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Pressure Vessel Inspections Using Ultrasonic Phased Arrays By Michael Moles, Noël Dubé and Frédéric Jacques

Abstract Phased arrays offer significant technical advantages for weld inspections over conventional ultrasonics. The phased array beams can be steered, scanned, swept and focused electronically. Beam steering permits the selected beam angles to be optimized ultrasonically by orienting them perpendicular to the predicted defects, especially Lack of Fusion. Electronic (linear) scanning permits very rapid coverage of the welds. Beam steering (usually called sectorial or azimuthal scanning) can be used for mapping welds at appropriate angles to optimize Probability of Detection of defects. Electronic focusing permits optimizing the beam shape and size at the expected defect location, also to optimize Probability of Detection. Overall, the use of phased arrays permits optimizing defect detection while minimizing inspection time. The paper describes the application of phased arrays for inspecting pressure vessel welds. Phased arrays offer significant practical advantages over conventional automated inspections. Thick section weld inspections typically use the established "top, side, end" or "top, side, TOFD" views of the weld. Other displays can be used, e.g. strip charts for zone discrimination scanning of narrow gap welds. Special inspections can be easily performed with phased arrays, e.g. additional beams for extra coverage, multiple angles or inspection set-ups simultaneously, or special scans such as tandem probes. Different delivery systems and instrumentation can be assembled for any required scan. FitnessFor-Service inspections requiring high PoD and accurate sizing can be performed using upscale systems. These phased array inspections can be tailored to any known code requirements. Introduction: A wide variety of pressure vessels are used in a similarly wide variety of industries: petrochemical, defense, oil and gas, energy, general manufacturing. These vessels are typically made from sections (usually ferritic steel) and welded together. Unfortunately, welding often produces defects, which can propagate with time and fail, sometimes catastrophically. The welds are (usually) inspected for defects, normally using the ASME code. For pressure vessel inspections, ASME Section V (ASME 2001) is the relevant code, with NDT covered under Articles 4 and 5. The ASME code has been the mainstay of PV inspections for decades.

02 a 06 de Junho de 2003 / June 2 to 6 2003 Rio de Janeiro - RJ - Brasil

Why Ultrasonics? Radiography has been the mainstay of pressure vessel inspections for many years. However, radiography has significant technical disadvantages. First, it is hazardous; personnel have to be moved away, or the vessel must be moved to a safe location for inspection. Second, radiography typically delays production or delays inspection, potentially leading to bottlenecks or unexpected repairs. Third, radiography only detects critical lack of fusion and cracking defects when they are suitably oriented. Fourth, radiography cannot size the vertical height of defects, which eliminates any application of Engineering Critical Assessment (ECA) (also called Fracture Mechanics). Fifth, it is subjective. Six, film costs and exposure times are high. The competing inspection technology is ultrasonics. In contrast to radiography, ultrasonics is safe, can be done on site, can be performed as soon as the weld is cool enough, should minimize any production delays, and can size the vertical height of defects with some accuracy and confidence. Until the nuclear industry started developing automated ultrasonic techniques a couple of decades ago, ultrasonic testing was performed manually. Manual UT is time-consuming and highly operator-dependent. Round robin testing showed significant variations in ability and results, including human factors. Mechanized or automated ultrasonics (AUT) has been available for some time, but has been slow and relatively expensive for day-today use. However, speeds have increased and costs reduced with time. Generally, AUT has proved a lot more repeatable than manual UT. The advent of diffraction-based techniques like TOFD have significantly improved defect sizing for ECA applications. More recently, ultrasonic phased arrays have become commercially available. Phased arrays now offer the considerable flexibility needed for the wide variety of pressure vessel welds on the market, plus increased inspection speed. Phased arrays have been penetrating the market for key applications, but still suffer from the disadvantage of being a new technology, and requiring skilled operators. Equipment purchase costs are still comparatively high, but are decreasing. However, operating (i.e. variable) costs are lower than radiography. The ASME Code and AUT: ASME Section V is normally used for manual ultrasonic testing (MUT), but has provisions for AUT as well. For example, Article 4 T-436 refers to "Computer Imaging Techniques", while Article 5 requires details of data recording and scanning mechanisms to be included in the procedure description. For example, ASME V permits alternate techniques to the usual DAC (Distance Amplitude Correction) curves, calibration block design and targets. In fact, Section V Article B-4 Appendix B-6 specifically states that other techniques can be used, while Section V Article 1 in T-150 indicates that alternate techniques should be equal or superior to the standard techniques (i.e. MUT). An Authorized Inspector must approve any such (documented) procedures.

Ultrasonic inspections are required in several of the ASME pressure vessel and power piping Codes and Code Sections. These Sections; I, III, VIII and XI as well as the piping requirements of B31.1 and B31.3; invariably reference Section V for details of requirements in methods, procedures and qualifications. These inspections usually specify "raster scanning" in pulse-echo mode, using two suitable angles at least 10o apart, with specifications on coverage etc. ASME Code Case 2235 More recently, ASME published Code Case 2235-4 (ASME 2001), which permits "nonamplitude, computer recorded" inspection techniques, which are widely interpreted as Time-Of-Flight Diffraction (TOFD). Code Case 2235 is an option for replacing radiography for fitness-for-service acceptance under specific circumstances (ASME 2001 a). This Code Case requires a performance demonstration, showing detection on three flaws, as a minimum. Other Codes There are other Codes or Standards regulating the requirements of pressure vessels, including API 510 (pressure vessel code), API 570 (piping), API 572 (inspection of vessels) and a variety of European codes. Ultrasonic phased arrays

How phased arrays work

Ultrasonic phased arrays are similar in principle to phased array radar, sonar and other wave physics applications. However, ultrasonic development is behind the other applications due to a smaller market, shorter wavelengths, mode conversions and more complex components. Several authors have reviewed applications of ultrasonic phased arrays (Clay et al, 1999, Wustenberg et al., 1999, Lafontaine and Cancre, 2000), though industrial uses have been limited until the last few years. From a practical viewpoint, ultrasonic phased arrays are merely a method of generating and receiving ultrasound; once the ultrasound is in the material, it is independent of generation method, whether generated by piezoelectric, electromagnetic, laser or phased arrays. Consequently, many of the details of ultrasonic inspection remain unchanged; for example, if 7.5 MHz were the optimum inspection frequency with conventional ultrasonics, then phased arrays would typically use the same frequency, focal length, and incident angle. Phased arrays use an array of elements, all individually wired, pulsed and time-shifted. These elements are typically pulsed in groups from 4-32, usually 16 elements for pipeline welds. In order to make a user-friendly system, a typical set-up calculates the time-delays from operator-input, or uses a pre-defined file containing inspection angle, focal distance, scan pattern etc (see Figure 1). The time delay values are back calculated using time-of-

flight from the focal spot, and the scan assembled from individual "Focal Laws". Time delay circuits must be accurate to around 2 nanoseconds to provide the accuracy required.

Figure 1: Schematic showing generation of linear and sectorial scans using phased arrays. While it can be time-consuming to prepare the first set-up, the information is recorded in a file and only takes seconds to re-load. Also, modifying a prepared set-up is quick in comparison with physically adjusting conventional transducers. Types of scans Using electronic generation and receiving provides significant opportunities for a variety of scan patterns.

Electronic (Linear) Scans

Multiplexing along an array produces electronic scans (see Figure 2). Typical arrays have up to 128 elements, pulsed in groups of 8 to 16. Electronic scanning permits rapid coverage with a tight focal spot. If the array is flat and linear, then the scan pattern is a simple B-scan. If the array is curved, then the scan pattern will be curved. Linear scans are straightforward to program. For example, a phased array can be readily programmed to inspect a weld using 45o and 60o shear waves, which emulate conventional manual inspections.

Figure 2: Schematic illustration of electronic (linear) scanning.

Sectorial (Azimuthal) Scans

Sectorial scans use the same set of elements, but alter the time delays to sweep the beam through a series of angles (see Figure 3). Again, this is a straightforward scan to program. Applications for sectorial scanning typically involve a stationary array, sweeping across a relatively inaccessible component like a turbine blade root (Ciorau et al, 2000) to map out the features (and defects). Depending primarily on the array frequency and element spacing, the sweep angles can vary from + 20o up to + 80o.

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Figure 3: Schematic showing sectorial scanning used on turbine rotor.

Combined Scans

Combining electronic scanning and sectorial scanning with precision focusing leads to a practical combination of displays (see Figure 4 for an example). Optimum angles can be selected for welds and other components, while scanning permits fast and functional inspections. A related approach applies to weld inspections, where specific angles are often required for given weld geometries; for these applications, specific beam angles are programmed for specific weld facets at specific locations. This is the principle of zone discrimination (see below).

Figure 4: top, ultrasonic scanning pattern using sectorial and electronic scanning; bottom, ultrasonic image using all data merged together.

Many multiprobe systems and phased arrays use a "linear scanning" approach. Here the probe pan is scanned linearly round or along the weld, while each transducer sweeps out a specific area of the weld. This makes AUT with linear scanning much faster than single transducer techniques. Phased arrays for linear weld inspections operate on the same principle as the multiprobe approach, though the probe pans are much smaller; additionally, phased arrays offer considerably greater flexibility than conventional AUT. Typically, it is much easier to change the set-up electronically, either by modifying the set-up or reloading another; often it is possible to use many more beams (equivalent to conventional transducers) with phased arrays; special inspections can be implemented simply by loading a set-up file. Evaluating the ASME Code for Ultrasonics: Not surprisingly, several attempts have been made to determine how well ultrasonics performs on the detection and sizing of defects. These have been primarily driven by the nuclear industry, which has both the need and resources. The most extensive and expensive trial was the PISC II program, which involved over thirty teams and many thick samples (Nichols and Crutzen, 1988). Many of the inspections were performed using ASME ­related procedures, though alternative techniques and procedures were also used. The data permitted some general conclusions on Probability of Detection, and earmarked some novel techniques, such as TOFD for defect sizing. The PISC and other trials were aimed at evaluating techniques and procedures. In contrast, phased arrays are a technology, which can use any technique or procedure. AUT technology has advanced significantly since the nuclear trials in the early 1980's: focused transducers, improved procedures and techniques (TOFD and tandem), better data collection and display. Typical Inspection Approaches Due to their flexibility, phased arrays can perform any of the ASME code inspections. It is possible to divide the inspections into three approximate categories: 1. ASME CC 2235 TOFD scans: The simplest solution is to perform linear TOFD scans using phased arrays. To properly inspect the known TOFD dead zones on the inside and outside of the vessel, R/D Tech also believes in combining TOFD with pulse-echo. These inspections can be performed with a limited amount of equipment, but are essentially "single technique" approaches. 2. General phased array inspections: This approach uses the flexibility of phased arrays to perform raster scans to fulfill ASME Section V requirements. Thus linear scanning is used along the weld, with the arrays performing electronic scanning to give multiangle coverage. Besides conventional scans and TOFD, it is

possible to perform specialized scans for specific defects, e.g. transverse scans or pitch-catch for vertical defects. 3. Premium phased array inspections for ECA: Highly reliable inspections are required for some fitness-for-purpose inspections. With phased arrays, it is possible to develop custom systems which can use two or more independent inspection techniques for all areas (outside surface, inside surface, mid-wall region, transverse defects), plus use alternate diffraction techniques to improve sizing. These systems offer maximized Probability of Detection (PoD) with high inspection speeds. Figure 5 below shows the electronic scanning approach for fulfilling the ASME Section V code requirements. In this instance, traditional 45 and 60o raster scans are used, with the axial movement mechanical and the rastering movement electronic. By collecting all waveform data, full image reconstruction can be performed by merging the data.

Figure 5: rastering at two angles using a single array and linear scanning. Typical results on a medium pressure vessel weld inspection (around 50 mm) are shown in Figure 6 below. These results show merged 45o and 60o scan results in "top, side" format, plus an additional double TOFD display from twin TOFD pairs.

Figure 6: Left, "top, side" view of weld with defects. Right; TOFD scans of the same area.

The scan speed is primarily dictated by the specifications, specifically the type of scan (raster, TOFD or zone discrimination), amount of data collected (amplitude only, or full waveform), number of pulses etc. For zone discrimination, scanning speed is 100 mm/sec, while thick pressure vessels are usually scanned at speeds of 10-20 mm/sec. Delivery Systems Handscanners Like pressure vessels themselves, there are many different types of delivery systems. The simplest system is a handscanner, as shown below. Handscanners will work for a pair of arrays, plus some additional TOFD or other transducers. This is adequate for ASME CC2235 inspections, or raster inspections of thinner vessels.

Figure 7: Typical handscanner configuration. Belt Scanners Belt scanners work well for pipes and vessels up to around 1.5 m diameter. Accurately positioning them on larger vessels becomes difficult. Figure 8 below shows a welding band in use for pipeline girth weld inspections. Belt scanners permit much faster travel speeds than handscanners.

Figure 8: Welding band for pipeline girth weld inspections. Rotate the Vessel For larger vessels, the simplest and cheapest solution is to rotate the vessel. This is shown schematically in Figure 9 below. However, it is important to be able to track the weld with known accuracy.

Figure 9: Schematic showing rotating the vessel for weld inspections.

Magnetic Wheel Scanners Another alternative is to use magnetic wheel scanners, such as TRAKER shown below in Figure 10. TRAKER can follow a magnetic strip, so that any straight weld, nozzle or bend can be tracked simply by adjusting the strip.

Figure 10: TRAKER magnetic wheel scanner inspecting vertical weld. Summary Automated ultrasonics (AUT) is now permitted for many pressure vessel weld inspections, and offers significant advantages over radiography: no safety issues, inspections shortly after welding, better Probability of Detection, plus defect vertical sizing capability for ECA applications. Phased arrays offer excellent potential for pressure vessel weld inspections: both TOFD and ASME V raster scans can be performed, inspection speeds are significantly higher than older mechanized conventional ultrasonics, there is minimal operator subjectivity,

and inspections can be custom tailored. The main problems are initial equipment costs and the availability of trained operators. Conclusions 1. Automated ultrasonics can overcome the limitations of radiography for pressure vessel inspections: safety, high probability of defect detection, vertical defect sizing, low data storage costs, plus offers the practical ability of inspecting right after welding. 2. Phased arrays offer significantly improved inspections over conventional ultrasonic systems: great flexibility for different components, high inspection speeds, custom-tailored scans for specific defect types and geometry's, tailored imaging, rapid set-ups using electronic files. 3. The main limitations of phased arrays are the high (but dropping) initial cost, technology awareness, and operators. Keywords: welds, pressure vessels, ultrasonics, phased arrays, beam steering, electronic scanning, sectorial scanning. References ASME 2001, ASME Boiler and Pressure Vessel code, Section V, Nondestructive Examination, 2001 Edition, July 1, 2001, The American Society of Mechanical Engineers. ASME 2001, ASME Code Case 2235-4, November 30, 2001, The American Society of Mechanical Engineers. ASME 2001 a, ASME Code Case 2235-4, November 30, 2001, The American Society of Mechanical Engineers. See Section I, para. PW-11; Section VIII, Division 1, para. UW11(a); and Section VIII, Division 2, Table AF-241.1. Ciorau P., D. MacGillivray, T. Hazelton, L.Gilham, D. Craig and J.Poguet, "In-situ examination of ABB l-0 blade roots and rotor steeple of low-pressure steam turbine, using phased array technology", 15th World Conference on NDT, Rome, Italy, October 11-15, 2000. Clay A.C., S-C. Wooh, L. Azar and J-Y. Wang, "Experimental Study of Phased Array Beam Characteristics", Journal of NDE, vol 18, no. 2, June 1999, page 59. Lafontaine G. and F. Cancre, "Potential of Ultrasonic Phased Arrays for Faster, Better and Cheaper Inspections", NDT.net, vol 5, no. 10, October 2000 http://www.ndt.net/article/v05n10/lafont2/lafont2.htm

R.W. Nichols and S. Crutzen, "Ultrasonic Inspection of Heavy Section Steel Components - The PISC II Final Report", Kluwer Academic Publishers, 1988. Wüstenberg H, A. Erhard and G. Shenk, "Some Characteristic Parameters of Ultrasonic Phased Array Probes and Equipments", NDT.net, vol 4, no. 4, http://www.ndt.net/article/v04n04/wuesten/wsuesten.htm

The Authors 1. Michael D. C. Moles (Author for correspondence) R/D Tech 5205 Tomken Road Mississauga, Ontario Canada L4W 3N8 Tel: (905) 629-0220 Fax: (905) 629-8383 E-mail:[email protected] 2. Noël Dubé VP, Industrial Division R/D Tech 505, boul du Parc Technologique Québec, PQ Canada G1P 4S9 Tel: (418) 872-1155 Fax: (418) 877-0141 E-mail : [email protected] 3. Frédéric Jacques Manager, Heavy Industry R/D Tech 505, boul du Parc Technologique Québec, PQ Canada G1P 4S9 Tel: (418) 877-9535 Fax: (418) 877-0141 E-mail:[email protected]

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