Phased Array Ultrasonic Detection & Sizing of In-Service Cracks in Heavy Walled Reactor Nozzles - Part 1

Transcription

1 Phased Array Ultrasonic Detection & Sizing of In-Service Cracks in Heavy Walled Reactor Nozzles - Part 1 Chris Chartier - Advanced NDE Services Acuren Group Inc., Oakville, ON, Canada cchartier@acuren.com Abstract Ultrasonic crack detection and sizing is an essential tool in determining the asset integrity of pressure vessels and piping. Conventional UT shear wave methods have proved to be time consuming, requiring high skill levels and frequently resulting in unreliable and inconsistent data. Phased array UT technology can provide novel solutions to these problems with reliable results, imaging of the cracks showing direction or propagation and reduced inspection times. 1. Introduction This paper describes some advanced PAUT techniques used in circumferential crack detection and sizing of heavy walled Category "D" nozzles and nozzle welds. Nozzle welds described in this paper represent several heavy wall pressure vessels ranging in thickness from 50.8mm to 110mm. Compositions from these inspections were of ASTM A516 Gr.70 material or similar metallurgy. Part 2 of this paper will describe detection and sizing techniques of transverse cracking in nozzles. Complex geometries composed of single piece forged nozzles with butt joint welded connections are also described. The ID and OD surfaces were not parallel with thickness ranging from 110mm to 180mm posing unique and extraordinary challenges for conventional UT techniques. 2. Part Descriptions Typical Cat "D" Nozzle Unparallel Complex Geometry - Nozzle with Butt Groove Weld Figures 1 & 2: Left side shows a typical set-through Cat "D" nozzle groove weld configuration, left is conventional design and right is an improved forged design for increased crack resistance. Thicknesses ranged from 50.8mm to 110mm (2.0" to 4.40").

2 Figure 3: Cross-sectional view of a complex geometry forged bottom nozzle, crack locations as identified by Finite Element Analysis. Complex geometry forged bottom nozzles were extremely difficult to examine with any reliability using conventional UT. Ultrasonic sound paths were approaching mm (10-12 inches). These long distances hindered reliable detection with virtually no sizing abilities from the OD (the only accessible surface). Phased array provided an advanced alternative with reliable detection and independently verified depth results within 1% of reported. 110mm 170mm Figure 4: Crack susceptible area on the ID radius surface in red identified by Finite Element Analysis (FEA). Conventional UT was difficult with long sound paths approaching 300mm (12 inches) using 60º shear waves due to beam spread. Phased array provided a solution with variable focal depths, apertures and beam steering facilitating novel detection and sizing techniques.

3 Figure 5: Overview of a typical Cat "D" set-through nozzle and probe placement for detection and sizing of circumferential cracks. 3. Equipment Figures 6 & 7: Above images show a scanners and probe placement for PAUT examination. Modified scanners for conventional AUT were used for PAUT data collection. Modifications include encoder patch cables, forks and general hardware set-up. Scanners allowed the use of variable offsets; this was required as nozzles on heads interacted with knuckle curvatures. Table 1.0 Probe # Frequency # of Elem. Pitch Comments 1 5.0MHz mm TW Rexolite wedge MHz mm LW Direct Contact, -45 to MHz mm TW Rexolite Wedge 4 6.0MHz mm LW Direct Contact, -45 to +45 Phased Array Acquisition Module: RD Tech Omniscan 32:128

4 4. Conventional Ultrasonic Figure 8: Sketch showing conventional ultrasonic technology with A-Scan presentation. Past conventional ultrasonic examinations with conventional UT was tedious and time consuming. In the field, cracks were manually plotted on full scale cross-sectional sketches to characterize, located position, verify ID connection, orientation and depth. Furthermore, back-scattering tip signals were absent with conventional UT. Crack interaction with weld anomalies presented further complications in efforts to verify separation or crack interaction. If separation could not be confirmed, potentially overly conservative values resulted in erroneous data for crack growth rates and hasty repairs. 5. Phased Array Ultrasonic Figure 9: Sketch showing Phased Array technology with corresponding Sectorial Scan image. Phased array UT images illustrate the crack face, providing the ability to distinguish and characterise welding discontinuities from cracks. Strong backscatter tip-diffraction signals also become apparent (4-5). Confirmation of proper probe position is also confirmed by the image geometry and ID surface. Scanners allowed for variable offsets as some nozzles interacted with head knuckle curvatures. Phased array imaging shows crack propagation angle, ID connections, depth and orientation. This has subsequently reduced the inspection time to approx 50% to that previously found with conventional UT, including supplemental sizing for Nozzles from 12"Ø to 30"Ø in wall thickness of 52mm to 100mm.

5 C-SCAN SECTORIAL SCAN B-SCAN Figure 10: Phased Array UT view descriptions of C-Scan, B-Scan and S-Scans (Sectorial). C-Scan is a top view, B-Scan is a side view and the S-Scan is a cross-sectional view. This is data presented in data analysis software - volume corrected. 6. Results Data was collected and reviewed on the instrument after the scan. Multiple probes and focal laws were used for sizing depending on the depth of the cracks. Additional free running scans were used for post-data analysis using Tomoview software. Depth sizing was accomplished by using last significant tip & delta time-of-flight (TOF). Sectorial scans were superimposed and plotted on to 3D scaled sketches to report crack depths, positions and orientations (1-3). This provided crucial details to the reliability engineers to assess whether to shut the vessel down or to continue running and determine a maintenance date later on to coincide with other necessary repairs. Radius Figure 11: Volume corrected C-Scan or top view image of bottom nozzle scan. Intermittent shallow cracking was noted around the circumference.

6 Drawings were made of the components in 2D and 3D views allowing the placement of sectorial scans over the components. This was helpful in the presentation of results and reporting. Figure 12: Example of a nozzle weld showing a deep thermal-fatigue crack, propagation direction is perpendicular towards shell. Figure 13: 3D Sketch showing probe position and sectorial scan for the detection of circumferential cracks.

7 LW TW Figures 14 & 15: Images showing crack face, tip signals and crack orientation. Both figures were nozzle thermal-fatigue cracks. Left image is LW and right image is TW. Figure 16: Fatigue crack noted on fillet weld toe, shell side. Note weld discontinuities in weld on fusion face. Weld area is exaggerated by previous weld repairs. Figures 17 & 18: Example of weld discontinuities encountered during inspections, this was an example of incomplete cross-penetration.

8 Figures 19 & 20: Left figure shows detection with T-Waves, right figure shows sizing with L- Waves. T-Waves tended to oversize at long sound paths and high angles due to beam spread. Figure 21: Example on the importance of orientation; direction of propagation is likely towards the nozzle bore, height was inflated on the image by specular reflection and beam spread. Subtle metallurgy differences between the weld and base materials were visible indicated by the colour palette. Again, the weld zone was exaggerated by previous weld repairs.

9 7. Conclusions In conclusion, PAUT showed significant reductions in scanning times and crack imaging proved to be a valuable asset for complex geometries, indicating the direction of propagation via imaging. The resulting data was accurate, proven and reliable. Conventional UT was frequently questionable and tip diffraction signals were not apparent therefore limiting sizing techniques. However, with composite phased array transducers and using multiple angles via sector scans, tip signals were easy to identify (4-5). The crack depth results were far more reliable and accurate and at the same time the examinations were recorded and archived for future trending and auditing. The resultant data is reliable and likely suitable for ECA. Phased array inspection times were typically quicker - at least 50% time reduction over conventional UT techniques, resulting in less down time for vessels passing cost savings to the owner. New techniques in detection and sizing where previous conventional UT was not feasible. Data is stored for trending, reporting, auditing, peer review and comparison purposes. Focal laws can be changed during inspection for each situation, thus optimizing tip signals and accuracy of depth measurements. Data can be projected onto 2D and 3D sketches for interpretation, thereby allowing easier communication and reporting of results to the customer (1-3). Imaging shows the crack face, tip and direction of propagation; all valuable information for the reliability engineer. Similar approach is reported by Ciorau (6). References 1. Ciorau, P.: A Contribution to Phased Array Ultrasonic Inspection of Welds Part 1: Data Plotting for S- and B-Scan Displays, ndt.net vol. 12, no.6 (June 2007), CINDE, vol. 28, no. 5 (Sep/Oct 2007), pp Ciorau, P.: A Contribution to Crack Sizing by Phased Array Ultrasonic Techniques. Part 2: Comparison with Optical, Magnetic Particles, Fracture Mechanics and Metallography for Last Significant Crack Tip. ndt.net vol.12,n.2 (Feb 2007) / 6 th NDE Nucl-Budapest-Oct Ciorau, P., Chartier, C., Mair, K.: A Contribution of Phased Array Ultrasonic Technology (PAUT) to Detection and Sizing Stress Corrosion Cracks ndt.net vol.13,n.10 (Oct 2008) / 7 th NDE Nucl.Conf.-Japan - May Davis, J.M. and Moles, M.C.D: Resolving capabilities of phased array sectorial scans (Sscans) on diffracted tip signals, Insight, Vol 48, No 4, pp 3,6 & 7, April Jacques, F., F. Moreau, and E. Ginzel. Ultrasonic backscatter sizing using phased array-developments in tip diffraction flaw sizing. Insight, vol. 45, no. 11 (Nov.2003): pp Ciorau, P., Gray, D., Daks, W.: Phased Array Ultrasonic Techniques Contribution to Engineering Critical Assessment (ECA) of Economizer Piping Welds, ndt.net vol. 11, no.5 (May 2006) /6 th NDE Nucl-Budapest-Oct.2007