Other Major Component Inspection I Mechanized UT inspections on complex nozzle geometries S. Farley, R. Jansohn, Westinghouse Electric Germany, Germany; H. Ernst, Schweizerischer Verein für technische Inspektionen, Germany; D. Kerr, Eskom Holdings Limited (Koeberg Nuclear Power Station), Germany ABSTRACT In 2007 and 2008 Eskom Holdings Limited (Eskom) performed automated ultrasonic inspection of forged austenitic branch welds in the primary piping in both units 1 and 2 of the Koeberg Nuclear Power Station. The nozzle size varied between 4 to 14. The examination volume was the inner 1/3 of the pipe wall thickness as defined by ASME XI 1992 edition. The inspection technique had to be able to detect, length and depth size inside surface connected service induced planar flaws as well as subsurface flaws, typically from manufacturing. As part of the project the essential parameters had to be evaluated following the guidelines posed in ENIQ recommended practices 1 and 2. Due to the complex geometrical conditions, modelling was performed to determine optimal scanning angle and position. A special scanner was used which enables 3 axis movements to follow previously calculated scanning curve and rotation. Practical trials on a 14 full size test specimen, with natural defects, confirmed the orientation angles as calculated by theoretical modelling. This paper describes the modelling, practical demonstration and the inspection process that were developed and successfully deployed in the field. INTRODUCTION Nuclear components in light-water cooled plants have to be periodically inspected based on regulations defined in industrial codes like e.g. ASME Boiler and Pressure Vessel Code [1]. Ultrasonic examination is a frequently used technique to comply with the requirements for volumetric inspections. Worldwide different regulations or guidelines are in place to direct and ensure the qualification process of non-destructive examination techniques. One guideline to be mentioned is the ENIQ Report European Methodology for Qualification [2]. The major principle behind this qualification guideline is to ensure by a thorough process that the used techniques and personnel involved are capable to detect certain sizes of flaws based on a certain failure mechanism. Therefore numerous essential parameters with its influence on the inspection reliability have to be considered and its variability has to be investigated. For reasons of traceability, repeatability, documentation purpose and personnel radiation exposure more and more complex ultrasonic examinations are performed in an automated way. With respect to geometrical influences the inspection of nozzle to shell welds is one of the most challenging due to its complex mathematical description needed. Eskom Holdings Limited (Eskom) requested to perform automated ultrasonic inspection of forged austenitic branch welds in the primary piping in both units 1 and 2 of the Koeberg Nuclear Power Station in the years 2007 and 2008. The nozzle size varied between 4 to 14. The inspection was implemented as state of the art ultrasonic testing (UT) with a practical demonstration following the principals of the ENIQ guideline but without formal qualification e.g. with additional blind tests. Figure 1 shows a typical branch weld. The examination zone covers the cross sectional volume for the full circumference of the component. The volume to be examined is the inner 1/3 of the pipe wall thickness as defined in Ref. [1]. The objective of the examinations was to accurately detect, length size and through wall / depth size inner surface (ID) connected service induced planar flaws. Sub-surface flaws typically from manufacturing had also to be reported and sized. The set-in nozzle examinations were performed from the outside diameter (OD) piping surface as far as physical limitations (geometry, material, etc.) allowed. For the sake of definition, the planar flaws will be described as follows:
- Transverse flaw = transverse or perpendicular to the subject weld - Longitudinal flaw = parallel to the subject weld Figure 2 shows one point of a postulated transverse flaw on the inner surface within the inspection volume. For each point within the inspection volume the ultrasonic beam shall hit the flaw under optimal scan angle β and misorientation angle Φ. To obtain optimal specular reflection the plane of the ultrasonic beam related to the misorientation angle Φ should hit the flaw surface as perpendicular as possible. Inner Surface Nozzle and Pipe Central Beam Figure 1 - Examination Volume C-D-E-F N T B β Φ σ Surface Normal Vector Tangential Vector Flaw Normal Vector Flaw Scan Angle Misorientation Angle Flaw Skew Angle Figure 2 - Beam Angles
As the head can just follow the contour of the outside diameter piping surface the criteria perpendicularity (Φ = 0) alone would require a numerous variation of the position, scan angle and skew angle. Therefore a maximum allowed misorientation angle will be introduced. This maximum misorientation angle had to be proven by practical demonstration on real test blocs. For this project a maximum misorientation angle of 20 deg has been found acceptable. Figure 3 demonstrates the principal in case of a postulated longitudinal flaw. The intersection between nozzle and pipe outer diameter has the form of a saddle (yellow curve). The green plane is the normal plane to the intersection curve. The inspection of the weld has to be performed from the pipe side. In an ideal case is the central beam part of the normal plane. It should be mentioned, that the nozzle axis not part of the normal plane is. The intersection curve of the normal plane with the pipe outer diameter results in an elliptical curve (blue curve). This is the way the head should move. Figure 3 - Geometrical Conditions for longitudinal Flaws MODELLING Mathematical modelling was used to reduce the numbers of practical trials. Existing simulation software was adapted to this specific project. The numbers of simulation runs were narrowed for practical reasons. Simulation was performed for the examination zone limits E and F. The scan angles were defined with 45 deg, 55 deg and 60 deg. The skew angle was defined with 38 deg, 47 deg and 55 deg and its corresponding negative values. With this input data it was possible to calculate the actual position on the outside diameter piping surface and the misorientation angle towards the flaw plane. Figure 4 shows an example for a transversal flaw on a 6 inch nozzle. This calculation needed then to be iterated to get a complete coverage of the inspection volume with the foreseen inspection technique.
inspection point theta [ ] inspection point radius [mm] approximate angle of misorientation examination zone limit angle [ ] skew [ ] theta [ ] radius R [mm] 67,5 185,00 20 E 55 55 91 256 Figure 4 - Beam modelling example for 6 inch nozzle geometry MANIPULATOR In compliance with the theoretically proven approach, a suitable scanner had to be made available which can follow the scan paths according the geometrical requirements. With the JNA-Scanner- System Westinghouse has developed a specific scanner system for automated UT inspections of nozzles in the main coolant pipe in pressurized water reactors. The system permits to scan the primary pipe from the outer surface with UT transducers. The JNA scanner consists on carriageway mounted parallel to the axis of the main coolant pipe and a system carriage on which a bent boom is mounted perpendicular, see figure 5. The carriageway is mounted on the main coolant pipe with 2 straps placed at each extremity. A system to tight the straps by mechanical buckle hold the manipulator in place on the primary pipe.
Figure 5 - JNA scanner system The JNA system allows 2 types of translational displacements: A displacement in regards with X axis (parallel to the primary pipe axis): this movement is performed through a toothed belt powered by a drive unit mounted on the end of the rail. The unit for the boom drive is mounted on the system carriage. A displacement in accordance with Y axis (circumferentially to the primary pipe): this movement is performed trough gears powered by a drive unit mounted on the unit for the boom. In addition to the two translational displacements a rotating unit is used which consists of: A frame that permits to interface with universal joint suspension. A Plexiglas shoe shaped according the primary pipe OD is screwed on the lower part in order to be in contact with inspected component. A rotating device in integrated in the frame. In consists in a toothed wheel that ensures the rotation and permits to integrate the A drive unit is fixed on the frame to generate the rotating movement. A universal joint suspension system permits to press the device on the component surface through a string system. Forks ensure the holding of the frame. The holder system is capable to carry either phased array heads or standard heads with e.g. pulse-echo transducers. PERFORMANCE DEMONSTRATION Finally the whole inspection system and the procedure have been proven by practical trials. Full scale specimen where available for a 14 inch and partially for a 6 inch nozzle. Both nozzles were with respect to material, welding process and geometry an exact copy of the on-site configuration. Upfront manual testing using a varied range of types with different refraction angles, frequencies, crystal sizes and wave modes were initially performed to find the optimal inspection
technique to detect and size the expected flaw. The types that gave the best results were then used together with the JNA scanner to perform automated examination in the laboratory. The full inspection system was used as is later on during in-service-inspection. Probe skew angles were varied as well to find optimum orientation. The optimum orientation was found in line with the results of the theoretical modelling. In general the inspection system was able to detect these flaws introduced into the specimen notches and real cracks which were in line the inspection target. SUMMARY For automated inspections of primary pipe nozzle to shell welds a complex mathematical description to position the ultrasonic with respect to postulated failure orientation is needed. Mathematical modelling has been used to optimize the inspection approach and to reduce the number of practical trials. A specific scanner had to be used to follow the required scan paths on a complex geometry. Based on practical trials on full scale specimen the complete inspection system has been proven and the applicability of the modelling technique has been validated. In the meantime the described technique has been successfully applied in field applications in the Koeberg units 2 in 2007 and unit 1 in 2008. REFERENCES 1) ASME Boiler and Pressure Vessel Code: Section XI, Rules for Inservice Inspection of Nuclear Power Plants 2) ENIQ Report EUR 17299: European Methodology for Qualification