Abstract:
A method of and apparatus for securing tubes within a steam generator against vibration with a metallic anti-vibration bar in a tube bundle. The tube bundle has a plurality of tubes, arranged in rows and columns, with lanes between the tube columns. The method comprises the steps of: inserting the anti-vibration bar in the tube bundle; plastically expanding the wall of the anti-vibration bar from an unexpanded position to an expanded position. In the unexpanded position there are a plurality of gaps between the anti-vibration bar and the tubes of the tube bundle. The gaps decrease in size as the wall of the anti-vibration bar moves from the unexpanded position to the expanded position.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from and claims the benefit of U.S. Provisional Patent Application Ser. No. 61/720,491, filed Oct. 31, 2012. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The disclosed concept pertains generally to a method of securing tubes in a steam generator against vibration, and in particular to a method of securing tubes against vibration with an anti-vibration bar. The disclosed concept also pertains to a steam generator including anti-vibration bars. 
         [0004]    2. Background Information 
         [0005]    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. 
         [0006]    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 heat exchanger 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. 
         [0007]    The portion of the steam generator primarily including the bend portion of the heat exchanger tubes and 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. 
         [0008]    The heat exchanger tubes are supported at their open ends by conventional means whereby the ends of the tubes are seal welded to the tube sheet which is disposed 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 heat exchanger tube configurations, for example wherein the bend portion is curved or U-shaped, or wherein the vertical components of the heat exchanger tubes each bend at sharp angles, forming a relatively horizontal shaped bend portion. 
         [0009]    Located within the bend portion of the tubes are a plurality of anti-vibration bars which are typically disposed between each column of the bend portion of the 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. The vibrations in question are caused by the flow of water and steam past the tubes. These flow-induced vibrations 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. While the advent of the anti-vibration bars has materially reduced the magnitude and presence of vibrations, they have not in all cases completely eliminated damage which is caused by vibrations. 
         [0010]    The mechanical aspects of the tube bundle are major obstacles to finding a mechanical solution to this problem. The heat exchanger tubes of the tube bundle have dimensional tolerances associated with their outer diameter. There can also be variations caused by ovalization of some tubes as a result of the bending in the bend portion. Furthermore, the spatial relationship between adjacent tubes is a variable, albeit, within design limits. Thus, there is a dimensional tolerance associated with the nominal spacing between the steam generator tubes. Additionally, current anti-vibration bar designs require a small clearance to facilitate assembly, which results in post-assembly gaps. 
         [0011]    These gaps are undesirable because they allow vibration of the tubes and relative motion between the tubes and the anti-vibration bars. The relative motion can cause wear and subsequent damage and possibly failure of the tubes. Therefore, it is important to control the spacing between the heat exchanger tubes and the anti-vibration bars for vibration control purposes. Accordingly, it is an object of this invention to decrease the size of the gaps between the anti-vibration bars and the tubes of the steam generator. 
       SUMMARY 
       [0012]    These needs and others are met by the disclosed concept in which the wall of a metallic anti-vibration bar is adapted to plastically expand from an unexpanded position to an expanded position. 
         [0013]    In accordance with one aspect of the disclosed concept, a method is provided for securing tubes within a steam generator against vibration with a metallic anti-vibration bar in a tube bundle, wherein the tube bundle has a plurality of tubes, arranged in rows and columns, with lanes between the tube columns. The method comprises the steps of: inserting the anti-vibration bar in the tube bundle; and plastically expanding the wall of the anti-vibration bar from an unexpanded position to an expanded position. In the unexpanded position, there are a plurality of gaps between the anti-vibration bar and the tubes of the tube bundle, the gaps decreasing in size as the wall of the anti-vibration bar moves from the unexpanded position to the expanded position. 
         [0014]    As another 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 having an axial dimension, for circulating a fluid to be heated by the heated fluid circulating in the primary side. The steam generator comprises: 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, with lanes between the tube columns, the tube bundle extending from the channel head, through the tube sheet and through at least a portion of the secondary side; and at least one metallic anti-vibration bar disposed within the tube bundle. The wall of the at least one anti-vibration bar is structured to plastically expand from an unexpanded position to an expanded position. In the unexpanded position there are a plurality of gaps between the at least one anti-vibration bar and the tubes of the tube bundle. The gaps decrease in size as the wall of the at least one anti-vibration bar moves from the unexpanded position to the expanded position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    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: 
           [0016]      FIG. 1  is a perspective view, partially cut away, of a vertical tube and shell steam generator; 
           [0017]      FIG. 2  is a side view of an anti-vibration bar in an unexpanded position in accordance with an exemplary embodiment of the disclosed concept; 
           [0018]      FIG. 2A  is a side view of an anti-vibration bar in accordance with an alternative embodiment of the disclosed concept; 
           [0019]      FIG. 3  is a front view of the anti-vibration bar of  FIG. 2 ; 
           [0020]      FIG. 4  is a cross-sectional view of the anti-vibration bar of  FIG. 2  situated between two heat exchanger tubes of a steam generator; 
           [0021]      FIG. 5  is a cross-sectional view of the anti-vibration bar of  FIG. 4  in an expanded position situated between the two heat exchanger tubes; 
           [0022]      FIG. 6  is a schematic view of a portion of a tube bundle of a steam generator with anti-vibration bars in an unexpanded position situated between heat exchanger tubes in accordance with an alternative embodiment of the disclosed concept; 
           [0023]      FIG. 7  is a schematic view of the portion of the tube bundle of  FIG. 6  with the anti-vibration bars in an expanded position; 
           [0024]      FIG. 8  is a side view of a portion of a steam generator including two anti-vibration bars in a bend portion of a tube bundle in accordance with the disclosed concept; 
           [0025]      FIG. 9A  is a side view of an anti-vibration bar tree configuration in accordance with the disclosed concept; 
           [0026]      FIG. 9B  is a side view of another anti-vibration bar tree configuration in accordance with the disclosed concept; 
           [0027]      FIG. 9C  is a side view of a further anti-vibration bar tree configuration in accordance with the disclosed concept; and 
           [0028]      FIG. 9D  is a side view of an additional anti-vibration bar tree configuration in accordance with the disclosed concept. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    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 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  and  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 . 
         [0030]    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  and  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 . 
         [0031]    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. 
         [0032]    As previously mentioned, the tube bundle  4  has a plurality of anti-vibration bars (not shown in  FIG. 1 ) located between the tubes  3 . As will be discussed below in connection with  FIGS. 2 through 9D , the size of the gaps between anti-vibration bars and heat exchanger tubes can be decreased by plastically expanding the walls of anti-vibration bars from unexpanded positions to expanded positions. 
         [0033]    Referring to  FIGS. 2 and 3 , a generally V-shaped anti-vibration bar  100  in accordance with one embodiment of the disclosed concept is shown. As seen, two chambers  102 , 104  are disposed within the anti-vibration bar  100  between the ends. The anti-vibration bar  100  may be made by any suitable mechanism known in the art (e.g., without limitation, a weldment of a tube that has been plastically compressed). Schematically shown, the chambers  102 , 104  terminate at an end  110  that is sealed. The end  110  may be sealed by any suitable mechanism known in the art (e.g., without limitation, being welded shut). The other end of the anti-vibration bar  100  includes a coupling assembly (schematically shown as  106 ) which may remain on the anti-vibration bar  100  while the steam generator (not shown) is in operation. The coupling assembly  106  may be, for example and without limitation, a quick connect pressure fitting. The coupling assembly  106  is adapted to be removably coupled to a pressure source  108  which may be, for example and without limitation, a hydraulic or pneumatic pressure source. 
         [0034]    During fabrication of the anti-vibration bar  100 , the bend region may become sealed.  FIG. 2A  shows an alternative embodiment of the disclosed concept in which a generally V-shaped anti-vibration bar  100 ′, similar to the anti-vibration bar  100  shown in  FIGS. 2 and 3 , is adapted to be coupled to two pressure sources  108 ′, one at each end of the anti-vibration bar  100 ′. 
         [0035]    Referring to  FIG. 4 , the chambers  102 , 104  are oval shaped. However, it is within the scope of the disclosed concept for the chambers to be other shapes (e.g., without limitation, round or rectangular shaped). Additionally, although the anti-vibration bar  100  has two chambers  102 , 104 , it is within the scope of the disclosed concept for an anti-vibration bar (not shown) to have one chamber or more than two chambers. The anti-vibration bar  100  is inserted into the tube bundle of a steam generator (not shown).  FIG. 4  shows the anti-vibration bar  100  located between two heat exchanger tubes  150  of a steam generator (not shown) after it has been inserted. As seen, the anti-vibration bar  100  has a thickness  152  that is smaller than a distance  154  between the heat exchanger tubes  150 . In other words, there is a gap between the heat exchanger tubes  150  and the anti-vibration bar  100 . 
         [0036]      FIG. 4  shows the wall of the anti-vibration bar  100  in an unexpanded position  103 . As seen the anti-vibration bar  100  is substantially flattened in the unexpanded position  103 . Referring to  FIGS. 2 through 5 , the pressure sources  108 , 108 ′ plastically expand the walls of the anti-vibration bars  100 , 100 ′. The pressure source  108  plastically expands the wall of the anti-vibration bar  100  from the unexpanded position  103  to an expanded position  103 ′, seen in  FIG. 5 . Referring to  FIG. 2A , the pressure sources  108 ′ likewise plastically expand the wall of the anti-vibration bar  100 ′. The pressure sources  108 , 108 ′ are removed before the steam generator (not shown) is placed into service. The pressure required to plastically expand the wall of any given anti-vibration bar varies depending on many factors such as material, size, and tube pitch. However, the pressure has to be sufficient to plastically expand the material so that the expanded section does not return to the original geometry once the pressure is removed. If a pneumatic pressure source is employed, the pressure required to plastically expand the wall of the anti-vibration bar  100 , 100 ′ is preferably less than 1,000 psi, more preferably being less than 500 psi. In other words, the expansion has to be irreversible rather than an elastic expansion. Regarding material, the anti-vibration bar  100  is metallic and may be made of any material suitable for plastically expanding the wall from the unexpanded position  103  to the expanded position  103 ′ in a steam generator (e.g., without limitation, stainless steel). 
         [0037]    As the wall of the anti-vibration bar  100  is plastically expanded to the expanded position  103 ′, the gaps between the heat exchanger tubes  150  and the anti-vibration bar  100  are decreased in size. This elimination of space between the heat exchanger tubes  150  and the anti-vibration bar  100  reduces vibrations which, in turn, advantageously reduces wear and damage of the heat exchanger tubes  150  during the operation of the steam generator (not shown). As seen in  FIG. 5 , there is a substantially tangential contact between the anti-vibration bar  100  and the heat exchanger tubes  150 . 
         [0038]    However, the disclosed concept is not limited to situations where the gaps are merely decreased in size, resulting in less space or a tangential contact. For example and without limitation, it is also within the scope of the disclosed concept for gaps to be eliminated such that a residual preload is created between the anti-vibration bar  100  and the heat exchanger tubes  150  of the tube bundle when the wall of the anti-vibration bar  100  is in the expanded position  103 ′. In this manner, the elastic rebound of the heat exchanger tubes  150  exceeds the rebound of the chambers  102 , 104 , resulting in more than a tangential contact between the anti-vibration bar  100  and the heat exchanger tubes  150 . 
         [0039]    Referring to  FIGS. 6 and 7 , an alternative embodiment of the disclosed concept is provided. Schematically shown, a plurality of linear anti-vibration bars  200  are located between a plurality of heat exchanger tubes  250 . Unlike the anti-vibration bars  100 , 100 ′, each anti-vibration bar  200  is an individual tube, distinct from the heat exchanger tubes  250 , that is plastically compressed, becoming substantially flattened, and sealed at one end before being inserted into the tube bundle of the steam generator (not shown). Furthermore, unlike the anti-vibration bars  100 , 100 ′, which are generally V-shaped, the anti-vibration bars  200  are generally linear, being located along a longitudinal axis between the ends. 
         [0040]    Schematically shown, one end of each anti-vibration bar  200  includes a coupling assembly  206  which may be any suitable assembly known in the art (e.g., without limitation, a quick connect pressure fitting). The coupling assembly  206  may be left on while the steam generator (not shown) is in operation. Furthermore, the coupling assembly  206  is adapted to be removably coupled to a pressure source  212  which may be, for example and without limitation, a hydraulic or pneumatic pressure source. Schematically shown, the anti-vibration bars  200  have opposite ends  214  that are sealed. The ends  214  may be sealed by any suitable mechanism known in the art (e.g., without limitation, being welded shut). The anti-vibration bars are substantially flattened in unexpanded positions  203 , seen in  FIG. 6 , and the pressure source  212  plastically expands the walls to expanded positions  203 ′, seen in  FIG. 7 . If a pneumatic pressure source is employed, the pressure required to plastically expand the walls of the anti-vibration bars  200  is preferably less than 1,000 psi, more preferably being less than 500 psi. Furthermore, the pressure source  212  is removed before the steam generator (not shown) is placed into service. 
         [0041]    When the walls of the anti-vibration bars  200  are in the unexpanded positions  203 , the heat exchanger tubes  250  have a distance  216  between any given pair of adjacent columns that is greater than a thickness  218  of the anti-vibration bars  200 . In other words, there is a gap between the heat exchanger tubes  250  and the anti-vibration bars  200  when the walls of the anti-vibration bars  200  are in the unexpanded positions  203 . As the walls of the anti-vibration bars  200  plastically expand from the unexpanded positions  203  to the expanded positions  203 ′, the size of the gaps decreases. This elimination of space between the heat exchanger tubes  250  and the anti-vibration bars  200  reduces vibration of the heat exchanger tubes  250  which, in turn, advantageously reduces wear and damage of the heat exchanger tubes  250  during operation of the steam generator (not shown). 
         [0042]    It is also within the scope of the disclosed concept for gaps to be eliminated such that a residual preload is created between the anti-vibration bars  200  and the heat exchanger tubes  250 .  FIG. 7  shows the contact between the anti-vibration bars  200  and the heat exchanger tubes  250  when the walls of the anti-vibration bars  200  are in the expanded positions  203 ′. As seen, the anti-vibration bars  200  become partially sinuous such that more than a tangential contact is created with the heat exchanger tubes  250  of the tube bundle. 
         [0043]    Continuing to refer to  FIGS. 6 and 7 , the exemplary heat exchanger tubes  250  have a triangular pitch. Seen in  FIG. 6 , any two adjacent heat exchanger tubes  250  have an equal distance  220  between their centers. The disclosed concept however, is not limited to arrangements wherein the heat exchanger tubes  250  have a triangular pitch (e.g., without limitation, the disclosed concept may be employed with heat exchanger tubes (not shown) that have a square pitch or a rotated square pitch (e.g., diamond shape)). 
         [0044]    The disclosed concept has been described in association with generally V-shaped anti-vibration bars  100 , 100 ′ that have chambers (see for example chambers  102 , 104 ) along the edges, and linear anti-vibration bars  200  that are plastically compressed tubes that are substantially flat when the walls are in the unexpanded positions  203 . However, the disclosed concept may be employed with alternative configurations of anti-vibration bars (e.g., without limitation,  FIGS. 8 through 9D ).  FIG. 8  shows two anti-vibration bars  302 , 304  located in a bend portion  306  of a tube and shell steam generator  300 . As seen, the anti-vibration bars  302 , 304  are generally V-shaped, the first anti-vibration bar  302  having an angle  310  and the second anti-vibration bar  304  having an angle  312 . 
         [0045]    The walls of the anti-vibration bars  302 , 304  are structured to plastically expand between unexpanded positions and expanded positions. The anti-vibration bars  302 , 304  may have chambers (not shown) similar to the chambers  102 , 104  of the anti-vibration bar  100 . It is also within the scope of the disclosed concept for the anti-vibration bars  302 , 304  to be individual tubes that have been bent to be generally V-shaped and plastically compressed to be substantially flat, before being placed into the steam generator  300 . In this manner, when the walls are plastically expanded, gaps between the anti-vibration bars  302 , 304  and the heat exchanger tubes  308  are advantageously decreased in size. The heat exchanger tubes  308  generally have a U-shaped curvature in the bend portion  306  of the steam generator  300 . The disclosed concept also applies to other steam generators (not shown). For example and without limitation, steam generators wherein the heat exchanger tubes each bend at sharp angles in the bend portion, forming a relatively horizontal shaped bend portion. 
         [0046]      FIGS. 9A through 9D  show a few of the many alternative, non-limiting configurations of anti-vibration bars  400 , 420 , 440 , 460 , which may be employed in the tube bundle of a steam generator (not shown). The walls of the anti-vibration bars  400 , 420 , 440 , 460  are structured to plastically expand between unexpanded positions and expanded positions. As the walls of the anti-vibration bars  400 , 420 , 440 , 460  expand, the size of the gaps between anti-vibration bars  400 , 420 , 440 , 460  and the surrounding heat exchanger tubes (not shown) advantageously decrease in size. 
         [0047]    The anti-vibration bars  400 , 420 , 440 , 460  are each weldments of numerous bars and have at least one sealed end and at least another end that is coupled to a pressure source (not shown). The individual bars that make up the anti-vibration bars  400 , 420 , 440 , 460  may have one or more chambers (not shown) similar to the chambers  102 , 104  of the anti-vibration bar  100 . The individual bars that make up the anti-vibration bars  400 , 420 , 440 , 460  may also be tubes that are plastically compressed and/or bent before being placed into a given steam generator (not shown). 
         [0048]    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. 
         [0049]    As employed herein, the statement that two or more parts or components are “coupled” together shall mean that the parts are joined or operate together either directly or through one or more intermediate parts or components.