Patent Number: 062367005
Section: description

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a perspective view with parts cut away of a reactor pressure vessel (RPV) 10. RPV 10 includes a shroud 12, core spray line laterals 14A and 14B, downcommer pipes 20A and 20B, a T-box junction 24, a T-box 28, and core spray spargers (not shown in FIG. 1). Downcommer pipes 20A and 20B include a lower elbow 32A and 32B. Lower elbows 32A and 32B are typically welded to shroud 12. As shown in FIG. 2, downcommer pipe 20A is located between shroud 12 and a wall of reactor pressure vessel 10. Particularly, downcommer pipe 20A is located about sixty feet under water, in an annulus between shroud 12 and the RPV wall having a width of about twelve to fifteen inches, and in a high radiation zone. Until now, the limited accessibility makes welding of replacement downcommer pipes impractical. A downcommer coupling apparatus constructed in accordance with the present invention facilitates replacement of downcommer pipes. Specifically, and referring to FIGS. 2, 3, 4A, 4B, and 5, a downcommer coupling apparatus 36 constructed in accordance with one embodiment of the present invention includes a wedge flange 40, a wedge 44, a wedge housing 48, a pipe seal 50, a substantially cylindrical pipe 52, an elbow 56, a T-box seal 60, a lower flange 62, a nutbar 64, and a clamp 68. Downcommer coupling apparatus 36 further includes wedge flange bolts 72A and 72B, dowel bolts 74A and 74B, and lower flange bolts 76A, 76B, and 76C. Wedge flange 40 includes wedge flange bolt openings (not shown) and an opening (not shown) sized to receive downcommer pipe 20A. Wedge 44 is a split tapered sleeve having, in one embodiment, flexible thinned segments 84A, 84B, 84C, 84D, 84E, 84F, 84G, and 84H extending substantially lengthwise and a substantially circular opening (not shown) sized to receive downcommer pipe 20A. Wedge housing 48 includes a frustro-conical portion 92 and a wedge housing flange 96 at a larger diameter end 100 of frustro-conical portion 92, and dowel bolt openings 102A and 102B (only one shown). Wedge housing flange 96 includes two wedge flange bolt openings (not shown) sized to receive wedge flange bolts 72A and 72B. Pipe seal 50 is a double diaphragm type seal sized to be positioned within wedge housing 48 adjacent frustro-conical smaller end 106 so that downcommer pipe 20A clamps seal 50 against wedge housing 48. Flexible seal 50 is self-energizing so that the pressure difference tending to cause leakage also increases the seal surface contact force, tightening the seal. Seal 50 minimizes the added joint load required to accomplish sealing, so the sizing of required respective dowel and wedge flange bolts 74A, 74B, 72A and 72B, dowel bolt openings 102A and 102B, and the wedge flange bolt openings is minimized to fit within the available access. Pipe 52 is substantially cylindrical having a first end 108 and a second end 112. Pipe first end 108 is coupled to wedge housing smaller diameter end 106. Pipe second end 112 is coupled to a lower elbow first end 116. Lower elbow second end 118 is coupled to lower flange 62. Lower flange 62 includes a T-box opening (not shown), eight lower flange bolt openings (not shown), and a circular lip 120 having taper pin openings 122A, 122B, and 122C (only two shown). Lower flange T-box opening is sized to receive a portion of T-box 28. Each of the eight lower flange bolt openings has a square recess (not shown) to mate with bolt locking collars (not shown). T-box seal 60 is a double diaphragm type seal identical to pipe seal 50. Lower flange bolts 76A, 76B, and 76C are sized to extend through three of the lower flange bolt openings depending on the available access. Taper pin openings 98A, 98B, and 98C extend radially through lower flange lip 120 and are sized to receive taper pins 124A, 124B, and 124C (only two shown). Taper pins 124A, 124B, and 124C are sized to extend through lower flange taper pin openings 122A, 122B, and 122C and hold T-box seal 60 in proper position during installation. Nutbar 64 includes a member 126 and nuts 128A and 128B. Member 126 includes two openings (not shown) sized to receive lower flange bolts 76A and 76B. Clamp 68 is substantially L-shaped and includes a bolt opening (not shown) extending through one leg 132 sized to receive lower flange bolt 76C. Wedge flange bolts 72A and 72B are configured to extend through wedge flange 40 and wedge housing 96 wedge flange bolt openings. Dowel bolts 74A and 74B are configured to extend through wedge housing dowel bolt openings 102A and 102B and downcommer pipe dowel bolt openings 136A and 136B (only one shown). In replacing the lower portion of downcommer pipe 20A, including lower elbow 32A, downcommer pipe 20A is cut-off between core spray line lateral 14A and lower elbow 32A. Lower elbow 32A is then removed from shroud T-box 28. Dowel bolt openings 136A and 136B are formed, typically by machining, near the end of downcommer pipe 20A to be coupled with coupling apparatus 36. Circular groove 154 is then formed in the outer diameter of shroud 12 around T-box 28, typically by spotface machining to the correct depth. Groove 154 provides shear constraint for coupling apparatus 36 and is sized to receive lower flange lip 120 so that lower flange 62 is adjacent to shroud 12. A portion of T-box 28 extending from shroud 12 is cut to the proper length, for example by spot face machining. Three lower flange bolt openings (not shown) are then formed, typically by machining, in shroud 12 at accessible locations aligned with three of the eight lower flange bolt openings (not shown). The replacement continues by inserting downcommer pipe 20A into wedge housing 48 so that pipe seal 50 is between downcommer pipe 20A and wedge housing 48. Dowel bolts 74A and 74B extend into respective threaded wedge housing dowel bolt openings 102A and 102B and into respective downcommer pipe dowel bolt openings 136A and 136B. Lower flange 62 is positioned adjacent shroud 12 so that T-box 28 is received by lower flange 62. Lower flange bolts 76A, 76B, and 76C are extended through lower flange bolt openings in lower flange 62 and shroud 12. Nutbar 64 is then positioned inside shroud 12, and lower flange bolts 76A and 76B are extended through nutbar member 124 and engaged to nutbar nuts 128A and 128B. Clamp 68 is then positioned so that lower flange bolt 76C extends into the clamp bolt opening and clamp 68 is positioned adjacent to T-box 28. Lower flange bolt 76C is then secured with a nut 140 to constrain T-box 28. Wedge flange bolts 72A and 72B are then torqued to clamp wedge 44 between wedge flange 40 and wedge housing 48 thereby forming a rigid joint between coupling apparatus 36 and downcommer pipe 20A. After torquing dowel bolts 74A and 74B and lower flange bolts 76A, 76B, and 76C, locking collars 160A, 160B, 160C, 160D, 160E, 160F, and 160G (only four shown) are positioned in square recesses and crimped into grooves (not shown) in heads of bolts 72A, 72B, 74A, 74B, 76A, 76B, and 76C preventing rotation. As explained above, coupling apparatus 36 is mechanically joined to shroud 12 and downcommer pipe 20A with sufficient strength and stiffness to react the originally specified piping loads from seismic, weight, thermal expansion and hydraulic conditions without significantly changing the piping system loads. Wedge 44 and wedge housing 48 grip downcommer pipe 20A tightly over a sufficient length to develop the full bending strength and stiffness of a continuous pipe. Dowel bolts 74A and 74B are sized to react the axial twisting and vertical loads. The bolted connection of lower flange 62 to shroud 12 is a similarly stiff connection, with the bolt preload sufficient to prevent separation of lower flange 62 from shroud 12. Placement of lower flange bolts 76A, 76B, and 76C outside pipe 20A minimizes the preload change due to temperature transients in the fluid inside downcommer pipe 20A. These transients can be significant, as downcommer pipe 20A is used to inject emergency cooling water at low temperature (about 50 degrees F) into shroud 12 which is, typically, above 500 degrees F. Temperatures of lower flange bolts 76A, 76B, and 76C thus tend to follow that of attached lower flange 62 and shroud 12 so that lower flange to shroud joint is neither over stressed or loosened by thermal differential expansion. Coupling apparatus 36 is constructed, typically, of type 316 stainless steel, matching the thermal properties of attached downcommer pipe 20A and shroud 12. Coupling apparatus 36 is given a solution heat treatment to remove weld residual stress and provide optimum corrosion resistance. Surfaces of wedge 44 are given a nitride hardening treatment to facilitate assembly without galling or sticking, which could prevent full tightening. Seals 50 and 60 and bolts 72A, 72B, 74A, 74B, 76A, 76B, and 76C are typically age hardened Ni--Cr--Fe alloy X-750 to provide high strength to minimize their size in the limited available access space. The alloy X-750 also provides good corrosion resistance and resistance to galling during engagement of the threaded connections. From the preceding description of various embodiments of the present invention, it is evident that the objects of the invention are attained. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. Accordingly, the spirit and scope of the invention are to be limited only by the terms of the appended claims.