Abstract:
An improved support for a tube bundle of a nuclear steam generator has a pair of plates separated by spacers and holding slotted disc springs used to mount tubes through tube holes of the pair of plates. The slotted disc springs deform upon installation and substantially eliminate tube vibration during use, thereby extending the useful life of the tubes. At the same time, the springs limit the pressure drop between sides of the support and enhance the heat transfer from the tubes.

Description:
FIELD AND BACKGROUND OF INVENTION 
     The present invention relates generally to the field of nuclear power generators and in particular to a new and useful device for supporting heat transfer tubes in an array in a steam generator of a nuclear plant. 
     Pressurized water vapor generators or heat exchangers, associated with nuclear power stations and which transfer the reactor produced heat from the primary coolant to the secondary coolant that drives the plant turbines may be as long as 75 feet and have an outside diameter of about 12 feet. Within one of these heat exchangers, straight tubes through which the primary coolant flows may be no more than ⅝ inch in outside diameter, but have an effective length of as long as 52 feet between the tube-end mountings and the imposing faces of the tube sheets. Typically, there may be a bundle of more than 15,000 tubes in one of these heat exchangers. It is clear that there is a need to provide structural support for these tubes in the span between the tube sheet faces to ensure tube separation, adequate rigidity and the like. 
     U.S. Pat. No. 6,498,827 to Klarner, assigned to Babcock &amp; Wilcox Canada, Ltd. and the entirety of which is hereby incorporated by reference, describes an hour-glassed, broached tube support plate (HBTSP) for use supporting these primary coolant tubes in tube bundles. The plate in the prior application supports and aligns the tubes, and minimizes deposit buildup, while providing gaps for reducing pressure drops across the plate, among other benefits. 
     Using the HBTSP of the prior application typically results in a tube-to-support gap between the tubes and the support holes through the HBTSP. The tube-to-support gap is in addition to the planned gap between the tube and support hole opening for relieving pressure drop differences. The tube-to-support gap is caused by tolerances of manufacture and is required for assembly of the tube bundles. 
     In use, the flow of steam and water through the tubes induces vibrations in the tube. The vibration may not be effectively restrained because of the tube-to-support gap. Continual vibration will result in fretting and other vibration damage and reduce the life expectancy of the tube considerably. The small tube-to-support gap also provides a limit flow area. 
     An HBTSP is expensive to manufacture because of the number of holes and the preferred cross-sectional shape, with three or four recesses and a corresponding number of contact or protruding members in each support hole. Complex and expensive broaching machines and tools are needed to make the plates. 
     A less expensive and simpler tube support plate in which the vibrations can be eliminated is very desirable. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to provide a support plate for nuclear steam generator tubes which eliminates vibrations in the tubes during use. 
     It is a further object of the invention to provide a tube support plate which is more easily and economically manufactured than presently known support plates, while maintaining the benefits of the known support plates. 
     Accordingly, a support for a tube bundle is provided having upper and lower plates with circular opening through holes for a plurality of tubes, and slotted disc springs mounted between the upper and lower plates in each through hole for supporting and contacting a tube inserted in the through hole. The upper and lower plates are bolted together and may be separated by spacing washers. The slotted disc springs each have a frusto-conical shape, with a center opening through which one of the tubes of the tube bundle can be inserted. 
     The slotted disc springs provide gaps for reducing pressure drops between sides of the support, while having deformable direct contact with the tube sides, which reduces or eliminates tube vibration. The support with slotted disc springs provides the same benefits as known supports, while also reducing stress on the tubes in certain conditions, requiring less complex tooling and manufacture, and thereby the manufacturing cost. By reducing the vibrations in the tubes during use, the supports also help extend the useful life of the tubes in the bundle, and the springs tend to act as fins and enhance the heat transfer effect. 
    
    
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a top plan view of a slotted disc (Belleville) spring used in the support of the invention; 
     FIG. 2 is a sectional side view of the spring taken along line  2 — 2  of FIG. 1; and 
     FIG. 3 is a partial sectional side elevation view of a tube support of the invention and tubes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, in which like reference numerals are used to refer to the same or similar elements, FIGS. 1 and 2 illustrate a slotted disc, or Belleville-type, spring  10  used to support tubes  50  (shown in FIG. 3) in a tube bundle of a nuclear steam generator. As shown, the slotted disc spring  10  has a center opening  30  formed by tabs  15  and slots  20  around the circumference. The slots  20  permit the tabs  15  to bend or deform, giving the spring  10  its intended effect. The side walls  25  of the spring  10  are initially formed conical, so that the slotted disc spring  10  has a frusto-conical appearance in its normal state prior to installation. 
     FIG. 3 shows how the slotted disc springs  10  are used to support tubes  50  in a manner which permits their installation, while limiting vibration in the tubes  50  during use. 
     Upper and lower plates  60 ,  70  are provided which are substantially mirror images of each other on their facing sides  67 ,  77 . Each of the plates  60 ,  70  has a plurality of tube holes  65 ,  75  bored through the respective plate  60 ,  70 . The tube holes  65 ,  75  are arranged to align with each other when the plates  60 ,  70  are joined as shown. The tube holes  65 ,  75  have a larger diameter than the outside diameter of the tubes  50  being supported, so that a gap is formed between the sides of the tube holes  65 ,  75  and the outer wall of tubes  50 . 
     The facing sides  67 ,  77  of the support plates  60 ,  70  each have a small annular recess  63 ,  73  adjacent the tube holes  65 ,  75  for receiving a slotted disc spring  10 . The support plates  60 ,  70  are joined to have a space  80  between them as well, as described below. The slotted disc springs  10  are held in the recesses  63 ,  73 , with the spring  10  center opening  30  being substantially coaxial with the tube holes  65 ,  75 . When the support plates  60 ,  70  are joined, the springs  10  will be flattened slightly from their original shape, as the facing sides  67 ,  77  converge on the springs  10 . 
     The support plates  60 ,  70  are preferably joined using bolt  95  and nut  97 , with a spacer washer  90  interposed between the facing sides  67 ,  77  of the plates  60 ,  70 . The bolts  95  are inserted through aligned bolt holes in the plates  60 ,  70  and tightened using nuts  97 . Using appropriate bolt-preloading, the tabs  15  of the slotted disc springs  10  are elastically deformed until they touch the adjacent tube inserted though the center opening  30 . The side walls  25  of the slotted disc springs  10  will tend to take on a concave shape when they are deformed. The washer  90  prevents the springs from overloading. 
     The slotted disc springs  10  used in the support  100  are preferably commercially available Belleville type springs made of INCONEL alloy or stainless steel. Other slotted disc springs having similar characteristics can be used as well. The springs should be capable of resisting the heat and moisture conditions inside a steam generator in the same manner as stainless steel or INCONEL alloy. 
     The springs  10  are not deformed initially during installation until the bolt pre-load is applied. This permits suitable clearance for the tubes  50  to ensure ease of installation. Applying the bolt pre-load deforms the tabs  15  at the sides of the tubes  50 , thereby restraining them and ensuring contact between the support  100  and tubes  50 . The amount of restraint force applied to the tubes  50  by the bolt pre-load is controllable, and, more importantly, determinable using Finite Element Methods. 
     Further, the slots  20  in the slotted disc springs  10  provide a relatively larger flow area than known supports. As a result, the pressure drop is reduced further than previously possible, and as well, water flow through the tube plates is smoothed and less turbulent. The larger flow area also results in increased space for sludge loading before local flow channels are blocked. 
     The tabs  15  contacting the sides of the tubes  50  help to prevent vibration in the tubes  50  during use, as they are held relatively rigidly compared to prior designs. The reduction or elimination of vibration eliminates damage from fretting caused by vibration, and thus extends the life of the tubes  50 . The tabs  15  also act as heat fins, thereby improving the heat transfer qualities of the tube bundle where the support  100  is used. 
     The allowance for rotational interference between the tubes  50  and hole  65 ,  75  is increased by the support  100 . An entrance taper formed in the lower plate  70  can be provided to guide the tubes  50  during installation. The springs  10  provide reduced stress under conditions of Level D loading due to larger deflection provided by the springs over prior tube support contacts. 
     The cost of manufacturing and installing tube bundles using the support  100  is significantly reduced, because tooling the tube holes  65 ,  75  in the plates is much simpler. The tube holes  65 ,  75  are simply straight cylindrical bores, without complex side wall geometry to machine. Special broaching machines and tools are not required. At the same time, the support  100  does not change the overall assembly of a conventional steam generator, so it is easily adaptable to new installations or replacements. 
     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.