Abstract:
A probe for transporting a nondestructive inspection sensor through a tube, that employs wheels to reduce friction. The radial travel of the wheels are mechanically linked through a cam and axially reciprocal plunger arrangement that centers the probe at tube diameter transitions. Internal wire bending is minimized and a dynamic seal is provided to facilitate an insertion force at the probe and reduce or eliminate compressive load buckling of the flexible cable carried by the probe. Like the wheel arrangement, radial travel of the seal segments are mechanically linked to provide probe centering.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119(e) from Provisional Application Ser. No. 61/661,441, entitled “Eddy Current Inspection Probe,” filed Jun. 19, 2012. 
    
    
     BACKGROUND 
     1. Field 
     This invention pertains generally to the nondestructive examination of tubular specimens and, more particularly, to an inspection probe for the nondestructive examination of the structural integrity of heat exchanger tubing. 
     2. Related Art 
     In pressurized water reactor nuclear power plants, steam generators convert the thermal energy of water from the reactor coolant to steam to drive turbine electric generators. In order to transfer the heat while maintaining separation between the high pressure water that flows through the reactor core and the lower pressure water that is converted to steam, steam generators are constructed of thousands of small diameter tubes which provide a large surface area for heat transfer. The number of tubes in a steam generator range from about 3,000 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 of 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. 
     Periodic inspection with eddy current probes is widely utilized to ensure the structural integrity of steam generator tubing. Due to the elevated radiation fields, robotics and remote controlled motorized devices are used to position and translate eddy current probes. The cost of equipment, labor, plant down time, and the benefit of minimizing personnel radiation exposure make it highly desirable to optimize the performance and capability of eddy current inspection probes. 
     One problem with the prior art eddy current probes is that a single probe does not access all the tubes in a steam generator. The larger diameter probes used to inspect the majority of the tubes will not pass through the small radius bends in the tubing. To access the small radius bends, a small probe with less resolution may be used. Additionally, in order to inspect the entire tube, only one half of the tube may be accessed from one side of the steam generator and the second half of the tube may require access from the opposite end of the tube. To maximize productivity, this usually requires the use and disposal of additional probes. 
     A second problem with current eddy current probes is the probe centering mechanism. Typically, the probes are centered employing compliant pads that extend out radially at equally spaced circumferential locations around the probe. The relatively small surface area of contact between the probes and tube increases radial material loss due to wear. To compensate for wear, the pads are slightly oversized which increases friction between the tube and the pads. A further drawback of the probe centering pads is that each pad is compressed, the amount of deflection is independent from one pad to another pad. Side loads developed as the probe traverses bends in the tubing can adversely impact the centering of the probe. Additionally, the axial location of probe centering pads relative to the probe inspection coil can cause the coil to contact the tube surface as the probe traverses bends, which can adversely affect proper interpretation of the sensor signals. 
     Another problem currently experienced is probe electrical signal failures. While the cause of electrical failures can be ambiguous, controlling the amount of flexure of the probe is expected to decrease stress in the probe wiring and reduce electrical failures. 
     An additional difficulty currently encountered during eddy current inspection is in regard to inserting the probe into the tube. Typically, an eddy current probe is pushed into the tube by mechanical means such as wheels or belts which engage the probe&#39;s flexible cable. Probe friction with the tube, gravity, and rubbing of the flexible cable attached to the probe induce forces that oppose insertion of the probe. The flexible cable attached to the probe tends to buckle and the side loads imparted on the tube further increase the friction force opposing insertion. In many cases, the friction due to buckling will continue to increase the buckling load and further increase friction until it is not possible to insert the probe regardless of the force applied. Since the probe position is measured externally by encoding the flexible cable displacement, buckling of the flexible cable also causes loss of probe position accuracy. 
     As previously described, there are a number of problem areas with current eddy current inspection techniques. It is an object of this invention to provide a single eddy current probe to access the entire steam generator tubing including the smallest radius U-bends. 
     It is a further object of this invention to provide such a probe that has wheels to reduce friction in all areas of the tube including the point of insertion, diameter transitions, dents and bends. 
     It is an additional object of this invention to provide such a probe that will keep the sensor centered through diameter transitions, bends and other anomalies in the tube. 
     It is a further object of this invention to provide such an inspection probe that limits internal wire bending to enhance probe life. 
     It is an additional object of this invention to provide such a probe that has a dynamic seal that will enable an insertion force at the probe and move the probe along while maintaining it centered. 
     Additionally, it is an object of this invention to provide such a probe that has enhanced axial position accuracy. 
     SUMMARY 
     These and other objects are achieved by an elongated nondestructive sensor inspection probe having a central axis running along the elongated dimension of the probe, for nondestructively examining the walls of tubing. The inspection probe includes a nose section having at least three sets of rollers substantially equidistantly spaced around a circumference of the nose section, the sets of rollers being biased in a radially outward direction to contact an interior wall of the tubing with a substantially equally applied force biasing each of the sets of rollers. A nondestructive sensor section is suspended at one axial end from the nose section and coupled to the nose section with a pivot coupling that enables the nondestructive sensor section a limited degree of rotation relative to the nose section. A tail section is coupled to the nondestructive sensor section at another axial end with a pivot coupling that enables the nondestructive sensor section a limited degree of rotation relative to the tail section. The tail section has a centering device biased radially outward from a central body of the tail section. The centering device has a plurality of contact points with the interior wall of the tubing with each of the contact points being biased outward around the circumference of the tail section, with substantially equal pressure. 
     In one embodiment, the nose section includes a plunger reciprocally moveable in the axial direction, a cam coupled between each of the rollers and the plunger and means for biasing the plunger in one direction that rotates each cam to bias the rollers radially outward with substantially equal force. Preferably, the plunger is coaxially supported within the nose section and the plunger is spring biased. The tail section may similarly include a plunger reciprocally moveable in the axial direction, a cam coupled between each of the contact points and the plunger and means for biasing the plunger in one direction that rotates each cam to bias the contacts radially outward with substantially equal force. In one embodiment, the plunger in the tail section includes an axial passage through which a signal cable extends from the nondestructive sensor section to a rear of the tail section. In the latter embodiment, preferably the pivot coupling between the tail section and the nondestructive sensor section includes an axial passage through which the signal cable passes from the nondestructive sensor section to the tail section. 
     In another embodiment, the contact points are rollers that are substantially equidistantly spaced around a circumference of the tail section. In another embodiment, the contact points are sections of an annular circumferential seal that extend around and project outwardly from a central body of the tail section and are biased against the interior wall of the tubing. In the latter embodiment, preferably the sections of the annular circumferential seal overlap circumferentially to accommodate varying diameters of the tubing. In the latter embodiment, the sections of the annular circumferential seal may include a fluid path extending from the rear of the tail section to a radially facing interior surface of the sections of the annular circumferential seal so that a pressure buildup to the rear of the tail section forces the seal in a radially outward direction. 
     Generally, the nondestructive sensor section of the nondestructive sensor inspection probe of this invention is supported substantially equidistantly spaced around the circumference of the nondestructive sensor section from the pipe wall as the nondestructive sensor inspection probe traverses a bend in the pipe. Preferably, the equidistance spacing around the circumference of the nondestructive sensor section from the interior wall of the pipe does not vary substantially more than from +20% to −20% of the nominal spacing. 
     In still another embodiment, each set of rollers includes two rollers that move radially together. Additionally, in one given embodiment, a stop is provided for limiting the degree of rotation of the pivot coupling between the nondestructive sensor section and the tail section. Preferably, the stop limits rotation of the pivot coupling between the nondestructive sensor section and the tail section by preventing axial rotation of the tail section relative to the nondestructive sensor section. 
    
    
     
       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 a    is a plan view of an eddy current inspection sensor incorporating the principles of one embodiment of this invention; 
         FIG. 1 b    is a cross sectional view of the sensor shown in  FIG. 1 a    taken along the lines B-B thereof, positioned within a tube to be inspected; 
         FIG. 1 c    is a frontal view of  FIG. 1 a    taken along the lines A-A thereof; 
         FIGS. 2 a -2 e    are cross sectional views of the sensor embodiment illustrated in  FIG. 1 a    positioned in a pipe to be inspected starting with  FIG. 2 a    with the sensor positioned just at a U-bend with the sensor gradually moved around the U-bend as shown in  FIGS. 2 b   - 2   e;    
         FIG. 3  is a cross sectional view of a second embodiment of the probe of this invention; 
         FIG. 4 a    is a plan view of a third embodiment of this invention; 
         FIG. 4 b    is a cross sectional view of the probe shown in  FIG. 4 a    taken along the lines C-C thereof, positioned within a section of tubing to be inspected; and 
         FIG. 4 c    is a cross sectional view of the embodiments shown in  FIG. 4 a    taken along the lines D-D thereof. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1 a    shows a plan view of an eddy current probe assembly  1  that incorporates the principles of this invention though it should be appreciated that a probe assembly incorporating these principles can employ any one of a number of nondestructive sensors and should not be limited to just the use of an eddy current probe. The three main parts of the probe assembly  1  are the nose assembly  2 , coil assembly  3  and tail assembly  4 . Flexible cable  5  is used to translate the probe assembly  1  along the interior of the tube. 
       FIG. 1 b    shows a cross sectional view of the probe assembly  1  shown in  FIG. 1 a    with section B-B taken through the center of the probe assembly, positioned within a tube  6  to be inspected. While various coil configurations are used in eddy current probes, a typical bobbin probe configuration is shown with circumferential coils  7  along with permanent magnet  8  fixed by means of coil support  9 . Unique to this invention is the means to position the coils  7  relative to the tube  6  as the probe is translated along the axis of the tube. It is also highly desirable to maintain an equidistant radial gap (“c”) between the coils  7  and the tube  6 . 
     Maintaining the equidistant gap is a greater challenge as the probe translates through curved sections of tubing. The function of the tail assembly  4  and moreover the nose assembly  2  is to maintain the probe centered in the tube  6 . This is for both to provide a more accurate eddy current signal and to avoid contact resulting in friction and wear between the coil  7  and the tube  6 . 
     The coil assembly  3  is coupled to the nose assembly  2  and tail assembly  4  with pivot couplings with spherical ended shafts  10  and  11  which thread into the coil support  9 . The threaded retainers  12  provide the means to couple each spherical shaft to the corresponding nose body  13  or tail body  14 . A small amount of clearance between each spherical shaft and a corresponding spherical recess or socket in the retainer  12  and body  13 ,  14  permits spherical translation between the coil assembly  3  and the nose/tail assemblies  2 ,  4 . As will be shown later, the axial locations of the spherical centers provides probe centering as the probe translate through bends in the tubing. Pins  15  and  16  provide two functions. After retainer  12  are threaded in place, the pins prevent the retainers from unthreading. Pins  15  and  16  can also be used to provide a fixed radial orientation between the tail body  14  and the coil support  9  about the probe axis  17 . This is important as not to twist the wiring between cable  5  and coil  7 . As shown in the tail assembly  4 , pin  16  is partially engaged into a circular slot  18  in the spherical shaft  11  which maintains radial orientation between the tail assembly  4  and the coil assembly  3 . 
     To negotiate bends in the tubing  6 , the nose assembly  2 , coil assembly  3  and tail assembly  4  can rotate up to the angle  19 . Rotation beyond angle  19  is prohibited as contact is made between the coil support  9  and the tail body  14 . The angle  19  limits the bending stress applied to the wiring within the probe tail to coil joint. Present eddy current designs have flexible connections which may not prohibit excessive stress on the internal wiring leading to probe failure. Tapered opening  20  in the wiring exit from the pivot coupling to the tail section assembly  4  permits spherical shaft  11  to rotate without impinging on wires located within. 
     Since there is no wiring in the nose assembly  2 , the orientation function of pin  15  is not necessarily required but desire to maintain alignment of the nose and tail for the probe type shown. For some types of eddy current probes, it is desirable to have different configurations of coils  7  that rotate about probe axis  17  as the probe is translated along the tube axis. In this case, pin  15  would not be engaged in the spherical shaft  10 . Rotational compliance between spherical shaft  10  and nose body  13  permits rotation of coil assembly  3  relative to nose body  2  and tube  6 . 
     Both the nose assembly and tail assembly contain wheels  21  that contact and provide a rolling coupling between the probe assembly and the tube  6 .  FIG. 1 c    shows a front view A-A of the nose assembly  2  with at least three sets of wheels  21  needed to provide centering of the probe. As shown in  FIG. 1 b   , the wheel arrangement is very similar in the nose assembly  2  and tail assembly  4 . Referring to the nose assembly  2  of the probe, wheels  21  rotate about axles  22  which are fixed to cams  23 . Wheel pairs are separated by distance “X”. The cams  23  are free to rotate about pins  24  that are fixed to the nose body  13 . Each cam  23  is engaged into opening  47  of plunger  25 . Plunger  25  is free to slide co-linearly along the axis of the nose body  13  and is biased to the right by means of compression spring  26 . The reaction force of spring  26  is to the cap  27  then through spring pins  28  back to the nose body  13 . As can be seen, the radial motion of each wheel toward the tube  6  is closely coupled by means of cams  23  and plunger  25 . All six wheels travel the same radial distance outward with the same pressure. This feature maintains the nose body  13  equidistance from the tube  6  inside surface and wheel separation (“X”) forces the axis of the nose body co-linear with the axis of the tube. When the probe is outside the tube, it is desirable to limit the maximum outward radial travel of the wheels. The travel is limited by the gap  29  between the plunger  25  and the nose body  13 . The configuration is very compact as the inside diameter of tubing can be quite small. 
       FIGS. 2 a  through 2 e    show the centering ability of the eddy current probe  1 . The tube  6  in the figures is typically the smallest outside diameter of 11/16 th  inch (1.75 cms.) and the smallest radius of 2.2 inch (5.59 cm.) used in nuclear steam generator U-bends.  FIG. 2 a    shows the probe just prior to entering the tube end. The wheels maintain a symmetrical radial gap “C” between the tube and the probe. The probe geometry permits free passage through the U-bend.  FIGS. 2 b  and 2 d    show the positions where the probe coils are at the closest radial gap (0.8 c) from the tube surface. While the probe coils are not exactly coincident with the tube center line when passing through sections of the U-bend, this small offset is well within the limits of the probe and an improvement over existing probes. Typically, for other probes used in the industry, the coil diameter must be reduced to permit passage of the probe through the smaller radius U-bends. This reduction in probe diameter decreases the resolution and performance of the probe. 
       FIG. 3  is an optional configuration of the probe with one wheel to each set in the tail. This configuration provides more space for securing the flexible cable  5  with some decrease in the centering ability of the coils. 
       FIGS. 4 a , 4 b  and 4 c    show an eddy current probe that uses compressed air to aid insertion and reduce or eliminate flexible cable buckling. Nose assembly  2  and coil assembly  3  are the same as used in  FIG. 1 . The difference is the tail assembly  30 . Similar to the wheels used in the nose, there are at least three seal pads  31  that engage sliding contact with the side of the tube  6 . Each seal pad is coupled to a cam  32  with axle  33  and is free to pivot on the axis of the axle. The cams are coupled to the tail body  34  and rotate about pins  35 . Each cam  32  is engaged into opening  36  of plunger  37 . Plunger  37  is free to slide co-linearly along the axis of the tail body  34  and is biased to the left by means of compression spring  38 . The reaction force of spring  38  is the sleeve  39  then through spring pins  40  secured to the tail body  34 . As can be seen, the radial motion of each seal pad towards the tube  6  is closely coupled by means of cams  32  and plunger  37 . Like the wheels in the nose assembly  2 , all seal pads  31  travel the same radial distance outward, applying the same pressure keeping the probe  1  centered in the tube  6 . 
     Another advantage is that for varying tube inside diameters, a near constant radial outward force is achieved with the cam/plunger/spring geometry. A relatively larger amount of radial seal pad travel results from only a small amount of plunger travel. Locating the spring axially along the axis of the probe provides more space allowing a relatively small spring displacement per spring length. 
     While the seal pads  31  keep the probe centered in the tube, the main function is to provide a motive force along the tube axis during probe insertion. A compressed fluid such as air  41  is injected between the tube  6  and the probe tail section  4 . Since the seal pads  31  provide a pressure boundary between the probe and the tube, the higher pressure fluid exerts an insertion force (to the left) on the probe. In order to limit leakage past the seal pads, the outward force of seal pads  31  against the inside diameter of the tube  6  increases with increasing fluid pressure. Shown in  FIGS. 4 a , 4 b  and 4 c    are openings  42  which permit compressed air to act on the inner pad radial surface  43 . Since the outer pad radial surface  44  is at a lower pressure (open tube) than the compressed air, there is a net outward radial force that is directly proportional to the compressed air pressure. Seal pads  31  will be forced radially outward and move along slidable interface  45 . This system creates a dynamic seal. During translation of the probe into the tube, resistance to probe motion will give rise to the air pressure and hence increase seal outward radial force. When the probe is withdrawn from the tube, the applied air pressure can be minimized. At the low air pressure, seal friction and wear will predominantly be from the applied force of the spring  38 . 
     It should also be realized that the overlap  46  between seal pads provides the ability for the seal to function in varying tube inside diameters. While the dynamic seal is shown as part of the probe design with its centering features, the dynamic seal can be used as a standalone addition to enhance the mobility of existing eddy current probe designs. In addition, as previously mentioned, while the probe was shown as a vehicle for translating an eddy current sensor, it can also be used to transport other nondestructive sensors such as cameras, ultrasonic probes, etc. 
     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.