Patent Document

[0001]    This application is a divisional of U.S. patent application Ser. No. 11/940,446, filed Nov. 15, 2007, pending, the entire contents of which is hereby incorporated by reference in this application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention relates generally to a weld joint and, more particularly to an apparatus and method for repairing or replacing a core spray line weld joint in a boiling water nuclear reactor. 
         [0003]    A core spray piping system in operating boiling water reactors is typically of welded construction. The welds in the core spray system piping, however, are susceptible to intergranular stress corrosion cracking (IGSCC). As a consequence, welded pipe joints in the core spray line can become cracked. 
         [0004]    Common to most operating reactors, the core spray cooling water is delivered to the reactor core region by piping internal to the reactor vessel. A portion of this internal piping is a horizontal segment that follows the radius of curvature of the reactor vessel wall. The proximal end of the horizontal piping is connected to a T-box at the core spray nozzle penetration. This weld is designated as the P 3  weld. The distal end of the horizontal core spray line is welded to a short radius elbow. This weld joining the distal end of the core spray line to the short radius elbow is designated as the P 4   a  weld. In at least one boiling water reactor design, there is another weld in the core spray line in close proximity to the T-box. This weld is designated as the P 3   a  weld. An exemplary piping configuration near the vessel T-box is shown in  FIG. 1 . 
         [0005]    In the event that cracking should occur in the P 3   a  weld, the structural integrity of the core spray line, which delivers cooling water to the reactor core, would be lost. A preemptive repair would be desirable to prevent separation of the P 3   a  weld in the event that circumferential through-wall cracking should occur at this weld location in the core spray line. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In an exemplary embodiment, a clamping device supports or structurally replaces a weld joint between connected pipes. The clamping device includes an upper clamp body and a lower clamp body securable on opposite sides of the connected pipes in facing relation, and at least one clamp bolt connecting the upper and lower clamp bodies and extendable through the connected pipes. One of the upper and lower clamp bodies has at least one shaped depression, which receives a complementary shaped clamp bolt nut engageable with the clamp bolt. The shaped depression prevents rotation of the clamp bolt nut. 
         [0007]    In another exemplary embodiment, the clamping device supports or structurally replaces a weld in a core spray line in close proximity to a T-box in a boiling water nuclear reactor. In this context, the clamping device includes an upper clamp body and a lower clamp body securable on opposite sides of the spray line in facing relation. The upper and lower clamp bodies include a trimmed section adjacent the T-box to ensure clearance from the T-box. At least one clamp bolt connects the upper and lower clamp bodies and extends through the spray line. One of the upper and lower clamp bodies has at least one shaped depression, which receives a complementary shaped clamp bolt nut engageable with the clamp bolt. The shaped depression prevents rotation of the clamp bolt nut. 
         [0008]    In yet another exemplary embodiment, a method of supporting or structurally replacing a weld joint between connected pipes includes the steps of forming at least one aperture through the connected pipes in a joint area; securing the upper clamp body and the lower clamp body on opposite sides of the connected pipes in facing relation; connecting the upper and lower clamp bodies by a clamp bolt extendable through at least one aperture and securing the clamp bolt with a clamp bolt nut, the clamp bolt nut having a non-circular shape; and preventing rotation of the clamp bolt nut by engaging the clamp bolt nut in a correspondingly shaped depression in one of the upper and lower clamp bodies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows an exemplary piping configuration near the vessel T-box in a boiling water nuclear reactor; 
           [0010]      FIG. 2  is a perspective view of the core spray line with a clamping device installed; 
           [0011]      FIGS. 3 and 4  are isometric views of the clamping device; 
           [0012]      FIG. 5  shows the upper clamp body; 
           [0013]      FIG. 6  shows the lower clamp body; 
           [0014]      FIG. 7  shows the clamp bolt; and 
           [0015]      FIG. 8  shows a step in the installation method. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    A clamping device is described herein that structurally supports or replaces a welded connection between connected pipes and, in particular, replaces the P 3   a  weld that joins a short section of horizontal piping to the remainder of the horizontal piping in the core spray line. The clamping device is applicable to reactor plants with varying sized core spray lines. 
         [0017]      FIG. 2  shows the clamp assembly  10  installed on the core spray line.  FIGS. 3 and 4  are isometric views of the clamp assembly  10 . The clamp assembly  10  includes an upper clamp body  12  and a lower clamp body  14 , which interface with the core spray line. The clamp bodies  12 , are held in position on the horizontal pipe by at least one clamp bolt  16 , preferably two, which pass through holes formed in the horizontal pipe. A clamp bolt nut  18  is threaded on an end of each clamp bolt  16 . 
         [0018]    The outside diameter of the core spray line can vary within specified manufacturing tolerances. Also, a curved pipe that has been formed will most likely be slightly oval in cross-section. As such, the radius of curvature machined into the upper and lower clamp bodies  12 ,  14  is slightly smaller than the nominal radius of curvature of the piping. This ensures that the clamp bodies  12 ,  14  will interface properly with the core spray line. The upper and lower clamp bodies  12 ,  14  are preferably machined to interface with a curved pipe (i.e. the machined surface follows or mimics the curvature of the pipe in the plane defined by the curved pipe). 
         [0019]    The upper and lower clamp bodies  12 ,  14  feature spherical seating surfaces  20 , which mate with spherical seating surfaces of the clamp bolt nut  18  and the clamp bolt  16 , respectively (see  FIGS. 5 and 6 ). In addition, the upper clamp body  12  incorporates a shaped (non-circular, preferably square) machined depression  22 , which interfaces with each clamp bolt nut  18  to prevent rotation of the clamp bolt nut  18 . 
         [0020]    Both the upper and lower clamp bodies  12 ,  14  include a trimmed section  26  on the side of the respective clamp body  12 ,  14  to ensure clearance with the core spray T-box and provide future inspection visibility of the P 3  weld (see  FIGS. 3 and 4 ). Additionally, both the upper and lower clamp bodies  12 ,  14  include a machined groove  28  to ensure clearance with any possible P 3   a  weld crown. 
         [0021]    The lower clamp body  14  houses a clamp bolt keeper  24  ( FIG. 4 ), which resides in a machined depression  25  in the lower clamp body  14 . One clamp bolt keeper  24  is preferably provided for each clamp bolt  16 . The clamp bolt keeper  24  is held captive at three separate locations by interfacing features shared by the keeper  24  and the lower clamp body  14 . The function of the clamp bolt keeper  24  is to prevent rotation of the clamp bolt  16  and thus retain clamp bolt pre-load (described in more detail below). 
         [0022]    The clamp bolt nut  18  internal threads mate with external threads of the clamp bolt  16 . The nut  18  has a generally preferably square shape and a spherical seating surface, which interface with the upper clamp body  12 . A distal end of the clamp bolt  16  is machined to a diameter slightly smaller than the minor thread diameter of the clamp bolt nut  18  in order to facilitate remote installation of the clamp bolt nut  18 . In order to minimize core spray coolant leakage, the outside diameter of the clamp bolt nut  18  is slightly smaller than the machined hole in the core spray line. 
         [0023]    With reference to  FIG. 7 , a proximal end of the clamp bolt  16  incorporates a spherical seating surface  34  and ratchet teeth  36 , which interface with the lower clamp body  14  and teeth  38  of the clamp bolt keeper  24 , respectively. In addition, the clamp bolt  16  has a shoulder diameter slightly smaller than the machined hole in the core spray line (see  FIG. 8 ) in order to minimize core spray coolant leakage. The clamp bolt keeper  24  is preferably shaped like a hairpin, which consists of essentially two cantilever beams joined at one end. There are retaining features at the free end of the first and second cantilever beams and also at the common end where both beams are joined together. In addition, the retaining feature at the end of the first cantilever beam also incorporates the teeth  38  that interface with the teeth  36  of the clamp bolt  16  and function to limit rotation of the clamp bolt  16  to the direction that increases bolt pre-load. 
         [0024]    Installation of the clamp assembly  10  is performed by first machining holes  46  via EDM (electric discharge machining) or the like in the piping segment as shown in  FIG. 8 . Subsequently, the clamp bolt keepers  24 , lower clamp body  14 , and clamp bolts  16  are brought together as an assembly on the underneath side of the core spray line. Distal ends of the clamp bolts  16  are inserted through the holes  46  provided in the underneath side of the piping and finally emerging from the holes  46  provided on the top side of the piping. The upper clamp body  12  and clamp bolt nuts  18  are then positioned over the distal end of the clamp bolts  16 . The clamp bolts  16  are rotated to engage the threads of the clamp bolt nuts  18 . The clamp bolts  16  are then tightened to a nominal pre-load. Finally, the clamp bolts  18  are pre-loaded to their final specified values by following an approved torque sequence. 
         [0025]    The described clamp assembly supports or structurally replaces the P 3   a  weld between a short section of horizontal piping to the remainder of the horizontal piping in the core spray line. The clamp assembly can be remotely installed and is applicable to reactor plants with varying sized core spray lines. 
         [0026]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Technology Category: g