Abstract:
A self-supporting system for positioning and restraining the U-bend tubes in the U-bend region of a nuclear steam generator includes arrays incorporating unique support bars having nubs projecting in the out-of-plane direction of the tube planes. The system also includes assemblies for spacing the arrays, tie bars to prevent the arrays from splaying and saddle bar assemblies to support the outermost tube layers. The system provides positive restraint to nub engaged tubes in both the in-plane direction of the tube planes and the out-of-plane direction.

Description:
FIELD AND BACKGROUND OF INVENTION 
       [0001]    The present invention relates generally to the field of heat exchanger tube supports, and in particular to a new and useful U-bend support system for positioning and restraining the U-bends of water tubes within a nuclear steam generator against flow-induced vibration. 
         [0002]    In a pressurized water nuclear power station, steam generators, which are large heat exchangers, transfer heat produced via nuclear reactions in the reactor core, from a primary water coolant to a secondary water coolant that drives the steam turbine. The primary coolant is pressurized, which allows the primary water coolant to be heated in the reactor core with little or no boiling. For example, in a light water reactor, the primary coolant is pressurized to about 2250 psia and heated to about 600 deg F. in the reactor core. From the reactor, the primary water coolant flows to a steam generator, where it transfers heat to the secondary coolant. In a U-tube, or recirculating steam generator, the primary coolant enters at the bottom of the steam generator, flows through tubes having an inverted U-shape transferring heat to the secondary coolant, and then exits at the bottom of the steam generator. The secondary coolant is pressurized only to a pressure below that of the primary side, and boils as it flows along the outside of the tubes, thereby producing the steam needed to drive the turbine. Nuclear steam generators must be capable of handling large quantities of two-phase secondary coolant moving at high flow rates, and are therefore very large structures. For example, a nuclear U-tube steam generator can weigh more than 450 tons, with a diameter exceeding 12 feet and an overall length of greater than 70 feet. It may contain as many as 9,000 or more of the long, small diameter, thin-walled U-shaped tubes. For a general description of the characteristics of nuclear steam generators, the reader is referred to Chapters 46, 48 and 50 of  Steam/Its Generation and Use,  41st Edition, The Babcock &amp; Wilcox Company, Barberton, Ohio, U.S.A., ©2005, the text of which is hereby incorporated by reference as though fully set forth herein. 
         [0003]    Nuclear steam generators require tube restraints or supports, to position the tubes and to restrain the tubes against flow induced vibration forces. In the U-bend region of a nuclear steam generator, a large flow of steam and water mixture passes upwards through the tube array, in a general direction which locally is normal to the axis of the individual U-bend tubes. This large two phase flow is able to cause excitation of the U-bend tubes via the turbulent and other flow forces imparted by the flow. As a result, the tubes tend to vibrate in both the out-of-plane and in-plane directions relative to the U-bend plane. Typically this restraint function is provided by an array of flat U-bend support bars. While such flat bars provide positive restraint in the U-bend out-of-plane direction, they provide restraint only by friction in the in-plane direction. 
         [0004]    One known type of nuclear steam generator U-bend support assembly, depicted in  FIG. 1 , and in greater detail in  FIG. 2 , is manufactured by Babcock &amp; Wilcox Canada Ltd.  FIG. 1  shows a nuclear steam generator  80  having a plurality of U-bend tubes  102 , referred to as a tube bundle, which are fixed at their ends to a heavy tubesheet  90 . The U-bend tubes  102  are arranged in layers or columns. Each layer or column incorporates a set of tubes of successively larger radius, which are nested, from innermost tube to outermost tube, to create the layer or column of tubes in the particular plane. The tubes are further arranged in rows, with each row containing all tubes of a particular U-bend radius. For purposes of illustration, however,  FIG. 1  shows only a limited number of U-bend tubes  102 , and  FIG. 2  shows only the outermost tubes of the center U-bend layers. The straight leg portions of the U-bend tubes  102  are supported at several locations by vertically spaced apart tube support plates  120  as shown in  FIG. 1 . 
         [0005]    The U-bend portions  103  of tubes  102  extend beyond the uppermost tube support lattice (or plate)  124  and sweep through  180  degrees of arc. The relatively long U-bend region  103  of each U-tube  102  requires supports to keep them in position and to restrain against flow-induced vibration (FIV) excitation due to the very large upward flow of two-phase steam/water mixture. 
         [0006]    As shown in  FIG. 1 , and in greater detail in  FIG. 2 , the U-bend tubes  102  are positioned and restrained in the U-bend region  103  of U-bend tubes  102  by a U-bend support assembly  100 , which includes a number of U-bend support bar arrays  180 . Each U-bend support bar array  180  is comprised of flat U-bend support bars  160 , which are positioned in sets between layers of tubes within the U-bend region of the steam generator. 
         [0007]    As shown in  FIG. 2 , the flat U-bend support bars  160  fan out from the center of the U-bend such that individual bar sets are assembled into a U-bend support bar array  180 , or “fan” bar array, in which the inner ends of the individual bars are interconnected to collector bar  114  by a mechanical or welded joint  190 . U-bend support bar array  180  is referred to as a “half-fan” array, since collector bar  114  covers only half the U-bend region (i.e. either the cold leg or the hot leg) of tubes in a particular plane. 
         [0008]    Each U-bend support bar array  180  incorporates about  4  to  12  of the flat U-bend support bars  160 . The flat U-bend support bars  160  are positioned so as to provide support to the U-bend tubes  102  at certain points along the arc of each U-bend tube in the array. The angular separation of the flat U-bend support bars  160  depends upon the U-bend size and flow conditions; the flat U-bend support bars  160  are located to minimize unsupported tube lengths. The individual flat U-bend support bars  160  are typically made of stainless steel, and are about 1″ to 1.5″ wide and about 0.1″ to 0.2″ thick. A U-bend support assembly  100  may incorporate between about 100 to about 200 of the fan-shaped U-bend support bar arrays  180 , with one such array located between each plane of U-bend tubes. 
         [0009]    The outer ends of the flat U-bend support bars  160  are collected, restrained and supported by arch bar support structures, which extend in the out-of-plane direction, perpendicular to the columns or layers of U-bend tubes  102 . Each arch bar structure is made up of arch bars  170  and clamping bars  175 . Each arch bar  170  is a single continuous piece. The clamping bars  175  are segmented and affix the J-tabs  176  and the upper ends of the flat U-bend support bars  160  to arch bars  170 . Each arch bar support structure positions the flat U-bend support bars  160  of a U-bend support bar array  180 , carrying the weight of the bars and redistributing the weight of the U-bend support assembly  100  back to the peripheral U-bend tubes via J-tabs  176 . Tie tubes  150 , arranged horizontally above arch bars  170  and interconnecting the arch bar support structures at selected locations, restrain the fan bar arrays in position on the U-bends. 
         [0010]    The U-bend support bar arrays  180  position the planes of U-bend tubes  102  in space, and most importantly, restrain the individual U-bend tubes against flow induced vibration. Restraint against out-of-plane motion is provided by the physical presence of the flat U-bend support bars  160 , which are situated immediately adjacent to the U-bend tubes  102 . The bar-to-tube clearance is purposely quite small, with individual bar-to-tube diametral clearances varying from about 0 to 0.010″ or more. The flat U-bend support bars  160 , with their small bar-to-tube clearances, thus prevent significant motion of the tubes in the out-of-plane direction  140 . In the in-plane direction  130 , however, the U-bend tubes  102  are not positively restrained, but instead depend solely upon friction between the U-bend tubes  102  and the flat U-bend support bars  160  to restrict and dampen the flow induced motion of the tubes in their in-plane direction. Depending on the design details and flow conditions, the effect of the friction in providing in-plane restraint may not be fully adequate in providing effective in-plane restraint. 
         [0011]    U.S. Pat. No. 6,772,832, which is assigned to the assignee of the present invention, discloses a corrective retrofit tube support structure having rows of concave pockets located on diagonally opposite surfaces of the bar. 
       SUMMARY OF INVENTION 
       [0012]    The present invention is drawn to an improved U-bend tube support system which is particularly suited for the U-bend region of a U-tube nuclear steam generator. The system includes arrays of unique support bars having nubs projecting in the out-of-plane direction of the tube planes. The system also includes assemblies for spacing the arrays, tie bars to prevent the arrays from splaying and saddle bar assemblies to support the outermost tube layers. 
         [0013]    The system of the present invention positions the U-bend region of the U-tubes and provides positive restraint in both the in-plane and out-of-plane directions. The system advantageously is self-supporting, requiring no additional structure or external restraints, and provides improved access for maintenance and repair. 
         [0014]    Accordingly, one aspect of the invention is drawn to a support bar for supporting the U-bend region of U-tubes in a nuclear steam generator comprised of an elongated body having a plurality of nubs projecting in the out-of-plane direction, from at least one side of the body. 
         [0015]    Another aspect of the invention is drawn to a nubbed support bar array for supporting the U-bend region of U-tubes in a nuclear steam generator. The nubbed support bar array includes a plurality of flat elongated bars, and a plurality of nubbed support bars. Each nubbed bar is an elongated body with a plurality of nubs projecting in the out-of-plane direction from at least one side of the body. The nubbed bar may include an integral spacer block. 
         [0016]    Yet another aspect of the invention is drawn to a support bar assembly for supporting the U-bend region of U-tubes in a nuclear steam generator. The assembly includes a plurality of nubbed support bar arrays with nubbed bar arrays arranged between adjacent layers of U-tubes. The nubbed support bar arrays include a plurality of flat elongated bars, a plurality of nubbed support bars, a generally flat elongated connector bar connected to the inner ends of the plurality of flat elongated bars and directly or indirectly connected to the inner ends of the plurality of nubbed support bars. The connector bar extends across both the hot leg and the cold leg of the associated tube layer. Each nubbed bar is an elongated body with a plurality of nubs projecting in the out-of-plane direction, from at least one side. Each nub has a generally rectangular longitudinal cross-section and tube contact faces generally parallel to the intrados or extrados of the U-tubes. The assembly also includes spacer blocks or spacer clips for spacing the outer bar ends in the out of plane direction, and arcuate tie bars for spacing each nubbed support bar array in the in-plane direction. Each tie bar is spaced in parallel with the extrados of the outermost tube of an associated tube layer, and has an out-of-plane thickness about twice the cross-sectional radius of the U-tubes. 
         [0017]    The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. For a better understanding of the present invention, and the operating advantages attained by its use, reference is made to the accompanying drawings and descriptive matter, forming a part of this disclosure, in which a preferred embodiment of the invention is illustrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    In the accompanying drawings, forming a part of this specification, and in which reference numerals shown in the drawings designate like or corresponding parts throughout the same: 
           [0019]      FIG. 1  is a schematic view of a nuclear steam generator having U-bend heat exchanger tubes; 
           [0020]      FIG. 2  is a partially cut away perspective view of a known U-bend support assembly; 
           [0021]      FIG. 3  is a sectional front elevation view of an improved U-bend tube support system of the present invention; 
           [0022]      FIG. 4  is a partial perspective view of a nubbed support bar employed in the present invention according to a first embodiment; 
           [0023]      FIG. 5A  is a partial perspective view of a nubbed support bar employed in the present invention according to a second embodiment; 
           [0024]      FIG. 5B  is a partial perspective view of a nubbed support bar extension; 
           [0025]      FIG. 6A  is a partial sectional elevation view of a spacer assembly suitable for use in the present invention; 
           [0026]      FIG. 6B  is a partial perspective view of a spacer assembly suitable for use in the present invention; 
           [0027]      FIG. 6C  is a partial perspective view of a spacer assembly and tie bar arrangement suitable for use in the present invention; 
           [0028]      FIG. 6D  is a partial sectional view of a spacer assembly and tie bar arrangement suitable for use in the present invention; 
           [0029]      FIG. 6E  is a partial sectional view of a clip assembly and tie bar arrangement; 
           [0030]      FIG. 7  is a sectional elevation view showing elements of an improved U-bend tube support bar array of the present invention according to a second embodiment; 
           [0031]      FIG. 8  is a sectional elevation view of an improved U-bend tube support bar array of the present invention according to a second embodiment; 
           [0032]      FIG. 9A  is a partial perspective view of a spacer clip connector suitable for use in the present invention; 
           [0033]      FIG. 9B  is a sectional view of a spacer clip connector suitable for use in the present invention; 
           [0034]      FIG. 9C  is a sectional view of a spacer clip and ladder component 
           [0035]      FIG. 10  is a perspective view of a saddle bar assembly suitable for use in the present invention; 
           [0036]      FIG. 11  is a cross sectional view of the saddle bar assembly taken along line  11 - 11  of  FIG. 3 ; and 
           [0037]      FIG. 12  is a partial perspective view of an improved U-bend support assembly according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    Referring to the drawings in which reference numbers are used to refer to the same or functionally similar elements,  FIGS. 3 and 4  depict the improved U-bend support assembly  200  of the present invention, for use in a U-tube nuclear steam generator, which incorporates nubbed fan bars  210  arranged in nubbed fan bar arrays  280 . 
         [0039]    Nubbed fan bar array  280  is a welded array of nubbed fan bars  210  and flat elongated bars  260 , running upward from collector bar  214 . 
         [0040]    Each nubbed fan bar  210  is an elongated body having multiple “nubs”  212 , which project from a flat face or side  240  of nubbed fan bar  210  and have a generally rectangular longitudinal cross section. Nubs  212  may be machined or otherwise created on a face  240  of nubbed fan bar  210 , down to a nub-initiation radius  276 , which is typically about 30% of the largest tube radius of curvature. The reverse face  250 , opposite face  240  of nubbed fan bar  210 , is typically flat, but may also have nubs. 
         [0041]    Nubs  212  typically fill the radial spaces between successive tubes (e.g. tubes  202 ,  204 ,  206  within a particular tube column  203 ), with provision for assembly clearance. Nubs  212  project in the out-of-plane direction (perpendicular to the flat face  240 ) for a distance greater than the cross-sectional radius  217  of the tubes in the adjacent tube column  203 . The tube contact faces  230 ,  231  of nub  212  are thus perpendicular to the U-bend in-plane direction (defined by a tube column such as tube column  203 ). Tube contact faces  230 ,  231  preferably of convex and flat or concave shape respectively are relatively parallel to the tube intrados and extrados, respectively (but relieved to avoid the possibility of tube distress from the nub corners.) 
         [0042]    Nubbed fan bars  210  are preferably arranged in opposing pairs having a radial orientation with respect to the center of curvature  219  of the U-bend of the tubes of a tube column or layer such as tube column  203 . 
         [0043]    As shown in  FIG. 3 , the inner ends of flat fan bars  260  are welded to collector bar  214 , which runs generally horizontally across the inner ends of fan bars  260 . The inner ends of nubbed fan bar  210  are likewise affixed by welding, directly or indirectly, to collector bar  214 . Bars  210 ,  214 , and  260  are arranged as even numbers of bars, typically from about 4 to about 12 bars total. Collector bar  214  is preferably made up of two elongated flat bars welded together. Collector bar  214  of nubbed fan bar array  280  runs across all the tubes of the entire tube layer or column  203 , i.e. from the outermost hot leg tube to the outermost cold leg tube, so that nubbed fan bar array  280  is a full fan bar array covering the entire U-bend region of tubes in a particular column  203 . 
         [0044]    Where employed, nubs  212  provide in-plane tube restraint against flow-induced vibration excitation. Nub coverage for nubbed fan bars  210  with nubs  212  may extend from the outer surface of outermost tube  202  of a particular tube layer and down to a tube at some nub-initiation radius  206 . The nub-initiation radius  206  is selected to provide in-plane restraint coverage to the smallest possible radius, without encountering excessive tube stress due to in-plane nub-induced constraint related to differential tube-to-tube motion within the particular tube layer. Nubs  212  of each nubbed fan bar  210  preferably extend over a range from just beyond the outermost tube of a tube column or layer (e.g. tube  202 ) down to a nub-initiation radius (e.g. tube  206 ) to cover approximately the outer 70% of the maximum tube bundle radius, i.e. the largest tube radius of curvature in the U-bend region. 
         [0045]    Referring now to  FIGS. 6A  thru  6 E, the outer ends  218 ,  268  of fan bars  210 ,  260  are preferably interconnected and spaced in the out-of-plane direction by a system of spacer block assemblies  400  comprised of spacer blocks  405 , studs  402 , retention pins  430 , nuts  404  and nut locking features. Collector bars  214  preferably have no spacer assemblies  400 . 
         [0046]    Spacer block assemblies  400  include spacer blocks  405  having a thickness preferably exactly equal to the tube out-of-plane pitch, i.e. the distance between adjacent tube planes  209 . Studs  402  interconnect the spacer blocks  405 . The tips or ends  218 ,  268  of fan bars  210 ,  260  are positioned within a slot  407  in the back face of each spacer block  405 , and engage the stud  402  passing through one or more drilled holes  215 ,  216  near bar ends  218 ,  268  and through aligned holes  415 ,  416  in the adjacent spacer block  405 . 
         [0047]    As shown in  FIG. 6A , spacer blocks  405  form a plurality of built-up arch assemblies  270  over the top of the tube bundle at the locations of the fan bar ends  218 ,  268 . 
         [0048]    Spacer block assemblies  400  are preferably designed to allow a progressive bottom to top (bundle and U-bend plane horizontal) assembly process, i.e. spacer blocks  405  are of similar shape and the same orientation throughout (and are not symmetrical about the center plane.) 
         [0049]    In an alternate embodiment, shown in  FIGS. 5A ,  7  and  8 , ladder-block nubbed bar  710  may be used in place of the nubbed fan bar  210  and spacer block  405  combination of nubbed fan bar array  280 , to form a nubbed ladder-block fan bar array  780 . 
         [0050]    Ladder-block nubbed bar  710  has a flat fan bar  760  and, similar to spacer block  405 , has a slot or channel  707  on its back face to engage flat fan bar  760 . Slot  707  is sized so that block assembly stack build up is block-to-block; i.e. the fan bar  760  has a slight clearance within slot  707  to ensure that stack-up is block-to-block and not block-to-bar-to-block. The block portion of nubbed ladder-block  710  preferably has all of the features of spacer block  405  including one or two stud holes ( 715 ,  716 ), stud retention pin hole  730 , etc. 
         [0051]    Nubbed ladder-block  710  has nubs  712  on ladder rails  740  which engage the U-bend regions of U-tubes  203 . As shown in  FIG. 7 , nubbed ladder-blocks  710  are preferably positioned at the outer ends  768  of pairs of radially oriented flat fan bars  760 , so as to engage the U-tubes in the U-bend region  203 . Regular spacer blocks  405  are preferably used at other, un-nubbed bar locations  260 . After positioning the tubes  203  and the nubbed ladder-blocks  710  and spacers  405  during assembly, the fan bar array  780  for that tube column is next placed on top of the tubes  203 , nubbed ladder-blocks  710  and spacers  405 . 
         [0052]    Where nubbed ladder-block bars  710  are used in place of nubbed bars  210 , the inner extent of nub coverage is limited by the length of the ladder rails  740  of nubbed ladder block  710 . That leaves tubes in the region between the nub-initiation radius  206  and the inner end  717  of nubbed ladder block  710  without in-plane restraint. As shown in  FIG. 8 , in-plane restraint is provided for such regions by nubbed ladder extensions  770 . The combination of nubbed ladder-blocks  710  and nubbed ladder extensions  770  thus provides nub coverage along the desired length. 
         [0053]    Nubbed ladder extensions  770 ,  FIG. 5B , are comprised of two ladder rails  741  of appropriate length, with transverse nubs  712  at each inter-tube space along their length, on at least one side, similar to the rail  740  and nub  712  detail of the ladder portion of nubbed ladder-blocks  710  shown in  FIG. 5A . 
         [0054]    Ladder lengths for a particular tube layer are preferably arranged so that all inter-tube spaces within each tube column, down to the nub initiation radius  206 , have nubs  712 , either from nubbed ladder-block  710  or one or more nubbed ladder extensions  770 . 
         [0055]    Referring now to  FIGS. 5B and 8 , nubbed ladder extensions  770  are positioned on the associated fan bar  760  with nubs  712  engaging the respective tubes. Nubbed ladder extensions  770  are preferably not positively affixed to each other, to the nubbed ladder blocks  710 , or to anything else. They are positioned entirely by engagement with their associated fan bars and tubes, and have no fasteners. Any ladder induced tube-to-tube interaction is thus within the span of the particular nubbed ladder extension  770 . 
         [0056]    The lower ends  717  of the rails  741  for ladder extensions  770  as well as those of the rails  740  of the ladder blocks  710  are generally positioned to avoid co-incidence with the line of tube contact at the rails, thereby limiting wear at the rail corners. The upper ends  718  of the rails  741  of ladder extensions are preferably positioned to allow a small clearance  719  between the respective rail ends, such that ladder blocks  710  and ladder extensions  770  remain unconnected and independent of one another. 
         [0057]    As shown in  FIGS. 7 and 8 , nubbed ladder-block fan bar array  780  is a full fan bar array having a plurality of flat fan bars  260 ,  760 . As the ladder blocks  710  and ladder extensions  770  are separate from the array, nubbed ladder-block fan bar array  780  is flat and devoid of any out-of-plane features. 
         [0058]    As shown in  FIGS. 9A through 9C , a spacer clip end connector  805  may be used as an alternative to the spacer block  405  and stud  402  arrangement of spacer block assembly  400 . Spacer clip  805  has a first slot  807  to engage a first fan bar  860  in its plane adjacent to tube layer  203 . Spacer clip  805  is fixed to fan bar  860  by a “dog” or other gripping means  804  which engages a hole or notch near the bar end and prevents the spacer clip  805  from sliding endwise along the bar  860 . 
         [0059]    Spacer clip  805  also has a second slot  808  to engage fan bar  861  in the adjacent plane. Bar  861  is free to slide end-wise within its slot  808 . 
         [0060]    As shown in  FIG. 9B , a spacer clip  805  is installed at the end of each fan bar  210 ,  260  so as to create a built-up arch  870  over the U-bend assembly at each fan bar location, similar to built-up arch assembly  270  comprised of spacer blocks  405 . 
         [0061]    The fan and U-bend layers are precisely spaced relative to their adjacent neighbors by the tolerance control of the spacer clips  805 . The layers of fans and tubes are, however, free to slide over each other so that the U-bundle is free to sway without layer-to-layer constraint (as may occur with clamping of the bar ends). Such constraint may cause higher forces and stresses in a fan bar, etc. With spacer clip  805 , the motion of the U-bends/fan layers is coordinated and moderated by the fan bars, but is not rigidly constrained. The resultant sway motions are greater that for a clamped arrangement, but local stresses due to rigid constraint are avoided. 
         [0062]    Referring now to  FIG. 9C , an alternative to spacer clips  805  is shown. Ladder clip  880  comprise clip sections  881 , similar to clip  805 , and ladder section  882  similar to ladder section  731  of ladder blocks  710 . 
         [0063]    Referring now to  FIG. 6E , where clips  805  are used, tie bars  220  are connected to clips  805  by projections  224  on the tie bars  220  which engage with notches  412  on the clips in a manner similar to the engagement of tie bars  220  with spacer blocks  405 . 
         [0064]    Referring now to  FIGS. 3 ,  6 A through  6 E,  7 ,  8 ,  9 B,  11  and  12 , tie bars  220  are preferably used to position the nubbed fan bar arrays  280 ,  780  in the in-plane direction and to keep the fan bars, e.g.  210 ,  260  and  760  from “splaying” apart (i.e. to keep the nubbed fan bar arrays  280  and  780  from spreading in the direction of the tube axes, and sliding down the two sides of the U-bend). 
         [0065]    As shown in  FIGS. 6A through 6E , tie bar  220  is an arcuate bar disposed in the plane of an associated tube layer or column  203  adjacent to, and spaced in parallel with, the extrados of the outermost U-tube  202 . Tie bars  220  preferably have the same out-of-plane thickness  222  as the tube diameter (i.e. twice the length of tube cross-sectional radius  217 , shown in  FIG. 4 ) and lie entirely within the plane of the associated tube column or layer  203 . In this way tie bars  220  are totally transparent to possible future inter-tube bundle access for service work in the field. 
         [0066]    As shown in  FIGS. 6C through 6E , the tie bar profile, having projections  224 , engages notches  411 ,  412  in the spacer blocks  405 ,  805  (or alternatively projections  711  of nubbed ladder-block  710 ), and is captured between successive fan bars, e.g. nubbed fan bar  210  and adjacent nubbed fan bar  211 , or fan bar  260  and adjacent fan bar  261 , so that no fasteners are required to keep tie bars  220  in place or to perform their function. 
         [0067]    Preferably about five to eight pairs of tie bars  220  may be required, distributed across the U-bend support assembly  200 . 
         [0068]      FIGS. 10 and 11  show outer fan bar arrays  380  located on the outer edges of the tube bundle, adjacent layers of U-tubes comprised of U-tubes having a small bend radius of curvature. Outer fan bar arrays  380  are therefore not captured between tube layers and must be spaced and connected to other fan bar arrays, e.g.  280 ,  780  within the bundle. In the present invention, this is accomplished using saddle bar assemblies  300 , which sit over the outer one, two or three tube layers of the tube bundle so as to properly position outermost fan bar arrays  380 . 
         [0069]    Saddle bar assembly  300  is comprised of fan bar arrays  380  (four shown in  FIG. 10 ) made up of flat fan bars  360  and space pieces  305  arranged to allow the saddle bar assembly  300  to nest over the respective U-bend regions of outermost U-tubes  303 , and to provide properly controlled support clearances at all tube contact locations. Fan arrays  380  are generally connected by welds at space pieces  305 . 
         [0070]    The innermost fan array  381  of the saddle bar assembly  300  is connected to the rest of the U-bend assembly by studs passing through fan bar stud holes  306  or other connection means, and joining the innermost fan array  381  to adjacent fan bar array  280  or nubbed ladder-block array  780 . The weight of the saddle bar assembly  300  is thus transferred to adjacent arrays  280 ,  780  having nubs  212 , thereby transferring the weight of saddle bar assembly  300  to the tube columns supporting nubbed fan bar arrays  280 ,  780 , by means of studs  308  passing through stud holes  306 . 
         [0071]      FIG. 12  is a partial perspective view of a U-bend support assembly  200 , where only selected U-tubes and nubbed fan bar arrays are shown for the sake of clarity. Support assembly  200  is made up of saddle bar assemblies  300 , tie bars  220 , fan bar arrays  280  or nubbed ladder-block arrays  780 , and spacer block assemblies  400  having spacer blocks  405 . 
         [0072]    Referring now to  FIG. 12 , spacer blocks  405 , and nubbed ladder-blocks  710 ,  780  form a plurality of built-up arch assemblies  270  over the top of the tube bundle at the locations of the fan bar ends such as  218 ,  268 ,  768 . Tie bars  220 , lying within associated tube planes or layers, in turn interconnect and control the position of built-up arch assemblies  270  and fan bar ends  218 ,  268 ,  768 . 
         [0073]    U-bend support assembly  200  is self-supporting to the tube layers through nubs  212  of the nubbed support bar arrays  280 ,  780 , and is spaced in the in-plane direction by tie bars  220  and by spacer blocks  405 , spacer clips  805 , or ladder-blocks  710  in the out-of-plane direction. No additional external U-bend support structure is needed. 
         [0074]    The individual tube and fan bar layers may optionally be made to slip relative to each other, so that the U-tubes and support assembly  200  as a whole may sway freely out-of-plane due to seismic or handling loads (including situations where the tube bundle is oriented horizontally) without excessive stress due to rigid local restraints. Optional free swaying condition may be achieved by limiting tension on studs  402  ( FIG. 6B ), or by use of clips  805  or ladder clips  881 . 
         [0075]    U-bend support assembly  200  ( FIG. 3 ) is preferably used in nuclear U-tube steam generators where all U-tubes in all columns  203  have the same center of curvature  219 , and thus have the same tangent point elevation  213 . Such steam generators are free of cross-over tubes, and are also free of tube layers having expanded U-bend pitch with vertically offset centers of curvature. 
         [0076]    While specific embodiments and/or details of the invention have been shown and described above to illustrate the application of the principles of the invention, it is understood that this invention may be embodied as more fully described in the claims, or as otherwise known by those skilled in the art (including any and all equivalents), without departing from such principles.