Abstract:
A means of offsetting semi-circular tube support plates typically present in heat exchangers with cross flow baffles, such as axial flow economizers, utilizing the motive force of steam generator pressurization. The offset slightly flexes the tubes, thereby providing a preload which minimizes the potential for tube vibration and wear.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to tube support arrangements for steam generators and more particularly to a tube support arrangement for a tube and shell steam generator that imparts a preload on the tubes. 
     2. Description of Related Art 
     A pressurized water nuclear reactor steam generator typically comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a dividing plate that cooperates with the tube sheet and a channel head forming 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 a 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 U-like curvature end of the tube bundle. 
     The primary fluid having been heated by circulation through the reactor 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 U-tube bundle, out the primary fluid outlet header, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced into the steam generator secondary side, i.e., that is the side of the steam generator interfacing with the outside of the tube bundle above the tube sheet, through a feedwater nozzle which is connected to a feedwater ring inside the steam generator. In one embodiment, upon entering the steam generator, the feedwater mixes with water returning from moisture separators. This mixture, called the downcomer flow is conducted down the annular chamber adjacent the shell until the tube sheet located at the bottom of the annular chamber causes the water to change direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the water is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to water surrounding the tubes causing a portion of the water surrounding the tubes to be converted to steam. The steam then rises and is conducted through a number of moisture separators that separate entrained water from the steam, and the steam vapor then exits the steam generator and is typically circulated through turbine and electrical generating equipment to generate electricity in a manner well known in the art. 
     Since the primary fluid contains radioactive materials and is isolated from the feedwater only by the U-tube walls, the U-tube walls form part of the primary boundary for isolating these radioactive materials. It is, therefore, important that the U-tubes be maintained defect free so that no breaks will occur in the U-tubes that will cause radioactive materials from the primary fluid to enter the secondary side; an undesirable result. 
     Vibration due to fluidelastic excitation of the heat exchanger tubes can result in wear of the walls of the tubes and breach of the pressure barrier between the primary and secondary fluid systems at the locations where the heat exchanger tubes pass through holes in support plates which are axially spaced along the tube bundle to support the tubes. This is especially a problem in axial flow preheaters, that employ a partition plate to separate the secondary side flow into hot leg and cold leg sides to minimize mixing of the warmer recirculating water with the cooler feedwater. This separation is necessary for the feedwater to be heated on the cold leg side of the unit to increase the unit&#39;s heat transfer efficiency. However, due to differences in secondary fluid densities, cross flow occurs at the top of the partition plate, with flow generally streaming from the cold leg side towards the hot leg side. This site has been the location of tube wear in several types of preheat steam generators including axial flow, cross flow and counter flow type steam generators. 
     Accordingly, it is an object of this invention to control tube bundle vibration to avoid wear of the heat exchanger tubes. 
     Furthermore, it is an object of this invention to control tube vibration in a manner that does not complicate the loading of the heat exchanger tubes through the support plates and into the tube sheet during manufacture or in a cool, depressurized condition. 
     SUMMARY OF THE INVENTION 
     These and other objectives are achieved in accordance with this invention by providing a tube and shell steam generator having 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 includes a channel head for receiving the heated fluid and a tube sheet that separates the channel head from the secondary side. A plurality of heat exchanger tubes respectively extend from the channel head, through the tube sheet and through a portion of the secondary side. A plurality of axially spaced tube support plates are supported in the secondary side approximately perpendicular to the tube axis and have through holes that respectively surround at least some of the heat exchanger tubes extending into the secondary side and through which the corresponding heat exchanger tubes pass, with the holes surrounding each heat exchanger tube, of at least some of the heat exchanger tubes, substantially axially aligned when the steam generator is in a cold condition. A displacement means is provided for laterally offsetting at least one of the tube support plates from one other of the tube support plates when the steam generator is in a hot condition to place a lateral load on the corresponding heat exchanger tubes sufficient to prevent liftoff and, thus, restrain vibration of the tubes. 
     In one embodiment, at least one of the tube support plates includes two semi-circular support plate halves that are separated by a vertical partition that extends in the axial direction. The displacement means is preferably supported by the vertical partition between the two semi-circular support plate halves. Desirably, the displacement means is supported near or at the upper end of the vertical partition and preferably at the upper end. In the one embodiment, the displacement means is a sealed flexible cavity containing a fluid or a gas/liquid mixture, wherein the cavity is connected to one or both of the semi-circular support plate halves and contracts or expands with changes in pressure inside the steam generator secondary side. Preferably, the displacement means imparts an equal load on the two diametrically opposed halves of the support plates on either side of the vertical partition. In one instance, the sealed flexible cavity is a bellows which may be formed from two concentric corrugated tubes with an annular opening between the corrugated tubes sealed at each end and the corrugated tubes having a central axis that extends substantially orthogonally to the tube axis. The sealed flexible cavity may also have a pressure relief valve and/or a mechanical stop to control the amount of pressure exerted on the tube support plates. Desirably, the displacement means is supported in the tube lane of the heat exchanger tubes and is responsive to a pressurization of the secondary side of the steam generator to laterally offset at least one of the tube support plates. More particularly, the displacement means deflects in response to the pressurization of the secondary side of the steam generator to laterally offset at least one of the two support plates. 
     In another embodiment, the displacement means is a screw thread or worm gear activated jack. 
     In still another embodiment, at least some of the support plates are at least in part supported by stay rods that axially extend through openings in the corresponding support plates. Preferably, the stay rod openings are slotted in a direction in which a force is applied by the displacement means for laterally offsetting at least one of the tube support plates so that a strain is not imparted to the stay rods when the support plate is offset. Preferably, movement of the displacement means in the lateral direction is limited to a predetermined distance to control the force applied by the displacement means and desirably that force is limited between one and seven pounds (0.45-3.2 kilograms) and preferably between approximately two and five pounds (0.9-2.3 kilograms) with a displacement desirably between 0.12 and 0.5 inch (3.0 and 12.7 millimeters) and preferably, approximately 0.25 inch (6.4 millimeters). 
     In still another embodiment, at least one of the tube support plates comprises two support plate halves that are separated by a vertical partition extending in the axial direction and the displacement means is supported by the vertical partition between the two support plate halves. In the latter embodiment, the two support plate halves need not surround all of the plurality of heat exchanger tubes. Preferably, the tubes that are not surrounded by the two support plate halves are on an outer periphery of the secondary side of the steam generator. In the latter embodiment, the heat exchanger tubes are arranged in a tube bundle having a generally circular cross section and the vertical partition divides the tube bundle into hot and cold sides extending a width of the tube bundle with the two support plate halves extending over said width and outward from the vertical partition in a direction transverse to the tube axis to a cord parallel to the partition. 
     In one embodiment, the displacement means imparts a lateral offset that is elastic so that at least one of the tube support plates returns to its original lateral position when the displacement means force is removed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the invention 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 representation of the tube bundle portion of the tube and shell steam generator illustrated in  FIG. 1  showing a preheat partition in the secondary side of the steam generator that incorporates one embodiment of the displacement mechanism of this invention; 
         FIG. 3  is an enlarged view of the displacement mechanism portion of  FIG. 2 , partially in section; 
         FIG. 3A  is a schematic cross-sectional view of the tube bundle of  FIG. 3  taken at the anti-vibration plate elevation; 
         FIG. 4  is a further enlarged view of a portion of  FIGS. 2 and 3  illustrating one embodiment of the displacement mechanism of this invention, partially in section; 
         FIG. 5  is a schematic representation of the tube bundle portion of the tube and shell steam generator illustrated in  FIG. 1  with a preheater partition incorporating a second embodiment of this invention; 
         FIG. 6  is an enlarged view of  FIG. 5  in the area of the displacement mechanism, partially in section; 
         FIG. 7  is a further enlarged view of the displacement mechanism portion of  FIG. 6 ; and 
         FIG. 8  is a cross sectional view of the displacement mechanism illustrated in  FIG. 7  sandwiched between two halves of a tube support plate. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings,  FIG. 1  shows a steam or vapor generator  10  that utilizes a plurality of U-shaped heat exchanger tubes which form a tube bundle  12  to provide the heating surface required to transfer heat from the primary fluid to vaporize or boil the secondary fluid. The steam generator  10  comprises a vessel having a vertically oriented tubular shell portion  14  and atop enclosure or dished head  16  enclosing the upper end and a generally hemispherical-shaped channel head  18  enclosing the lower end. The lower shell portion  14  is smaller in diameter than the upper shell portion  15  and a frustoconical-shaped transition  20  connects the upper and lower portions. A tube sheet  22  is attached to the channel head  18  and has a plurality of holes  24  disposed therein to receive ends of the U-shaped heat exchanger tubes  13 . A dividing plate  26  is centrally disposed within the channel head  18  to divide the channel head into two compartments  28  and  30 , which serve as headers for the tube bundle  12 . Compartment  30  is the primary fluid inlet compartment and has a primary fluid inlet nozzle  32  in fluid communication therewith. Compartment  28  is the primary fluid outlet compartment and has a primary fluid outlet nozzle  34  in fluid communication therewith. Thus, primary fluid, i.e., the reactor coolant, which enters fluid compartment  30  is caused to flow through the tube bundle  12  and out through outlet nozzle  34 . 
     The tube bundle  12  is encircled by a wrapper  36  which forms an annular passage  38  between the wrapper  36  and the shell and cone portions  14  and  20 , respectively. The top of the wrapper  36  is covered by a lower deck plate  40  which includes a plurality of openings  42  in fluid communication with a plurality of riser tubes  44 . Swirl vanes  46  are disposed within the riser tubes 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. After flowing through the primary centrifugal separator, the steam passes through a secondary separator  48  before reaching a steam outlet nozzle  50  centrally disposed in the dished head  16 . 
     The feedwater inlet structure of this generator includes a feedwater inlet nozzle  52  having a generally horizontal portion called feedring  54  and discharge nozzles  56  elevated above the feedring. Feedwater, which is supplied through the feedwater inlet nozzle  52 , passes through the feedwater ring  54 , and exits through discharge nozzle  56  and, in one prior art embodiment, mixes with water which was separated from the steam and is being recirculated. The mixture then flows down above the lower deck plate  40  into the annular downcomer passage  38 . The water then enters the tube bundle  12  at the lower portion of the wrapper  36  and flows among and up the tube bundle where it is heated to generate steam. 
     As previously mentioned, control of heat exchange tube vibration in the tube bundle  12  is a key requirement in a steam generator and other heat exchanger designs. Vibrations due to fluidelastic excitation can be avoided in accordance with this invention by providing a preload force at least one tube support plate location of sufficient magnitude to prevent tube liftoff. The tube support plates are illustrated by reference character  58  in  FIG. 1  and are typically supported by stay rods which extend from the tube sheet in which the ends of the stay rods are threaded, through tubular spacers that extend between adjacent tube support plates, and through openings in each of the axially spaced support plates. The stay rods typically have diameters larger than the heat exchanger tubes and limit deflection of the tube support plates in the unlikely event of an accident, e.g., break loadings of a steamline or feedline of the steam generator. The heat exchange tubes pass through additional openings in each of the tube support plates. Vibration of the heat exchange tubes  13  within the tube support plate openings is the cause of the wear that was previously mentioned, that if unchecked can breach the pressure barrier of the heat exchange tubing. 
     Although the usefulness of this invention may be evident in many types of heat exchangers, the preferred embodiment described herein is for an axial flow preheat unit for which this invention has particular benefit. Preheat steam generators have a different feedwater inlet structure than is shown in  FIG. 1 , such that the feedwater is not mixed with the water separated from the steam. In axial flow preheaters, a partition plate  60 , shown in  FIGS. 2-7  is used to separate shell side flow into hot leg and cold leg sides, to minimize mixing of the warmer recirculation water with the cooler feedwater. This separation is desirable for the feedwater to be heated more efficiently on the cold leg side of the unit. However, due to differences in secondary fluid densities, cross flows occur at the top of the partition plate  60 , with flow generally streaming from the cold leg side towards the hot leg side. This site has been the location of tube wear in several types of preheat steam generators. 
     This invention provides a means of offsetting at least one of the anti-vibration plates  64  or semi-circular tube support plates  66  to provide a preload on at least some of the heat exchanger tubes  13 . In one preferred embodiment the mechanism for offsetting the anti-vibration plates  64  or semi-circular tube support plate  66  is a “box” which deforms under pressurization. Alternatively, a number of other mechanism can be employed, such as mechanical screw-type adjusters activated through ports located along the tube lane, or commercially available bellows arrangements can also be used. 
     One preferred configuration for establishing such a preload in accordance with this invention for an axial flow-type preheat steam generator is shown in  FIGS. 2, 3 and 4 .  FIG. 2  illustrates the approximate elevation of anti-vibration plates  64  within the lower shell of a steam generator. The anti-vibration plates are employed in this embodiment to impart the preload on the heat exchange tubes. The partition plate  60  in the  FIG. 2  example extends to above the fifth elevation of semi-circular tube support plates  66 . However, it should be appreciated that the number of tube support plates may vary depending upon the size of the steam generator. The anti-vibration plates  64  are located in this embodiment between the fourth and fifth half tube support plates. Semi-circular tube support plates  66  are used at all elevations where the partition plate is present, in this example, that is through the fifth tube support plate, counting from the bottom. 
       FIG. 3  is a closer view of  FIG. 2 , though it should be appreciated that only three of the many U-shaped heat exchanger tubes  13  within the tube bundle  14  are shown and only two of the several stay rods  68  are illustrated so as not to obscure the anti-vibration plates  64  and displacement mechanism  62  of this invention. The anti-vibration plates  64  extend laterally, approximately the full width of the partition plate  60  and each extends laterally to a chord  70  (shown in  FIG. 3A ) parallel to the partition plate  60 . Each of the anti-vibration plates  64  need not be a full semi-circle, since cross flow velocities are rapidly attenuated in the region of the tube bundle  12  supported by the anti-vibration plates  64 . The anti-vibration plates  64  are supported vertically by the stay rods  68 . If needed, the holes in the anti-vibration plates  64  may be slotted for the stay rods  68  as figuratively illustrated by reference character  72  in  FIG. 3A . Aside from the slotted holes  72  for the stay rods  68 , the anti-vibration plates  64  have similar material, hole size and hole shape as the standard tube support plates  58  and semi-circular tube support plates  66 . The upper portion  76  of the partition plate  60  is open to the secondary side environment, and is provided with drain holes and internal stiffening elements, etc., as required. 
       FIG. 4  shows details of the anti-vibration plate  64  and a preloading box  62  which is one embodiment of the displacement mechanism. The preloading box is welded all around, and filled with air, nitrogen or another gas or other compressible fluid, and is supported from the partition plate  60 . Slots  78  in the partition plate  60  permit access to attachment blocks  80 . The attachment blocks are welded along the length of the preloading box  62  and transmit the lateral load from the compression of the box  62  to the anti-vibration plate  64 . The transfer of a lateral load is accomplished through connector bars  82 , installed prior to tubing installation. The connector bars  82  are attached to both the anti-vibration plate  64  and the attachment blocks  80  by connector pins  84 . Spacers  86 , internal to the preloading box  62 , limit deflection to preset limits, and thus limit the extent of the load imparted to the anti-vibration plate  64 . Preferably, all elements of the design are welded to prevent loose parts. Various changes to enhance the assembly are possible. 
     A pressure relief valve  88  may be included to vent the box  62  in the unexpected case of a leak into the box, which would allow the box to vent during a depressurization transient. In the given embodiment, the preloads are statically balanced, i.e., an equal total preload occurs on the hot leg side of the heat exchanger tubes as occurs on the cold leg side of the heat exchanger tubes  13 . Should there be a later desire to defeat the preloading of the tubes, this may readily be accomplished by venting the preloading box  62 . In this example, the preloading per tube imparted by the preloading box is anticipated to be between approximately one and seven pounds (0.45-3.2 kilograms) per tube or preferably between approximately two and five pounds (0.9-2.3 kilograms) per tube, which should be sufficient to prevent liftoff. The lateral offsets to achieve the foregoing preloads are between approximately 0.12 and 0.5 inch (3.0 and 13 millimeters) and more preferably about 0.25 inch (6.4 millimeters). The heat exchanger tube fatigue and heat exchanger tube bending stress contribution from this preload will be negligible. 
     Another embodiment of this invention is illustrated in  FIG. 5  which shows the schematic of the steam generator that was previously illustrated in  FIG. 2  except that the anti-vibration plates are not used and the displacement box  62  has been moved upward.  FIG. 5  illustrates the approximate elevations of the semi-circular tube support plates  66  and the full circular tube support plates  58  within the lower shell  14  of the steam generator  10 . The partition plate  60  in  FIG. 5  extends to the fifth elevation of semi-circular tube support plates  66 . The anti-vibration displacement mechanism  62  is located between the two tube support half plates  66  at the fifth tube support elevation. As before, semi-circular tube support plates  66  are used at all elevations where the partition plate  60  is present. 
       FIG. 6  is a closer view of  FIG. 5 , showing the tube support plates  58  and  66  in the vicinity of the displacement box  62 . In this embodiment, the displacement box  62  is cylindrical with approximately a 4.2 inch (107 millimeters) diameter, and an overall length of approximately six inches (150 millimeters), and thus fits into the tube lane region. The displacement box&#39;s size and diameter allows that it could be installed and/or serviced, if needed, through six inch (150 millimeters) diameter ports at each end of the tube lane. The displacement box  62  may be attached to either the partition plate  60  or to the tube support plate halves  66 , or specially configured attachments may be provided. Since the displacement box  62  is not active during shop assembly, the heat exchanger tubes  13  can be installed through all the tube support plates  58  in line, thereby avoiding scratching of the heat exchanger tubes  13 . 
       FIG. 7  shows a further detail of the displacement box  62  outline. On the right side of  FIG. 7 , the plunger  90  contacts a tube support plate half plate  66 , and on the left side, the displacement box  62  body contacts a tube support plate half plate  66 . As previously mentioned, the stay rod holes in the half plate  66  may be slotted to permit lateral movement of the plate without inducing bending stresses in the stay rods  68 . 
       FIG. 8  shows a sectional view of the displacement box  62  through the center line of the tube support plate halves  66 . The displacement box in this embodiment has two metal bellows, i.e., an inner metal bellows  92 , and an outer metal bellows  94 , concentrically positioned. At one end  96  both metal bellows are attached by welding to the plunger pin  90 . At the other end  98  the metal bellows are both attached to the displacement box  62  enclosure body by welding. This effectively seals the region between the two bellows, which is filled with atmospheric air or inert gas. Upon steam generator secondary side pressurization, the external pressure acts to compress the air in the annular region  100 , producing an axial movement of the pair of bellows (in a direction transverse to the heat exchanger tube axis) and with it the plunger  90  against the semi-circular half support plate  66 . The force produced is equal to the secondary pressure times the annular area between the two bellows, minus the spring force acting to compress the bellows. 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. For example, more than one displacement mechanism may be employed at different elevations of the partition plate as shown in  FIG. 2 . Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breath of the appended claims and any and all equivalents thereof.