Patent Publication Number: US-2015083365-A1

Title: Steam generator and method of securing tubes within a steam generator against vibration

Description:
BACKGROUND 
     1. Field 
     The disclosed concept pertains generally to a steam generator, and in particular to a steam generator including anti-vibration bars. The disclosed concept also pertains to a method of securing tubes in a steam generator against vibration with a number of anti-vibration bars. 
     2. Background Information 
     Heat exchangers having tube bundles are commonly employed in pressurized water nuclear reactor systems. A steam generator generally comprises a vertically oriented shell, a tube bundle formed of tubes which each comprise two vertical components that meet at a bend portion, a tube sheet for supporting the tubes at the ends opposite the bend portion, a dividing plate that cooperates with the tube sheet and a hemispheric channel head to form a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with the primary fluid outlet header. The steam generator secondary side comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber made up of the shell on the outside and the wrapper on the inside, and a feedwater ring disposed above the bend portion of the tube bundle. 
     The primary fluid having been heated by circulation through the reactor core enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid is conducted through the primary fluid inlet header, through the inside of the tube bundle, out the primary fluid outlet header, through the primary fluid outlet nozzle to the reactor coolant pump for recirculation. At the same time, feedwater is introduced to the steam generator secondary side through a feedwater nozzle which is connected to the feedwater ring inside the steam generator. Upon entering the steam generator, the feedwater mixes with water returning from moisture separators positioned above the tube bundle referred to as the recirculation stream. This mixture, called the downcomer flow, is conducted down the annular chamber between the shell and the wrapper until the tube sheet near the bottom of the annular chamber causes the water to change direction, passing in heat exchange relationship with the outside of the tubes and up through the inside of the wrapper. While the water is circulating in heat exchange relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the water surrounding the tubes, causing a portion of the water outside the tubes to be converted to steam. The steam-water mixture then rises and is conducted through a number of moisture separators that separate any entrained water from the steam, and the steam vapor then exits the steam generator and is circulated typically through a turbine generator to generate electricity in a manner well known in the art. 
     The portion of the steam generator primarily including the bend portion of the tubes and below to the channel head is typically referred to as the evaporator section. The portion of the steam generator above the tubes that includes the moisture separators is typically referred to as the steam drum. Feedwater enters the steam generator through an inlet nozzle which is disposed in the upper portion of the cylindrical shell. The feedwater is distributed and mixed with water removed by the moisture separators and then flows down the annular channel surrounding the tube bundle. 
     The tubes are supported at their open ends by conventional means whereby the ends of the tubes are welded to the tube sheet which is disposed generally transverse to the longitudinal axis of the steam generator. A series of tube support plates or grids arranged in an axial spaced relationship to each other are provided along the straight portion of the tubes in order to support the straight section of the tubing. Regarding the tube bundle, various steam generators utilize different tube configurations, for example wherein the bend portion is curved or U-shaped, or wherein the vertical components of the tubes each bend at sharp angles, forming a relatively horizontal shaped bend portion. 
     Located within the bend portion of the tubes are a plurality of anti-vibration bars which are typically disposed between each column of tubes. The anti-vibration bars provide support and do not substantially interfere with the flow of the moisture laden steam. The anti-vibration bars are intended to prevent excessive vibrations of the individual tubes of the entire tube bundle; vibrations which can potentially damage the tubes. It is well known that the bend portion of the tube bundle is more severely affected by the vibrations, and, because of the bend configuration, more difficult to adequately support in order to eliminate the vibrations. 
     Typical motion of the tubes experiencing normal vibration is transverse to the plane of the U-bend and therefore such vibration is referred to as out-of-plane vibration. Under unusual conditions, tubes can also experience in-plane vibration. In such situations, adjacent tubes in a given column can contact one another, resulting in severe damage to the tubes. The manufacturing and assembly of the tube bundle are major obstacles to a mechanical solution to this problem. Hence, current anti-vibration bar assembly designs do not significantly restrict in-plane motion of the tubes. 
     SUMMARY 
     These needs and others are met by the disclosed concept in which a solid anti-vibration bar having an increased thickness is structured to be located within a tube bundle. 
     In accordance with one aspect of the disclosed concept, a steam generator is provided. The steam generator has a primary side for circulating a heated fluid and a secondary side for circulating a fluid to be heated by the heated fluid circulating in the primary side. The steam generator includes: a channel head for receiving the heated fluid; a tube sheet that separates the channel head from the secondary side; a tube bundle having a plurality of tubes, arranged in rows and columns, the tube bundle extending from the channel head, through the tube sheet and through at least a portion of the secondary side; and a first number of solid anti-vibration bars. The plurality of tubes includes a first column of tubes, the first column of tubes comprising a first tube having a curved center line disposed in a first plane. The plurality of tubes further includes a second column of tubes, each of the first number of anti-vibration bars being disposed between the first column of tubes and the second column of tubes. The second column of tubes comprises a second tube having a curved center line disposed in a second plane, the second plane being parallel to and spaced a distance from the first plane. Each of the tubes has a tube outer diameter. Each of the first number of anti-vibration bars has a thickness generally transverse to the first and second planes. The thickness of each of the first number of anti-vibration bars is greater than the distance between the first and second planes minus the tube outer diameter. 
     In accordance with another aspect of the disclosed concept, a method is provided for securing tubes within a steam generator against vibration, the tubes being disposed in a tube bundle and arranged in rows and columns, with lanes between the columns. The method comprises: providing a first column of tubes, the first column of tubes comprising a first tube having a curved center line disposed in a first plane; providing a first number of solid anti-vibration bars; and providing a second column of tubes, each of the first number of anti-vibration bars being disposed between the first column of tubes and the second column of tubes, the second column of tubes comprising a second tube having a curved center line disposed in a second plane, the second plane being parallel to and spaced a distance from the first plane. Each of the tubes has a tube outer diameter. Each of the first number of anti-vibration bars has a thickness generally transverse to the first and second planes. The thickness of each of the first number of anti-vibration bars is greater than the distance between the first and second planes minus the tube outer diameter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full understanding of the disclosed concept can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is a perspective view, partially cut away, of a vertical tube and shell steam generator; 
         FIG. 2  is a schematic, cross-section of a portion of a tube bundle of a steam generator with anti-vibration bars; 
         FIG. 3  is a schematic, cross-section of a portion of a tube bundle of a steam generator with anti-vibration bars in accordance with an embodiment of the disclosed concept; 
         FIG. 4A  is schematic front view of a number of tubes of the tube bundle of  FIG. 3 ; 
         FIG. 4B  is a schematic side view of the tubes of  FIG. 4A ; 
         FIG. 4C  is a schematic isometric view of the tubes of  FIG. 4A ; 
         FIG. 5  is a schematic, cross-section of a portion of a tube bundle of a steam generator with anti-vibration bars in accordance with an alternative embodiment of the disclosed concept; 
         FIG. 6A  is a schematic, cross-section of a portion of a tube bundle of a steam generator with anti-vibration bars in accordance with a further embodiment of the disclosed concept; 
         FIG. 6B  is a schematic, cross-section of the tube bundle of  FIG. 6A  with the anti-vibration bars being displaced; and 
         FIG. 7  is a schematic, cross-section, of a portion of a tube bundle of a steam generator with anti-vibration bars in accordance with an additional embodiment of the disclosed concept. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings,  FIG. 1  shows a steam generator  2  that utilizes a plurality of heat exchanger tubes  3  which form a tube bundle  4  to provide the heating surface required to transfer heat from the primary fluid to vaporize or boil the secondary fluid. The steam generator  2  comprises a vessel having a vertically oriented tubular shell portion  6  and a top enclosure or dished head  8  enclosing the upper end and a generally hemispherical-shaped channel head  10  enclosing the lower end. The lower shell portion  6  is smaller in diameter than the upper shell portion  12  and a frustoconical-shaped transition  14  connects the upper portion and lower portions. A tube sheet  16  is attached to the channel head  10  and has a plurality of holes  18  disposed therein to receive ends of the tubes  3 . A dividing plate  22  is centrally disposed within the channel head  10  to divide the channel head  10  into two compartments  24 , 26 , which serve as headers for the tube bundle  4 . Compartment  26  is the primary fluid inlet compartment and has a primary fluid inlet nozzle  27  in fluid communication therewith. Compartment  24  is the primary fluid outlet compartment and has a primary fluid outlet nozzle  28  in fluid communication therewith. Thus, primary fluid, i.e., the reactor coolant, which enters fluid compartment  26  is caused to flow through the tube bundle  4  and out through outlet nozzle  28 . 
     The tube bundle  4  is encircled by a wrapper  30  which forms an annular passage  32  between the wrapper  30  and the shell and transition portions  6 , 14 , respectively. The top of the wrapper  30  is covered by a lower deck plate  34  which includes a plurality of openings  36  in fluid communication with a plurality of riser tubes  38 . Swirl vanes  40  are disposed within the riser tubes  38  to cause steam flowing therethrough to spin and centrifugally remove some of the moisture contained within the steam as it flows through this primary centrifugal separator. The water separated from the steam in this primary separator is returned to the top surface of the lower deck plate  34 . After flowing through the primary centrifugal separator, the steam passes through a secondary separator  42  before reaching a steam outlet nozzle  44  centrally disposed in the dished head  8 . The water separated from the steam in the secondary separator  42  is returned to mix with the water returned from the primary separator above the lower deck plate  34 . 
     The feedwater inlet structure of this steam generator  2  includes a feedwater inlet nozzle  46  having a generally horizontal portion called a feedring  48  and discharge nozzles  50  elevated above the feedring  48 . Feedwater, which is supplied through the feedwater inlet nozzle  46 , passes through the feedwater ring  48 , exits through the discharge nozzles  50  and mixes with water which was separated from the steam and is recirculated. The mixture then flows down above the lower deck plate  34  into the annular downcomer passage  32 . The water then enters the tube bundle  4  at the lower portion of the wrapper  30  and flows among the tubes  3  and up the tube bundle  4  where it is heated to generate steam. 
     As previously mentioned, the tube bundle  4  has a plurality of anti-vibration bars (not shown in  FIG. 1 ) located between the tubes  3 .  FIG. 2  shows a portion of a tube bundle  100  that includes a number of columns of tubes  110 , 130 , 150 . Located between the first column of tubes  110  and the second column of tubes  130  is an anti-vibration bar  120 . Located between the second column of tubes  130  and the third column of tubes  150  is an anti-vibration bar  140 . The anti-vibration bar  120  has a thickness  122  and the anti-vibration bar  140  has a thickness  142 . As seen, because the anti-vibration bars  120 , 140  are linear, the thicknesses  122 , 142  are restricted by a distance  101  between the columns of tubes  110 , 130 , 150 . As a result, in operation the anti-vibration bars  120 , 140  do not significantly reduce the amount of possible in-plane motion within the columns of tubes  110 , 130 , 150 . 
     As will be discussed in connection with  FIGS. 3 through 7 , in-plane vibration can be significantly reduced by including a number of improved anti-vibration bars  220 , 240 , 320 ,  460 , 480 , 520 . Referring to  FIG. 3 , the cross section of a portion within a U-shaped bend of a tube bundle  200  of a steam generator (not shown) is shown. The tube bundle  200  includes a number of columns of tubes  210 , 230 , 250 , wherein any two adjacent tubes have an equal distance  203  (subject to manufacturing tolerance) between their centers (e.g., a triangular pitch). Although the disclosed concept will be described in association with a triangular pitch, it will be appreciated that the disclosed concept could be employed with alternative orientations (e.g., without limitation, a tube bundle (not shown) with tubes having square pitch rotated 45 degrees). 
     The first column of tubes  210  may be either in the middle of the tube bundle  200  or may be at an end. Located between the first column of tubes  210  and the second column of tubes  230  is an anti-vibration bar  220 . The anti-vibration bar  220  is solid and has a thickness  222 . Referring to  FIGS. 3 through 4C , the first column of tubes  210  includes a tube  212  that has a curved center line  214  located in a plane  216 . Similarly, the second column of tubes  230  includes a tube  232  that has a curved center line  234  located in a plane  236 . As seen in  FIG. 3 , the plane  216  and the plane  236  are parallel and are spaced apart by a distance  206 . The distance  206  is substantially equal to two times an outer radius  202  (e.g., a tube outer diameter  204 ) plus a distance  201 . The distance  201  corresponds to the distance  101  shown in  FIG. 2 . 
     As seen in  FIG. 3 , the thickness  222  of the anti-vibration bar  220  is generally transverse to the planes  216 , 236  and is greater than the distance  201  between the columns of tubes  210 , 230 . Continuing to refer to  FIG. 3 , located between the second column of tubes  230  and the third column of tubes  250  is a second anti-vibration bar  240 . Similar to the anti-vibration bar  220 , the anti-vibration bar  240  is solid and has a thickness  242 . Referring to  FIGS. 3 through 4C , the third column of tubes  250  includes a tube  252  that has a curved center line  254  located in a plane  256 . The plane  256  is parallel to and spaced a distance  208  from the plane  236 . The distance  208  is substantially equal to two times the radius  202  (e.g., the tube outer diameter  204 ) plus the distance  201 . 
     Similar to the thickness  222  of the anti-vibration bar  220 , the thickness  242  of the anti-vibration bar  240  is generally transverse to planes  236 , 256  and is greater than the distance  201  between the columns of tubes  230 , 250 . In operation, this increased thickness prevents significant in-plane (see, for example, planes  216 , 236 , 256 ) motion in the columns of tubes  210 , 230 , 250 , advantageously corresponding to a significant decrease in in-plane vibration within the tube bundle  200 . As seen in  FIG. 3 , the anti-vibration bar  220  includes a number of bends  224  that are curved and are structured to wind between the first column of tubes  210  and the second column of tubes  230 . 
     Similarly, the anti-vibration bar  240  includes a number of bends  244  that are curved and are structured to wind between the second column of tubes  230  and the third column of tubes  250 . The bends  224 , 244  enable the thicknesses  222 , 242  of the anti-vibration bars  220 , 240  to be greater than the thicknesses  122 , 142  of the anti-vibration bars  120 , 140 . Furthermore, while the thicknesses  122 , 142  of the anti-vibration bars  120 , 140  are no greater than the distance  101 , the thicknesses  222 , 242  of the anti-vibration bars  220 , 240  are only limited by the distance  203  between adjacent centers minus two times the radius  202  (e.g., the tube outer diameter  204 ). 
       FIG. 5  shows a portion within a U-shaped bend of a tube bundle  300  of a steam generator (not shown) in accordance with an alternative embodiment of the disclosed concept. As seen, the tube bundle  300  includes an anti-vibration bar  320  that is located between a first column of tubes  310  and a second column of tubes  330 . The first column of tubes  310  may be either in the middle of the tube bundle  300  or may be at an end. Furthermore, the first column of tubes  310  includes a tube  312  that has a curved center line (not shown) that is located in a plane  316 . The second column of tubes  330  includes a tube  332  that has a curved center line (not shown) that is located in a plane  336 . The plane  336  is parallel to and spaced a distance  306  from the plane  316 . Similar to the anti-vibration bars  220 , 240 , the anti-vibration bar  320  has a thickness  322 . The thickness  322  is generally transverse to the planes  316 , 336  and is greater than a distance  301  between the columns of tube  310 , 330 . 
     As seen, the distance  301  corresponds to the distance  306  minus two times a radius  302  (e.g., a tube outer diameter  304 ). In a similar manner as the anti-vibration bars  220 , 240 , the anti-vibration bar  320  is structured to wind between the first column of tubes  310  and the second column of tubes  330 . However, while the anti-vibration bars  220 , 240  include a number of bends  224 , 244  that are curved, the anti-vibration bar  320  includes a number of bends  324  that are substantially jagged. The bends  324  of the anti-vibration bar  320 , like the bends  224 , 244  of the anti-vibration bars  220 , 240 , allow the anti-vibration bar  320  to have the increased thickness  322 . Furthermore, similar to the anti-vibration bars  220 , 240 , in operation, the increased thickness  322  of the anti-vibration bar  320  prevents significant in-plane (see, for example, planes  316 , 336 ) motion with the columns of tubes  310 , 330  advantageously corresponding to a significant decrease in in-plane vibration within the tube bundle  300 . 
       FIG. 6A  shows a portion within a U-shaped bend of a tube bundle  400  which includes a number of anti-vibration bars  460 , 480 . The anti-vibration bar  460  is located between a first column of tubes  410  and a second column of tubes  430 . The first column of tubes  410  may be either in the middle of the tube bundle  400  or may be at an end. The anti-vibration bar  480  is located between the second column of tubes  430  and a third column of tubes  450 . Similar to the anti-vibration bars  220 , 240 , the anti-vibration bars  460 , 480  include a number of bends  464 , 484  that are structured to wind between the columns of tubes  410 , 430 , 450 . However, the anti-vibration bars  460 , 480  are less thick than the anti-vibration bars  220 , 240 . 
     As seen in  FIG. 6A , the first column of tubes  410  includes a tube  412  and the second column of tubes  430  includes a tube  432 , the anti-vibration bar  460  being situated adjacent the tubes  412 , 432 . Since the anti-vibration bar  460  is less thick, there are gaps (see, for example, gap  467 ) between the anti-vibration bar  460  and the tubes  412 , 432 . Similarly, the anti-vibration bar  480  is situated adjacent the tube  432  and each of the tubes in the third column of tubes  450 . Since the anti-vibration bar  480  is less thick, there are gaps (see, for example, gap  487 ) between the anti-vibration bar  480  and the tubes in the second column of tubes  430  and the third column of tubes  450 . 
     As seen, the anti-vibration bar  460  is substantially located along a longitudinal axis  465  and the anti-vibration bar  480  is substantially located along a longitudinal axis  485 .  FIG. 6B  shows a portion of a tube bundle  400 ′ in which the anti-vibration bars  460 , 480  have been displaced along the longitudinal axes  465 , 485 . As seen in  FIGS. 6A and 6B , the anti-vibration bar  460  is displaced in a first direction  461  along the longitudinal axis  465 . The anti-vibration bar  480  is displaced in a second direction  481  along the longitudinal axis  485 . The first direction  461  and the second direction  481  are substantially parallel to and opposite each other. The anti-vibration bars  460 , 480  may be displaced by being pulled and/or pushed after fabrication of the tube bundle by an operator or by the use of a suitable mechanism known in the art. 
     As seen in  FIG. 6B , as the anti-vibration bar  460  moves in the first direction  461  along the longitudinal axis  465 , the anti-vibration bar  460  engages the tube  412  such that there is no gap (or the gap  467  seen in  FIG. 6A  substantially decreases in size). Similarly, as the anti-vibration bar  480  moves in the second direction  481  along the longitudinal axis  485 , the anti-vibration bar  480  engages the tube  432  such that there is no gap (or the gap  487  seen in  FIG. 6A  substantially decreases in size). In this manner, gaps (see, for example, gaps  467 , 487  in  FIG. 6A ) between the anti-vibration bars  460 , 480  and tubes in the columns of tubes  410 , 430 , 450  decrease in size, further reducing the amount of possible in-plane motion. 
       FIG. 7  shows a portion within a U-shaped bend of a tube bundle  500  of a steam generator (not shown) in accordance with an alternative embodiment of the disclosed concept. As seen, the tube bundle  500  includes a number of columns of tubes  510 , 530 , 550 . The first column of tubes  510  may be either in the middle of the tube bundle  500  or may be at an end. Located between the first column of tubes  510  and the second column of tubes  530  is an anti-vibration bar  520 . The anti-vibration bar  520  is substantially similar to the anti-vibration bars  220 , 240 , having a thickness  522  generally transverse to planes  516 , 536  and greater than a distance  501  between the columns of tubes  510 , 530 . 
     Located between the second column of tubes  530  and the third column of tubes  550  is an anti-vibration bar  540  that is substantially similar to the anti-vibration bars  120 , 140  seen in  FIG. 2 . The anti-vibration bar  540  has a thickness  542  that is generally transverse to the plane  536 , which is parallel to and spaced a distance  508  from a plane  556 . The thickness  542  of the anti-vibration bar  540  is less than the thickness  522  of the anti-vibration bar  520 . Similar to the thicknesses  122 , 142  of the anti-vibration bars  120 , 140 , the thickness  542  is restricted by the distance  501  and may be substantially equal to, but no more than the distance  501 . As seen, the anti-vibration bar  540  is substantially linear, having no bend or curvature along its longitudinal axis. 
     In this manner, in-plane vibration within the tube bundle  500  can be significantly reduced by including the anti-vibration bar  520 , while costs can advantageously be saved by including the anti-vibration bar  540  in accordance with existing designs.  FIG. 7  shows one of many alternative embodiments that are within the scope of the disclosed concept. For example and without limitation, it is within the scope of the disclosed concept to have any number of the anti-vibration bars  220 , 240 , 320 , 460 , 480 , 520  that are arranged in any configuration with existing anti-vibration bars  120 , 140 , 540 . Additionally, it is further understood that the anti-vibration bars  220 , 240 , 320 , 460 , 480 , 520 , 540  are secured to a structure or structures (not shown) extending around the tube bundle bends in one of several manners known in the art. 
     The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art. 
     As employed herein, the term “solid” shall mean being without an internal cavity or opening. As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality).