Abstract:
A method of repairing a slip joint on a jet pump assembly between an inlet mixer and a diffuser, with the diffuser having an opening that receives the inlet mixer with a given spacing between an outside diameter of the inlet mixer and an inside diameter of the opening in the diffuser forming an annulus whose spacing is a product of manufacture and vibration wear. The method comprises the steps of remotely accessing the annulus and narrowing a radial dimension of the annulus.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/715,367, filed Oct. 18 2012, entitled JET PUMP REPAIR FOR A NUCLEAR POWER PLANT. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The present invention relates to a method for repairing a jet pump and more particularly to a method for repairing the slip joint between an inlet mixer and a diffuser of a jet pump with particular benefit to jet pumps employed in boiling water reactors. 
         [0004]    2. Related Art 
         [0005]    As can be appreciated from  FIG. 1 , in conventional boiling water reactors  10 , jet pump assemblies  18  are located in the reactor vessel&#39;s annulus region  12 , between the core shroud  14  and a wall of the reactor vessel  16 . The primary function of the jet pump is to pump coolant below the reactor core where the coolant then flows up through the fuel assemblies to extract heat from the fuel assemblies. The jet pumps are also relied upon to maintain two-thirds of the water level height in the reactor core during postulated accident conditions. For these reasons, the jet pump assemblies  18  are considered safety related components. 
         [0006]    In a typical arrangement, twenty jet pumps are paired in ten assemblies  18 , as illustrated in  FIG. 2 . Each jet pump assembly  18  is driven by flow from a common riser pipe  22 . Jet pump flow is then directed to the lower plenum region  24  below the core supported within the shroud  14 . Jet pump flow is roughly one-third driven by the reactor coolant system pump flow through the riser pipe  22 , and the last two-thirds of jet pump flow is due to venturi action of the jet pump suction inlet that pulls in fluid from the annulus region  12 . The jet pump assemblies  18  are circumferentially spaced around the shroud  14 , supported on a shroud support ledge  20  near the bottom of the shroud. 
         [0007]    Industry operating experience has encountered numerous instances of damage or accelerated wear to critical components of the jet pump assemblies  18  which has affected a large population of boiling water reactors both in the United States and globally. The most common jet pump components to experience damage are the main wedge and rod  26 , restrainer bracket pad in the restrainer bracket  28 , riser pipe  22  welds and riser brace  30 , all of which components can be observed in  FIG. 3 , which shows an enlarged perspective view of a jet pump assembly  18 . These common types of damage are all in critical supporting structures or features of the jet pump assemblies. Damage and wear are largely attributable to flow induced vibration due to normal operation. In addition to flow induced vibrations, many plants experience excessive leakage in the slip joint region  32 , which can exacerbate the vibration experienced in the jet pump assemblies  18 . This leakage can greatly accelerate the degradation and damage done to the jet pumps. 
         [0008]    Plants that have shown signs of accelerated wear and component movement, both indicators of flow induced vibration issues, have historically attempted to address the problem by adding additional hardware to help support the jet pump components and reinforce the components against flow induced vibration. These solutions have generally been in the area of the restrainer bracket  28  and include larger main wedges  26 , supplemental auxiliary wedges, and slip joint clamps. These solutions do not address the root cause of flow induced vibrations, have been ineffective for some plants, and their effectiveness overall is questionable. 
         [0009]    In addition to these typical solutions, a few boiling water reactor plants have added labyrinth seals to the inlet mixer  34  original equipment manufacturer design. These labyrinth seals are intended to reduce bypass flow in the slip joint region  32  of the jet pumps; however, it appears that under certain operating conditions in some plants these seals have been ineffective and the seal geometry has been damaged on the inlet mixer&#39;s outer diameter and has caused damage to the inner diameter of the collar  38  on the diffuser  36 . 
         [0010]    In the slip joint region  32 , the inlet mixer  34  is unsupported or floats, allowing the mixer to thermally expand along its length during plant startup and shutdown. The inlet mixer&#39;s bottom section fits into the collar  38  of the diffuser assembly  36 , forming a slip joint. The inlet mixer  34  is laterally supported by a three-point contact at the restrainer bracket  28 . This three-point contact is maintained with a sliding (main) wedge  26  and two set screws that are tack welded in place. The main wedge  26  is held in place by gravity, theoretically resulting in three-point contact. The very upper portion of the inlet mixers are supported by a pre-tensioned beam bolt assembly  40  that presses down on the inlet mixer  34  where it is seated in the transition seat  42 . 
         [0011]    Because of how the inlet mixers  34  are supported, small lateral loads on the bottom of the inlet mixer (within the slip joint  32 ) can create large reaction moments at the restrainer bracket  28 . As previously mentioned, the main wedge  26  is held in place by its weight, typically about eight pounds, which can be overcome and lifted by small lateral forces. Since the inlet mixer  34  weighs significantly more than the wedges  26 , its mass can easily overcome the holddown force of the main wedge  26  with small lateral displacements at the outlet of the inlet mixer  34  within the diffuser collar  38 . Once the wedge is temporarily displaced, three-point contact is lost, and, in severe cases, the bottom of the inlet mixer may hammer against the inside of the diffuser collar  38 . This hammering of the inlet mixer  34  and diffuser  36  can also be excited at particular frequencies of vibrations, potentially caused by drive flow or bypass flow in the slip joint  32 . 
         [0012]    Thus, a new solution to flow induced vibrations is desired that will address the root cause of the vibrations. 
         [0013]    Furthermore, a solution to the flow induced vibration wear is desired that will minimize such wear and require little or no disassembly of the jet pump assembly  18 . 
         [0014]    Further, such a repair is desired that can be performed remotely, under water. 
       SUMMARY 
       [0015]    These and other objects are achieved by employing a new method of repairing a slip joint on a jet pump assembly between an inlet mixer and a diffuser that has an opening that receives the inlet mixer with a given spacing between an outside diameter of the inlet mixer and an inside diameter of the opening in the diffuser forming an annulus; with the given spacing a product of manufacture and vibration wear. The method comprises the steps of remotely accessing the annulus and narrowing a radial dimension of the annulus. 
         [0016]    In one embodiment, the method includes the step of measuring a dimension of the outside diameter of the inlet mixer that fits within the slip joint. A clamp is then fabricated having a generally circular collar clamp opening with a design diameter that is larger than the outside diameter of the inlet mixer and smaller than a maximum extent of the inside diameter of the diffuser opening. A collar clamp is then fitted around the inlet mixer and at least partially over and above the diffuser opening with the collar clamp supported by the diffuser. The collar clamp is then attached to a portion of the diffuser housing below the diffuser opening. Preferably, the measuring step measures dimensions around the diffuser opening in addition to the outsider diameter of the inlet mixer. In one embodiment, the diffuser has guides spaced circumferentially around a housing of the diffuser, with the guides extending above the opening in the diffuser that receives the inlet mixer. In the latter embodiment, the method includes the steps of forming notches in an underside of the collar clamp, in line with the guides; and fitting the notches over the guides wherein the guides restrain rotation of the collar clamp. In these embodiments, the collar clamp effectively optimizes the insertion depth of the inlet mixer within the diffuser opening. Desirably, the collar clamp is fabricated in at least two circumferential sections with each of the sections fastened together to form the generally circular opening. In this embodiment, the attaching step clamps the collar clamp to the portion of the diffuser housing, which is preferably a radially outwardly extending collar on the diffuser housing. In this latter arrangement, the collar clamp has at least two radially, outwardly extending segments that extend out radially further than the diffuser collar and the outwardly extending segments have a vertical opening therethrough. A tie bar having a radially, inwardly extending lip at a lower end positioned under the diffuser collar and a second end of the tie bar extending through one of the openings in the segments is captured on another side of the opening in the segments to tighten the collar clamp down against the diffuser collar. Preferably, the attaching step clamps the collar clamp to the portion of the diffuser housing at a plurality of discrete circumferential locations around the housing. In this arrangement, the method does not require the step of removing the inlet mixer from the diffuser. 
         [0017]    In still another embodiment, the collar clamp has an axially extending convergent surface that faces an outer surface of the inlet mixer when the collar clamp is fitted around the inlet mixer and the collar clamp rests on a lip of the diffuser opening. In one arrangement, the collar clamp has an annular circumferential groove adjacent the generally circular clamp opening, the groove having a generally “L” shape in the radial direction with one leg of the “L” extending in a horizontal direction and resting on a lip of the diffuser opening. Preferably, the second leg of the “L” contacts an outer wall of the diffuser. The method may also insert a gasket between the collar clamp and a lip of the diffuser opening to minimize leakage. 
         [0018]    In still another embodiment, the step of narrowing the radial dimension of the annulus comprises the step of removing the inlet mixer from the diffuser. Then, the inside surface of the diffuser opening is machined and material damage on the inlet mixer outer surface that is to be inserted into the diffuser opening is resurfaced. The method then inserts an internal collar having an outside diameter substantially equal to an inside diameter of the machined inside surface of the diffuser opening and has an inside diameter that narrows the annulus gap when the inlet mixer is inserted into the diffuser opening so that the annulus has a radial dimension that is less than the given spacing. Preferably, the internal collar is fabricated to have an axially convergent contour on a surface that opposes the outer surface of the inlet mixer. 
         [0019]    In each of the foregoing embodiments, the radial dimension of the annulus is narrowed to be equal to or smaller than a corresponding original equipment manufacturer specification. 
         [0020]    Alternately, in a separate embodiment, the step of narrowing the radial dimension of the annulus includes the step of cutting a collar portion of the diffuser that surrounds the inlet mixer from the remainder of the diffuser. The collar portion of the diffuser is then removed from the rest of the diffuser and the inlet mixer. A spool piece is then fabricated having a replacement opening with a desired inside diameter to replace the collar portion of the diffuser; and the spool piece is secured to the rest of the diffuser with an end of the inlet mixer within the replacement opening. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    A further understanding of the invention claimed hereafter can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
           [0022]      FIG. 1  is a perspective view of a boiling water reactor with the reactor vessel cut away to show the core shroud and the general placement of the jet pump assemblies; 
           [0023]      FIG. 2  is a perspective view of the core shroud of  FIG. 1  with a better view of the placement of the jet pump assemblies; 
           [0024]      FIG. 3  is an enlarged perspective view of one of the jet pump assemblies illustrated in  FIGS. 1 and 2 ; 
           [0025]      FIG. 4  is a close-up perspective view of a prior art slip joint; 
           [0026]      FIG. 5  is a close-up perspective view of a slip joint incorporating one embodiment of this invention; 
           [0027]      FIG. 6  is a cross section of a slip joint incorporating the embodiment illustrated in  FIG. 5  showing the optimization of insertion depth of the inlet mixer; 
           [0028]      FIG. 7  is a perspective view of the clamp employed in the embodiment shown in  FIG. 5 ; 
           [0029]      FIG. 8  is a close-up perspective view of a portion of the collar clamp employed in the embodiment of  FIG. 5  supported on a lip of the diffuser opening adjacent an outer surface of the inlet mixer showing an axially convergent interface of one form of the embodiment shown in  FIG. 5 ; 
           [0030]      FIG. 9  is a perspective view of a diffuser collar illustrating another embodiment of this invention; 
           [0031]      FIG. 10  is a sectional view of the embodiment shown in  FIG. 9 ; 
           [0032]      FIG. 11  is a perspective view of a third embodiment of this invention; and 
           [0033]      FIG. 12  is a sectional view of the embodiment shown in  FIG. 11 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0034]      FIG. 4  shows a close-up perspective view of the slip joint region  32  with the diffuser  36  having a radially outward projecting shoulder  44  just below the diffuser collar  38  that defines the opening  46  in the diffuser in which the inlet mixer  34  is inserted. The diffuser collar  38  has guides (sometimes referred to as ears) that extend radially outward and upward from the opening  46  to guide the inlet mixer  34  into the opening  46 . According to one embodiment of the present invention, the current inlet mixer  34  and diffuser collar  38  are supplemented by stacking an additional collar clamp  48  on top of the diffuser, over and around the diffuser collar  38  as shown in  FIG. 5 . In addition to other benefits, the collar clamp  48  optimizes the overall insertion depth for the slip joint  32 . The insertion depth of the inlet mixer  34  into the diffuser  36  has been recognized as one of several critical parameters that lead to the onset of inlet mixer vibration. In another embodiment, the collar clamp  48  is structured to create an axially convergent slip joint geometry relative to the diffuser and/or inlet mixer. 
         [0035]    Rather than replace the existing inlet mixer, the design of the present embodiment retains and creates a new slip joint region  32  by the addition of hardware onto the top of the diffuser  36 . The present invention addresses flow induced vibration issues by either: (1) using a convergent slip joint design, (2) optimizing the effective insertion depth, or (3) both using a convergent slip joint design and optimizing the effective insertion depth.  FIG. 5  shows this design concept developed according to one embodiment of the invention. The embodiment shown in  FIG. 5  allows for any existing damage to both the mixer and the diffuser to be left in place. A new slip joint area is created directly above the old slip joint (see  FIG. 6 ). This design approach has the following advantages: (i) it is able to be installed in situ with no inlet mixer removal required; (ii) does not require surface repairs; (iii) provides tight tolerance control of the slip joint gap; (iv) creates an optimal insertion depth; (v) reduces overall repair time and costs; (vi) enables a convergent slip joint configuration; and (vii) provides a flow-induced vibration solution that addresses a root cause. 
         [0036]    Generally, there are two basic options for implementing an axially convergent slip joint design onto the existing diffuser components. The first option entails modifying the exiting diffuser surface by removing or adding material on its collar  38 . The second option entails adding additional hardware and creating a new slip joint area above the old slip joint area, e.g., as described with respect to  FIG. 5 . The present invention contemplates both options.  FIG. 6  is a cross section of the embodiment shown in  FIG. 5 ; i.e., the second option mentioned above. Bracket A on the right shows the original insertion depth prior to the collar clamp  48  being installed. Bracket B, just to the left of bracket A shows the original slip joint area. Bracket C on the left shows the new improved slip joint area achieved by adding the collar clamp  48 . 
         [0037]      FIG. 7  shows one embodiment of the collar clamp  48  that is created from two semi-circular segments  52  and  54  which are joined by dovetail joints  56  and  58 , though it should be appreciated that other means of joining the segments are available and the clamp  48  may be constructed out of two or more such segments. Each segment has a radially outwardly extending arm  60  and  62  through which holes  64  and  66  are formed that will be used to clamp the collar clamp  48  to the diffuser housing  36  as will be described hereafter.  FIG. 8  is an enlarged partial sectional view of the inlet mixer  34 , the diffuser  36  and the collar clamp  48  embodiment shown in  FIG. 5 , uncovering the convergent slip joint at the intersection between collar clamp segments. The design utilizes a convergence geometry, i.e., the inner face of the collar clamp  48  that faces the outer surface of the inlet mixer  34  converges toward the outer surface of the inlet mixer as one progresses from the upper and lower ends to the center of the inside face of the collar clamp  48 . The convergence geometry works off an unmodified inlet mixer original equipment manufacturer outer surface design. The actual dimensions and angles can be fine tuned for each slip joint (since the existing slip joint geometry is left in place and new differential pressure conditions are created in the slip joint). 
         [0038]    According to one embodiment of this invention, digital measurements are taken and three D models rendered of the inlet mixer  34  and diffuser collar slip joints  32 , for example, using a three-D laser scanner. These measures are taken since the as-found conditions of the diffuser and mixer may differ between jet pumps (i.e., components will vary dimensionally from one another, and actual as-built dimensions are unknown). Also, the tight tolerance for the slip joint gap requires the added hardware to have high tolerance requirement for fit-up. The three-D laser scanner technology provides very accurate measurements, approximately plus/minus 0.005 inch (0.013 cm). Also, the rendered three-D model may be saved as a compatible AutoCAD file type, which allows a machine shop to use the CAD file to automatically program CNC mills and lathes to machine from hardware blanks which meet these tight tolerances. 
         [0039]    According to the current embodiment, the collar clamp configuration uses two stack halves  52 ,  54  that interlock the dovetail joints  56 ,  58  formed at their circumferential ends; see  FIG. 7 . To ensure the collar clamp  48  cannot be raised up off of the diffuser upper lip, two tie bars  70 ,  72  clamp down on the collar clamp  48 ; leveraging off of the bottom edge of the diffuser shoulder  44 . Each tie bar has a laterally inwardly extending projection  74  that seats under the diffuser shoulder  44  against which the tie bars  70 ,  72  react to maintain the collar clamp  48  pressed against the upper lip of the diffuser opening  46  ( FIG. 5 ). The upper portion of the tie bars is threaded so that nuts can tighten down the tie bars, applying a slight preload. Preferably, these nuts are crimped in place by crushable material built in to the nut or collar clamp  48 . 
         [0040]    A crushable gasket may be employed if needed between the diffuser stack  48  and the diffuser lip to ensure there is no leakage at their interface. In order to prevent the collar clamp from rotating, the diffuser guides  50  are used as support surfaces. The diffuser guides (often called ears) purpose is to help align and aid in the insertion of the inlet mixer  34  during jet pump reassembly. Notches  76  are formed in the underside of the collar clamp that allow the external collar clamp  48  to fit over the ears  50  and down onto the diffuser lip. These notches also prevent the collar from rotating. The ear recesses in the collar clamp may allow some leakage, but only small amounts of bypass flow are likely. 
         [0041]    The hardware shown in the embodiment illustrated in  FIG. 5  is light enough that the two stack halves can be delivered remotely using tool poles. Much of the tooling necessary for installation exists, and minimal if any new hardware handling tooling is required. While the current embodiment illustrates one design for clamping the stack halves together, it should be appreciated by those skilled in the art that this invention is not limited to this particular embodiment. 
         [0042]      FIG. 9  illustrates another embodiment for repairing damaged jet pump surfaces and/or reducing/eliminating flow induced vibration. According to the embodiment shown in  FIG. 9 , damaged material on the inside diameter of the diffuser is resurfaced.  FIG. 10  is a sectional view of the embodiment shown in  FIG. 9 . A new internal collar  78  is inserted into the diffuser collar  38 , restoring it to at least its original designed inside diameter, or even narrowing the annular gap between the inlet mixer and the diffuser collar. The originally engineered manufactured tolerance gap for the slip joint between the inlet mixer outside diameter and the diffuser collar is very tight, plus/minus 0.010 inch diametrically. It should be appreciated that the new internal collar may also be structured to form a convergent geometry relative to the outside diameter of the inlet mixer. 
         [0043]    According to another embodiment of the present invention, the inlet mixer is left in place, but the diffuser collar portion  38  of the diffuser is cut and removed. A new casting or spool piece  80  is then secured to the diffuser  36  ( FIGS. 11 and 12 ). This allows the slip joint geometry to be tightly controlled. This spool piece can be a single section (which may require the removal of the inlet mixer for installation) or multiple sections (i.e., like a clam shell) which may allow for the inlet mixer to in situ install. Again, the geometric relationship between the inlet mixer and the diffuser can be structured such that the inlet mixer outside diameter surface and the diffuser inside diameter surface converge. 
         [0044]    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. 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 breadth of the appended claims and any and all equivalents thereof.