Patent Publication Number: US-2009217991-A1

Title: Pipe

Description:
TECHNICAL FIELD 
     The present invention relates to a pipe through which a fluid flows, and more particularly relates to a closed pipe branched from a main pipe through which a high-temperature fluid flows. 
     BACKGROUND ART 
     In general, various kinds of branch lines are connected to a main pipe, through which water flows, of power generation plants and other plants. Among these, there are pipes that are only used during plant startup or maintenance/inspection and are then closed by a gate valve provided in the branch line after the plant starts normal operation. 
     In the branch pipe closed by a gate valve or the like as described above, a so-called cavity flow, which is a spiral vortex flow induced by a water flow flowing through the main pipe, is formed. When heat radiation through a pipe wall of the closed branch pipe occurs in addition to the formation of the cavity flow, a thermal stratification interface is formed at a front end of the cavity flow. The thermal stratification interface is an interface where the temperature of water in the branch pipe rapidly changes, and hence, a large thermal stress is generated in a pipe located in the vicinity of the thermal stratification interface due to the temperature difference. 
     When such a thermal stratification interface stagnates at a predetermined position, such as a pipe bent portion of the branch pipe, thermal fatigue may be generated in the pipe in some cases. Hence, various techniques have been proposed to eliminate adverse influences, such as damage caused by thermal fatigue in the branch pipe, by controlling an intrusion depth of the cavity flow into the branch pipe (for example, see Patent Document 1). 
     Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2002-928925 (pages 4 to 5, FIG. 4, etc.) 
     DISCLOSURE OF INVENTION 
     In the above Patent Document 1, for example, a technique has been disclosed in which plate members formed in a cross shape are disposed in a branch line. 
     According to this technique, it is disclosed that intrusion into a branch pipe of the cavity flow occurring in the branch line is suppressed and that adverse influences on the pipe, such as thermal fatigue, can be eliminated. 
     However, according to the technique disclosed in Patent Document 1, since the cavity flow is not sufficiently controlled, a thermal stratification interface, which is a portion of the fluid exhibiting a rapid temperature change, may be generated at the above predetermined position of the pipe in some cases. As a result, thermal fatigue of the pipe occurs at the predetermined position due to the thermal stratification interface, and adverse influences on the pipe may result in some cases. 
     The present invention has been conceived in order to solve the problems described above, and an object of the present invention is to provide a pipe eliminating an adverse influence on a branch pipe caused by a cavity flow. 
     To this end, the present invention provides the following solutions. 
     In accordance with a first aspect of the present invention, there is provided a pipe which is branched from a main pipe through which a high-temperature fluid flows and which is configured to be closable, wherein dividing part for dividing a channel inside the pipe into a plurality of small channels is provided in the pipe. 
     According to the first aspect of the present invention, since the channel inside the pipe is divided into the plurality of small channels by the dividing part, channel resistance against a spiral flow is increased in a region where the dividing part is disposed. Hence, intrusion of the cavity flow, which is a spiral vortex, into the pipe can be suppressed by the dividing part, and a rapid temperature change portion is prevented from being formed at a predetermined portion of the pipe (such as a pipe bend). As a result, an adverse influence on the pipe caused by the rapid temperature change portion can be prevented. 
     In addition, since the plurality of small channels are formed, the heat radiation area of the fluid is increased, and the heat thereof can be easily radiated outside; hence, the rapid temperature change portion is less likely to be formed. 
     In the first aspect described above, the dividing part is preferably configured to include a first pipe connected to the main pipe, a second pipe separated from the first pipe, and a plurality of connection pipes connecting the first pipe and the second pipe so as to enable the fluid to flow therebetween. 
     According to this configuration, since the dividing part is formed by connecting the first pipe and the second pipe, which are separated from each other, by the plurality of connection pipes, the channel resistance against a circumferential component of the flow in the pipe is increased, and hence, intrusion of the cavity flow into the pipe is suppressed. In addition, the heat radiation area of the fluid in the plurality of connection pipes is increased, and hence the heat of the fluid can be easily radiated outside. 
     Also in the first aspect described above, it is preferable that the dividing part include a plurality of cylindrical members, provided in the pipe, each having a smaller diameter than that of the pipe, and that the plurality of cylindrical members be disposed side-by-side in a circumferential direction of the pipe. 
     According to this configuration, the plurality of cylindrical members, which collectively form the dividing part, are disposed side-by-side in the circumferential direction of the pipe, and hence the channel of the pipe can be divided into the plurality of small channels. Hence, the channel resistance against the circumferential component of the flow in the pipe is increased, and hence, intrusion of the cavity flow into the pipe can be suppressed. 
     In addition, since the plurality of cylindrical members are disposed side-by-side in the pipe in the circumferential direction, the rigidity of the pipe can be increased. Furthermore, since existing pipes or the like are used for the cylindrical members, the pipe can be easily manufactured. 
     In the first aspect described above, it is preferable that the dividing part include a porous body having a plurality of penetrating holes, that the porous body be disposed inside the pipe, and that axial lines of the plurality of penetrating holes be along a central axis direction of the pipe. 
     According to this configuration, since the dividing part includes the porous body, the plurality of penetrating holes form a plurality of small channels. In addition, since the axial lines of the penetrating holes are disposed along the central axis direction of the pipe, the channel resistance against the circumferential component of the flow in the pipe is increased, and hence, intrusion of the cavity flow into the pipe is suppressed. 
     In accordance with a second aspect of the present invention, there is provided a pipe which is branched from a main pipe through which a high-temperature fluid flows and which is configured to be closable, wherein disturbing part for disturbing a vortex flow swirling in the pipe in a circumferential direction is provided in the pipe. 
     According to the second aspect of the present invention, since the vortex flow (in the spiral shape) swirling in the circumferential direction of the pipe is disturbed by the disturbing part, intrusion of a cavity flow, which is a spiral vortex, into the pipe is suppressed. Hence, the rapid temperature change portion is prevented from being formed at a predetermined position (such as a pipe bent portion) of the pipe, and as a result, an adverse influence on the pipe caused by the rapid temperature change portion can be prevented. 
     In the second aspect described above, it is preferable that the disturbing part include a first spiral member twisted in one rotary direction and a second spiral member twisted in another rotary direction, and that the first spiral member and the second spiral member be disposed in the pipe. 
     According to this configuration, since the disturbing part includes the first spiral member and the second spiral member, which are twisted in the directions opposite to each other, intrusion of the cavity flow, which is a spiral vortex in the pipe, into the pipe can be suppressed. 
     For example, a cavity flow swirling in said one rotary direction is suppressed by the second spiral member twisted in said another rotary direction, and a cavity flow swirling in said another rotary direction is suppressed by the first spiral member twisted in said one rotary direction. Hence, regardless of whether the swirling direction of the cavity flow is said one rotary direction or said another rotary direction, the cavity flow can be suppressed by the disturbing part. 
     In the above second aspect, the disturbing part is preferably configured to include a channel resistance portion increasing channel resistance against a central axis direction flow in the pipe. 
     According to this configuration, since the disturbing part includes the channel resistance portion increasing the channel resistance against the central axis direction flow in the pipe, intrusion of the cavity flow, which is a spiral vortex, into the pipe can be suppressed. 
     In the above second aspect, it is preferable that the disturbing part include a plate member extending in a central axis direction of the pipe, and that the plate member protrude inside and outside the pipe. 
     According to this configuration, since the plate member extends in the central axis direction of the pipe and protrudes inside the pipe, intrusion of the cavity flow, which is a spiral vortex, into the pipe can be suppressed. 
     Since the plate member protrudes outside the pipe, the heat of the fluid can be efficiently radiated outside. 
     In accordance with a third aspect of the present invention, there is provided a pipe which is branched from a main pipe through which a high-temperature fluid flows and which is configured to be closable, wherein a cylindrical member is provided in the pipe, and the cylindrical member is disposed in a region of the pipe where a rapid temperature change portion of the fluid is formed. 
     According to the third aspect of the present invention, since the cylindrical member is disposed in the pipe, a cavity flow, which is a spiral vortex, intrudes into the cylindrical member. Hence, the rapid temperature change portion is formed in the cylindrical member, and as a result, the cylindrical member is thermally deformed. In addition, because the pipe is isolated from the rapid temperature change portion by the cylindrical member, and thermal fatigue formed by the rapid temperature change portion is reduced. Accordingly, an adverse influence on the pipe caused by the rapid temperature change portion can be prevented. 
     In addition, since the thermal stress generated is reduced, the thickness of the cylindrical member is preferably decreased. 
     In the above third aspect, it is preferable that a concave portion or a convex portion spirally extending on the inner surface of the cylindrical member be provided. 
     According to this configuration, the concave portion or the convex portion spirally extending on the inner surface of the cylindrical member is formed, and hence the formation of the rapid temperature change portion can be disturbed. That is, turbulence of a low-temperature fluid flowing toward the main pipe is promoted by the concave portion or the convex portion of the cylindrical member. In addition, turbulence of a high-temperature fluid flowing from the main pipe to an opposite side thereto is promoted by the concave portion of the cylindrical member. Hence, stirring of the low-temperature fluid and the high-temperature fluid is facilitated, and as a result, the formation of the rapid temperature change portion is disturbed. 
     In accordance with a fourth aspect of the present invention, there is provided a pipe which is branched from a main pipe through which a high-temperature fluid flows and which is configured to be closable, wherein in a region of the pipe where a rapid temperature change portion of the fluid is formed, an extendible portion which is extendible in a central axis direction of the pipe is provided. 
     According to the fourth aspect of the present invention, since the extendible portion is formed in the region of the pipe where the rapid temperature change portion is formed, thermal deformation caused by the difference in temperature of the rapid temperature change portion is absorbed in the extendible portion. Hence, the generation of thermal fatigue in the pipe can be prevented, and an adverse influence on the pipe can be prevented. 
     In the above fourth embodiment, it is preferable that the extendible portion include an accordion portion having a concave and a convex part in the radial direction of the pipe, and that the distance between the concave and convex parts along the central axis direction of the pipe be determined in accordance with fluctuation in position where the rapid temperature change portion is formed. 
     According to this configuration, since the extendible portion is the accordion portion having the concave and the convex parts in the radial direction of the pipe, thermal stress caused by the difference in temperature of the rapid temperature change portion is absorbed in the accordion portion, and hence, the generation of thermal fatigue in the pipe can be reduced. In addition, since the distance between the concave and the convex parts along the central axis direction of the pipe is determined in accordance with the fluctuation in position where the rapid temperature change portion is formed, even when the position where the rapid temperature change portion is formed fluctuates, the generation of thermal fatigue in the pipe can be prevented. 
     According to the pipe of the present invention, since the channel of the pipe is divided into the plurality of small channels by the dividing part, in the region where the dividing part is disposed, the channel resistance against the spiral flow is increased. Hence, intrusion of the cavity flow, which is a spiral vortex, into the pipe can be prevented by the dividing part, and as a result, the rapid temperature change portion is prevented from being formed at a predetermined position (such as a pipe bent portion) of the pipe. As a result, an advantage is afforded in that an adverse influence on the pipe caused by the rapid temperature change portion can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view illustrating the structure of a branch pipe according to a first embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of the branch pipe shown in  FIG. 1  taken along the line A-A. 
         FIG. 3  is a schematic view illustrating the structure of a branch pipe according to a second embodiment of the present invention. 
         FIG. 4  is a schematic view illustrating the structure of a branch pipe according to a third embodiment of the present invention. 
         FIG. 5  is a schematic view illustrating the structure of a branch pipe according to a fourth embodiment of the present invention. 
         FIG. 6  is a cross-sectional view of the branch pipe shown in  FIG. 5  taken along the line B-B. 
         FIG. 7  is a schematic view illustrating the structure of a branch pipe according to a fifth embodiment of the present invention. 
         FIG. 8  is a cross-sectional view of the branch pipe shown in  FIG. 7  taken along the line D-D. 
         FIG. 9  is a schematic view illustrating the structure of a branch pipe according to a sixth embodiment of the present invention. 
         FIG. 10  is a schematic view illustrating the structure of a branch pipe according to a seventh embodiment of the present invention. 
         FIG. 11  is a schematic view illustrating the structure of a branch pipe according to an eighth embodiment of the present invention. 
         FIG. 12  is a schematic view illustrating the structure of a branch pipe according to a ninth embodiment of the present invention. 
         FIG. 13  is a schematic view illustrating the structure of a branch pipe according to a tenth embodiment of the present invention. 
     
    
    
     EXPLANATION OF REFERENCE SIGNS 
     
         
           1 ,  51 ,  101 ,  151 ,  201 ,  251 ,  301 ,  351 ,  401 ,  451 : branch pipe (pipe) 
           3 : main pipe 
           5 : first pipe (dividing part) 
           7 : second pipe (dividing part) 
           9 : connection pipe (dividing part) 
           55 : first spiral vane (first swirling member, disturbing part) 
           57 : second spiral vane (second swirling member, disturbing part) 
           105 : labyrinth-structured portion (channel resistance portion, disturbing part) 
           155 : fin (plate member) 
           205 : cylindrical member (dividing part) 
           255 : inner cylinder (cylindrical member) 
           305 : inner cylinder (cylindrical member) 
           307 : concave portion 
           355 : accordion portion (extendible portion) 
           405 : honeycomb portion (porous body) 
           407 ,  457 : penetrating hole 
           455 : porous plate 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     First Embodiment 
     Hereinafter, a branch pipe according to a first embodiment of the present invention will be described with reference to  FIGS. 1 and 2 . 
       FIG. 1  is a schematic view illustrating the structure of the branch pipe of this embodiment. 
     A branch pipe (pipe)  1  is a pipe branched from a main pipe  3  through which high-temperature water flows, as shown in  FIG. 1 . 
     The branch pipe  1  has a first pipe (dividing part)  5  connected to the main pipe  3 , a second pipe (dividing part)  7  separated from the first pipe  5 , and connection pipes (dividing part)  9  which are a plurality of small pipes connecting the first pipe  5  and the second pipe  7 . 
     At an end portion of the first pipe  5  at the second pipe  7  side (lower side in  FIG. 1 ), a connection portion  11 A having a diameter larger than that of the other portion is formed. The connection pipes  9  are connected to a circumferential surface of the connection portion  11 A. At an end portion of the second pipe  7  at the first pipe  5  side (upper side in  FIG. 1 ), a connection portion  11 B having a diameter larger than that of the other portion is formed. The connection pipes  9  are connected to a circumferential surface of the connection portion  11 B. 
     In addition, the connection portions  11 A and  11 B may be formed in the first and second pipes  5  and  7 , as described above, or they may not be formed at all; that is, the configuration is not particularly limited. 
       FIG. 2  is a cross-sectional view of the branch pipe shown in  FIG. 1  taken along the line A-A. 
     The connection pipes  9  are pipes that are narrower than the diameters of the first and second pipes  5  and  7  and are disposed side-by-side at regular intervals around a circle centered at a central axis C of the branch pipe  1 . 
     Ends of the connection pipes  9  are each bent inward in the radial direction toward the circumferential surface of the connection portion  11 B and are connected to the connection portion  11 B, as shown in  FIG. 2 . The ends at the connection portion  11 A side are configured similarly. In addition, central portions of the connection pipes  9  are disposed approximately parallel to the central axis C, as shown in  FIG. 1 . 
     In addition, the shape of each connection pipe  9  is not limited to that in which only the ends are bent, as described above; various shapes may be used. 
     Next, the effect of the branch pipe  1  having the above structure will be described with reference to  FIG. 1 . 
     When high-temperature water flows through the main pipe  3  while the branch pipe  1  is closed, a cavity flow F, which is a spiral vortex swirling around the central axis C, is formed in the first pipe  5 , as shown in  FIG. 1 . While the cavity flow F spirally swirls, it intrudes into the first pipe  5  toward the second pipe  7  side (lower side in  FIG. 1 ) and further intrudes into the connection portion  11 A. 
     The cavity flow F intruding into the connection portion  11 A is attenuated due to an increase in channel resistance against a circumferential component of the flow by the connection pipes  9 . In addition, high-temperature water intruding into the branch pipe  1  from the main pipe  3  as the cavity flow F radiates its heat to the connection pipes  9 , and hence the temperature is decreased. 
     Next, the case in which the closed state of the branch pipe  1  is released, for example, the case in which water flows from the main pipe  3  into the branch pipe  1 , will be described. 
     The water flowing into the first pipe  5  from the main pipe  3  flows into the connection portion  11 A toward the second pipe  7  side. The water flows from the connection portion  11 A into the connection portion  11 B through the connection pipes  9  and then flows into the second pipe  7 . The water flowing into the second pipe  7  is lead, for example, to a water-quality inspection portion. 
     According to the above structure, since the first pipe  5  and the second pipe  7 , which are separated from each other, are connected with the plurality of connection pipes  9 , the channel resistance of the branch pipe  1  against the cavity flow F is increased, and hence the intrusion of the cavity flow F into the branch pipe  1  can be stopped at the position at which the connection pipes  9  are located. Hence, a thermal stratification interface (rapid temperature change portion) can be prevented from being formed in a predetermined portion (such as a pipe bent portion) of the branch pipe  1 , and as a result, an adverse influence on the branch pipe  1  due to the thermal stratification interface can be prevented. 
     In addition, since a heat radiation area of the water is increased in the plurality of connection pipes  9 , the water temperature can be decreased, and hence the thermal stratification interface is less likely to be formed. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described with reference to  FIG. 3 . 
       FIG. 3  is a schematic view illustrating the structure of a branch pipe of this embodiment. 
     In this embodiment, the same constituent elements as those in the first embodiment are designated by the same reference numerals, and a description thereof is omitted. 
     A branch pipe (pipe)  51  is a pipe branched from a main pipe  3  through which high-temperature water flows, as shown in  FIG. 3 . 
     The branch pipe  51  has a pipe  53  connected to the main pipe  3 , a first swirling vane (first spiral member, disturbing part)  55  disposed in the pipe  53 , a second swirling vane (second spiral member, disturbing part)  57 , and a support member  59  supporting the first and the second swirling vanes  55  and  57 . 
     The first swirling vane  55  is a spiral-shaped vane twisted in one swirling direction (for example, clockwise in the direction away from the main pipe  3 ). The second swirling vane  57  is a spiral-shaped vane twisted in another swirling direction (for example, anticlockwise in the direction away from the main pipe  3 ). The first swirling vane  55  and the second swirling vane  57  are disposed in series along a central axis C. 
     The support member  59  is a member formed to have a bar shape and is fixed to the pipe  53 . In addition, the first swirling vane  55  and the second swirling vane  57  are provided on the support member  59  and are supported thereby. 
     Next, the effect of the branch pipe  51  having the above structure will be described with reference to  FIG. 3 . 
     When high-temperature water flows through the main pipe  3  while the branch pipe  51  is closed, a cavity flow F, which is a spiral vortex swirling around the central axis C, is formed in the pipe  53 , as shown in  FIG. 3 . While the cavity flow F spirally swirls, it intrudes into the pipe  53  in the direction away from the main pipe  3  (downward in  FIG. 3 ) and reaches the first and the second swirling vanes  55  and  57 . 
     In the case in which the cavity flow F is a spiral vortex swirling anticlockwise in the direction away from the main pipe  3 , the cavity flow F is disturbed by the first swirling vane  55 . Hence, the cavity flow F is attenuated by the first swirling vane  55 . On the other hand, in the case in which the cavity flow F is a spiral vortex swirling clockwise in the direction away from the main pipe  3 , the cavity flow F is disturbed by the second swirling vane  57 . Hence, the cavity flow F is attenuated by the second swirling vane  57 . 
     The swirling direction of the cavity flow F is not stable and always varies between clockwise swirling and anticlockwise swirling, and clockwise swirling and anticlockwise swirling cavity flows F may be present at the same time in some cases. 
     According to the structure described above, since the first swirling vane  55  and the second swirling vane  57 , which are twisted in directions opposite to each other, are provided in the pipe  53 , the cavity flow F, which is a spiral vortex, is suppressed from intruding into the pipe  53 . 
     In particular, the clockwise swirling cavity flow F is attenuated by the second swirling vane  57  twisted in the anticlockwise direction, and the anticlockwise swirling cavity flow F is attenuated by the first swirling vane  55  twisted in the clockwise direction. Hence, regardless of whether the swirling direction of the cavity flow F is clockwise or anticlockwise, the cavity flow F can be attenuated by the first and the second swirling vanes  55  and  57 . Hence, a thermal stratification interface (rapid temperature change portion) can be prevented from being formed in a predetermined portion (such as a pipe bent portion) of the branch pipe  51 , and as a result, an adverse influence on the branch pipe  51  due to the thermal stratification interface can be prevented. 
     Third Embodiment 
     Next, a third embodiment of the present invention will be described with reference to  FIG. 4 . 
       FIG. 4  is a schematic view illustrating the structure of a branch pipe of this embodiment. 
     In this embodiment, the same constituent elements as those in the first embodiment are designated by the same reference numerals, and a description thereof is omitted. 
     A branch pipe (pipe)  101  is a pipe branched from a main pipe  3  through which high-temperature water flows, as shown in  FIG. 4 . 
     The branch pipe  101  has a pipe  103  connected to the main pipe  3 , and a labyrinth-structured portion (channel resistance portion, disturbing part)  105  disposed in the pipe  103 . 
     The labyrinth-structured portion  105  has a plurality of plate members  107  extending from the inner wall of the pipe  103  inward in the radial direction. The plurality of plate members  107  block part of the channel of the pipe  103  and cause water to flow through remaining open portions  109 . 
     In addition, the plate members  107  are disposed side-by-side in a central axis C direction, and adjacent open portions  109  are disposed opposite each other with the central axis C interposed therebetween. That is, the plate members  107  are disposed so that the open portions  109  are disposed in a staggered manner. 
     Next, the effect of the branch pipe  101  having the above structure will be described with reference to  FIG. 4 . 
     When the high-temperature water flows through the main pipe  3  while the branch pipe  101  is closed, a cavity flow F, which is a spiral vortex swirling around a central axis C, is generated in the pipe  103 , as shown in  FIG. 4 . While the cavity flow F spirally swirls, it intrudes into the pipe  103  in a direction away from the main pipe  3  (downward in  FIG. 4 ) and reaches the labyrinth-structured portion  105 . 
     In the labyrinth-structured portion  105 , the water flows meanderingly through the open portions  109  disposed in a staggered manner, and hence the channel resistance is increased. Accordingly, intrusion of the cavity flow F is suppressed by the labyrinth-structured portion  105 , and as a result, the cavity flow F is attenuated. 
     According to the structure described above, since the labyrinth-structured portion  105 , which increases the channel resistance against the flow in the central axis C direction, is provided in the pipe  103 , intrusion of the cavity flow F into the pipe  103  is suppressed, and hence the cavity flow F can be attenuated. 
     Fourth Embodiment 
     Next, a fourth embodiment of the present invention will be described with reference to  FIG. 5 . 
       FIG. 5  is a schematic view illustrating the structure of a branch pipe of this embodiment. 
     In this embodiment, the same constituent elements as those in the first embodiment are designated by the same reference numerals, and a description thereof is omitted. 
     A branch pipe (pipe)  151  is a pipe branched from a main pipe  3  through which high-temperature water flows, as shown in  FIG. 5 . 
     The branch pipe  151  has a pipe  153  connected to the main pipe  3 , and fins (plate members)  155  provided in the pipe  153 . 
       FIG. 6  is a cross-sectional view of the branch pipe shown in  FIG. 5  taken along the line B-B. 
     The fins  155  are plate members extending in a central axis C direction and also in a radial direction of the pipe  153 . In addition, the fins  155  are disposed to penetrate a circumferential surface of the pipe  153 , an inner edge portion  157  at the central axis C side is located inside the pipe  153 , and an outer edge portion  159 , provided on the opposite side of the central axis C of the fins  155 , is located outside of the pipe  153 . 
     In this embodiment, the number of fins  155  is four, as shown in  FIG. 6 , and they are provided in the pipe  153  at an angular interval of approximately 90°. 
     In addition, the number of fins  155  may be four, as described above, or any other number and is not particularly limited. 
     Next, the effect of the branch pipe  151  having the above structure will be described with reference to  FIGS. 5 and 6 . 
     When high-temperature water flows through the main pipe  3  while the branch pipe  151  is closed, a cavity flow F, which is a spiral vortex swirling around the central axis C, is generated in the pipe  153 , as shown in  FIG. 5 . While the cavity flow F spirally swirls, it intrudes into the pipe  153  in a direction away from the main pipe  3  (downward in  FIG. 5 ) and reaches a region where the fins  155  are disposed. 
     A circumferential component of the cavity flow F is prevented from flowing by the inner edge portions  157  of the fins  155 , as shown in  FIG. 6 . Hence, the cavity flow F is attenuated in the region where the fins  155  are disposed. 
     In addition, after the heat of the water forming the cavity flow F is transmitted to the inner edge portions  157  in contact with the water, the heat is further transmitted to the outer edge portions  159  from the inner edge portions  157  and is then radiated therefrom. 
     According to the structure described above, since the fins  155  extend in the central axis C direction and also have the inner edge portions  157  protruding inside the pipe  153 , the cavity flow F, which is a spiral vortex, is blocked by the inner edge portions  157 , and as a result, intrusion of the cavity flow F into the pipe  153  is prevented. 
     Since the outer edge portions  159  of the fins  155  protrude outside the pipe  153 , the heat of the water can be efficiently radiated outside, and hence the formation of a thermal stratification interface can be prevented. 
     Fifth Embodiment 
     Next, a fifth embodiment of the present invention will be described with reference to  FIG. 7 . 
       FIG. 7  is a schematic view illustrating the structure of a branch pipe of this embodiment. 
     In this embodiment, the same constituent elements as those in the first embodiment are designated by the same reference numerals, and a description thereof is omitted. 
     A branch pipe (pipe)  201  is a pipe branched from a main pipe  3  through which high-temperature water flows, as shown in  FIG. 7 . 
     The branch pipe  201  has a pipe  203  connected to the main pipe  3 , and cylindrical members (dividing part)  205  provided in the pipe  203 . 
       FIG. 8  is a cross-sectional view of the branch pipe shown in  FIG. 7  taken along the line D-D. 
     The cylindrical members  205  are each formed of a pipe having a diameter smaller than that of the pipe  203 , as shown in  FIG. 8 . In this embodiment, four cylindrical members  205  are disposed side-by-side in the pipe  203  in the circumferential direction. 
     Next, the effect of the branch pipe  201  having the above structure will be described with reference to  FIGS. 7 and 8 . 
     When high-temperature water flows through the main pipe  3  while the branch pipe  201  is closed, a cavity flow F, which is a spiral vortex swirling around a central axis C, is generated in the pipe  203 , as shown in  FIG. 7 . While the cavity flow F swirls, it intrudes into the pipe  203  in a direction away from the main pipe  3  (downward in  FIG. 7 ) and reaches a region where the cylindrical members  205  are disposed. 
     The cavity flow F is attenuated due to an increase in channel resistance against a circumferential component of the flow by the cylindrical members  205 , as shown in  FIG. 8 . Hence, the cavity flow F is attenuated in the region where the cylindrical members  205  are disposed. 
     According to the structure described above, since the four cylindrical members  205  are disposed in the pipe  203  side-by-side in the circumferential direction thereof, the channel of the pipe  203  is divided into a plurality of small channels. Hence, the channel resistance in the pipe  203  against the circumferential component of the flow is increased, and as a result, intrusion of the cavity flow F into the pipe  203  can be suppressed. 
     In addition, since the four cylindrical members  205  are disposed in the pipe  203  side-by-side in the circumferential direction, the rigidity of the pipe  203  can be increased. Furthermore, since existing pipes and the like can be used for the cylindrical members  205 , the branch pipe  201  can be easily manufactured. 
     Sixth Embodiment 
     Next, a sixth embodiment of the present invention will be described with reference to  FIG. 9 . 
       FIG. 9  is a schematic view illustrating the structure of a branch pipe of this embodiment. 
     In this embodiment, the same constituent elements as those in the first embodiment are designated by the same reference numerals, and a description thereof is omitted. 
     A branch pipe (pipe)  251  is a pipe branched from a main pipe  3  through which high-temperature water flows, as shown in  FIG. 9 . 
     The branch pipe  251  has a pipe  253  connected to the main pipe  3 , and one inner cylinder (cylindrical member)  255  provided in the pipe  253 . 
     The inner cylinder  255  is formed of a pipe having a diameter smaller than that of the pipe  253  and is supported thereby. The inner cylinder  255  is disposed at a front end of a cavity flow F, that is, at a position where a thermal stratification interface BL is formed. 
     In addition, the thickness of the inner cylinder  255  is preferably decreased in order to reduce the thermal stress applied to the inner cylinder  255 . 
     Next, the effect of the branch pipe  251  having the above structure will be described with reference to  FIG. 9 . 
     When high-temperature water flows through the main pipe  3  while the branch pipe  251  is closed, the cavity flow F, which is a spiral vortex swirling around a central axis C, is generated in the pipe  253 , as shown in  FIG. 9 . While the cavity flow F spirally swirls, it intrudes into the pipe  253  in a direction away from the main pipe  3  (downward in  FIG. 9 ) and reaches the inner cylinder  255 . 
     The cavity flow F intrudes into the inner cylinder  255  and forms a thermal stratification interface therein. On the other hand, the cavity flow F does not intrude between the inner cylinder  255  and the pipe  253 , and hence the pipe  253  is not in contact with the thermal stratification interface BL. 
     According to the structure described above, since the inner cylinder  255  is disposed in the pipe  253 , the cavity flow F, which is a spiral vortex, intrudes into the inner cylinder  255 . Hence, the thermal stratification interface BL, which is a rapid temperature change portion, is formed in the inner cylinder  255 , and hence the inner cylinder  255  is deformed by heat. On the other hand, since the pipe  253  is separated from the thermal stratification interface BL by the inner cylinder  255 , thermal fatigue formed in the pipe  253  by a rapid change in temperature is reduced. As a result, an adverse influence on the pipe  253  due to the thermal stratification interface BL can be prevented. 
     Seventh Embodiment 
     Next, a seventh embodiment of the present invention will be described with reference to  FIG. 10 . 
       FIG. 10  is a schematic view illustrating the structure of a branch pipe of this embodiment. 
     In this embodiment, the same constituent elements as those in the first embodiment are designated by the same reference numerals, and a description thereof is omitted. 
     A branch pipe (pipe)  301  is a pipe branched from a main pipe  3  through which high-temperature water flows, as shown in  FIG. 9 . 
     The branch pipe  301  has a pipe  303  connected to the main pipe  3 , and one inner cylinder (cylindrical member)  305  provided in the pipe  303 . 
     The inner cylinder  305  is formed of a pipe having a diameter smaller than that of the pipe  303  and has a spirally extending concave portion  307  in the inner surface. In addition, the inner cylinder  305  is supported by the pipe  303  and is disposed at a front end of a cavity flow F, that is, at a position where a thermal stratification interface BL is formed. 
     In addition, the spirally extending concave portion  307  may be formed in the inner surface of the inner cylinder  305 , as described above, or a spirally extended convex portion may be formed; hence, the inside structure of the inner cylinder  305  is not particularly limited. 
     Next, the effect of the branch pipe  301  having the above structure will be described with reference to  FIG. 10 . 
     When high-temperature water flows through the main pipe  3  while the branch pipe  301  is closed, the cavity flow F, which is a spiral vortex swirling around a central axis C, is generated in the pipe  303 , as shown in  FIG. 10 . While the cavity flow F spirally swirls, it intrudes into the pipe  303  in a direction away from the main pipe  3  (downward in  FIG. 10 ) and reaches the inner cylinder  305 . 
     The cavity flow F intrudes inside the inner cylinder  305  and is mixed with low-temperature water in the inner cylinder  305 . 
     According to the structure described above, since the spirally extending concave portion  307  is formed in the inner surface of the inner cylinder  305 , the thermal stratification interface BL, which is a rapid temperature change portion, is prevented from being formed. That is, the generation of turbulence of the low-temperature water in the inner cylinder  305  flowing toward the main pipe  3  (from lower side to upper side in  FIG. 9 ) is facilitated by the concave portion  307 . In addition, the generation of high-temperature water in the inner cylinder  305  flowing from the main pipe  3  toward a side opposite thereto (from upper side to lower side in  FIG. 9 ) is also facilitated by the concave portion  307 . Hence, stirring and mixing of the low-temperature water and the high-temperature water are facilitated by the concave portion  307 , and hence the formation of the thermal stratification interface BL is disturbed. 
     Eighth Embodiment 
     Next, an eighth embodiment of the present invention will be described with reference to  FIG. 11 . 
       FIG. 11  is a schematic view illustrating the structure of a branch pipe of this embodiment. 
     In this embodiment, the same constituent elements as those in the first embodiment are designated by the same reference numerals, and a description thereof is omitted. 
     A branch pipe (pipe)  351  is a pipe branched from a main pipe  3  through which high-temperature water flows, as shown in  FIG. 11 . 
     The branch pipe  351  has a pipe  353  connected to the main pipe  3 , and an accordion portion (extendible portion)  355  provided in the pipe  353 . 
     The accordion portion  355  formed in an inner surface of the pipe  353  is a concave-convex portion folded in the radial direction and is formed at a front end of a cavity flow F, that is, at a position where a thermal stratification interface BL is formed. 
     The pitch between a concave and a convex part of the accordion portion  355  is formed larger than the fluctuation in position where the thermal stratification interface BL is formed. 
     Next, the effect of the branch pipe  351  having the above structure will be described with reference to  FIG. 11 . 
     When high-temperature water flows through the main pipe  3  while the branch pipe  351  is closed, the cavity flow F, which is a spiral vortex swirling around a central axis C, is generated in the pipe  353 , as shown in  FIG. 11 . While the cavity flow F spirally swirls, it intrudes into the pipe  353  in a direction away from the main pipe  3  (downward in  FIG. 11 ) and reaches the accordion portion  355 . The cavity flow F forms the thermal stratification interface BL at the accordion portion  355 . 
     The accordion portion  355  is thermally deformed according to the temperature of the water in contact therewith, and the folded concave-convex portion is extended, so that the thermal deformation is absorbed. 
     According to the structure described above, since the accordion portion  355  is formed in the region in which the thermal stratification interface BL is formed, thermal stress caused by the difference in temperature of the thermal stratification interface BL is absorbed in the accordion portion, and hence the generation of thermal fatigue in the pipe  353  can be reduced. 
     In addition, since the pitch (distance) between the concave and convex parts along the central axis C direction of the pipe  353  is formed larger than the fluctuation in position where the thermal stratification interface BL is formed, even when the position where the thermal stratification interface BL is formed fluctuates, the generation of thermal fatigue in the pipe can be prevented. 
     Ninth Embodiment 
     Next, an eighth embodiment of the present invention will be described with reference to  FIG. 12 . 
       FIG. 12  is a schematic view illustrating the structure of a branch pipe of this embodiment. 
     In this embodiment, the same constituent elements as those in the first embodiment are designated by the same reference numerals, and a description thereof is omitted. 
     A branch pipe (pipe)  401  is a pipe branched from a main pipe  3  through which high-temperature water flows, as shown in  FIG. 12 . 
     The branch pipe  401  has a pipe  403  connected to the main pipe  3 , and a honeycomb member (porous body)  405  provided in the pipe  403 . 
     The honeycomb member  405  has a so-called honeycomb structure in which a plurality of penetrating holes  407  each having a polygonal cross-section are formed. In addition, in the honeycomb member  405 , central axes of the penetrating holes  407  are disposed substantially parallel to a central axis C. 
     Next, the effect of the branch pipe  401  having the above structure will be described with reference to  FIG. 12 . 
     When high-temperature water flows through the main pipe  3  while the branch pipe  401  is closed, a cavity flow F, which is a spiral vortex swirling around the central axis C, is generated in the pipe  403 , as shown in  FIG. 12 . While the cavity flow F spirally swirls, it intrudes into the pipe  403  in a direction away from the main pipe  3  (downward in  FIG. 12 ) and reaches the honeycomb member  405 . 
     When passing through the penetrating holes  407  of the honeycomb member  405 , the cavity flow F is attenuated by an increase in channel resistance against a circumferential component of the flow due to the honeycomb member  405 . Hence, in the region in which the honeycomb member  405  is disposed, the cavity flow F is attenuated. 
     According to the structure described above, since the honeycomb member  405  is disposed in the pipe  403 , and the central axes of the penetrating holes  407  are disposed along the central axis C, the channel resistance against the circumferential component of the flow in the pipe  403  is increased. Hence, the cavity flow F is attenuated in the region in which the honeycomb member  405  is disposed and is prevented from intruding into the pipe  403 . 
     Tenth Embodiment 
     Next, a tenth embodiment of the present invention will be described with reference to  FIG. 13 . 
       FIG. 13  is a schematic view illustrating the structure of a branch pipe of this embodiment. 
     In this embodiment, the same constituent elements as those in the first embodiment are designated by the same reference numerals, and a description thereof is omitted. 
     A branch pipe (pipe)  451  is a pipe branched from a main pipe  3  through which high-temperature water flows, as shown in  FIG. 13 . 
     The branch pipe  451  has a pipe  453  connected to the main pipe  3 , and a porous plate (porous body)  455  provided in the pipe  453 . The porous plate  455  is a plate member in which a plurality of penetrating holes  457  are formed. 
     Next, the effect of the branch pipe  451  having the above structure will be described with reference to  FIG. 13 . 
     When high-temperature water flows through the main pipe  3  while the branch pipe  451  is closed, a cavity flow F, which is a spiral vortex swirling around a central axis C, is generated in the pipe  453 , as shown in  FIG. 13 . While the cavity flow F spirally swirls, it intrudes into the pipe  453  in a direction away from the main pipe  3  (downward in  FIG. 13 ) and reaches the porous plate  455 . 
     When passing through the penetrating holes  457  of the porous plate  455 , the cavity flow F is attenuated by an increase in channel resistance against a circumferential component of the flow due to the porous plate  455 . Hence, in the region in which the porous plate  455  is disposed, the cavity flow F is attenuated. 
     According to the structure described above, since the porous plate  455  is disposed in the pipe  453 , the channel resistance against the circumferential component of the flow in the pipe  453  is increased. Hence, the cavity flow F is attenuated in the region in which the porous plate  455  is disposed and is prevented from intruding into the pipe  453 . In addition, compared to the case in which an orifice is disposed, in the case in which the porous plate  455  is disposed, the channel resistance against the circumferential component of the flow is increased more, and a swirling flow can be reliably attenuated.