PROPOSED ASME SECTION III CODE CASE REDUCTION OF NDE WELD REPAIRS

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Jane Harper
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1 Proceedings of the ASME 2011 Pressure Vessels & Piping Division Conference PVP2011 July 17-21, 2011, Baltimore, Maryland, USA PVP PVP2011- PROPOSED ASME SECTION III CODE CASE REDUCTION OF NDE WELD REPAIRS Steven L. McCracken, Steven M. Swilley, Yoshihisa Sekinuma Electric Power Research Institute Charlotte, NC Owen Hedden Consultant Fort Worth, TX Dave Cowfer Consultant Aiken, SC Sampath Ranganath XGEN engineering San Jose, CA ABSTRACT Section III of the ASME Boiler and Pressure Vessel Code requires radiographic testing (RT) of pressure boundary welds. RT is performed to detect flaws that might be created in welds as they are fabricated. Current Section III acceptance standards require rejection and repair of flaw indications characterized as cracks, lack of fusion, or incomplete penetration regardless of the size of the indication or the structural significance of such indications on fitness for service (FFS). The current Section III requirements have been effective in meeting the design objective of preventing pressure boundary failures. However, the rules are sufficiently conservative that not only are structurally significant flaws excluded, but they also exclude more benign indications that have no impact on structural integrity. This approach has resulted in repairs for even minor flaws that have no FFS impact. In addition to the cost of performing these unnecessary repairs, the repairs may have contributed to service induced cracking because of the higher residual stresses from the repair. Clearly, there is a need to revisit the Section III inspection and repair rules so as to distinguish between structurally unacceptable flaws and benign flaws that have no FFS impact. This paper describes the technical basis for the proposed Section III Code Case that uses the FFS approach to eliminate the need for weld repairs for minor flaws that have been shown to have no structural impact. Specifically, the Code Case will provide the option to use qualified volumetric inspection to size the flaw indications accurately and define acceptance criteria to determine flaw sizes that are judged to have little structural significance. In addition to describing the requirements of the proposed Code Case, this paper also describes the technical basis for the flaw acceptance criteria and the results of ultrasonic (UT) qualification testing to demonstrate the capability to detect and characterize fabrication flaw indications. INTRODUCTION The design and fabrication of nuclear pressure vessels and piping components are governed by the rules of Section III of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code [1]. The Code requires radiographic testing (RT) of pressure boundary welds. The objective is to ensure the high levels of structural integrity needed for the safe operation of nuclear power plants. RT is performed to detect flaws that might be created in welds as they are fabricated. Current Section III acceptance standards require rejection and repair of flaw indications characterized as cracks, lack of fusion, or incomplete penetration regardless of the size of the indication. In addition, volumetric indications such as porosity and slag that exceed the specified acceptance criteria are rejectable and require repair. Section III does not currently provide for analysis and acceptance of flaws not meeting the acceptance standards. The requirements in Section III are referred to as a workmanship standard because they are not based on detailed structural integrity or fracture mechanics evaluations. The current Section III requirements that preclude flaw indications have been effective in that they have helped meet the objective of preventing pressure boundary failures. However, the rules are sufficiently conservative that not only are structurally significant flaws excluded, but they also exclude more benign indications that have no impact on structural integrity. This broad-base approach has resulted in repairs for even minor flaws. This over-conservative approach was somewhat justified because the early inspection techniques were not very sensitive and there were no reliable methods such as fracture mechanics techniques to evaluate the significance of flaws. Notwithstanding the success of the Section III design and fabrication rules in ensuring structural 1 Copyright 2011 by ASME

2 reliability, it is appropriate to revisit the Section III inspection and repair rules for the following reasons: Because small flaws are inherent even to high-quality welding processes, flaw-free welds are not a realistic goal. Operating experience has shown a high level of reliability, with service failures rarely being attributed to fabrication flaws. Ultrasonic testing (UT) methods have improved since the 1960s; current UT methods can now ensure reliable flaw detection and accurate characterization of flaws to an extent not possible with RT. Section XI [2] has developed fracture mechanics approaches that allow more realistic evaluations of the significance of detected flaws on structural integrity. Local repair of flaws can result in high residual stresses that can aggravate stress corrosion cracking (SCC). In fact, a correlation has been found between local weld repairs in Ni- Cr-Fe welds and SCC in dissimilar metal welds in both boiling water reactors (BWRs) and pressurized water reactors (PWRs). In addition to field experience that indicates the likelihood of cracking in repair welds, analytical studies [3] have shown that weld repair can increase the probability of failure. Codes and standards used outside the nuclear power industry (e.g. the petrochemical industry) have been moving to flaw evaluations based on the FFS approach. For example, API 579 [4] is based on the FFS approach and provides the fracture mechanics methods to evaluate the structural significance of crack-like flaws in pressure boundary components. Clearly, this discussion supports the use of the FFS approach for the fabrication, inspection, and flaw acceptance of pressure boundary welds in pressure vessels and piping. The FFS approach entails the use of UT as an alternative to RT when the acceptance standards of Section III NB or NC-5000 are not satisfied. CURRENT CODE SECTION III STATUS Section III requires RT of fabrication welds. If unacceptable indications are found, weld repair would be required. Although radiography is an acceptable (but with some limitations on depth sizing) volumetric inspection technique, there are circumstances in which UT might be the preferred choice for the inspection of fabrication welds. Because UT is the overwhelming choice for volumetric inspection for in-service inspection (ISI), the use of UT for the inspection of fabrication welds will allow one-to-one comparison of pre-service and in-service inspection results. In addition, significant improvements have been made in UT techniques for both detection and sizing that make UT the better alternative for fabrication welds. The UT techniques and transducers are qualified with mockups that simulate actual flaws in welds and are therefore more reliable. Code Case N provides the option of performing ultrasonic examination in lieu of radiography. This is an alternative to the RT requirement in Section III, NB-5200 and NC N defines the specific conditions and limitations under which UT is allowed as an alternative to radiography. For example, it requires: (i) 100% of the volume of the entire weld plus 1 2 in. (13 mm) of each side of the weld to be inspected, (ii) scanning by angle beam examination in four directions, two directions perpendicular to the weld axis and two directions parallel to the weld axis, (iii) use of automated computer data acquisition system, (iv) qualification of the procedure and personnel using mock-ups that are similar to the component weld being examined. An important caveat is that flaws that exceed the Section III acceptance criteria (specifically the exclusion of flaw indications characterized as cracks, lack of fusion, or incomplete penetration) would require weld repair or flaw removal. Thus, Code Case N offers the alternative of using a different inspection technique (UT instead of RT) but does not eliminate the need for unnecessary weld repair. Therefore, the most important disadvantage of the current Section III rules remains: that crack-like indications and lack-of-fusion defects would be unacceptable, regardless of whether there is any impact on structural integrity. Clearly, Code Case N is an improvement, but it ignores the issue of unnecessary weld repairs. The proposed Code Case not only allows as an alternative, the use of UT for inspection of Section III fabrication welds, but also specifies acceptance criteria for any flaws that may be discovered during the inspection. PROPOSED CODE CASE The objective of proposed Code Case Use of Fitness-for- Service Approach for Acceptance of Full Penetration Butt Welds in Lieu of Weld Repair [5] is to provide alternative options for the acceptance of full penetration butt welds that may not meet the overly conservative Section III requirements which would mandate weld repair in such cases. The Code Case provides the option to use qualified volumetric inspection to size the flaw indications accurately and defines acceptance criteria to determine flaw sizes that are judged to have little structural significance. Appendix 1 of the Code Case describes the requirements for the UT procedure and personnel qualification. The inspection has to be consistent with Section XI Appendix VIII examination and performance demonstration requirements. Appendix 2 of the Code Case describes the flaw acceptance criteria. The Code Case considers two types of flaws: subsurface flaws and surface flaws. Subsurface flaws that do not exceed the size limits in the Code Case are acceptable as is, that is, without repair. Surface fabrication flaws are not acceptable because of the potential for environmentally assisted crack growth in surface flaws exposed to the water environment. However, surface indications can be removed by grinding or machining and need not be repaired by welding if the specific requirements in the Code Case are met. 2 Copyright 2011 by ASME

3 The specific requirements of the Code Case are described in [5]. The Code Case is in the process of being voted on by the ASME Code committees at this point. The key aspects (but not all) of the Code Case are described here: Applicability i) Materials are limited to Section III NB or NC-2000 with P- No. 1 and P-No. 3 ferritic steel meeting NB or NC-2331, and Appendix G-2110 toughness requirements, P-No. 8 and P-No. 43 material and associated weld filler metal. ii) The ultrasonic (UT) method specified in this Case shall be applied in lieu of the RT method specified in NB-5200 or NC-5200 to examine welds in vessels and piping and characterize flaw indications. The UT method shall be qualified in accordance with Appendix 1. Acceptance of flaws shall be in accordance with Appendix 2. iii) The ultrasonic examination shall include 100% of the volume of the entire weld, plus 1 2 in. (13 mm) of base material on each side of the welds. iv) The ultrasonic examination volume shall be scanned by angle beam examination in four directions, two directions perpendicular to the weld axis and two directions parallel to the weld axis. v) The ultrasonic examination shall be performed using a device with an automated computer data acquisition system. vi) The UT procedure and personnel shall be qualified using mock-ups that are similar to the component weld being examined. vii) Flaws exceeding the acceptance criteria referenced in this Case shall be repaired or reduced to an acceptable size, and the weld subsequently reexamined using the same examination procedure that detected the flaw. Appendix 1: UT Procedure and Personnel Qualification Program UT personnel and procedures shall be qualified in accordance with the following. The main criteria (but not all) are described here: Specimen Requirements The qualification test specimens shall have a minimum of ten flaws and shall include the minimum and maximum pipe diameters and thickness for which the examination procedure is applicable. The specimen set shall include examples of the fabrication conditions e.g. Counterbore or weld root conditions, cladding, weld buttering, remnants of previous welds, adjacent welds in close proximity, weld crowns etc. Flaw Location: Flaws should be located in the butt weld joint and its heat affected zone. At least 10% of flaws should be distributed in the upper third and middle third of the through wall thickness respectively, and at least 50% of flaws should be located within the lower third of the test specimens. Flaw Classification: At least 25% of flaws shall be surface flaw, and at least 40% of flaws shall be subsurface flaw. A flaw is considered as a surface flaw if the nearest distance from the surface, S, is less than 0.4 d where 2d is the depth of the flaw (Fig. 1). Flaw Type: At least 50% of flaws shall be planar flaws, such as lack-of-fusion (LOF), incomplete penetration or crack. At least 30% of flaws shall be volumetric flaws, such as slag inclusion, porosity or lack-of-bond. Flaw Depth: Flaw depth shall be within 5% to 50% of nominal wall thickness. At least 20% of flaws located in lower third of the test specimens shall be smaller than 10% of wall thickness, but equal to or not less than 5% of wall thickness. Flaw Orientation: At least 10% but no more than 20% of flaws shall be axially oriented, and rest of flaws shall be circumferentially oriented. Performance Demonstration Personnel and procedure performance demonstration tests shall be conducted according to the following requirements: Detection test: The performance demonstration test shall be conducted as a blind test. Specimens shall be divided into grading units with specific criteria for each grading unit as stated in the Code Case. The number of unflawed grading units shall be at least 1 ½ times the number of flawed grading units. Flawed and unflawed grading units shall be randomly mixed. Examination equipment and personnel are qualified for detection when personnel performance demonstrations satisfy the acceptance criteria for both detection and false calls as stated in the Code Case. Flaw evaluation test: In addition to the flaw detection tests, the Code Case also requires flaw evaluation tests. In addition to flaw depth sizing, each flaw shall be classified as either surface flaw or sub-surface flaw. A flaw is considered as a surface flaw if the nearest distance from the surface, S, is less than 0.4 d where 2d is the depth of the flaw (Fig. 1). The acceptability of the flaw is based on the acceptance criteria in Appendix 2 of the Code Case. Procedure Qualification The Code Case specifies procedure qualification requirements using specimen tests similar to those specified for performance demonstration. Appendix 2 UT Flaw Acceptance for FFS Code Case This Appendix provides standards for acceptance of subsurface flaws. Flaws exceeding these acceptance standards shall be repaired, and the weld subsequently reexamined using the same UT procedure that detected the flaw. Because of environmental crack growth concerns, surface flaws are not acceptable. However, the flaws can be removed (by machining or grinding) in accordance with the requirements of Section III, NB-2538 or NC No weld repair is needed if 3 Copyright 2011 by ASME

4 the thickness after defect removal meets the minimum thickness requirement in Section III NB-3641 and NC Acceptance Standards for Full Penetration Butt Welds in Ferritic and Austenitic Piping and Vessel Nozzles-to-Piping A fabrication subsurface flaw (depth 2a in Fig. 1) shall not exceed 20% of maximum wall thickness regardless of length. Acceptance Standards for Pressure Retaining Vessel and Nozzleto-Vessel Welds A fabrication subsurface flaw (depth 2a in Fig. 1) shall not exceed 4% of wall thickness regardless of length. UT Demonstration The Electric Power Research Institute (EPRI) has performed extensive UT work to demonstrate the ability to detect and size flaws. Because Appendix 1 of Code Case requires UT performance demonstration, EPRI implemented a major effort to demonstrate that there are available transducers and UT techniques that can be used by the industry to meet the demonstration requirements of the Code Case. The EPRI NDE demonstration included the following key segments: Flaw Detection Demonstration of the flaw detection capability must be based on both flawed and unflawed specimens. Both the ability to detect flaws and the discrimination to avoid false calls in unflawed specimens must be demonstrated. The UT demonstration program described here addresses this. Flaw Evaluation to specify the through-thickness location of the flaw (i.e. surface or subsurface flaw) According to the proposed Code Case and Section XI, IWB- 3500, the flaw acceptance criteria are significantly different for surface and subsurface flaws. For example, surface flaws are not acceptable under the Code Case because of environmental crack growth concerns, but a subsurface flaw would be acceptable depending on its size. Therefore, accurately specifying the through thickness location of the flaw is critical for this project. Flaw type (i.e. planar or non-planar) In addition to specifying the through-thickness location of the flaw, the acceptance criteria of the Code Case are different depending on the flaw types. Identifying the flaw type is important because the resolution of the indication is different. For the demonstration project, flaws were classified into two indication types: planar and non-planar. Laminar flaws, porosity, and slag inclusions were classified as non-planar. Flaw Sizing (both length and height) In addition to flaw detection, accurate flaw depth sizing is necessary to determine the acceptability of the weld, that is, whether the flaw is acceptable as is or requires removal or repair. To verify the NDE techniques and capabilities, test samples (of dissimilar metal weld configuration) were designed and fabricated for flaw detection and sizing. Details of the UT demonstration data acquisition and analysis, UT instrumentation and examination parameters, preparation of the weld mockups, and comparison of the UT findings relative to the actual flaws are described here. EPRI designed and fabricated a total of fifteen test samples. All of the samples were from dissimilar metal weld joints with outside diameter ranges of 14 to 32 in. and wall thickness ranges of 1.13 to 2.46 inches. The weld configuration details are shown in Table 1. The scope of the UT demonstration covered detection, flaw evaluation (i.e. surface vs. subsurface, planar vs. non-planar) and depth sizing. Table 2 shows a comparison of the UT results in comparison with the actual as-built data. The comparison is described below: A total of 65 weld defects were inserted in the 15 test specimens. It included 9 surface flaws and 56 subsurface flaws. 35 flaws were planar and 30 flaws were non-planar (e.g. slag, porosity, lack of bond, incomplete penetration). The UT demonstration was performed using an automated UT Phased Array system (Fig. 2). The UT equipment and scanning parameters are described in Fig. 3. A flow chart describing the process for characterization and classification of the weld defect is shown in Fig. 4. The characterization of flaw type planar vs. non-planar flaw and the process for the classification of weld defects surface vs. subsurface flaw are shown Fig. 5 and Fig. 6. The results of the UT demonstration were excellent. 64 of the 65 weld defects were detected successfully. 7 of the 9 surface flaws were evaluated correctly but two surface flaws were evaluated as subsurface. 52 of the 56 subsurface flaws were evaluated correctly (four were classified as surface). 34 of the 35 planar flaws and 25 of the 30 non-planar flaws were characterized correctly. Justification of Flaw Acceptance Standards in Appendix 2 of the Code Case The acceptance criteria in Appendix 2 of the Code Case are based on the flaw acceptance standards in Section XI, IWB Flaw sizes that are less than the acceptance standards in IWB-3500 are considered to be benign and are acceptable for continued operation without the need for evaluation or reinspection. Tables 3 and 4 show the Section XI Acceptance Standards for Piping and Vessel Flaws respectively. As shown in Fig. IWB , Section XI designates d as the crack depth for an individual surface flaw (2d for a subsurface flaw), but uses the term a for the crack depth for a surface flaw after flaw combination (2a for a subsurface flaw). Both a and d are the same and represent the crack depth for a surface flaw and similarly, 2a and 2d are the corresponding depths for a subsurface flaw. They are used interchangeably in this paper. 4 Copyright 2011 by ASME

5 Justification for Flaw Acceptance Criteria for Piping Welds The Section XI piping surface flaw acceptance standards in Table 3 show that for a/l = 0 where l is the crack length (or very long flaws relative to the depth), the acceptance value of a is 10% of wall thickness. Because the depth for a subsurface flaw is 2a, the acceptance flaw depth is 20% of wall thickness. The acceptance standard for shorter flaws (a/l > 0) is somewhat higher, so the acceptance flaw size of 20% in the Code Case is conservative. Justification for Flaw Acceptance Criteria for Vessel Welds The Section XI vessel flaw acceptance standards in Table 4 show that for a/l = 0.05 where l is the crack length (or very long flaws relative to the depth), the acceptance value of a is 2% of wall thickness. Because the depth for a subsurface flaw is 2a, the acceptance flaw depth is 4% of wall thickness for vessels with wall thickness 4 to 12 in. nominal. Somewhat higher percent depth may be justified for vessel thickness lower than 4 inches. The acceptance standard for shorter flaws (a/l > 0) is somewhat higher, so the acceptance flaw size of 4% in the Code Case is conservative. Consideration of Surface Flaws As discussed earlier, because of environmental crack growth concerns, surface flaws are not acceptable. However, the flaws can be removed (by machining or grinding) in accordance with the requirements of Section III, NB-2538 or NC No weld repair is needed if the thickness after defect removal meets the minimum thickness requirement in Section III NB-3641 and NC References [1] Section III, ASME Boiler and Pressure Vessel Code Rules for Construction of Nuclear Facility Components, Subsection NB, American Society of Mechanical Engineers, 2010 Edition. [2] Section XI, ASME Boiler and Pressure Vessel Code Rules for Inservice Inspection of Nuclear Power Plant Components, American Society of Mechanical Engineers, 2010 Edition. [3] F. A. Simonen, G. J. Schuster, and S. R. Doctor, Effects of Weld Repairs on Calculated Failure Probabilities for Reactor Pressure Vessels and Piping, PVP , PVP Volume 463, American Society of Mechanical Engineers, New York, NY. [4] API 579-1, Fitness for Service, American Petroleum Institute, June [5] ASME C&S Record , Proposal File, 2011, American Society of Mechanical Engineers, New York, NY. CONCLUSIONS Current ASME Section III acceptance standards use RT and require the rejection and repair of flaw indications characterized as cracks, lack of fusion, or incomplete penetration regardless of the size of the indication. The Code Case described here provides the use of UT examinations as an alternative to RT. Section III RT can only provide flaw length. The EPRI UT demonstration results described here confirm that UT can accurately detect and provide the flaw location, flaw length, and size in the through-wall dimension to support detailed flaw assessment. The flaw acceptance criteria based on the fitnessfor-service approach using fracture mechanics methods allows the justification for acceptance of benign flaws that have no structural significance. This in turn avoids the need for weld repair that is required under current Section III rules. The high residual stresses resulting from weld repair can result in stress corrosion cracking. Field experience has shown a clear correlation between stress corrosion cracking in dissimilar metal Nickel-base welds and local weld repair in operating reactors. Clearly, the Code Case provides the alternative of using UT (instead of RT which is ineffective in crack sizing) and allows the acceptance of structurally benign flaws. 5 Copyright 2011 by ASME

TABLE 1: WELD CONFIGURATIONS USED IN THE EPRI UT DEMONSTRATION STUDY TABLE 2:

7 TABLE 3: SECTION XI ACCEPTANCE STANDARDS FOR PIPING FLAWS (FROM 2010 EDITION) Allowable Planar Flaws Materials: Ferritic steels that meet the requirements of NB-2300 and the specified minimum yield strength of 50 ksi (350 MPa) or less at 100 F (40 C) and Austenitic steels that meet the specified minimum yield strength of 35 ksi (240 MPa) or less at 100 F (40 C) Aspect Ratio, a/l Volumetric Examination Method, Wall Thickness 1,2, t, in. (mm) (8) and less 1.0 (25) 2.0 (50) 3.0 (75) and over Surface Flaw, Subsurface Surface Flaw, Subsurface Surface Flaw, Subsurface Surface Flaw, Subsurface Preservice and Inservice Examination Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y to Y Y Y Y 0.96 General Note This table is not applicable to planar surface connected flaws that are in contact with the reactor coolant environment during normal operation and are detected by inservice examination in materials that are susceptible to stress corrosion cracking as defined for PWRs in IWB-3514(a)(1) and for BWRs in IWB-3514(a)(2) and IWB-3514(b). For planar surface connected flaws that are in contact with the reactor coolant environment during normal operation and are detected by preservice examination in these materials, the requirements of IWB shall be satisfied. Notes 1. For intermediate flaw aspect ratios a/l and thickness t, linear interpolation is permissible. 2. t is the nominal wall thickness of actual wall thickness if determined by UT examination. 3. The total depth of a subsurface flaw is 2a. 4. Y = [(S/t)/(a/t)] or (S/a). Y is flaw to surface proximity factor and S is defined in Figure 1. If S < 0.4d, the subsurface flaw is classified as a surface flaw. If Y > 1.0, use Y = Copyright 2011 by ASME

8 TABLE 4: SECTION XI ACCEPTANCE STANDARDS FOR VESSEL FLAWS (FROM 2010 EDITION) Allowable Planar Flaws Material: Ferritic steels that meet the requirements of NB-2331 and G-2110(b) from Section III Volumetric Examination Method, Nominal Wall Thickness 1,2, t, in. (mm) 2 ½ (650) and less 4 (100) to 12 (300) 16 (400) and greater Aspect Surface Flaw, Subsurface Surface Flaw, Subsurface Surface Flaw, Subsurface 5 Flaw, Ratio, a/l Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y 1.0 Notes 1. For intermediate flaw aspect ratios a/l and thickness t, linear interpolation is permissible. 2. Component thickness t, is measured normal to the pressure retaining surface of the component. Where the section thickness varies, the average thickness over the length of the planar flaw is the component thickness. 3. The total depth of a subsurface flaw is 2a. 4. Y = [(S/t)/(a/t)] or (S/a). Y is flaw to surface proximity factor and S is defined in Figure 1. If S < 0.4d, the subsurface flaw is classified as a surface flaw. If Y > 1.0, use Y = Applicable to flaws in the surface region B-E shown in Figure IWB only if the maximum postulated defect of Appendix G of section III is justified. 8 Copyright 2011 by ASME

S 2d Flaw considered as a surface flaw if S<0.