Patent Number: 051942143
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention described herein provides a plugging device, such as a tube plug, and a method for plugging a tubular member, such as a nuclear steam generator tube, which utilizes a locking cup for securing together the individual components comprising the tube plug, thereby reducing the possibility of a separation of the plurality of members of the tube plug due to vibration during the operation of the steam generator. Referring to FIG. 1, the steam generator 10 is schematically shown having a plurality of tubes 20 (only two of which are shown) disposed therein. One of the tubes 20, for example a tube 22 having an inner wall 24, may have become degraded due to vibration or primary water stress corrosion cracking, which may have occurred during operation of the steam generator 10. The tube 22 may have a crack which will allow the commingling of the radioactive primary fluid in the tube 22 with the nonradioactive secondary fluid outside the tube 22. To prevent any such fluid commingling, the steam generator 10 is taken out of service in a manner well known in the art and a tube plug 40 is disposed in both open ends of the tube 22 for plugging the degraded tube 22. After the tube 22 is plugged, the steam generator 10 is returned to service. The tube plug 40 prevents the flow of primary fluid through the tube 22. However, the stress corrosion cracking which caused the tube 22 to degrade may also affect the tube plug 40 and thus cause the tube plug 40 to likewise degrade. Referring to FIG. 2, the tube plug 40 comprises a first member, such as a shell 52, an expander member 54, a second member, such as bolt means 56, and a locking cup 58. The shell 52 is a substantially cylindrical member manufactured from a metal, such as Inconel 600. The metal Inconel is an alloy composed substantially of nickel, chromium, and iron and a trace amount of cobalt. The shell 52 has a conical inner surface 60 of an outer wall 62, which has a larger diameter at a closed end 64 of the shell 52 and a smaller diameter at an open end 66 of the shell 52. The shell 52 also has a bore 68 at least partially therethrough and a plug face 69 at the open end 66. The closed end 64, open end 66 and outer wall 62 define a chamber 70 in the shell 52. Disposed in the chamber 70 is the expander member 54, which may have a threaded cylindrical bore 72 therethrough, for expanding the outer wall 62 of the shell 52 into sealing engagement with the inner wall 24 of the tube 22 for plugging the tube 22. The inner surface 60 is formed such that the expander member 54 is captured within the shell 52 so that movement of the expander member 54 relative to the inner surface 60 causes the shell 52 to expand without allowing the expander member 54 to be removed from the shell 52. The shell 52 has a threaded cylindrical skirt portion 74 near the open end 66, which has a diameter larger than the smallest diameter of the inner surface 60 of the outer wall 62 for allowing an apparatus (not shown), such as a tool for manipulating the expander member 54, to be threadedly engaged in the skirt portion or to be inserted through the threaded skirt portion 74 and into the chamber 70. The skirt portion 74 has inside diameter threads 75 for threadedly engaging the locking cup 58. The skirt portion 74 also has a recess 76 positioned adjacent to and inward of the inward end 77 of the inside diameter threads 75 of the skirt portion 74 for providing thread relief to the inside diameter threads 75. The shell 52 also has a substantially uniform wall thickness in the portion of the shell 52 that is expanded by the expander member 54. A plurality of outer lands 78 may be integrally formed on the outer wall 62 of the shell 52 in a manner such that the height of each land 78 increases from the closed end 64 to open end 66 while the outer surfaces of all the lands 78 are maintained at approximately the same external diameter and while the wall thickness of the shell 52 remains substantially constant throughout the portion of the shell 52 where the lands 78 are located. The lands 78 provide an enhanced seal between the shell 52 and the tube 22. The shell 52 is also constructed such that the skirt portion 74 near the open end 66 has a thicker wall section than the remainder of the shell 52 to provide stability in extracting the tube plug 40. Prior to the completion of manufacture of the shell 62, the expander member 54 is installed through the open end 66 of the shell 52 and into the chamber 70 of the shell 52. Then, the shell 52 is swaged so that the expander member 54 is captured within the shell 52. Still referring to FIG. 2, the expander member 54 may be manufactured from a hardenable metal, such as a Carpenter Custom 455 and is formed, in this embodiment, such that it has a leading end 80, which has a tangentially blended radius that minimizes "plowing" or pushing metal ahead of the expander member 54 when the expander member 54 is pulled through the shell 52. The expander member 54 also has a polished exterior surface that enhances its movement relative to the shell 52. The expander member 54 also has a trailing edge 82, which is formed to have a sharp edge such that it provides a self-locking mechanism. While the sharp edge feature of the trailing edge 82 retains the expander member 54 from moving toward the closed end 64 of the shell 52, thereby preventing inadvertent diametral contraction of the shell 52, the construction of the expander member 52 is such that the sharp edge feature of the trailing edge 82 does not prevent the expander member 52 from being pushed toward the closed end 64 by a plug removal tool (not shown). The expander member 54 is also provided with a conical outer surface 84 that is arranged such that its outside diameter is smaller near the leading end 80 and larger near the trailing edge 82. The shape of the outer surface 84 provides a mechanism for expanding the shell 52 when the expander member 54 is moved relative to the shell 52. The expander member 54 also has inside diameter threads 86, which may be used for gripping a plug installation tool (not shown) during the expansion process and can be used to engage the bolt means 56. As described hereinabove, the shell 52 and the expander member 54 may be of the type disclosed in U.S. Pat. No. 4,390,042 issued Jun. 28, 1983 to Kucherer et al. entitled "Tube Plug" and assigned to the assignee of the present invention. Still referring to FIGS. 2 and 3, the bolt means 56 has a bolt head 90 at one end, a shaft 91 at an opposite end, and a surface 92 and a taper 93 positioned between the bolt head 90 and the shaft 91. When disposed through the open end 66 of the shell 52, the bolt head 90 is defined as that end portion of bolt means 56 nearer open 66 of the shell 52 and the shaft 91 is defined as that end portion of bolt means 56 nearer closed end 64 of the shell 52. The shaft 91 may have outside diameter threads 94 corresponding to inside diameter threads 86 of the expander member 54 for threaded engagement of the bolt means 56 and the expander member 54. Bolt means 56 may be manufactured from a metal, such as Inconel 690, for resisting stress corrosion cracking. The bolt head 90 has a scalloped portion 98 with at least one recess 99, but preferably at least two recesses for providing a stronger locking feature for the tube plug 40, for engaging the locking cup 58 to secure together the shell 52, the expander member 54, the bolt means 56, and the locking cup 58. The bolt head 90 of bolt means 56 also has a cavity 100, such as a hexagonal hole, for receiving an apparatus (not shown) capable of inserting the bolt means 56 into and retrieving bolt means 56 from the shell 52 and the expander member 54. The bolt head 90 of the bolt means 56 is sized to be received within the locking cup 58 and has a larger diameter than the shaft 91 of the bolt means 56 for providing the surface 92 and taper 93 therebetween. Because the exact position of the expander member 54 within the shell 52 may not be known, the length of the bolt means 56 should be sufficient for the outside diameter threads 94 of bolt means 56 to engage with the inside diameter threads 86 of any installed expander member 54. By installing the bolt means 56 into the shell 52, fluid is prevented from corrosively attacking the inner surface 60 of the shell 52. Also, the bolt means 56 occupies space within the chamber 70, thereby reducing the liquid volume within the chamber 70 so that any stored energy in the chamber 70 is minimized. Sealing the chamber 70 and reducing the liquid volume within the chamber 70 provides a tube plug 40 having reduced susceptibility to stress corrosion cracking. Also, by threading the bolt means 56 into the expander member 54, the expander member 54 is retained by the bolt means 56 in the unlikely event of a portion of the shell 52 cracking, thereby preventing the expander member 54 from moving loosely throughout the steam generator. Referring to FIGS. 2 and 4, tube plug 40 also comprises locking cup 58, which may be manufactured from Inconel 690 for reducing the susceptibility of the locking cup 58 to stress corrosion cracking. Locking cup 58 is an annular member having an inward open end portion 102, an outward open end portion 104, a wall 106, and an outside diameter surface 107 positioned between the outward open end portion 104 and the inward open end portion 102. The inward open end portion 102 has a smaller diameter than the diameter of the outward open end portion 104 for enabling the inward open end portion 102 to be installed within the skirt portion 74 of the shell 52 and the outside diameter surface 107 to abut shell 52. The diameter of the outward open end portion 104 is of a size to enable the locking cup 58 to be installed within tube 22, thereby enabling the locking cup 58 to be installed in a tube plug 40 positioned at any elevation within the tube 22. When the locking cup 58 is installed within the shell 52, the outside diameter surface 107 of the locking cup 58 abuts the plug face 69 of the shell 52 for sealing the chamber 70 of the shell 52. Wall 106 has outside diameter threads 108 at the inward open end portion 102 corresponding to the inside diameter threads 86 of the skirt portion 74 of the shell 52 for threaded engagement of the locking cup 58 and the shell 52. Referring to FIG. 5, the outside diameter threads 108 have a deformed portion 110 for resisting the threading and unthreading of the locking cup 58 into and out of the shell 52 for preventing the inadvertent separation of the shell 52, the bolt means 56, and the locking cup 58. The deformed portion 110 includes at least two segments of the outside diameter threads 108 and has outside diameter threads 112 with a deformed thread shape. A segment is the distance from a point on the outside diameter threads 108 to the corresponding point on the outside diameter threads 108 after one complete turn of the locking cup 58. The deformed portion 11O may comprise 4-5 segments maximum. Still referring to FIG. 5, the pitch of the outside diameter threads 112 of the deformed portion 110 is offset from the pitch of the outside diameter threads 108 of the locking cup 58 for providing the additional resistance to the unthreading of the locking cup 58 from the shell 52. As an example, the degree of the offset between the pitch of the outside diameter threads 112 and the pitch of the outside diameter threads 108 may be approximately 0.005 inches for obtaining the desired degree of interference between the inside diameter threads 75 of the shell 52 and the outside diameter threads 112 of the locking cup 58. Referring to FIGS. 5 and 6 and using the same reference characters to define like parts, FIG. 6 is a detailed enlarged view of the threads illustrated in FIG. 5. At least one portion of the root 113 of the outside diameter threads 112 is offset from the configuration of the root 113 of the outside diameter threads 108 so that the root 113 of the deformed portion 110 includes at least one wave 114. Within a segment, a root 113 with a plurality of offsets has the configuration of a wave-like pattern. As an example, the root 113 of the deformed portion 110 may be offset 0.005 inches from the position of the remainder of the root of the outside diameter threads 108 and 112. Referring again to FIGS. 5 and 6, positioned between the crest 116 and the root 113 is at least one protuberance 118. The protuberance 118 is axially aligned with each wave 114. Preferably, for each segment, the outside diameter threads 112 may have two protuberances 118 and the root 113 may have two waves 114. As an example, the protuberance 118 may have a width of approximately 0.005 inches, a height of approximately 0.005 inches, and a length of approximately 0.200 inches for obtaining the desired degree of resistance between the inside diameter threads 75 of the shell 52 and the outside diameter threads 112 of the locking cup 58, during the installation of the locking cup 58 into and the removal of the locking cup 58 from the shell 52. Referring to FIG. 5, an installation tool (not shown) and a removal tool (not shown) provides a torque sufficient to overcome the interference between the deformed portion 110 of the locking cup 58 and the inside diameter threads 75 of the shell 52, such as a torque of approximately 35-50 in-lb. The torque required to install a locking cup not having a deformed portion 110 is approximately 10 in-lb. Requiring the additional torque of approximately 25-40 in-lb, the locking cup 58 with the deformed portion 110 provides a locking cup with an additional resistance to the threading or unthreading of the locking cup 58 into or out of the shell 52. During the installation of the locking cup 58 into the shell 52, the inside diameter threads 75 of the shell 52 will be plastically deformed. During the removal of the locking cup 58 from the shell 52, the inside diameter threads 75 of the shell 52 will be plastically deformed, which will provide an additional resistance to the inadvertent unthreading of the locking cup 58 from the shell 52. Preferably, the outside diameter threads 108 and the outside diameter threads 1 12 of the threaded portion 110 of the locking cup 58 may be formed by the process of thread rolling for preventing high stress concentration at the root of the threads. The protuberance 118 may be formed by displacing a portion of the thread form between the crest 116 and the root 113. In addition to the wave-like pattern of the root 113 and the protuberances 118 of the deformed portion 110 providing additional resistance during the unthreading of the locking cup 58 from the shell 52, the deformed portion 110 also provides resistance to the inadvertent loosening of the locking cup 58 from the shell 52 due to vibrations occurring during the operation of the steam generator, thereby preventing loose parts from moving throughout the steam generator. Referring to FIGS. 2, 7 and 8, the inside diameter threads 75 of the shell 52 and the outside diameter threads 108 of the locking cup 58 have a larger pitch than the inside diameter threads 86 of the expander member 54 and the outside diameter threads 94 of the bolt means 56 for providing an additional locking means for securing together the bolt means 56, the locking cup 58, the expander member 54, and the shell 52. The locking cup 58 further comprises an inside diameter surface 120 positioned between the outward open end portion 104 and the inward open end portion 102 and positioned substantially parallel to the outside diameter surface 107. A taper 122 is positioned between the inside diameter surface 120 and the outward open end portion 104 at the inside diameter of wall 106 of the locking cup 58. The angle of the taper 122 corresponds to the angle of the taper 93 of bolt means 56 for providing an interference fit between the taper 122 and the taper 93 to seal the chamber 70 of the shell 52. The locking cup 58 also prevents inadvertent backward movement of the bolt means 56 relative to the shell 52. Referring to FIG. 7, the bore through the inward open end portion 102 of locking cup 58 may be a hexagonal bore 124 for receiving an apparatus (not shown) capable of inserting the locking cup 58 into and retrieving the locking cup 58 from the shell 52. Referring again to FIGS. 2, 7 and 8, the threads 108 and 112 may be sized and shaped for known forces such that the torque required for removal of the bolt means 56 and the locking cup 58 is higher than the torque expected to be applied to the bolt means 56 during service of the steam generator. By planning the size and shape of the threads 108 and 112, if the tube plug 40 needs to be removed from the tube 22, the tube plug 40 may be efficiently removed from the tube 22. The outward open end portion 104 of the locking cup 58 may be deformed or crimped into the recess 99 of the scalloped bolt head 90 of the bolt means 56 for securing the shell 52, the expander member 54, the bolt means 56, and the locking cup 58 together. Referring to FIGS. 9 and 10 and using the same reference characters to define like parts, as an alternative, a deformable portion 126 of a bolt means 128 may be deformed or crimped into a recess 130 of a locking cup 132. By utilizing locking cup 58 having the deformed portion 11O with the deformed thread shape and the crimpable outward end portion 104, the plurality of members comprising the tube plug 40 can be secured together and the chamber 70 will remain sealed to prevent fluid from entering the chamber 70 and corrosively attacking the inner surface 60 of the shell 52. The locking or fastening feature of the tube plug 40 also prevents a separation of the members, which may result in loose parts moving throughout the steam generator. OPERATION Referring again to FIG. 2, the shell 52 and the expander member 54 may be positioned within the tube 22, as described in U.S. Pat. No. 4,390,042 issued Jun. 28, 1983 to Kucherer et al. entitled "Tube Plug", and assigned to the assignee of the present invention. An installation tool (not shown) is inserted into the hexagonal bore 124 of the locking cup 58 to thread the locking cup 58 into the skirt portion 74 of the shell 52. The surface 107 of the locking cup 58 will seat on the plug face 69 of the shell 52. A torque, such as 35 in-lb, is applied during the installation of the locking cup 58 into the shell 52 for overcoming the resistance of the deformed portion 110 of the outside diameter threads 108 as the locking cup 58 is threaded into the skirt portion 74 of the shell 52 and for providing the interference fit between the plug face 69 of the shell 52 and the surface 107 of the locking cup 58 An installation tool (not shown) is inserted into the cavity 100 of the bolt head 90 to install the bolt means 56 in the locking cup 58 and to thread the bolt means 56 into the expander member 54 positioned within the shell 52. The taper 93 of the bolt means 56 will seat on the taper 122 of the locking cup 58. The installation tool (not shown) applies a torque to the bolt means 56, such as 90 in-lb, to provide the interference fit between the taper 122 of the locking cup 58 and taper 93 of the bolt means 56. The torque applied should be sufficient to prevent significant leakage of fluid into the chamber 70 of the shell 52 through the metal-to-metal abutment of the plug face 69 of the sell 52 and the outside diameter surface 107 of the locking cup 58 and through the metal-to-metal abutment of the taper 122 of the locking cup 58 and the taper 93 of the bolt means 56, sufficient to prevent loosening of the tube plug 40 while the tube plug 40 is in service, and sufficient to keep a compressive load on the closed end 66 of the shell 52 for reducing the possibility of a separation of the shell 52 due to stress corrosion cracking. Referring again to FIG. 8, after the locking cup 58 is installed within the skirt portion 74 of the shell 52 and the bolt means 56 is installed within the locking cup 58, the outward open end portion 104 of the locking cup 58 may be deformed or crimped into the recess 99 of the scalloped portion 98 of the bolt head 90 of the bolt means 56 for securing the shell 52, the expander member 54, the bolt means 56, and the locking cup 58 together. Referring again to FIGS. 9 and 10, as an alternative, a deformable portion 126 of the bolt means 125 may be deformed or crimped into a recess 130 of the locking cup 132 for securing together the individual components of the plug 40. Referring again to FIG. 8, to remove the tube plug 40 from the tube 22, the bolt means 56 and locking cup 58 must be removed from the shell 52, and then the shell 52 may be removed from the tube 22 by methods well known in the art. To remove the bolt means 56 from the shell 52, a removal tool (not shown) is inserted into the cavity 100 in the bolt head 90 and provides a torque sufficient to deform the crimped locking cup 58 so that the crimped outward open end portion 104 disengages from the recess 99 of the scalloped portion 98 of the bolt head 90. The bolt means 56 is unthreaded from the expander member 54. After the bolt means 56 is removed from the shell 52, a removal tool (not shown) is inserted into the hexagonal bore 124 of the locking cup 58, and then the locking cup 58 is unthreaded from the skirt portion 74 utilizing a torque sufficient to overcome the resistance from the deformed portion 110 of the outside diameter threads 108 of the locking cup 58. If the tube plug 40 is designed so that the locking cup 58 and the bolt means 56 must be removed as one unit, the torque must be sufficient to overcome the resistance from the difference in the pitch sizes of the threads 75 of the skirt portion 74 of the shell 52 and of the threads 86 of the expander member 54. Therefore, the invention provides a plugging device, such as a tube plug, and a method that utilizes a locking cup to lock or fasten together the plurality of members comprising the tube plug in a manner so that the plurality of members will not vibrate apart, thereby preventing loose parts from moving throughout the steam generator, a locking or fastening means that will lock or fasten the bolt means to the shell at any elevation of the shell within the tube, and a tube plug that has a reduced susceptibility to stress corrosion cracking due to the reduction of primary fluid within the chamber of the tube plug.