Abstract:
This patent discloses and claims a system for providing in-plane stabilization to the tubes of a heat exchanger. The system increases the friction between the tubes and the existing anti-vibration bars by pressing the tube bundle together in the out-of-plane direction. The invention involves a structure that develops forces by reacting either off the inner diameter of the tube bundle wrapper, or off an assembled stressor hoop, which is self-supporting on top of the tube bundle, or involves purposely deforming connecting hoops to bias the anti-vibration bars in a way that the tube U-bend is no longer planar. It imposes a controlled clamping and/or deforming force on the tube bundle, either by direct contact with the outer tubes or indirectly via the existing anti-vibration bar structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application No. 61/671,636 filed on Jul. 13, 2012, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a stabilization system, and, more particularly, the present invention relates to a system for stabilizing the in-plane flow-induced vibration of U-bend tubes of a heat transfer device. 
         [0004]    2. Description of the Related Art 
         [0005]    While the present invention may be used in a variety of industries, the environment of a pressurized water reactor (PWR) nuclear power plant will be discussed herein for illustrative purposes. There are two major systems utilized in a PWR to convert the heat generated in the fuel into electrical power. In the primary system, primary coolant is circulated past the fuel rods where it absorbs the emitted heat. The heated fluid, which is in liquid form due to the elevated pressure of the primary loop, flows to the steam generators where heat is transferred to the secondary system. After leaving the steam generators, the primary coolant is pumped back to the core to complete the primary loop. In the secondary loop, heat is transferred to the secondary coolant, or, feedwater, from the primary side in the steam generators, producing steam. The steam is used to rotate a turbine, generating electricity. The wet steam leaves the turbine, passes through a condenser to remove residual heat, and the liquid feedwater is pumped back to the steam generators. 
         [0006]    Inside of the steam generator, the hot reactor coolant flows inside of the many tubes and the feedwater flows around the outside of the tubes. There are two forms of steam generators: once-through steam generators, in which the tubes are straight, and U-bend steam generators, which are more common and in which the tubes contain a U-shaped bend. 
         [0007]    The typical heat exchanger, steam generators in the nuclear industry in particular, are susceptible to vibration-induced wear on the internals. Vibration is due to flow-induced forces on tubing during normal operation, particularly in the U-bend region of the tubes where flow is much more cross-flow than axial. The normal industrial practice is to analyze, design, and construct the heat exchanger with specific supports, called anti-vibration bars, that directly and positively act against instability in the out-of-plane direction (that is, against the plane defined by the U-bend tube). Commonly, anti-vibration bars, however, are not designed with specific features to prevent instability in the in-plane direction (that is, within the plane defined by the U-bend tube). 
         [0008]    Recently, tube-to-tube wear has been detected in the U-bend region of steam generators. This wear is indicative of tube-to-tube contact during power operation, and has been attributed to tube instability in the U-bend area. The tube motion is in the in-plane direction (movement back and forth parallel to the anti-vibration bars). It has been concluded that the in-plane instability is due to a lack of sufficient friction between the anti-vibration bars and the tubes, which renders the anti-vibration bars ineffective at preventing in-plane motion of the tube. 
         [0009]    Several classic repair approaches exist tier recovery from out-of-plane instability, but those approaches do not work optimally for in-plane repair. They can require extensive disassembly of the steam generator to effect the repair, and are very labor-intensive. Additionally, most U-bend steam generators are not currently designed and fabricated to guarantee resistance to in-plane instability by mechanical restraint. 
         [0010]    Thus, what is needed is a stabilization system that specifically counteracts induced in-plane vibration of heat exchanger tubes. 
       SUMMARY OF THE INVENTION 
       [0011]    The inventive tube stabilization system increases the friction between the steam generator tubes and the anti-vibration bars by pressing the tube bundle together in the out-of-plane direction and/or distorts the plane of the U-tubes. The invention involves a structure that reacts either off the inner diameter of the tube bundle wrapper, or off an assembled “hoop,” which is self-supporting on top of the tube bundle. it imposes a controlled pushing force on the tube bundle, either by direct contact with the outer tubes and/or indirectly via the existing anti-vibration bar structure. 
         [0012]    The invention creates a controllable distortion geometry and loading on the U-bends of a heat exchanger, providing a unique means to create stability against excessive tube vibration, whether it be in-plane and/or out-of-plane. By creating a controllable side load and/or elastic tube axis distortion, the tube is heavily damped against any vibration movement in the in-plane direction. It also becomes more damped against out-of-plane vibrations. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention is described with reference to the accompanying drawings, which illustrate exemplary embodiments and in which like reference characters reference like elements. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive. 
           [0014]      FIG. 1  shows an example of the tubes and anti-vibration bars of a known pressurized water reactor steam generator. 
           [0015]      FIGS. 2A ,  2 B,  2 C, and  2 D show examples of various anti-vibration bar support designs. 
           [0016]      FIG. 3  shows a first example of a system of the current invention in place atop the U-bend region of a steam generator. 
           [0017]      FIG. 4  shows a detailed view of the system of  FIG. 3 , illustrating the connection to the existing anti-vibration bars of the steam generator. 
           [0018]      FIGS. 5 ,  6 ,  7 , and  8  each show detailed views illustrating configurations of the load blocks and how the load can be distributed to a multiplicity of tubes and anti-vibration bars. 
           [0019]      FIG. 9  shows a rotated view of an anti-vibration bar hoop and illustrates another example of the current invention, 
           [0020]      FIG. 10  shows anti-vibration bars and steam generator tubes prior to being stabilized according to the present invention. 
           [0021]      FIG. 11  shows anti-vibration bars and steam generator tubes being stabilized according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The inventive tube stabilization system counteracts induced in-plane vibration of heat exchanger tubes. According to one aspect of the system, existing anti-vibration bars (AVBs) are used to counteract in-plane vibration.  FIG. 1  shows an example of the tubes and anti-vibration bars of a known pressurized water reactor steam generator, and  FIGS. 2A ,  2 B,  2 C, and  2 D show examples of various anti-vibration bar support designs uses in the industry. The view of  FIG. 1  is a partial cut-away showing a cross-section view collinear with the longitudinal axis of the steam generator tubes  10 . The AVBs  15  are bars that extend in between the tubes  10 , thereby resisting out-of-plane movement of the tubes  10  (only a representative portion of which are shown). The AVBs are connected and retained in place by a plurality of connecting devices  20 , or even singular rods welded to AVB terminators  16 , as seen in  FIG. 5 . Additionally, AVB hoops  17  (see  FIG. 3 ) extend across the U-bend at intervals to interconnect the connection devices  20  and support the AVBs as they span through and across the tubes  10 . As will be appreciated, the flat AVBS  15  do not resist in-plane motion of the tubes  10 . As the flow-induced forces imparted by normal use of the steam generator can be different for each tube  10 , adjacent ones of the tubes  10  may come into contact, potentially causing damage. 
         [0023]      FIG. 3  shows an overhead view of the U-bend region of a steam generator (for clarity, the tubes themselves are not shown in the figure, just some of the AVBs and the “mesh” of external structures that hold all of the AVBs in place are shown). The AVBs  15  are designated. by horizontal lines, only a few of which are shown in the bubbled region and with only these being referenced in the figure so that the figure will not be cluttered. (Similarly, only two AVB hoops  17  are referenced in  FIGS. 3 and 4 .) The inventive system  50  includes a stressor hoop  51  that is assembled by a plurality of connectors  52  (if the assembly must be constructed in place with limited access) and in turn is connected to load blocks  57 , which in turn connect to some of the AVBs  15  or connecting structures  17 ,  20 . Preferably, the number of connectors  52  is minimized. If multiple connectors  52  are used, they preferably are symmetrically spaced about the hoop  51 . By tightening the hoop  51  in known manner, such as forcing of load developing device  54 , inward-directed force  53  is imparted to the AVBs via the load blocks  57  and pressure-point tugs  55  and, as needed, load fingers  56 . The magnitude of this force  53  can be varied by adjusting the compressive load generated by device  54 ; increasing the compressive load will impart a force  53  of relatively greater magnitude. Alternate means of using the system  50  are also available; for example, shortening or lengthening the hoop  51  (like a turn-buckle) will impart a force  53  of relatively greater or lesser magnitude, respectively. Preferably, the hoop  51  may be chosen to safely withstand loads of 1000 to 9000 lbs or more, and be thermally compatible with the operating components. Differential thermal expansion of the various components is logically important to consider the net loads applied to the tube U-bend assembly at operating conditions. 
         [0024]      FIG. 4  shows a detailed view of the system  50  of  FIG. 3 , illustrating the connection thereof to the existing AVBs of the steam generator. The stressor hoop  51  is linked to the load block  57  by a sliding connection at the interface and by load developing device  54 . The load block  57  is the structure that holds and positions the pressure point lugs  55  that contact one or more AVBs  15  and/or tubes  10 . Preferably, each load block  57  contacts a plurality of AVBs  15  such that the force  53  imparted by the hoop  51  is transmitted to a great many of the AVBs  15 . This may be achieved, as shown in the illustrated example of  FIG. 4 , by providing multiple pressure point lugs  55 . The force imparted by the system  50  can be adjusted via load adjustment screw  54 . Tightening or loosening this screw  54  adjusts the force imparted by the load block  57  to the AVBs  15 . 
         [0025]      FIGS. 5 ,  6 ,  7 , and  8  contain more explicit sketches to illustrate some of the ways that loads can be readily imparted to multiple tubes  10  and AVBs  15  such that contact stress levels are reduced but the total loads per load block  57  can be amplified. In  FIG. 5  the pressure point lugs  55  are shown pressing on pairs of tubes  10 , taking advantage of any natural steps its the tube-bundle geometry. The geometry is chosen as desired to allow use near the AVB connecting hoops  17  and to provide for system elasticity. One can press directly on the AVBs  15 , or on tube-pairs which then press on the adjacent “underlying” AVB  15 .  FIG. 6  is provided to aid in comprehension of the concept of using pre-assembled load blocks  57  and how they may be installed.  FIG. 7  illustrates a further variation of loading which embodies load fingers  56  to develop direct contact on event more AVBs  15  and tubes  10 . Here, the lug  55  again rests on two tubes  10 , but the finger  56  projects through and dog-legs to rest on an AVB  15 . This approach has the advantage of further developing successful interlock with the tube bundle to aid in lift-off resistance during any main steam tine break event; such events develop high ejection loads on AVBs  15  and connecting structures. 
         [0026]      FIGS. 5 ,  6 ,  7 , and  8  further illustrate how the blocks  57  can be fashioned to accommodate and even take advantage of individual geometries of a given AVB support structure. AVBs  15 , AVB terminators  16 , AVB hoops  17 , and connecting devices  20  are shown since they are commonly used elements in steam generator U-bend configurations. ( FIG. 1  shows an alternate design used for connecting device  20  which would require appropriate modification to the load block  57 .) 
         [0027]      FIG. 8  shows example variations of the load block  57 . The upper sketch shows loading with offset load fingers  56 , and the log ser sketch shows loading on tubes  10 , balanced across the AVBs  15  to minimize bending stress, plus the offset load fingers  56 . All figures are used as example configurations that can be applied to develop loading on a multiplicity of tubes  10  and AVBs  15 , reducing peak point loading as needed. 
         [0028]    This imparted force  53  causes the AVBs to move, such that they make firm contact with the adjacent steam generator tubes  10 . Thus, the system  50  causes the existing AVBs to provide resistance to in-plane motion of the tubes  10 , such as vibration caused by normal operation of the steam generator when the PWR is in use. Furthermore, if the force applied to the AVBs  15  on one side of the U-bend is reacted by pushing on a different set of AVBs  15  on the opposite side of the tube bundle, then the U-bends of the tubes  10  are forced into a slight curvature. In-plane motion of those tubes  10  are thus further restricted because they are not allowed to move without elastically “turning the curves,” and thus must absorb more energy themselves, and impart desirable rubbing friction against the AVBs  15 . 
         [0029]    An important advantage gained by this use of non-collinear forces (that is, not applying the balance load to the same set of AVBs  15  on the opposite side of the tube bundle) is that the bundle is typically much less stiff so that significant bundle deflection can occur with less load, and the peak side loads on tubes are much reduced. This has the advantage of allowing some relative sliding of the tube  10  against the AVB  15 . Tubes  10  that are highly pinched between both adjacent AVBs  15  may be excessively locked and may then deform in-plane excessively at other areas where pinch loads are lower. 
         [0030]    The AVBs  15  further “domino” by the connectivity of the connecting devices  20 . This AVB-to-tube contact causes friction against and resistance to in-plane motion of a large number of the tubes  10 . This domino effect has the added effect of moving most of the AVBs  15  in essentially the same displacement such that even in the presence of pre-existing tube-to-AVB wear, there is still movement of every related AVB  15 , and there is no significant accumulation of the effects of wear on multiple columns of wear. This is the effect present when the entire column of tubes is not worn; it is observed that some level of wear may occur in five to perhaps fifteen or more consecutive tubes  10 , however the tubes  10  outboard of that, not showing wear, or the tubes  10  with minimal wear will transfer the load to the next AVB  15  once the nominal gap is taken up by displacement. The relatively short spans between those tubes  10  that have no or insignificant wear in a given column means that the unsupported span of the associated AVB  15  is also short, and thus remaining relatively quite stiff. The AVB  15  then has the ability to impart side load on the worn tubes  10  once further bundle displacement has occurred. 
         [0031]    A further advantage gained by the domino effect is that the tube  10  is not necessarily pinched between adjacent AVBs  15 , as excessive pinch could lock the tube U-bend, which may have undesirable effects under operating conditions. 
         [0032]    In further explanation of this feature,  FIG. 10  illustrates that simple compaction of stacks of tubes  10  and AVBs  15  does not impart side loading onto those tubes that have pre-existing wear. With true distortion of the U-bend and domino of the AVBs  15 , as shown in  FIG. 11 , all tubes  10  are pushed, regardless of wear and regardless of wear in adjacent columns of tubes. 
         [0033]      FIG. 3  illustrates the case where load blocks  57  are not a aligned on directly opposing ends of the same AVB  15  set, but rather are shown offset by three on each side. 
         [0034]    The components of system  50  may be interconnected and connected to the existing plant equipment such that any threat of it coming loose during plant operation is minimized or eliminated. It is also preferable that system- 50  is adjustable, such that added or reduced U-bend distortion can be made should there be a desire to change operating characteristics. This is readily achieved, for example, by unlockable bolting on the load developing device  54 . 
         [0035]    Additionally, it is preferable that the system  50  and its components be readily removable. These goals may be achieved, for example, by using tack welds, straps, or unlockable bolting to removably lock them in place. Rather than affixing the pressure point lugs  55  to each AVB  15  they are in contact with, the lugs  55  may simply hook into the AVBs  15  by being under the AVB terminators  16  and thus be held in place by geometry, and also the load block  57  would be positively engaged into the stressor-hoop  51 . Other means of retaining the system  50  and its components in place during plant operation will be apparent to those of skill in the art. 
         [0036]    According to another aspect of the invention, the forces  53  can be generated by “jacking” against structures inside the pressure vessel; in terms of a steam generator, a good surface would be the inside of the wrapper. In this aspect, the stressor hoop  51  need not be installed and is replaced and/or augmented by a foot which braces to the inside of the wrapper, and toad developing device  54  is modified to span the distance from the foot to the load blocks  57 . Such loading approaches may be more suitable for developing higher loads. 
         [0037]    According to another aspect of the invention, and as an alternative method to elastically deforming the AVBs  15  by installing the added hardware of structure  50 , small plastic deformations may be made to many or all of the AVB linking hoops  17 . The net effect is similar for peripheral tubes  10 , although it may not have much penetration effect deep into the bundle if the AVBs  15  are relatively thin, thus being relatively flexible over long spans. No new tooling is however left inside the steam genera or vessel. Preferably, the plastic deformations are implemented in a stepped fashion. This stepped deformation could be implemented, for example, by hydraulic pressing tools temporarily clamped onto the AVB hoop  17 . The installer can enter the steam generator vessel with several hydraulic forming tools, clamp them onto several places on one of the AVB hoops  17 , activate them to deform that AVB hoop  17 , and then move them to the next AVB hoop  17 . 
         [0038]    When finished, the entire AVB bundle ends up somewhat deformed but no tooling remains inside the steam generator. The tube-to-bundle deforming loads are generated by the sum of the small deformations on each of the AVB hoops  17 . A representative deformation value for each step might be small and on the order of 0.020 inch, for a total of about 0.06 inch for three steps. The range of peak total deformation targeted will be a function of the steam generator size and the needed amplitude of average tube-to-AVB contact force. 
         [0039]      FIG. 9  shows a rotated view of one of the AVB hoops  17 . The boxed numbers above the illustrated AVB hoop  17  correspond to sets of AVBs. It is contemplated that all or most of the AVB hoops  17  would be step-deformed to collectively build large forces. The impact of the stepping is to displace the sets of AVBs  15  indirectly attached to the AVB hoops  17  by connecting devices  20  and to create deformation of the central AVB sets  15  relative to the outer AVB sets  15 . In the illustrated embodiment of  FIG. 9 , AVB sets  4  through  9  have been displaced relative to the others. The original position of the AVB hoop  17  is shown by dashed lines and the AVB sets  15  by dotted lines. 
         [0040]    As will be appreciated by the illustrated embodiment of  FIG. 9 , the AVB hoop  17  is subjected to multiple deformations that, preferably, are made in pairs. This results in a stepped appearance of the AVB hoop  17  with each stepped portion being substantially parallel to the other stepped portions. Additionally, the inside portion of the AVB hoop  17  is more displaced than outside portions thereof, relative to a center of the heat exchanger. 
         [0041]    While being quite applicable to new steam generators, the aspects of this invention beneficially can be assembled inside an already-fabricated steam generator using normal manway openings. This eliminates expensive and time-consuming efforts to temporarily cut apart the steam generator shell or some of its internals. In this feature, the inventive system is highly useful as a repair tool for pre-installed steam generators, and may be applicable tier other heat exchangers which have adequate secondary side access. 
         [0042]    This system does not require removal of the existing AVBs, and thus likely eliminates extensive reanalysis of vibration and wear calculations. This also eliminates large costs and risks of damaging the steam generator tubing  10  during the removal and installation process of replacement AVBs  15 . 
         [0043]    This invention also provides a means to create controlled side forces on the tubing  10 . This eliminates the assumption of adequate side loads during operating conditions. 
         [0044]    The forces generated by the system  50  on the tubing  10  can be adjusted as described above after installation, for example should there be a need to increase or decrease loading or deformations. 
         [0045]    Local areas or large zones of the U-bend can be controlled by the system  50 . High void fraction regions or high velocity regions are typically quite localized, and may be deep in the tube bundle  10 . A single tool-system under this invention can impart the needed loading over a large volume of the U-bend. 
         [0046]    When applied to a new steam generator design, the inventive system  50  may allow reduction of the need and cost of high tolerance machining of AVBs and other AVB structure subcomponents, and assembly tools, since it will absorb and accommodate variable tolerances. 
         [0047]    The invention may significantly improve tube bundle stability during vessel handling and shipping and may eliminate the need for anti-liftoff devices if sufficient resistive friction is generated. 
         [0048]    The invention accommodates the existence of wear marks at tube-to-AVB intersections, which may otherwise interfere with many other repair concepts. Further, it is observed that this inventive system  50  does not loosen gaps to existing AVBs  15  within the compression/distortion zone. This removes concern that would arise otherwise: increased gaps can promote tube wear rates created by out-of-plane vibration, which is generally a stronger phenomenon than in-plane vibration. Careful attention would of course need to be taken for tube columns outboard of the compression zone, to assure those tubes are adequately supported for their flow conditions. 
         [0049]    While the preferred embodiments of the present invention ha re been described above, it should be understood that they have been presented by way of example only, and not of limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus the present invention should not be limited by the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Furthermore, while certain advantages of the invention have been described herein, it is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.