Patent Number: 055685277
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is described herein as applied to the examination of the circumferential weld which attaches a core spray T-box to a thermal sleeve inside the core spray nozzle and the circumferential weld which attaches the cover plate to the T-box. However, an artisan of ordinary skill in the art of nondestructive examination will readily appreciate that the method and apparatus of the invention are generally applicable to the detection of cracks in any piping system having a geometry similar to that of the core spray T-box assembly. To accommodate inspection of the core spray T-box attachment welds, it is necessary that reactor steam separator and steam dryer be temporarily removed during outages. A T-box attachment weld inspection tool 40 is then lowered through the reactor water from the refueling bridge 42 (see FIG. 1) and clamped on the T-box arms 38 (see FIG. 4). Referring to FIG. 4, the T-box weld inspection tool 40 is clamped onto the T-box arms 38 by a pair of clamping mechanism assemblies 50. Each clamping mechanism 50 comprises a strut 52, a clamp insert 54 and a lower jaw 56. The lower jaw 56 is pivotable relative to strut 52 via a jaw pivot plate 58 and is coupled to an air-actuated cylinder 60 via a jaw pin 62 and a jaw clevis 64. The cylinder 60 is mounted to strut 52 via a cylinder clevis 66. In response to activation of each cylinder 60 by a solenoid valve 61 (see FIG. 8), each lower jaw 56 is rotated into abutment with a corresponding T-box arm 38, thereby clamping the tool on the T-box arms. Each clamping mechanism assembly 50 is connected to and supports a fixed plate 72 of a rotation stage 70. The rotation stage 70 in turn rotatably supports the side slide stage 130 and the radial face stage 100,, as described in detail below. The clamping mechanisms are positioned so that the axis of rotation of the rotation stage is coaxial with the centerline axis of the thermal sleeve. Referring to FIG. 5, the fixed plate 72 has a central opening in which an inner flange 74 is installed. Inner flange 74 has an offset bore for receiving the stationary part of a duplex bearing 76, which is held in place by a bearing retaining plate 78. One end of a center spacer 80 is mounted to the rotatable part of the bearing 76 such that the center spacer is rotatable relative to the fixed plate 72. As described in detail hereinafter with reference to FIG. 6, a rotary shaft connected to the radial face stage is attached by way of a rotation nut 105 (see FIG. 4) to center spacer 80 for rotation therewith. Referring to FIG. 5, a radial face stage gear 82 is mounted on the outer circumference of center spacer 80 and is threadably coupled to a worm gear 84, which is driven to rotate by a drive motor 86 (shown in FIG. 8) mounted on fixed plate 72. In response to rotation of worm gear 84, the gear 82 and radial face stage mounted thereon are rotated about the centerline axis of the thermal sleeve. The other end of a center spacer 80 is mounted to a first part of a duplex bearing 88. An outer flange 90, mounted to a second part of duplex bearing 88, is installed in a central opening of a side slide mounting plate 92, on which the side slide stage is mounted. The first and second parts of duplex bearing 88 are rotatable relative to each other. Therefore outer flange 90 and the side slide stage coupled thereto are rotatable relative to the center spacer 80. A side slide stage gear 94 is attached to outer flange 90 and is threadably coupled to a worm gear 96, which is driven to rotate by a drive motor 98 (shown in FIG. 8) mounted on fixed plate 72. In response to rotation of worm gear 96, the gear 94 and side slide stage connected thereto are rotated about the centerline axis of the thermal sleeve. Referring to FIG. 6, the radial face stage 100 comprises a circular face stage plate 102 mounted on a rotation shaft 104. Rotation shaft 104, as previously mentioned, is seated inside and secured to the center spacer 80 (see FIG. 5). In response to rotation of center spacer 80, face stage plate 102 is rotated. Face stage plate 102 has a linear slide assembly mounted thereon. The linear slide assembly comprises a face stage slider 106 which slides along a pair of parallel linear shafts 108 (only one of which is visible in FIG. 7). The linear shafts 108 are connected to and supported by the face stage plate 102. The slider 106 is rigidly linked to a threaded coupling 110 which engages the threads of a lead screw 112. In response to rotation of lead screw 112, slider 106 slides along linear shafts 108. Conventional bearing means are provided. The lead screw 112 is driven to rotate by a drive motor 114 (depicted in FIG. 8) which is mounted on the face stage plate 102 and coupled to the lead screw by way of timing pulleys and a timing belt 116. One of the timing pulleys designated 118 is visible in FIG. 6. An angle bracket 120 is attached at one end to the slider 106. The other end of bracket 120 has a transducer mount 122 attached thereto. The transducer mount 122 carries a transducer pack 124 (shown in FIG. 8). These transducers are used to interrogate the core spray T-box to cover plate attachment weld 36 (see FIG. 3). The transducer mount 122 travels along a diameter of the face stage plate 102 when the lead screw 112 is rotated. Thus, the transducers can be moved both circumferentially (in response to rotation of the face stage plate 102) and radially (in response to rotation of lead screw 112). This allows the transducers in pack 124 to be optimally position to interrogate crevice weld 36 (see ultrasound beam path D in FIG. 3). In accordance with a further aspect of the preferred embodiment, a pair of diametrally opposed air-actuated cylinders 126 are mounted on the face stage plate 102. Cylinders 126, when actuated by a solenoid valve 127, will drive respective pistons into abutment with the opposing face of the front cover plate 30b (see FIG. 2). Since the pistons exert equal and opposite forces, they can be used to align the radial face stage 100 relative to the front cover plate 30b prior to clamping the inspection tool onto the T-box arms. Referring to FIG. 7, the side slide stage 130 comprises a slide support plate 132 which is attached to the side slide mounting plate 92 (see FIG. 5). By this attachment, the side slide stage 130 is rotatable about the axis of center spacer 80, which will be coaxial with the centerline axis of the thermal sleeve 32 (see FIG. 2) when the tool is properly installed. The side slide stage 130 further comprises a transducer pack 134 (depicted in FIG. 8) which is mounted on a side slide transducer mount 136. The transducer mount 136 is slidable in parallel with the axis of the thermal sleeve in response to activation of a drive motor 138, as explained in detail below. The drive motor 138 is mounted on slide support plate 132 by way of a motor support bracket 140. Drive motor 138 drives rotation of a drive shaft 142 which is supported at two points along its length by ball bearings 144a and 144b, which are mounted in respective support plates 146a and 146b rigidly connected to slide support plate 132. The drive motor 138 is coupled to the drive shaft 142 via a timing belt mounted on a pair of timing pulleys. One of the timing pulleys 148, visible in FIG. 7, is mounted on the drive shaft 142. The distal end of drive shaft 142 has a spur gear 150 mounted thereon. Spur gear 150 has teeth which engage the teeth of a gear rack 152 which forms part of an assembly. The slidable assembly further comprises a pair of linear shafts 154, only one of which is visible in FIG. 7. The linear shafts 154 and the gear rack 152 are supported at opposite ends by a shaft retainer block 156 and a rack support bar 158. Shaft retainer bar 156 is connected to an inspection slide plate 157, on which the transducer mount 136 is attached. Bars 156 and 158 also support a cable strap plate 160, which is provided with straps 162 for holding a plurality of coaxial cables 164. The coaxial cables 164 are connected to the transducers carried on transducer mount 136. The entire assembly comprising rack 152, linear shafts 154, bars 156 and 158, cable strap plate 160, inspection slide plate 157 and transducer mount 136 is slidable in a slide support block 166. The slide support block 156 houses four ball bushing bearings 168 separated by a spacer 170. Each pair of ball bushing bearings 168 slidably supports a respective one of the linear shafts 154. Slide support block 166 further houses a pair of ball bushing bearings 172 separated by a spacer 174, which facilitate sliding of rack 152 relative to slide support block 166. In response to actuation of drive motor 138, the spur gear 150, which engages the teeth of rack 152, is rotated, causing the rack to translate relative to slide support block 166. As a result of displacement of the rack 152, the entire slidable assembly can be translated relative to the T-box. In particular, the transducer mount 136 can be translated between a position whereat its transducers interrogate crevice weld 36 (see ultrasound beam paths A and B in FIG. 3) to a position whereat its transducers interrogate crevice weld 34 (see ultrasound beam path C in FIG. 3). The electrical and pneumatic connections for the T-box inspection tool in accordance with the preferred embodiment are schematically depicted in FIG. 8. Each drive motor (86, 98, 114, 138) has a resolver (176, 178, 180, 182) coupled thereto. Each resolver encodes the position of the respective motor drive shaft and sends coded position signals back to the central computer system 184 (see FIG. 1). The motors and encoders are connected to the central computer via twisted pairs of 24-gauge wire. The ultrasonic transducers are connected to the central computer via individual coaxial cables. The air-actuated cylinders are connectable to an air compressor via 1/8-inch ID air hoses. For each degree of freedom, a pair of limit switches 186 are provided. These limit switches are preferably microswitches having a contact arm which is rotated to a closed position when the respective transducer reaches its limit of allowable rotation or translation. The corresponding motor is deactivated in response to closure of each limit switch. Limit switches 186 and solenoid valves 61 and 127 are connected to the central computer via 24-gauge wire. Ultrasound is a common means of nondestructively inspecting materials for flaws and structural integrity. For steels, the preferred frequency used for inspection and sizing of flaws is in the range of 1 to 10 MHz with 2.25 to 5 MHz preferred. Ultrasonic transducers and associated electronics are conventional in the art of nondestructive examination. In accordance with conventional practice, pulsed ultrasound generated by a transducer propagates into the metal to be inspected via a coupling fluid, such as water, in contact with the surface of the metal. Discontinuities in the metal (e.g., cracks) produce ultrasonic pulse reflections, due to sudden changes in acoustic impedance, that are dependent on factors such as flaw size and shape, angle of incidence, and metal path length. These reflections are detected by ultrasonic transducers operating in a reception mode. All transducers incorporated in the inspection tool of the present invention are of the immersion type with frequency in the range of 2.25-10 MHz and have active diameters in the range of 0.25-1.0 inch. The above-described inspection tool performs ultrasonic scanning in three modes depicted in FIG. 3. In the first mode, transducer pack 124 scans crevice weld 34 and the heat-affected zone thereof. In the second mode, transducer pack 134 scans crevice weld 34 and the heat-affected zone thereof. In the third mode, transducer pack 134 scans crevice weld 36 and the heat-affected zone thereof. For inspection of the thermal sleeve attachment weld, the transducers are designed to produce angled beam refracted longitudinal and/or shear waves of 35.degree. to 70.degree. resulting in the ultrasonic beam being perpendicular to the thermal sleeve attachment weld, with allowable skew up to and including angles of plus or minus 20.degree. . When performing immersion tests the water gap distance should not be less than 0.25 inch. For inspection of the front cover plate attachment weld, the angle beam transducers include longitudinal wave transducers to detect indications lying perpendicular to the beam. The examinations may be conducted using multiple transducers, frequencies and angles in sequence or in parallel. These examinations are performed with single or multiple transducers which have a minimum of 50% beam overlap of the complete area to be examined, skew angles and multiple examinations considered. Examination coverage for the thermal sleeve attachment weld is limited to the maximum scanning radius area allowed by the restriction between the T-box piping arms, both top and bottom. The examination coverage for the T-box end plate is 360.degree. both from the top surface and the horizontal face. Less than 360.degree. scanning is allowed if mechanical restrictions are encountered by the piping attachments. All scanning equipment is remotely motor driven with appropriate encoders and motor controls to assure that the proper scanning motion will be achieved. In cases where raster scans are used, the motor controller has the capability of providing a rotational index which results in a minimum of a 50% scan overlap. The resolvers provide position information to the data acquisition system with regard to transducer position. The scanners have variable speed control so as to limit scanning speed to less than 6 inches/sec. The foregoing preferred embodiment has been disclosed for the purpose of illustration. Variations and modifications which do not depart from the broad concept of the invention will be readily apparent to those skilled in the design of ultrasonic inspection equipment. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.