken line. The outside diameter of the sidewall of the guide tube  43  above the flange portion  43   a  may be larger than the outside diameter of the flange portion  43   a . That is, the outside diameter of the sidewall of the guide tube  43  above the flange portion  43   a  is set equal to or larger than the outside diameter of the flange portion  43   a.    
     In the steam valve  15  including such a flow direction changing part  80 , the partial steam F of the steam F having passed between the valve element  32  of the steam control valve  30  and the valve seat  60  flows along the side surface of the guide tube  43 . The steam F flowing along the side surface of the guide tube  43  flows substantially vertically downward to collide with the bottom surface of the casing  20 . The steam F which has collided with the bottom surface of the casing  20  flows toward the steam outlet  22  together with other steam. 
     Accordingly, there occurs substantially no flow of the steam F toward the drain discharge hole  23 . Consequently, the pressure fluctuation of the flow of the steam F is not transmitted to the drain pipe  70  via the drain discharge hole  23 . This can prevent the abnormal temperature increase of the drain pipe  70 . 
     As described above, according to the steam valve  15  of the sixth embodiment, owing to the presence of the flow direction changing part  80 , there occurs substantially no flow of the steam F toward the drain discharge hole  23 . Accordingly, there is no transmission of the pressure fluctuation to the drain pipe  70  due to the steam flowing toward the drain discharge hole  23 . This can prevent the abnormal temperature increase of the drain pipe  70 , enabling to provide the highly reliable steam valve  15 . 
     Seventh Embodiment 
       FIG. 16  is a view illustrating a vertical cross section of a steam valve  16  of a seventh embodiment. The structure of the steam valve  16  of the seventh embodiment is the same as the structure of the steam valve  10  of the first embodiment except the structure of the flow direction changing part  80 . Therefore, the flow direction changing part  80  is mainly described here. 
     As illustrated in  FIG. 16 , the flow direction changing part  80  is constituted by making the height I of the flange portion  43   a  of the guide tube  43  three times or more the axial-direction distance J between the bottom surface of the casing  20  and the upper end of the drain discharge hole  23 . Note that the axial direction is synonymous with the axial direction of the steam control valve  30  and the main stop valve  40 . That is, the axial direction is the up and down direction in  FIG. 16 . 
     The height I of the flange portion  43   a , which is ordinarily set less than three times the distance J as described above, falls here within a range exceeding this range. That is, the height I of the flange portion  43   a  in the seventh embodiment is set larger than the aforesaid ordinary height I of the flange portion  43   a.    
     In the steam valve  16  including such a flow direction changing part  80 , the partial steam F of the steam F having passed between the valve element  32  of the steam control valve  30  and the valve seat  60  flows along the side surface of the guide tube  43 . The height I of the flange portion is larger than the ordinary height I of the flange portion  43   a . Accordingly, more upstream than ordinarily, the component of velocity directed outward is added and as illustrated in  FIG. 16 , the steam F flows outward. 
     Accordingly, there occurs substantially no flow of the steam F toward the drain discharge hole  23 . Consequently, the pressure fluctuation of the flow of the steam F is not transmitted to the drain pipe  70  via the drain discharge hole  23 . This can prevent the abnormal temperature increase of the drain pipe  70 . 
     As described above, according to the steam valve  16  of the seventh embodiment, owing to the presence of the flow direction changing part  80 , there occurs substantially no flow of the steam F toward the drain discharge hole  23 . Accordingly, there is no transmission of the pressure fluctuation to the drain pipe  70  due to the steam flowing toward the drain discharge hole  23 . This can prevent the abnormal temperature increase of the drain pipe  70 , enabling to provide the highly reliable steam valve  16 . 
     Eight Embodiment 
       FIG. 17  is a view illustrating a vertical cross section of a steam valve  17  of an eighth embodiment. The steam valve  17  of the eighth embodiment does not include the flow direction changing part  80  unlike those of the above-described embodiments. In the steam valve  17  of the eighth embodiment, the structure of a drain discharge hole  110  is different from the structure of the drain discharge hole  23  of the above-described embodiments. Therefore, the drain discharge hole  110  is mainly described here. 
     As illustrated in  FIG. 17 , the drain discharge hole  110  is formed in a bottom wall  20   b  of the casing  20 . The drain discharge hole  110  includes a vertical hole  111  extending vertically downward from an inner surface  20   c  of the bottom wall  20   b  and a lateral hole  112  communicating with the vertical hole  111  and laterally penetrating. The lateral hole  112  is formed in the horizontal direction, for instance. Alternatively, the lateral hole  112  may be formed so as to slant downward toward the side where the lateral hole  112  penetrates. 
     Here, the drain discharge hole  110  is formed with a hole diameter large enough for the drain to pass therethrough. In view of preventing the steam from entering the drain discharge hole  110 , the drain discharge hole  110  preferably has the minimum hole diameter allowing the passage of the drain. 
     On a side surface of the bottom wall  20   b  through which the lateral hole  112  penetrates, the drain pipe  70  is provided so as to communicate with the lateral hole  112 . The drain pipe  70  is provided with the shutoff valve  71 . 
     In the steam valve  17  including such a drain discharge hole  110 , the partial steam F of the steam F having passed between the valve element  32  of the steam control valve  30  and the valve seat  60  flows along the side surface of the guide tube  43 . When the steam F flows along the flange portion  43   a  of the guide tube  43 , the component of velocity directed outward is added, and as illustrate in  FIG. 17 , the steam F flows outward. 
     Accordingly, there occurs substantially no flow of the steam F toward an opening  111   a  of the vertical hole  111 . Consequently, there is no transmission of the pressure fluctuation to the drain pipe  70  due to the steam flowing toward the drain discharge hole  110 . Note that the flow of the steam F toward the drain discharge hole  110  (opening  111   a ) refers to a flow of the steam flowing into the opening  111   a  of the drain discharge hole  110  mainly due to the dynamic pressure of the flow. Further, for example, even if the partial steam F enters the vertical hole  111 , the steam F does not enter the lateral hole  112  bending perpendicularly from the vertical hole  111 , owing to a large pressure loss. For the above-described reasons, it is possible to prevent the abnormal temperature increase of the drain pipe  70  and provide the highly reliable steam valve  16 . 
     As described above, according to the steam valve  17  of the eighth embodiment, owing to the above-described structure of the drain discharge hole  110 , there occurs no flow of the steam F toward the drain discharge hole  110  (opening  111   a ). Accordingly, there is no transmission of the pressure fluctuation to the drain pipe  70  due to the steam flowing toward the drain discharge hole  110 . This can prevent the abnormal temperature increase of the drain pipe  70 , enabling to provide the highly reliable steam valve  15 . 
     Here, as an example of a steam valve, the above embodiments describe the steam valve which is provided in the main steam pipe and in which the steam control valve  30  and the main stop valve  40  are combined. However, the structures of the embodiments are applicable also to, for example, a steam valve which is provided in the high-temperature reheat pipe and in which an intercept valve and a reheat steam stop valve are combined. In this case, the same operation and effect as those when the structures of the embodiments are applied to the steam valve in which the steam control valve  30  and the main stop valve  40  are combined can be obtained. 
     According to the above-described embodiments, it is possible to prevent an abnormal temperature increase of a drain pipe to provide a highly reliable steam valve. 
     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.