Non-destructive testing of welds per ASME pressure vessel and piping codes [1-3] is outlined within the codes and additionally in ASME BPVC Section V. As it relates to pressure vessels, ultrasonic testing may be used in lieu of radiography for all welds in which the thinner of the members joined is 0.250” and greater [4]. The power piping code calls out volumetric examination, radiographic or ultrasonic, as minimum requirements for butt welds and welded branch The process piping code defaults to ASME Section V Article 4. This article discusses the ultrasonic and phased array ultrasonic testing (PAUT) options available during the fabrication and in-service inspection phases.
Introduction to AMSE BPVC Construction Code
The ASME BPVC (Boiler and Pressure Vessel Code) Section VIII covers rules for the construction of pressure vessels, and it is divided into Division 1 and Division 2 to suit various applications and design safety factors. A generic comparison of Division 1 and 2 is provided in the table below.
Table 1: ASME BPVC Division 1 and 2 Generic Comparison
ASME Construction Code | Division 1 | Division 2 |
Usage | This is the most commonly used division for pressure vessel construction. | Used when pressure vessels need to be designed with a higher precision to handle more demanding conditions, and where optimization of weight and material usage is important. |
Design Approach | Division 1 follows a simpler, prescriptive design approach, using thicker walls and conservative design factors to ensure safety. | Division 2 follows a more detailed and advanced design approach, incorporating more in-depth analysis (including finite element analysis and fatigue analysis). |
Safety Margins | It provides higher safety margins by applying more conservative stress limits (lower allowable stress). | It uses a lower factor of safety, meaning thinner walls and less material can be used compared to Division 1. However, the design methods used ensure a similar level of safety through more precise calculations. |
Applications | - Vessels with standard operational conditions and without the need for complex calculations. - Vessels used in industries such as oil and gas, chemical processing, and general manufacturing. - Preferred when cost is a consideration, especially for smaller, lower-pressure vessels.
| - High-pressure or high-temperature vessels. - Vessels that require fatigue analysis due to cyclic loading or demanding service conditions. - Situations where reducing vessel weight or material cost is important, such as offshore platforms or aerospace industries.
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The different scope between the two ASME construction code trickles down into the non-destructive testing requirements for pressure vessels constructed to the respective code. Division 1, in general, requires visual inspection and additional radiographic inspection depending on pressure vessel size, service and pressure rating. Part UW Requirements for Pressure Vessels Fabricated by Welding UW-51 (4) state that ultrasonic examination may be used in lieu of radiography in which the thinner of the material joined is at least 0.25”. UW-53 Ultrasonic Examination of Welded Joints (b) and (c) outline the requirements for ultrasonic examination and phased array manual raster examinations. The manual ultrasonic testing raster scan recommended requires a minimum 10% overlap. While a 45 degree angle is cited for the inspection, the actual angle should be selected based on the vessel thickness and weld reinforcement profile to ensure complete volumetric coverage of the weld and heat affected zone. A separate ultrasonic test using an ultrasonic beam angle in the 0 to 60 degree range is recommended for transverse reflectors.
Phased array ultrasonic testing may also be performed using a linear array in accordance with Division 1 and Division 2 code. In ASME Section VIII Division 1 code, phased array ultrasonic testing (PAUT) is deferred to ASME Section V, Article 4, Mandatory Appendices IV and V. The appendices outline the requirements to perform manual raster and encoded electronic (E-scan) or sectorial S-scans. The ASME code book cites the use of a linear array which can be confused with a linear scan, or E-scan. The linear array simply refers to a 1-D linear phased array probe which may be used to control E-scans or S-scans.
Ultrasonic Shear Wave Testing of Welds per AMSE BPVC
Manual and phased array ultrasonic testing of welds per ASME BPVC is a multi-dimensional scope that requires compliant equipment, personnel, calibration blocks, procedures, and reporting. Each of these sub-categories is also multi-dimensional with enough layers to warrant a separate in depth article. These following sections serve only as introduction to the end result, a scan plan, developed for a 30” diameter, 0.80” thick pipe joined together using a single vee butt weld for each ultrasonic option described in ASME Section V Division 1 and ASME Section V.
In the example below, the scan plan for a 45 and 60 degree shear wave inspection of the welded pipe is shown. While 45 degrees is the code cited shear wave angle, it does not provide any weld coverage in the first leg which is undesirable, especially for the root of a single vee. Alternatively, a 60 degree shear wave does provide some limited root coverage in the first leg and as the index offset is increased full weld and HAZ coverage is obtained.
Figure 1. Example 45 degree shear wave scan plan for 30” diameter 0.080” single vee butt weld showing limited roof coverage in the first leg.
Figure 2. Example 60 degree shear wave scan plan for 30” diameter 0.080” single vee butt weld showing root coverage in first leg.
Manual E-scan and S-scan Testing of Welds per AMSE BPVC
Manual phased array E-scan and S-scan guidelines are described in ASME BPVC Sec V Article 4 Mandatory Appendix IV in very general terms over only 2 pages and defers details to ASTM E2700-20 Standard Practice for Contact Ultrasonic Testing of Welds Using Phased Arrays [5]. ASTM E2700-20 presents example S-scans and E-scans for thin butt welds, thick butt welds, corner joints, T-joints from the flange, and T-joints from the web. The standard describes proper shear wave angle selection and index offset range to cover the weld volume and heat affected zones for thin and thick wall piping and vessels. Depending on the phased array transducer and wedge selected multiple groups and/or index offset position may be required. Figure 3 below shows inadequate weld root coverage using a 45 degree linear scan. Figure 4 shows that selecting a 60 degree linear scan provides some root coverage in the first leg. As the index offset is increased manually, the entire weld and HAZ weld volume is covered. Per ASTM E2700-20, the scan plan shall be duplicated on the opposite side of the weld.
Figure 3. Example 45 degree PAUT E-scan plan for 30” diameter 0.080” single vee butt weld showing poor coverage in first leg.
Figure 4. Example 60 degree PAUT E-scan plan for 30” diameter 0.080” single vee butt weld showing minimal coverage in first leg.
Encoded E-scan and S-scan Testing of Welds per AMSE BPVC
While not implicit in the title of ASME BPVC Sec V Article 4 Mandatory Appendix IV, Phased Array E-scan and S-scan Linear Scanning Examination Techniques, the scope of this section is encoded PAUT scans. Also similar to Mandatory Appendix IV, the inspection process is very generally described with a detailed outline describing PAUT encoded scanning procedure requirements but does not explicitly reference ASTM E2700-20. The standard discusses some of the key elements of automated or semi-automated scanning using a positional encoder. Some of the key parameters include confirming encoder position accuracy within 1%, guide mechanisms that ensure an accurate index is maintained, and maximum scan axis resolution. For example, at least three scan axis data points are required for the length of the smallest required detectable defect length but shall not exceed 2 mm (0.80”).
Example scan plans for encoded PAUT S-scans are shown below. In the top scan, the weld root is adequately covered in the first leg using a 45-70 degree S-scan. Full weld and HAZ coverage are achieved by increasing the index offset to -17 mm to -30 mm. Per code and standard requirements, the process must be duplicated on the opposite side of the weld with a second transducer and or a second scan. The examples provided below can be improved through intelligent selection of phased array probe, phased array wedge, aperture size, start element, and using multiple groups to enhance weld and HAZ coverage in the first and second legs.
Figure 5. Example 45 – 70 PAUT E-scan plan for 30” diameter 0.080” single vee butt weld showing root and weld volume coverage in the first and second legs.
Figure 6: Example 45 – 70 PAUT E-scan plan for 30” diameter 0.080” single vee butt weld showing full weld coverage.
Summary
ASME code compliant conventional shear wave and phased array ultrasonic testing requires a detailed understanding of pressure vessel construction code, non-destructive testing code, piping codes, and ASTM E2700 requirements. Exact code compliant requirements for personnel, equipment, calibration blocks, procedures, and reporting are embedded in the are initiated in the construction codes deferred to non-destructive testing code book. This article touches on some of the requirements of ASME BPVC Rules for Construction of Pressure Vessels Section VIII Division 1, 2023 and subsequent requirements in ASME Boiler and Pressure Vessel Code, Section V Non-destructive Examination, 2023 and ASTM E2700-20 Standard Practice for Contact Ultrasonic Testing of Welds Using Phased Arrays.
References
1. ASME Boiler and Pressure Vessel Code, Section V Non-destructive Examination, 2023
2. ASME B31.1 Power Piping, 2023
3. ASME B31.3 Process Piping, 2023
4. ASME BPVC Rules for Construction of Pressure Vessels Section VIII Division 1, 2023
5. ASTM E2700-20 Standard Practice for Contact Ultrasonic Testing of Welds Using Phased Arrays