METHOD AND APPARATUS FOR INSPECTION OF GAS TURBINE DISCS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of U.S. Provisional Application No. 61/023,702 filed on January 25, 2008, the contents of which are incorporated herein fully by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to the field non-destructive inspection of gas turbine engines and more specifically the ultrasonic inspection of gas turbine discs using a linear array transducer and three-dimensional reconstruction display.

SUMMARY OF THE INVENTION

[0003] The present invention is directed to an apparatus for ultrasonic inspection of an internal channel in a component. The apparatus comprises a frame, a plurality of support arms, a probe arm, an ultrasonic probe assembly, a spring loaded guide, and an encoder. The support arms arc pivotally connected to the frame and each comprise a connector for positioning the frame centrally relative of the internal channel. The probe arm is extendable from the frame into the internal channel and comprises a first end movably connected to the frame and a second end. The ultrasonic probe assembly is supported on the second end of the probe arm. The spring loaded guide is connectable to the second end of the probe arm and adapted to center and stabilize the ultrasonic probe assembly within the internal channel. The encoder is adapted to collect data from the ultrasonic probe at predetermined intervals. [0004] The present invention is further directed to a method for non-destructive examination of an internal channel of a component. The method comprises providing a frame and support arm assembly to position a probe arm at a central location relative to the internal channel and positioning a transducer on a face of the internal channel using the frame and support arm. The method further includes rotating and extending to probe arm into the internal channel to move the transducer about the face of the internal channel and transmitting ultrasonic waves into the component. Ultrasonic echoes indicative of an acoustic impedance interface within the component are received and transmitted to a

computer system. The computer system is used to construct a three-dimensional image of the transmitted and received ultrasonic echoes.

[0005] Further still, the present invention is directed to an apparatus for ultrasonic inspection of a gas turbine disc bore. The apparatus comprises a frame, a probe arm, a linear array ultrasonic probe assembly, a biasing assembly, and an encoder. The frame comprises a plurality of support arms. Each support arm comprises a connector for positioning the frame centrally relative to the disc bore. The connector is adapted to engage a turbine disc bolt hole. The probe arm is extendable from the frame into the disc bore and comprises a first end movably connected to the frame and a second end. The linear array ultrasonic probe assembly is supported on the second end of the probe arm and moveable in response to rotation and axial movement of the probe arm. The biasing assembly is connectable to the second end of the probe arm and is adapted to center and stabilize the linear array ultrasonic probe assembly within the disc bore during movement of the probe arm. The encoder is adapted to collect data from the linear array ultrasonic probe at predetermined intervals. [0006] The present invention is also directed to a system for ultrasonic inspection of a gas turbine disc. The apparatus comprises an ultrasonic inspection fixture, a probe assembly and a computer system. The ultrasonic inspection fixture comprises a frame, a probe arm, and a plurality of support arms for positioning the probe arm centrally within the disc bore. The probe assembly is supported on the probe arm and the computer system is remote from the probe assembly. The computer system is operatively connected to the probe assembly and adapted to receive scan data from the probe assembly. The computer system reconstructs a three-dimensional image of the disc based upon the scan data received from the probe assembly.

DESCRIPTION OF THE FIGURES [0007] Figure 1 is a diagrammatic representation of a common small bore gas turbine disc.

[0008] Figure 2 is a top view diagrammatic representation of a small bore ultrasonic scanner constructed in accordance with the present invention.

[0009] I igure 3 is a side view representative illustration of the small bore ultrasonic scanner of Figure 2

[0010] Figure 4 is a diagrammatic representation of the ultrasonic scanner device ot figure 2 positioned for scanning of a small bore turbine disc [0011] Figure 5 is a representative diagram of an alternative scanning device of the present invention I he apparatus of Figure 5 is adapted for ultrasonic scanning of large bore turbine discs

DESCRIPTION OF THE INVENTION [0012] Ultrasonic inspection techniques arc used to perform nondestructive testing on articles formed of materials having an intrinsically coarse grain structure, which results in anisotropic and non-uniform acoustic properties Examples of such articles include turbine discs (rotors) used in gas and steam turbines Due to the hostile operating environments ot gas and steam turbines, the structural integrity of the turbine disc is of great importance in view of the high mechanical stresses and temperatures turbine discs are subject to during turbine operation

[0013] Inspections for defects in turbine discs are routinely performed on steam turbine discs at maintenance intervals Stress corrosion cracking typically develops at the surface of the central bore, and such cracks can be located using ultrasonic signals are targeted at the bore Ultrasonic inspection or testing techniques are used lor detecting material flaws in a number of situations and are particularly valuable in detecting flaws in the material of turbine or generator rotors Turbine discs containing certain types of flaws in their material may fail abruptly and catastrophically, particularly under the start-up rotational and thermal stresses The flaws in the rotor material may grow over the course of the many start-up sequences and finally link up with each other to form cracks Cracks with dimensions greater than the critical size for unstable growth may cause the disc to burst or fracture under rotational and thermal stresses present during a final start up sequence of the turbine Thus, there remains a need for improved inspection techniques to examine a turbine discs, in particular the web portion of the turbine disc

[0014] The present invention provides methods and apparatus used in the ultrasonic inspection of turbine discs from the bore of the turbine disc. The method and apparatus of the present invention provide a three-dimensional representation of the turbine disc. [0015] Turning now to the figures and more specifically to Figure 1 , there is shown therein a component comprising a gas turbine disc 10. The disc IO generally includes a rim 12, a hub 14, and a web 16 between the Hm and hub 12 and 14. The rim 12 is configured for the attachment of turbine blades (not shown). A disc bore 18 in the form of an internal channel hole is centrally located in the hub 14, and therefore the axis of the disc bore 18 coincides with the axis of rotation of the disc 10. A plurality of bolt holes 20 are machined through the web 16 at locations that are equal radial distances from the disc bore 18 (center- to-center), as well as circumferentially equally spaced from each other (center-to-center). The axes of the bolt holes 20 are shown as parallel to the axis of the disc bore 18. [0016] Turning now to Figure 2 there is shown therein a positioning fixture 22 configured to be supported from the turbine disc 10 and to position a transducer (FlG. 3) within the disc bore 18. The positioning fixture 22 comprises a frame 24, a plurality of support arms 26, and a probe arm 28 (FIG. 3). The frame comprises a central access hole 30 through which the probe arm 28 passes and by which the probe arm is movably supported. The support arms 26 are pivotally connected to the frame 24 for movement about fasteners 32. Each support arm 26 comprises a connector 34 adapted to fit within bolt holes 20 (FIG. 1). The pivotal support arms 26 allow the positioning fixture 10 to be supported on the turbine disc such that the central access hole of the frame is centrally aligned relative to the central axis of the disc bore 18

[0017] With reference now to Figure 3 there is shown therein a side view of the positioning fixture 22 of Figure 2. The positioning fixture 22 is shown with probe arm 28 extended from the frame 24. The probe arm 28 may comprise an elongate cylindrical member made from a material suitable for withstanding the harsh operational environment of turbine inspections. The probe arm 28 is extendable from the frame into the disc bore and comprises a first end 36 and a second end 38. The first end 36 is movably connected to the frame 24. The second end 38 is configured to support the ultrasonic probe assembly 40. The

probe arm 28 may include an internal channel (not show) to allow wires 42 and optional couplant to pass along the probe arm to the probe assembly 40.

[0018] The probe assembly 40 comprises support plate 43 and a transducer 44 connected to the support plate, and a biasing member 46. The support plate 43 is connected to the second end 38 of the probe arm 28 and is adapted to support the transducer 44 and biasing assembly. The transducer 44 is configured for placement within the disc bore 18 and to transmit ultrasound waves into and receive ultrasound echoes from within the disc bore 18. The transducer 44 may comprise an ultrasonic probe having a plurality of ultrasonic transducers and wedges used to induce longitudinal or transverse waves. For example, a preferable probe may comprise a 128 element linear array probe designed to inspect one or more inches of the disc bore 18 in a single rotational pass. The ultrasonic transducers 44 may be supported by a body (not shown) having a semicircular cross-section with a radius of curvature approximately equal to a radius of curvature of the disc bore. [0019] The biasing member 46 is generally connected to the support plate 43 on a side opposite the transducer 44. The biasing member 46 may comprise a spring adapted to bias the transducer toward the face of the disc bore 18. The biasing member 46 functions to center and stabilize the transducer 44 within the disc bore to maintain contact between the transducer and the face of the disc bore during movement of the probe arm in directions λ, B, or C. [0020] As shown in Figure 3, the probe arm 28 may be moved in directions A, B, or

C using a manual manipulator 48 comprising a handle or an automatic manipulator configured to reposition the ultrasonic probe radially and axially along the disc bore 18. It will be appreciated by one skilled in the art that the connection between the probe arm 28 and the frame 24 may be threaded or otherwise constructed to allow for stepwise inspection of the disc bore.

[0021] As previously mentioned, wires 42 may extend from the handle 48 to an ultrasonic transmitter/receiver 50. One skilled in the art will appreciate the ultrasonic transmitter/receiver may comprise an ultrasound system manufactured and sold by such suppliers as Terason Ultrasound. As the transducer 44 is passed along the face of the disc

bore 18, the ultrasonic transmitter/receiver 50 generates ultrasonic pulses to excite transducer 44 and then receives echoes from the disc web 16 to facilitate detecting flaws, which may have developed within the web Data received from each scan position includes an axial and rotational position of the transducer 44, as measured by a transducer position encoder (not shown), along disc bore 18, and a distance from the test surface on the face oi bore 18 to each recorded reflector, which may include a structural edge of disc 10 The data is indicative of a structure of web 16 and/or a feature or flaw at which each echo was detected In the preferred embodiment, the data is transmitted to a computer 52 such as, but not limited to a laptop computer, a personal digital assistant (PDA), a data collector or a network connection In an alternative embodiment, the echo data and transducer position data may be received by separate processors In the exemplary embodiment, computer 52 includes a display 54 to monitor results of each scan and operation of the scan λs used herein the term ' processor ' also refers to microprocessors, central processing units (CPU), and application specific integrated circuits (λSIC), logic circuits, and any other circuit or processor capable of executing the inspection system, as described herein I he computer 52 may include visualisation software adapted to render a three-dimensional image of the inspected area Software used in accordance with the present invention may include visualization software provided by companies such as Avizo™ [0022] 1 urning now to Figure 4, the positioning fixture 22 of Figures 2 and 3 is shown supported on a turbine disc 10 The support arms 26 are shown disposed in a generally tripod arrangement to provide adequate support for the probe arm and probe assembly 1 he probe arm 28 and frame 24 are shown disposed such that the access hole 30 of the frame is substantially aligned with the disc bore 18 (FIG 1) [0023] With reference now to Figure 5, an alternative positioning device constructed in accordance with the present invention is shown therein I he device of Figure 5 comprises an ultrasonic inspection device for large bore turbine discs The device of Figure 5 comprises a truck 60 adapted to support an undercarriage comprising rolling members 62 adapted to translate the truck about the face (not shown) of a large diameter disc bore I he rolling

members 62 may comprise wheels, tracks, or other known devices for allowing movement of the truck about the face of the disc bore.

[0024] The truck 60 supports a transducer position encoder 64, and support housing 66, a probe arm 68 and the probe assembly 70. The transducer position encoder 64 is adapted to measure the position of the truck 60 within the disc bore. As previously discussed the probe assembly 70 may comprise a linear array ultrasonic probe 72 and a biasing member 74. The biasing member 74 is connected to the probe arm 68 and may comprise a spring (not shown) to allow movement of the probe 72 in the directions indicated in Figure 5 to compensate for slight variations in the surface dimensions of the disc bore face. Movement of the truck 60 is guided by edge of the disc bore. The linear array probe may induce longitudinal or transverse ultrasonic waves into the disc to inspect for features and flaws therein.

[0025] The present invention also includes method for ultrasonically testing a turbine disc having a disc bore. The method comprises scanning along a face of the disc bore in a substantially axial direction on an axis of rotation of the turbine disc using a linear-array ultrasonic transducer 44 (Fig. 3). Ultrasonic waves arc transmitted from the face of the disc bore into the web 16 of the turbine disc at a plurality of steering angles. Ultrasonic echoes are received from the web. The echoes may be indicative of web structure features and flaws. The ultrasonic echoes arc used to generate a three-dimensional representation of the disc web to determine a location and dimension of the features and flaws. The method may likewise include receiving transducer position information from the transducer position encoder. The information from the encoder is used to correlate the transducer position with echo data to determine a flaw location and to generate the three-dimensional representation of the turbine disc. Such three-dimensional reconstruction of the turbine disc is performed by the computer using software

[0026] The method of the present invention further includes a method for the nondestructive examination of an internal channel of a component. The method comprises first providing a frame and support arm assembly to position a probe arm at a central location relative to the internal channel. λ transducer supported at the distal end of the probe arm is

positioned on the face of the internal channel and ultrasonic waves are transmitted from the transducer into the component Ultrasonic echoes indicative of an acoustic impedance interface within the component are received and the transducer is repositioned Repositioning the transducer within the internal channel is accomplished by rotating and extending the probe arm within the internal channel The transducer may be repositioned by either manually or automatically advancing the transducer axially and radially along the face of the internal channel upon the completion of each scan

[0027] Each received echo is transmitted to a computer system The computer system displays and captures an image of the ultrasonic echoes By combining a series of ultrasonic images from position information provided by an encoder, a three-dimensional representation it the inspected area can be displayed by the computer system

[0028] One skilled in the art will appreciate additional inspection methods may be accomplished using the fixture and truck of the present invention For example, the probe assembly of Figure 3 may be configured to include an eddy current probe designed to scan the disc bore of the turbine disc using electromagnetic signals Further, the probe assembly may include both phased-array and linear-array probes to increase the area of inspection [0029] Various modifications can be made in the design and operation of the present invention without departing from the spirit thereof Thus, while the principal preferred construction and modes of operation of the invention have been explained in what is now considered to represent its best embodiments, which have been illustrated and described, it should be understood that the invention may be practiced otherwise than as specifically illustrated and described