Innovative Semi-Flexible Probe Design
For smaller turbine rotors and disks, conventional UT probes (typically 2 MHz, Ø 24mm or 0.9in) are used for straight-beam inspection. However, as the inspection path length increases, these standard probes fail to comply with code requirements due to their inadequate signal-to-noise ratio (SNR). This is mainly due to the relatively small active surface of the probe which limits the acoustic energy transmitted in the inspected part and leads to a larger divergence of the acoustic beam.
By increasing the active aperture of the UT probe, more acoustic energy will interact with the reflector. This is because a wider aperture reduces the divergence of the UT beam, allowing more energy to be transmitted through the inspected specimen.
Using phased array UT technology instead of a conventional UT probe has the additional benefit of being able to focus acoustic energy at various depths and to steer the acoustic beam. This is a significant advantage, since turbine rotors usually have sections or stages of different sizes. A single phased array probe can be used for the whole rotor length and the focal depth can be optimized for each section. Also, the applicable standards require inspection at different angles to increase the probability of “perfect” specular reflection on a given defect. A single phased array UT probe can be used to generate these different angles instead of several conventional probes mounted on wedges.
Figure 1: Semi-flexible QUAD probe assembly
Based on the above considerations, Eddyfi Technologies developed a new concept, the semi-flexible 2D array probe, consisting of four individual matrix arrays of 8 x 4 elements and an aperture of 16mm x 64mm (0.6in x 2.5in). The four matrices are mechanically linked and are used as a single array (see Figure 1). Due to the mechanical linking, the 2M8x4E16-64-QUAD probe assembly allows adequate direct coupling on diameters of 250mm (9.8in) and up, thus allowing for conducting efficient inspection of a complete rotor with multiple stages. It also provides the benefit of mechanically focusing the acoustic energy towards the centre of curvature of the inspected specimens.
Figure 2 shows the 64mm x 64mm active surface of the QUAD probe, approximately nine times larger than the typical conventional UT probe used for this application. The -6 dB cross section of the acoustic beam at 2,000mm (78.7in) sound path was measured at roughly 60mm (2.4in) for the QUAD probe, compared to 190mm (7.5in) for the conventional UT probe, thus illustrating the superior focusing of acoustic energy on the reflectors.
Figure 2: QUAD probe vs. standard UT probe: Comparison of active aperture and acoustic beam cross-section
The design was experimentally validated on several representative test specimens. Figure 3 compares the signals obtained with the 2M8x4E16-64-QUAD probe assembly and the standard conventional UT probe on a specimen with a 1.6mm (0.06in) diameter flat-bottom hole (FBH) at a sound path of approximately 1,800mm (70.9in). A considerable improvement of signal-to-noise ratio (15 dB) can be observed in favor of the new semi-flexible probe design.
Figure 3: Response of semi-flexible QUAD probe (left) and standard UT probe (right) on 1.6mm diameter reflector at 1,800mm depth in representative test specimen
Even with the very large active aperture, it was confirmed that the element size of the semi-flexible QUAD probe still allows for steering the acoustic beam from 0 to 35°LW on specimens from 250mm (9.8in) OD up to flat. Figure 4 illustrates the steering capability of the semi-flexible QUAD probe (128 elements in total) and the semi-flexible TRI probe (63 elements in total), designed for smaller rotor sizes.
Figure 4: Semi-flexible 2 MHz QUAD and TRI-probes allow for beam steering up to more than 35°LW
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Software Innovation
Eddyfi Technologies’ UltraVision Classic software is a complete inspection package that manages all phases of the inspection process, starting from probe design and validation to ultrasonic signal acquisition with real-time imaging of the signals, and finally online as well as offline data analysis, evaluation and reporting. Given the amount of data recorded when conducting an efficient inspection on large rotors and disks, the fact that the software can handle data files of unlimited size makes it a perfect fit for this type of work.
UltraVision allows us to design, perform acoustic beam simulations, and control custom arrays with multiple pitches like QUAD and TRI semi-flexible probes. The DGS sizing diagram simulation tool allows generating and storing DGS curves for conventional UT probes, as well as rigid and semi-flexible phased array UT probes. As soon as the appropriate set of focal laws has been prepared, the user interface allows the operator to define the parameters of the DGS curve simulation: required FBH diameters, sound path range, and number of points along the sound path axis. The simulated set of DGS curves can be visualized in the Advanced Calculator (see Figure 5).
Figure 5: 2D and 3D-views of acoustic beam simulation results (left) and DGS curve simulation (right) for semi-flexible TRI probe on rotor stage with OD = 1,000mm
During calibration and inspection, the DGS curves for each focal law can be displayed on the corresponding A-Scan views, and a dedicated set of information fields can be visualized for a given ultrasonic indication, to quantify and evaluate the inspection results in accordance with the Equivalent Reflector Size (ERS) method (see Figure 6).
Figure 6: UT signals from Ø 6mm FBH at 300mm depth with DGS curves and ERS evaluation
Interested in sharpening your NDT skills? The Eddyfi Academy offers a range of online training courses including this refresher on phased array ultrasonic testing.
Conclusion
The benefits of the innovative Eddyfi Technologies solution for the manufacturing inspection of heavy rotors can be summarized as follows:
- Large semi-flexible 2D-array probes provide superior performance for the inspection of large rotor forgings, compared to standard conventional UT probes and rigid PAUT probes. This is due to higher acoustic energy transmitted in the specimen (active aperture), better focusing, and more efficient coupling.
- It was experimentally validated that a single PAUT probe can be setup to meet the detectability requirements of the applicable codes for both straight beam examination and angle beam examination; this results in a drastic reduction of the number of required scanning sequences, and hence the total inspection time.
- Eddyfi Technologies’ high-performance phased array hardware, driven by UltraVision Classic, allows for efficient application of this improved inspection technique in industrial conditions.
- UltraVision Classic allows for designing and controlling the semi-flexible PAUT probes, generating the DGS curves for all focal laws, then recording the ultrasonic data and finally analyzing the inspection results according to code requirements.
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