Abstract:
A tubesheet anchor for suspending a tool from the underside of a heat exchanger tubesheet that inserts one end of two fingers into corresponding openings in the tubesheet and leverages one finger off the other to apply a frictional force to the sides of the tubesheet openings in which the fingers are inserted to clamp the fingers to the tubesheet.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119(e) from Provisional Application Ser. No. 61/645,117, entitled “Simplified Tubesheet Gripping Mechanism,” filed May 10, 2012. 
    
    
     BACKGROUND 
     1. Field 
     This invention generally concerns robotic systems and is specifically concerned with an improved gripping mechanism for lightweight robotic systems for servicing heat exchanger tubes of a nuclear steam generator. 
     2. Related Art 
     In a pressurized water nuclear power electric generating system, the heat generated by the nuclear reaction is absorbed by a primary coolant that circulates through the reactor core and is utilized to generate steam in a heat exchanger commonly referred to as a steam generator. The steam generator typically is an upright cylindrical pressure vessel with hemispherical end sections. A transverse plate called a tubesheet, located at the lower end of the cylindrical section, divides the steam generator into a primary side, which is the lower hemispherical section below the tubesheet, and a secondary side above the tubesheet. A vertical wall bisects the primary side into an inlet section and an outlet section. The tubesheet is a thick carbon steel plate with an array of thousands of holes into which are inserted the ends of U-shaped tubes. One end of each U-shaped tube is inserted into a hole within the tubesheet which communicates with the inlet section of the primary side and the other end is inserted in a hole within the tubesheet which communicates with the outlet section. The primary coolant is introduced under pressure into the inlet section of the primary side, circulates through the U-shaped tube and exits through the outlet section. Water introduced into the secondary side of the steam generator circulates around the U-shaped tubes and is transformed into steam by heat given up by the primary coolant. Typically, there are thousands of small diameter U-shaped tubes which provide a large surface area for heat transfer. The number of tubes in a steam generator range from about 4,000 to 15,000. Some steam generators utilize straight length tubes each about 60 feet long. Most of the steam generators are constructed of U-shaped tubing or long vertical sections with two 90° bends joined by a shorter horizontal length tubing. During plant operation, the high pressure water that flows through the reactor core transports some amount of radioactive particles through the steam generators and some particles become deposited on the interior surface of the tubes. After plant operation, the steam generators become a source of radiation. 
     Occasionally, during the operation of the steam generator, degradation occurs in some of the tubes. This is undesirable because the primary coolant is radioactive and any leakage of the reactor coolant into the secondary side of the generator contaminates the steam. It is generally not practical, however, to replace degraded tubing, but instead the steam generator is periodically inspected and the affected tubes are plugged at both ends. In view of the thousands of tubes in the steam generator, plugging of a few tubes does not appreciably affect the efficiency of the heat transfer. 
     Because of the radiation hazard present in steam generators used in a nuclear power utility, the heat exchanger tubes of such steam generators must be, for the most part, remotely serviced to avoid exposing maintenance personnel to potentially harmful radiation. Consequently, a number of robotic systems have been developed for remotely performing repair and maintenance operations on these heat exchanger tubes. These robotic systems typically include some sort of robotic delivery arm in combination with any one of a number of specialized tools designed to be carried by the robotic arm, which are known in the art as “end effectors.” Some of the common robotic systems for this task utilize the holes in the tubesheet to anchor the robot via number of camlocks (typically four or more), for example, as shown in U.S. Pat. No. 7,314,343, assigned to the Assignee of this invention. Each charlock consists of a cylindrical arrangement of flexible “fingers” that protrude into a single tube and are expanded by a central cam actuated to engage the tube inner diameter surface. They thereby achieve anchoring from the resulting frictional force of the fingers on the tube inside diameter. This anchoring method is effective, but the problems are that the camlocks are costly, complex devices and they may release unexpectedly if the actuation force is lost. 
     Accordingly, it is an object of this invention to provide a simpler gripper capable of anchoring a robot to the underside of a tubesheet without the use of camlocks. 
     It is a further object of this invention to provide a single mechanism that provides both anchoring and rotational alignment. 
     It is an additional object of this invention to provide such a mechanism that supplies a very high gripping force through a mechanical advantage. 
     It is a further object of this invention to provide such a mechanism that automatically locks in place and requires no actuation force to stay locked. 
     It is a further object of this invention to provide such a mechanism that can release and re-grip very quickly. 
     It is an additional object of this invention to provide such a mechanism that is self-aligning and provides accurate locating. 
     SUMMARY 
     These and other objects are achieved by a tool having an actuator for gripping a tubesheet of a heat exchanger having a plurality of heat exchange tubes extending at least partially through thru-holes in the tubesheet, with each of the heat exchange tubes having a central axis extending along a length thereof. The actuator includes a first elongated finger sized to have a first end of the first elongated finger inserted at least partially within a first of the thru-holes within the tubesheet. A second elongated finger is sized to have a first end of the second elongated finger inserted at least partially within a second of the thru-holes in the tubesheet. The second elongated finger is spaced from the first elongated finger to substantially align with the second of the thru-holes when the first elongated finger is substantially aligned with the first of the thru-holes. A tie rod is connected between the first elongated finger and the second elongated finger at a first elevation along the first elongated finger and the second elongated finger that is spaced from the first ends. The connection of the tie rod between the first elongated finger and the second elongated finger is configured to restrain movement at the first elevation of the first elongated finger and the second elongated finger in at least a first of two lateral directions, either toward each other or away from each other. The actuator also includes an actuation arm connected between the first elongated finger and the second elongated finger at a second elevation along the first elongated finger and the second elongated finger that is spaced from the first elevation and spaced from the first ends. The connection of the actuation arm between the first elongated finger and the second elongated finger is configured to move the first elongated finger in at least one of two lateral directions and cant at least one of the first elongated finger and second elongated finger relative to the axis of a corresponding tube or through a hole in which it is designed to be inserted to pressure the one of the first elongated finger and the second elongated finger against an inner wall of the corresponding tube or through a hole and hold that position until the actuation arm is positively released. 
     In one embodiment, the actuation arm cants both the first elongated finger and the second elongated finger relative to the axis of the corresponding tube or thru-hole in which it is designed to be inserted to pressure the first elongated finger and the second elongated finger against the corresponding tube in which it is inserted. Preferably, the actuation arm toggles between a locked position in which at least one of the first elongated finger and the second elongated finger is canted relative to the axis of the corresponding tube or thru-hole in which it is designed to be inserted and an unlocked position in which the first elongated finger and the second elongated finger are not pressured against the inner wall of the corresponding tube or thru-hole. In another embodiment, both the first elongated finger and the second elongated finger are pressured against the inner wall of the corresponding tube or thru-hole when the actuation arm moves in the at least one of the two lateral directions. 
     In still another embodiment wherein the first elevation is between the first ends and the second elevation, the tie rod restrains movement of the first elongated finger and the second elongated finger towards each other. In an alternate embodiment, the tie rod restrains movement of the first elongated finger and the second elongated finger away from each other. 
     In an additional embodiment, the second elevation is between the first ends and the first elevation and the tie rod restrains movement of the first elongated finger and the second elongated finger towards each other. Alternately, the tie rod restrains movement of the first elongated finger and the second elongated finger away from each other. 
     In a further embodiment the first elongated finger and the second elongated finger are configured to move a distance vertically independent of the actuation arm. Preferably, the actuation arm includes a compensator that is configured to accommodate a variation in spacing of the thru-holes while maintaining an approximately constant clamping force. The compensator may be an air spring, for example. 
     The invention also contemplates a method of supporting a tool from the underside of a heat exchange tubesheet having a plurality of openings extending through an underside. The method includes the step of inserting a portion of a first finger into a first opening in the underside of the tubesheet and inserting a portion of a second finger into a second opening in the underside of the tubesheet. The method leverages the first finger off the second finger to clamp at least a part of the portion of either the first finger or the second finger that is inserted into the corresponding opening against a wall of the opening and locks the first finger and the second finger in their clamped position. 
     In one embodiment the method leverages both the first finger and the second finger against the wall of the corresponding opening. Preferably the leveraging step cants either the first finger or the second finger or both relative to an axis of the corresponding opening in which it is inserted. The method also includes the step of suspending the tool from the first and second finger. The method may also include the step of moving the first finger and the second finger in a vertical direction independent of a mechanism for performing the leveraging step. Further, the method may additionally include the step of compensating for a variation in the distance between openings in the underside of the tubesheet while substantially maintaining a constant clamping force. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is a view in perspective and partial vertical section of a steam generator with parts removed in the interest of clarity; 
         FIG. 2  illustrates a probe carrier drive assembly disposed in a plenum chamber of the steam generator beneath a steam generator tube to be inspected and releasably connected to a remote service arm for positioning the drive assembly beneath the tube to be inspected; 
         FIG. 3  is a cross sectional view of a portion of a tubesheet showing the heat exchange tube ends extending therethrough and one embodiment of the gripper of this invention for supporting a robot, such as the remote service arm illustrated in  FIG. 2 ; 
         FIG. 4  is a cross sectional view of the portion of the tubesheet shown in  FIG. 1  with a second embodiment of the gripper of this invention shown inserted within two of the heat exchange tube ends; 
         FIG. 5  is a cross section of the portion of the tubesheet shown in  FIGS. 3 and 4  with a third embodiment of the gripper of this invention shown disposed within two of the heat exchange tube ends; 
         FIG. 6  is a cross sectional view of the portion of the tubesheet shown in  FIGS. 3 ,  4  and  5  with a fourth embodiment of the gripper of this invention shown disposed within two of the heat exchange tube ends; and 
         FIG. 7  is a cross sectional view of the portion of the tubesheet shown in  FIGS. 3 ,  4 ,  5  and  6  with a fifth embodiment of the gripper of this invention shown disposed within two of the heat exchange tube ends. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Occasionally it is necessary to inspect steam generator tubes for surface and volume flaws by using a robot that can position an inspection probe within the tubes to be inspected and support the equipment employed to facilitate the probe&#39;s travel through the tube. The invention claimed hereafter and the embodiments thereof described herein provide a simplified anchor for supporting such a robot from the underside of the tubesheet. 
     Referring to  FIG. 1 , a steam generator is referred to generally by reference character  10  and comprises a generally cylindrical outer shell  12  having a cylindrical upper portion  14  and a cylindrical lower portion  16 . Disposed in the upper portion  14  is moisture separating means  18  for separating a steam-water mixture so that entrained water is removed from the steam-water mixture. Disposed in lower portion  16  is an inner shell  20  which is closed at its top end except for a plurality of openings disposed in its top end for allowing passage of the steam-water mixture from the inner shell  20  to the moisture separating means  18 . Inner shell  20  is open at its bottom end, which inner shell  20  defines an annulus  21  between the inner shell  20  and the lower portion  16  of the outer shell  12 . Disposed in the inner shell  20  is a vertical steam generator tube bundle  22  having a plurality of vertical, U-shaped steam generator tubes  24  therein. Disposed at various locations along the length of the tube bundle  22  are a plurality of horizontal circular tube support plates  26 , having holes therein for receiving each tube of the tube bundle  22 , for laterally supporting the tubes and for reducing flow-induced vibration in the tubes. Additional support for the tubes in the tube bundle  22  is provided in the U-bend region of the tube bundle  22  by a plurality of anti-vibration bars  28 . 
     Still referring to  FIG. 1 , disposed in a lower portion  16  of the outer shell  12 , below a bottom most support plate  52  is a horizontal, circular tubesheet  30  having a plurality of vertical apertures  32  therethrough for receiving the ends of the tubes of the tube bundle  22 , which ends of the tubes extend a predetermined distance through the apertures  32 . Tubesheet  30  is sealingly attached, which may be by welding, around a circumferential edge to a hemispherical channel head  34 . Disposed in channel head  34  is a vertical, semi-circular divider plate  36  sealingly attached, which may be by welding, to the channel head  34  along the circumferential edge of the divider plate  36 . Divider plate  36  is also sealingly attached, which may be by welding, to tubesheet  30  along the flat edge of the divider plate  36 . Divider plate  36  divides the channel head  34  into an inlet plenum chamber  38  and an outlet plenum chamber  40 . 
     Referring again to  FIG. 1 , disposed on the outer shell  12  below the tubesheet  30  are a first inlet nozzle  42  and a first outlet nozzle  44  in fluid communication with inlet plenum  38  and with outlet plenum chamber  40 , respectively. A plurality of manway holes  46  are disposed on the outer shell  12  below the tubesheet  30  for providing access to the inlet plenum chamber  38  and outlet plenum chamber  40 . Disposed on the outer shell  12  above the tube bundle  22  is a second inlet nozzle  48 , which is connected to a perforated horizontal and generally toroidal feedwater ring  50  disposed in the upper portion  14  of the outer shell  12  for allowing entry of nonradioactive secondary fluid or feedwater into the upper portion  14  through inlet nozzle  48  and through the perforations (not shown) of feedwater ring  50 . A second outlet nozzle  54  is disposed on top of the upper portion  14  for exit of steam from the steam generator  10 . 
     During operation of the steam generator  10 , radioactive primary fluid from the reactor, which may obtain a temperature of approximately 620° F. (327° C.) enters inlet plenum  38  through first inlet nozzle  42  and flows through the tube bundle  22  to the outlet plenum  40  where the primary fluid exits the steam generator  10  through the first outlet nozzle  44 . The secondary fluid, which is water, enters the feedwater ring  50  through the second inlet nozzle  48  which is connected to the feedwater ring  50  and flows downwardly from the perforations (not shown) of the feedwater ring  50  through the annulus  21  until the secondary fluid is in fluid communication with the tubesheet  30 . The secondary fluid then leaves annulus  21  flowing upwardly by natural convection through the tube bundle  22  where the secondary fluid boils and vaporizes into a steam-water mixture due to conductive heat transfer from the primary fluid to the secondary fluid through the walls of the tube bundle  22  which functions as heat conductors. The steam-water mixture flows upwardly from the tube bundle  22  and is separated by moisture separating means  18  into saturated water and dry saturated steam which may obtain a minimal quality of approximately 99.75%. The saturated water flows downwardly from the moisture separating means  18  and mixes with the secondary fluid. Thus, as the secondary fluid enters second inlet nozzle  48  dry saturated steam exits steam generator  10  through the steam outlet nozzle  54 . In a manner well known in the art, the dry saturated steam is ultimately transported to perform useful work such as drive turbine generators for the production of electricity. Moreover, as previously mentioned, in a nuclear reactor, the primary fluid is radioactive; therefore, steam generator  10  is designed such that the primary fluid is nowhere in direct communication with the secondary fluid in order that the nonradioactive secondary fluid is not radioactively contaminated by intermixing with the radioactive primary fluid. 
     Occasionally, due to tube wall defects or tube wall cracking caused by stress and corrosion, some tubes within the tube bundle  22 , for example, a suspect steam generator tube (see  FIG. 2 ), may develop surface and volume flaws and thus may not remain leak tight. Therefore, it is customary to inspect the steam generator tubes such as tube  56  to detect the location and extent of flaws or irregularities so that corrective action may be taken, preferably before a leak develops. A determination of whether tube  56  has flaws or irregularities sufficient to require corrective action may be obtained by examining tube  56  using a nondestructive examination scanning device (not shown). Naturally, the scanning device should be suitably moved without slip or creep along the inside surface of the tube  56  so that the tube may be thoroughly scanned for flaws or irregularities. 
     Referring now to  FIG. 2 , there is illustrated a probe carrier drive assembly, generally referred to by reference character  58 , for suitably moving a probe carrier  60  in the tube  56 . As previously mentioned, a robotic arm  62  supported from two or more of the tubesheet holes  32  with the aid of camlocks has been employed to support the drive assembly  58  during this process. This invention, as claimed hereafter, several embodiments of which will be described herein, provides a single simplified mechanism for supporting such a robot on the underside of the tubesheet that provides lockable anchoring with rotational stability that would require at least two of the prior art camlocks. 
     One preferred embodiment is shown in  FIG. 3 . Instead of engaging a single tube hole with the camlock as in the prior art, this mechanism engages two separate tube holes  64  and  66  spaced one or more pitches apart. When the linkage mechanism  68  is actuated it spreads first and second gripper fingers  70  and  72  and engages one surface of each tube  64  and  66  (or corresponding hole where the tubes do not fully penetrate the hole) inside diameter. Due to the geometry, the finger engagement will align the mechanism with the two holes,  64  and  66 , thereby providing a fixed rotational reference, and will provide an anchoring force by way of friction with the tube inside diameter surface. The linkage mechanism  68  provides a significant mechanical advantage such that the engagement force is much larger than the actuation force, and with proper dimensioning and compliance, the linkage will toggle into a locked position such that the mechanism will stay gripped even after the actuation force is removed. As can be seen in  FIG. 3 , the fingers  70  and  72  have a first end  74  and  76  that are inserted at least partially within the tubesheet openings of heat exchange tubes  64  and  66 . The distal ends of the fingers  70  and  72  are restrained against laterally moving outward by a tie bar or rod  78  which extends through an opening  80  and  82 , respectively, in the distal ends of the fingers  70  and  72  and is captured by an enlarged end or nut  96  at either end. Preferably, the tie rod loosely fits through the openings  80  and  82  or the tie bar or rods  78  is flexible so that the fingers  70  and  72  can cant (slant) against the side walls of the openings in the tubes  64  and  66  when the actuation arm  68  is activated to the horizontal position in which it is locked. The robotic arm  62  that supports the tool (shown in  FIG. 2 ) can be supported from either finger  70 ,  72  or the tie rod  78 . The actuation arm is mainly formed by the two links  84  and  86  which are connected at the center by a pivot pin  92  and at the ends by pivot brackets  88  and  90 . An actuation grip  94  is provided that can be accessed using a remote tool, such as a pole, which is manipulated from outside of one of the manways  46 . 
       FIG. 4  shows an alternate embodiment in which the actuation linkages  84 ,  86  of the actuation arm  68  are arranged as tension members. As explained with regard to  FIG. 3 , the mechanism is self-aligning with the holes, has significant mechanical advantage in gripping force, and is capable of toggling into a locked state. Like reference characters are used to identify corresponding components among the various figures. In this embodiment, the ends of the tie rods  78  are screwed into openings  80  and  82  in the distal ends of the fingers  70  and  72 . The linkages  84  and  86  on the actuation arm  68  extend through openings  98 ,  100 , respectively, in the fingers  70  and  72 . The distal ends of the linkages  84  and  86  are captured by nuts  102 ,  104  on the other side of the openings  98  and  100 . Thus, when the actuation disk  106  is rotated in the counterclockwise direction, the linkages  84  and  86  will be placed in tension drawing the fingers  70  and  72  towards each other and bracing the distal ends  74  and  76  against the inner walls of the tube openings  64  and  66 . It should be appreciated that a similar result could be obtained by attaching the distal ends of the linkages  84  and  86  directly to the inside surfaces of the fingers  70  and  72  as was done in the embodiment illustrated in  FIG. 3  and the actuation disk rotated in a clockwise direction to place the linkages  84  and  86  in compression and cant the ends  74  and  76  of the fingers  70  and  72  outward against the walls of the openings  64  and  66 . 
       FIGS. 5 and 6  show other embodiments in which the fulcrum (tie rod  78 ) is moved to the upper position and the fingers  70  and  72  are operated as levers from actuation linkages  84 ,  86  at the bottom or distal ends of the fingers, arranged as either compression or tension members. As before, the mechanism is self-aligning with the tube holes, has a significant mechanical advantage in applying the gripping force, and is capable of toggling into a locked state. 
     It should be appreciated that a variety of support structures can house this mechanism and be supported from the tubesheet to perform any number of mechanical tasks such as inspecting the heat exchange tubes, plugging the ends of the tubes, rolling the ends of the tubes, welding tube sections, etc. In addition, if the connections to the fingers  70 ,  72  are slotted it will be possible to slide the fingers vertically into and out of the tubesheet when the mechanism is not actively gripped, thus providing a way to disengage the mechanism from the tubesheet, i.e., one or both of the tie rod and the actuation arm being connected to the fingers loosely through slots. An example of the latter arrangement is shown in  FIG. 7 . 
       FIG. 7  shows still another embodiment of this invention that includes two additional features not included in the above embodiments. The first additional feature provides clearance slots  108 , and  110 , respectively in the fingers  72  and  70 , that permit the fingers to move vertically, relative to one or both of the tie rod  78  and the actuation links  84  and  86 , into or out of the tubesheet  30 . This enables the tie rod  78  and/or the activation linkages  84 ,  86 , to be separately supported while the fingers move into the tubes in the tubesheet  30  prior to gripping or out of the tubesheet after gripping. The second feature includes an air spring  120  between the primary cam lever  116  and the clamping lever  118  to accommodate dimensional variations. 
     In the embodiment shown in  FIG. 7  the two fingers  70  and  72  can be lifted into the tubesheet  30  using the common bar  130 , while the tie rod  78  and actuation links  84  and  86  are separately supported, such as by a robot  62 . The fingers  70  and  72  can be moved vertically by a pneumatic or hydraulic cylinder  136 . As the fingers  70  and  72  are raised or lowered, the slots  110  and  108  move over the actuation linkages  84  and  86 , respectively. The linkages  84  and  86  are provided with notches  126  which are captured within the slots  110  and  108  by the enlarged ends  112  and  114  of the linkages  84  and  86 , i.e., as compared to the notches  126 . Similarly the tie rod  78  may ride in similar slots, though it is not necessary unless the tie rod  78  and the linkages  84  and  86  are supported together, for example by the back plate  148 . Alternately, the actuation linkages  84  and  86  can be provided with openings in their ends through which notches in the finger pass and move vertically in a similar manner. Alternately, in this latter embodiment, the actuation linkages  84  and  86  can be flat bars with clearance holes towards the peripheral ends of the bars for the passage of the fingers. In this embodiment the lower tie rod  78  is optional as the common lift bar  130 , used to raise and lower the fingers, may also be used to restrain the bottom of the fingers. 
     The actuation mechanism  68  comprises a cam and a series of interconnected actuation linkages. The actuation linkages  116  and  118  are connected to the back plate  148  respectively with pivot couplings  138  and  140  and the cam is connected to the back plate  148  with rotatable coupling  142 . A pin  132  protrudes from the surface of the cam and rides against the curve surface of the hook  134  at the lower end of the actuation linkage  116 , over at least of portion of the travel of the cam. The actuation linkage  116  is connected to one end of the actuation link  118  through an air spring  120  which is pivotally connected at its ends respectively to an intermediate segment of actuation link  116  and one end of actuation link  118 . Actuation link  84  is pivotally coupled to an intermediate segment of actuation link  118  through bracket  124  which is rigidly attached to actuation link  84 . Similarly actuation link  86  is pivotally coupled to an end portion of actuation link  118  through bracket  122  which is rigidly connected to actuation link  86 . Once the fingers are in the up position inside the appropriate tubesheet tubes, rotating the cam pushes the actuation linkage lever  116  to compress the air spring  120 , in turn moving the actuation link clamp lever  118  to inwardly tilt the ends  74  and  76  of the fingers  70  and  72  to engage the side walls of the tubesheet tubes  64  and  66 . As the cam  106  continues motion in a clockwise direction it compresses the air spring  120  thus providing the clamping action. The cam  106  rotation continues until it reaches the curved hook  134  on the actuation lever arm  116  at which point the geometry causes a detent to occur (i.e., the cam cannot be back driven by the air spring). The use of the air spring  120  allows the clamping mechanism to accommodate small dimensional variations in the tube spacing while achieving a nearly constant clamping force. 
     It should be appreciated that the fingers need not have a round cross-section and two contact points provide better self-alignment with the tubesheet openings. In addition, the tips of the fingers  70 ,  72  may be provided with elastomeric sheaths  150  (shown in  FIG. 7 ) to protect the heat exchange tubes. 
     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.