Abstract:
A method and system for inspecting a turbine wheel having axial slots along a perimeter thereof that are configured for mating with and securing airfoil members to the perimeter of the wheel, and an annular slot that intersects the axial slots. The method and system make use of one or more eddy current probes that are placed in the annular slot to electromagnetically inspect the annular slot and at least one of the axial slots for cracks in surfaces thereof. The probe is part of a probe assembly that includes a mounting member for engaging at least one of the annular and axial slots. The probe assembly is operable to maintain the probe at a fixed distance from surfaces of the annular slot as the probe travels through the annular slot.

Description:
BACKGROUND OF THE INVENTION 
   The present invention generally relates to nondestructive inspection methods and systems. More particularly, this invention relates to a method and system for scanning a turbine wheel with an eddy current probe, and particularly surface regions of slots within the wheel. 
   Various nondestructive examination (NDE) techniques have been used to perform nondestructive testing on articles. An example is eddy current probe inspection of turbine components, as disclosed in commonly-assigned U.S. Pat. Nos. 4,706,020, 6,426,622, and 6,545,467, whose disclosures pertaining to the construction, operation, and use of eddy current probes are incorporated herein by reference. A component of particular interest is industrial gas turbine wheels to which the buckets of the turbine are mounted. In the hostile operating environments of gas turbines, the structural integrity of the turbine wheels within its turbine section is of great importance in view of the high mechanical stresses that wheels must be able to continuously withstand at high temperatures. The regions of a wheel forming the slots into which the buckets are secured, typically in the form of what are known as dovetail slots, are known to eventually form cracks over time, necessitating monitoring of the wheel in these regions. In some wheel designs, such as the stages 1, 2, and 3 wheels of the General Electric 7FA gas turbine, cooling of the buckets and wheel perimeter is assisted by the presence of a cooling slot located near the perimeter of the wheel and into which the dovetail slots extend. Over extended periods of time under the severe operating conditions of a wheel, cracks may form at common edges formed where the dovetail slots and cooling slot intersect. The ability to detect cracks with lengths of as little as 60 mils (about 1.5 mm) and even less is desirable in order to provide sufficiently early detection to avoid catastrophic failure of turbine wheels. 
   While a turbine rotor can be completely disassembled to gain access to its individual wheels, inspection techniques that can be performed with limited disassembly are preferred to minimize downtime, such as to fit within outage schedules of a gas turbine employed in the power generating industry. Since buckets are typically removed for inspection, it would be preferred if the dovetail and cooling slots of a turbine wheel could be examined with only the buckets removed. However, access to the cooling slot is very limited, and any inspection technique using an eddy current probe must address the difficulty of bringing the probe into stable, near-proximity to the surfaces being tested. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a method and system for inspecting a turbine wheels having axial slots along a perimeter thereof that are configured for mating with and securing airfoil members to the perimeter of the wheel, and an annular slot that intersects the axial slots. The method and system make use of at least one eddy current probe that is sized and configured to be placed in the annular slot to electromagnetically inspect the annular slot and the axial slots for cracks in surfaces thereof. 
   The inspection system of this invention generally includes a probe assembly having an eddy current probe that is sized and configured to be received in the annular slot. The probe assembly further includes a mounting member having means for engaging at least one of the annular and axial slots to maintain the eddy current probe at a fixed distance from surfaces of the annular slot when the eddy current probe is caused to travel through the annular slot. The inspection further includes means for causing the eddy current probe to travel through the annular slot in a circumferential direction of the turbine wheel so as to enable the eddy current probe to electromagnetically inspect the annular slot and at least one of the axial slots. The method of this invention generally includes placing into the annular slot an eddy current probe configured as described above. Once in place, the eddy current probe is caused to travel through the annular slot in a circumferential direction of the turbine wheel to electromagnetically inspect the annular slot. 
   The eddy current probe can be individually inserted into the annular slot to perform an inspection on the immediate surfaces of the annular slot, or one of multiple eddy current probes interconnected together as a continuous unit that travels through the annular slot to perform inspections on surfaces of the annular slot. With any of the embodiments, it can be seen that the present invention is able to provide an inspection process for turbine wheels having dovetail slots that intersect an annular cooling slot, and in particular for inspecting the surfaces of the cooling slot. According to a preferred aspect of the invention, the eddy current probe is sized and configured to be placed in the cooling slot through one of the dovetail slots, such that the inspection process can be performed by removing the buckets from the dovetail slots to gain access to the cooling slot, without necessitating further disassembly of the wheel. 
   Other objects and advantages of this invention will be better appreciated from the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an eddy current probe assembly in accordance with a first embodiment of this invention. 
       FIGS. 2 and 3  are two perspective views from different angles of the probe assembly of  FIG. 1  placed in a cooling slot of a turbine wheel for inspection of dovetails slots within the vicinity of the cooling slot in accordance with the first embodiment of this invention. 
       FIG. 4  is an axial view of the turbine wheel showing an apparatus for moving the probe of  FIG. 1  through the cooling slot in accordance with the first embodiment of this invention. 
       FIGS. 5 and 6  are side views of an eddy current probe in accordance with a second embodiment of this invention. 
       FIG. 7  is an axial view of a portion of a turbine wheel showing the probe of  FIGS. 5 and 6  placed in a cooling slot of the turbine wheel in accordance with the second embodiment of this invention. 
       FIG. 8  is a perspective view of an eddy current probe assembly in accordance with a third embodiment of this invention. 
       FIG. 9  is a plan view of a support block of the probe assembly of  FIG. 8 . 
       FIG. 10  is an axial view of a portion of a turbine wheel showing the probe assembly of  FIG. 8  placed in a cooling slot of the turbine wheel in accordance with the third embodiment of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  depicts an eddy current probe assembly  10  configured for performing a nondestructive examination (NDE) of a turbine wheel  12  as depicted in  FIGS. 2 and 3  in accordance with a first embodiment of the present invention. The probe assembly  10  comprises a probe  14  to which two flexible cables  16  are attached at opposite longitudinal ends of the probe  14 . The cables  16  must be sufficiently strong to be used to pull the probe  14  through an annular cooling slot  18  formed in an axial face of the turbine wheel  12 , as represented in  FIGS. 2 and 3 . For this purpose, each cable  16  may be a single strand of wire, braided wires, or an electrical cable, the latter of which would also serve to electrically connect the probe  14  to appropriate instrumentation (not shown), which typically includes an electrical (e.g., Wheatstone) bridge. The cooling slot  18  is radially inward from radially inward-extending dovetail slots  20  that are circumferentially spaced along the perimeter of the wheel  12 , as is conventional for turbine wheels of industrial gas turbines. As is also conventional, the geometry of each dovetail slot  20  comprises contoured surfaces with facing lobes, and the plane of symmetry of each dovetail slot  20  is not parallel to the axis of the wheel  12 , but is offset by some acute angle from the wheel axis. The particular configuration represented in the Figures is that of the General Electric 7FA stage 1 wheel, though other wheel configurations are within the scope of this invention. The dovetail slots  20  intersect the cooling slot  18  at their radially inward extremities, such that cooling flow through the cooling slot  18  conducts heat away from buckets (not shown) secured in the dovetail slots  20 . 
   As is evident from  FIG. 2 , any tension applied to the cables  16  in the circumferential direction through the cooling slot  18  will produce a force component in a radially inward direction toward the axis of the wheel  12 , forcing the probe  14  out of the slot  18  through its radially inboard-facing slot opening  22 . Accordingly, the probe assembly  10  requires a structure capable of supporting the probe  14  within the cooling slot  18 . As depicted in  FIG. 2 , such a structure may be in the form of a panel  52  (a limited portion of which is shown in  FIG. 2 ) configured to at least close a sufficient portion of the slot opening  22  to support the probe  14  within the slot  18 . In a preferred embodiment, the panel  52  has an annular shape that is sized to fit snugly against a lip  26  that defines one half of the slot opening  22  and an axially-outboard wall  28  of the cooling slot  18 . A variety of materials could be used for the panel  52 , such as a stainless steel about 0.005 to 0.010 inch (about 127 to 254 micrometers) in thickness. 
   As shown in  FIGS. 1 and 2 , the cables  16  can be attached to the ends of the probe  14  with pins  34  mounted between devises defined at the ends of the probe  14 . In addition to pulling the probe  14  through the cooling slot,  18 , the opposing tensions applied to the probe  14  through the cables  16  serve to stabilize and hold the probe  14  in place within the cooling slot  18 . To promote stability, the probe  14  is preferably sized relative to the cooling slot  18  so that the side walls  28  and  30  of the cooling slot  18  maintain the orientation of the probe  14  within the slot  18 , while the panel  52  positions the probe  14  relative to the radially-outward wall  32  of the slot  18 . In this manner, the tension of the cables  16  and the close fit between the probe  14  and cooling slot  18  promote controlled motion of the probe  14  when it is moved with the cables  18  during scanning. 
   The body  36  of the probe  14  can be formed of a durable plastic or metal, preferred materials being those that will not scratch or mar the walls  28 ,  30 , and  32  of the cooling slot  18 . Suitable sizes for the probe  14  will depend in part on the cross-sectional dimensions and radius of curvature of the slot  18 . For use with the General Electric 7FA stage 1 wheel, a suitable length for the probe  14  is about 1.1 cm in length. The probe body  36  contains cavities  38  that contain test coils (not shown) of any type suitable for use in eddy current scanning, such as ferrite-shielded probe coils available from Staveley NDT Technologies and having coil diameters of about 0.110 inch (about 2.8 mm). As noted above, electrical connection to the test coils can be made through one or both of the cables  16 . In the configuration shown in  FIG. 1 , the test coils are arranged in an staggered two-dimensional array. The test coils can be operated in any suitable manner, such as being pulsed simultaneously or multiplexed to simulate movement in the circumferential direction through the cooling slot  18 . By appropriately orienting the test coils within the probe body  36 , the probe  14  can be used to scan all three walls  28 ,  30 , and  32  of the cooling slot  18 , as is evident from  FIG. 3 . The embodiment of  FIGS. 1 through 4  preferably employs reference coils, which are preferably located within the probe  14  though it is foreseeable that reference coils could be located externally in a junction box (not shown). 
   Because the electrical output signal of an eddy current probe is maximized by maintaining contact between the probe and the surface being scanned (thereby minimizing lift-off noise), the probe  14  is preferably biased into contact with the outward wall  32  of the cooling slot  18 . Another benefit of maintaining contact between the probe  14  and outward wall  32  is enhanced stability of the probe  14 , which reduces probe wobble noise in the output signal of the probe  14 . For maintaining contact,  FIG. 1  depicts the probe  14  as having a pair of pins  40  that are biased outwardly from the probe body  36  with springs (not shown) or other suitable biasing means. The pins  40  are shown as facing the panel  52 , such that sliding contact occurs between the pins  40  and a surface  54  the panel  52  facing the cooling slot  18 . To minimize friction between the pins  40  and panel surface  54 , the pins  40  may be formed of silicone or a composite containing graphite in a thermoset matrix, though a variety of other materials could be used as the pin material. The surface  42  of the probe body  36  opposite the pins  40  is urged into surface-to-surface contact with the outward wall  32  of the cooling slot  18 . For this reason, the contact surface  42  preferably has a radius of curvature approximately equal to that of the slot outward wall  32 . 
   With the embodiment of  FIGS. 1 through 3 , the cables  16  can be used to pull one or more probes  14  through the entire circumference of the cooling slot  18 . For example, the probe  14  can be coupled to a drive motor (not shown) with the cables  16 , which can be routed through two different dovetail slots  20  with support and mounting blocks  56  and  58  as depicted in  FIG. 4 . The mounting block  58  supports a shaft  60  that extends through the support block  56  and terminates with a pulley  62 , whose position within the cooling slot  18  can be adjusted with the shaft  60  to obtain to the desired radial position of probe  14 . One probe cable  16  would be used to pull the probe  14 , while the other is used to maintain tension in the cables  16  through a second motor against which the drive motor works. Using an electronic control system common in motion control systems, e.g., available from the Compumotor Division of Parker Hannifin, Inc., the tension in the cables  16  and the speed of the probe  14  through the cooling slot  18  can be accurately controlled. By monitoring the positions of the cables  16 , the location of the probe  14  within the cooling slot  18  can be measured, recorded and monitored so that any cracks detected can be related to a position in the cooling slot  18 . For this purpose, a computer system (not shown) can be used to remotely control the motion of the probe  14  and record the position and eddy current signals of the probe  14 . A preferred system includes analysis software for control, operation, and analysis of the eddy current data, along with suitable displays. The probe operation may be singly or multiplexed for array operation, or in a subset of these configurations. Multi-channel parallel operation is also possible. It is also foreseeable that a single probe  14  or multiple interconnected probes  14  could be configured for fully remote operation by incorporating a battery pack, motorized friction drive, eddy current test coils, eddy current reference coils, instrumentation and on-board controls, and a wireless interface for communication with a remote control unit. 
   In a second embodiment of the invention depicted in  FIGS. 5 ,  6 , and  7 , a probe assembly  110  is provided that does not require the panel  52  and omits the cables  16  of the first embodiment, while making possible an inspection process that can be remotely controlled. Instead of cables, the probe assembly  110  makes use of multiple pivotally interconnected probe segments  114  as shown in  FIG. 7 , one of which is shown in  FIGS. 5 and 6 . Similar to the probe  14  of  FIGS. 1 through 3 , the outer surface  142  of each segment  114  is curved to match the curvature of the cooling slot  18 . Also similar, the probe segments  114  are equipped with test coils (not shown) contained in a recess  138  in a sidewall of the segment  114 . As with the embodiment of  FIGS. 1 through 4 , the second embodiment preferably employs reference coils that may be located within the probe  114  or located externally. A pair of pads  140  project from the opposite sidewall of each segment  114  to assist with axial alignment of the segments  114  within the cooling slot  18 . 
   The length, width, and shape of each probe segment  114  is selected to enable the segments  114  to be placed in the cooling slot  18  through the bucket dovetail slot  20 . Adjacent probe segments  114  can be connected end-to-end using pins or clips  134 . Any number of probe segments  114  can be connected together in this manner. For example, a sufficient number of probe segments  114  can be assembled to encompass the entire 360 degrees of the cooling slot  18 . In a probe assembly  110  made up of multiple probe segments  114 , test coils can be omitted from some of the segments  114 , such that these inactive segments  114  serve only to promote the mechanical stability of the assembly  110 . However, it is within the scope of the invention that all segments  114  can be equipped with test coils and thereby operate as active eddy current probes. The interconnected segments  114  can be inserted into the cooling slot  18  through one of the dovetail slots  20 . The first segment  114  can be grasped with a standard three-prong flexible gripper inserted through an adjacent dovetail slot  20  to pull the probe assembly  110  through the cooling slot  18 . A flexible cable (not shown) may also be attached to the probe assembly  10  to assist with positioning the assembly  110  in the cooling slot  18 . The cable may also serve as the conduit for electrical connection for the assembly  110  to appropriate instrumentation (not shown). 
   To maintain each probe segment  114  in contact with the outward wall  32  of the cooling slot  18 , a radially-outward mechanical force is applied to the probe segments  114  with a support shaft  144 . A bearing support  156  is shown mounted at the lower end of the shaft  144  and oriented to extend under that portion of the probe segment  114  exposed within the dovetail slot  20 . The space between the pads  140  of the probe segment  114  provides a gap through which the shaft  144  is able to pass to the radially-inward surface of the segment  114 . The shaft  144  is held in a radially-outward direction through the combination of a spring  148  and nut  150  threaded onto the shaft  144 . The spring  148  is biased against a support block  152  whose shaped profile  154  is complementary to at least a portion of the dovetail slot geometry  24 , such that the support block  152  can be axially inserted into the dovetail slot  20  similar to when installing a bucket in the dovetail slot  20 . The spring-loaded shaft  144  and support block  152  ensure that the probe segment  114  remains in contact with the outward wall  32  of the slot  18 , while the support  156  supports the probe segment  114  in a manner that allows the segment  114  to move in the circumferential direction of the slot  18 . By spring loading the shaft  144 , the operator&#39;s hands are free to scan the probe assembly  110  through the cooling slot  18  while operating the instrumentation for the eddy current probe assembly  110 . 
   As evident from the above, the embodiment of  FIGS. 5 through 7  can be operated to scan a region of the cooling slot  18  in the immediate vicinity of the dovetail slot  20  from which the probe assembly  110  is supported. Instead of a bearing affixed to the shaft  144 , bearings could be provided on the probe segment  114  to reduce sliding friction between the probe segment  114 , support  156 , and surface being examined. Motorized remote control of the scanning operation can be achieved using essentially the same approach as that described for the embodiment of  FIGS. 1 through 4 . 
   As an alternative to the spring-loaded shaft  144  of the preceding embodiment, one or more of the segments  114  may be equipped with a spring or other biasing element (not shown) to apply a circumferential bias, so that a multi-segmented probe assembly  110  can be assembled to form a complete ring in which the circumferential bias generated by the one or more segments  114  causes the entire assembly  110  to radially expand outward into contact with the outward surface  32  of the cooling slot  18 . As an alternative, a linear actuator could be used in place of one or more of the segments  114 . Because the shaft  144  extends beneath the probe segment  114  through a gap created between the segment  114  and one of the walls of the cooling slot  18  that by the pads  140  on the segment  114  ( FIG. 7 ), the pads  140  will prevent full circumferential movement of a multi-segmented probe assembly  110  assembled as a complete ring. Therefore, in order to achieve greater movement, the pads  140  would be removed as needed during the scanning of the cooling slot  18 , or entirely eliminated. Another alternative is to replace the pads with a turnstile sprocket (not shown) that provides support and can allow the shaft  144  to push past the sprocket. 
     FIGS. 8 ,  9 , and  10  depict a simpler probe assembly  210  that permits the examination of the cooling slot  18  and the immediately surrounding surface regions of a dovetail slot  20  in which the probe assembly  210  is placed. The probe assembly  210  includes a probe  214  mounted to one end of a shaft  244 . Test coils are housed within cavities  238  formed in the probe  214  and wired to a connector block  240  mounted to the opposite end of the shaft  244 . An adjustment ring  242  is threaded onto the shaft  244  and enables the position of the probe  214  to be radially adjusted relative to the dovetail slot  20 , in which the assembly  210  is held by a support block  252 . Similar to the support block  152  of the second embodiment, the support block  252  has a profile  254  that is complementary to a portion of the dovetail slot geometry  24 , such that the support block  252  can be axially inserted into the dovetail slot  20 . As more readily seen from  FIG. 9 , which shows the support block  252  separate from the remainder of the probe assembly  210 , the support block  252  is assembled onto the shaft  244  by sliding the shaft  244  through a slot  234  defined at one end of the support block  252 . The slot  234  has a generally T-shaped profile when viewed in a direction parallel to the shaft  244 , such that once placed in the slot  234  the shaft  244  is able to travel in a transverse camming portion  256  of the slot  234 . As evident from  FIG. 10 , the support block  252  anchors the entire probe assembly  210  and establishes the positional reference when controlling the position of the probe  214  relative to the cooling and dovetail slots  18  and  20 . 
   A slide block  236  is mounted between the adjustment ring  242  and support block  252 , and is secured to the support block  252  with the ring  242 . The slide block  236  cams against a shoulder  258  on the upper surface of the support block  252 . The shoulder  258  is disposed at the same angle to the axial direction as the camming portion  256  of the slot  234 , such that sliding movement of the slide block  236  and the shaft  244  occur in the same direction. This movement of the slide block  236  and shaft  244  allows limited bidirectional scanning of the probe  214  through the cooling slot  18 . The angular offset of the camming portion  256  and shoulder  258  is intended to accommodate the angle of the dovetail slots  20  relative to the axis of the wheel  12 . In the General Electric 7FA stage 1 wheel, this angle is about 74.5 degrees to the wheel axis. 
   From  FIGS. 8 and 10 , it can be seen that the uppermost cavity  238  housing a test coil is disposed at an angle of about forty-five degrees relative to the other cavities  238 , enabling its test coil to inspect the cooling slot fillet closest to the radial plane of the wheel  12 . The connector block  240  is equipped with a staggered array of cavities  246  in which reference coils can be placed for the eddy current system. The reference coils are preferably operated as an array of eddy current sensors, as is conventional for a differential probe system. Another option is to place the reference coils near the instrument end of the electrical cable bundle (not shown) of the probe assembly  210 , which has the advantage of reducing the size of the cable bundle at its point of attachment to the probe assembly  210 . Once the reference coils are mounted in their desired location, the performance of the array of reference coils can be adjusted by covering the coils with a sheet formed of the same alloy from which the wheel  12  is formed. Ideally, the reference coils are recessed into their cavities  258  so as to be spaced apart from the sheet a distance equal to the mean gap that will exist between the test coils of the probe  214  and the surfaced  28 ,  30 , and  32  they are scanning. 
   While the invention has been described in terms of particular embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configurations of the probe assemblies  10 ,  110 , and  210  and of the wheel  12  being inspected could differ from that shown. Therefore, the scope of the invention is to be limited only by the following claims.