Phased Array Ultrasonics Now Replacing Radiography for Small Bore Piping Welds

Phased Array Ultrasonics Now Replacing Radiography for Small Bore Piping Welds

By Mark Carte and Michael Moles, Olympus NDT

Phased array ultrasonics is steadily replacing radiography for construction weld inspections, in particular, using encoded scanning. An encoder is a device that coordinates distance with recorded digital data for storage in a computer, unlike conventional ultrasonics. Encoding permits auditable, repeatable scanning with additional sizing capabilities. The encoder has a resolution of 32 steps/mm, which is plenty for inspection of welds. Portable phased arrays offer significant advantages over radiography for detection, sizing, imaging, and characterization of defects in welds. Unlike conventional ultrasonics, phased arrays (PA) use electronically-controlled time delays to sweep, steer, and focus beams. Arrays are similar to conventional transducers, but are sliced into small elements for phasing. It is easy to change either arrays or the contoured wedges.

 

Phased array probes typically consist of a transducer assembly with anywhere from 16 to as many as 256 small individual elements that can each be pulsed separately. These may be arranged in a strip (linear array), a ring (annular array), a circular matrix (circular array), or a more complex shape. As is the case with conventional transducers, phased array probes may be designed for direct contact use, as part of an angle beam assembly with a wedge, or for immersion use with sound coupling through a water path. Transducer frequencies are most commonly in the range from 2 MHz to 10 MHz. A phased array system will also include a sophisticated computer-based instrument that is capable of driving the multi-element probe, receiving and digitizing the returning echoes, and plotting that echo information in various standard formats. In general, performing a PA inspection is fairly straightforward. Equipment and training are available, so the major emphasis is on correct setups and interpreting the results.

 

The ASME Boiler and Pressure Vessel Code Section V, Article 4, has published rules for performing phased array ultrasonic inspections of welds. Besides being safer and more repeatable, PA systems are faster for high-volume weld  inspections. These inspection devices can be used for a number of different applications where small bore piping placed in close proximity creates an inspection challenge. In many cases, defects can be clearly identified and characterized, and locations mapped. This article will describe some results obtained during testing.

 

Advantages of PA over Traditional Ultra sonic Techniques

 

Phased arrays offer significant advantages in speed, imaging, data auditing, and flexibility over traditional ultrasonic techniques. Not surprisingly, encoded phased arrays, either mechanically or semi mechanically driven, have become a serious competitor to radiography testing (RT) for welds. Radiography testing has been thestandard for weld inspections for the last several decades; however, it has known limitations. Specifically, RT has major safety issues from radiation, licensing issues from the same problem, work disruptions, environmental and chemical wastes, large volumes of film, and film deterioration. Defect analysis can be subjective and quite slow. In addition, RT is unable to reliably size defects in the vertical plane for structural integrity, and is poor at detecting planar defects. In contrast, there are no safety or licensing issues, film deterioration, or storage issues with PA. Phased arrays can size defects in the vertical plane, within known errors. The inspection speed is generally a lot higher when using encoded linear scanning. Probability of detection (POD) is improved using multiple angles and imaging, and results are fully auditable when encoded^{(1, 2)}.

 

Time-of-flight diffraction (TOFD) is the other common automated ultrasonic testing (AUT) technology and has been covered by ASME ⁽³⁾ since 2004. TOFD uses two transducers in a pitch-catch arrangement and measures arrival times and defect sizes to high accuracy. However, the main problem with scanning small-diameter pipes occurs when wall thicknesses are less than ~10 mm (3/8"). When this happens, TOFD becomes less useful as the dead zones at the outside diameter (OD) and inside diameter (ID) dominate the wall thickness.

 

Industry Awareness

 

Most design engineers, procurement personnel, and outage planners have not recognized that compared to RT, PA has the ability to reduce overall cost in manufacturing facilities and at plant sites. Those who are using PA technology have recorded significant cost savings in all facets of construction, but even more so in reduction of downtime during outages of boilers, furnaces, and other projects involving small bore piping in close proximity to other pipes and

obstructions.

 

Inspection service providers have reported that more than 120 welds of 3" pipe butt welds can be scanned in one eight‑hour shift. Scanning can occur very soon after welding when the metal is sufficiently cool. Additionally,

testing of small bore pipe welds using ultrasonic testing instruments and manually driven, semi-automated scanners do not require A/C power. This adds a tremendous complement to safety.

 

Figure 1: Small, hand-held scanner in action.

 

Codes

 

One major issue facing automatic ultrasonic testing (AUT) is code acceptance, but that improved significantly in July 2010, when ASME published three Mandatory Appendices on AUT (4) and two on phased arrays (5). The three Mandatory Appendices on AUT should allow operators to inspect welded components using AUT with more clearly defined inspection criteria. In addition, ASME recently published a Code Case on calibration for pipes (6), which permits much greater flexibility in pipe diameter (0.9 to 1.5 times the nominal) and in wall thickness (+ 25%), in keeping with other global codes. The three AUT Mandatory Appendices effectively replace Code Case 2235(7). The two Mandatory Appendices on phased arrays cover both manual and encoded scanning. The Mandatory Appendix for encoded scanning requires fully automated or semi automated scans, with appropriate data recording, displays, reporting, and scanning conditions. Section V, Article 4, can be called upon by any number of ASME referencing codes, which includes Sections I, VIII, and XII. In addition, other non-pressure vessel codes can call for ultrasonics and use these set-up rules, e.g., ASME B31.1 and B31.3.

 

Equipment

 

Industry demand for off-the shelf equipment to provide complete application solutions for small-diameter pipes has driven manufacturers to design and produce small scanners (see Figures 1 and 2). These scanners are semi-automated (encoded and hand pushed around the weld).

 

Hand-propelling saves costs, is technically easier, and provides ultralow- profile design, which makes scanning between tubes convenient. The scanner itself can be adapted to a range of sizes matched to the pipe diameter. As it is spring-loaded, it can inspect both carbon steel and non-magnetic materials (e.g., stainless steels). Field experience has shown that the scanner provides good coupling for 360o around the pipe, which is essential. These low-profile scanners can inspect pipe diameters from 21 mm (0.84") OD to 115 mm (4.5") OD. Clearance, including the low-profile array, is only 12 mm, which permits it to inspect most small-diameter welds in most configurations. It is waterproof, rust-free, and CE compliant. Scanners can be configured to inspect both sides of unobstructed circumferential welds. (See Figure 2.)

Figure 2: Two-sided scan being performed on small-diameter vertical pipe.

 

For welds with one-sided access only (flanges or pipes-to-component), the scanner can be re-configured for single access. (See Figure 3.)

 

Figure 3: Phased array probes and scanner positioned to scan a weld with single-sided access.

 

Weld Quality Review "On the Fly"

 

In an effort to streamline weld inspection, the latest portable ultrasonic instruments are capable of viewing images of a weld using S-scans (side or swept angle scans) and C-scan (plan) views simultaneously. Such features allow inspectors to determine go/no-go very quickly; therefore, production of welds is not impacted due to inspection. In many cases, anomalies produced due to welding procedures are identified and corrected very early, which helps reduce the quantity of repairs and lost production.

 

Another major advantage of using PA is that there is no disruption to the production schedule. PA is "safe," so no clearing or local shutdowns are required to minimize safety issues.

 

Multigroup scanning (scanning up to eight channels of data at the same time), is yet another time-saving feature of PA. Multigroup effects are where two separate images are shown, one from each side of the weld. These images consist of two A-scans (waveforms), S-scans, and C-scans. The A-scans show the time-corrected gain (TCG) and angle-corrected gain (ACG) data, such that all points above the recording threshold at any scanning angle show the same color/palette. This is very convenient for analyzing results quickly. Devices such as Olympus' NDT portable PA instrument OmniScan have linked cursors, so any item selected by a cursor in one image will link to other images automatically.

 

Measuring Defect Depth

 

Measuring weld defect depth on thin-walled pipes is not always required by code. This type of inspection is usually more challenging, but also more critical for ensuring structural integrity. The traditional method of amplitude drop-off gives a reasonable estimate of defect depth. Zooming the image and recording the depth will give better defect sizing and more precise defect location. Both can be recorded in the defect tables. Sizing defects can also be based on diffraction rather than amplitude.

 

Data display with S-scan and C-scan (side and plain views).

 

Conclusions

 

Instruments, software, and scanners are available off-the-shelf for performing standard PA weld inspections on small-diameter pipes. Phased arrays can be performed manually or by using automated scanners. Codes are published and available for PA and TOFD if required. Interpreting the results can be challenging, but there is potential for better defect detection and accurate sizing because of this new technology for weld inspection.

 

References

1. Olympus NDT, "Introduction to Phased Array Technology Applications," by R/D Tech, 2004.

2. Olympus NDT, "Phased Array Testing: Basic Theory for Industrial Applications," November 2010.

3. ASME Section V, Article 4, Mandatory Appendix III, "Time-of-Flight Diffraction (TOFD) Technique," 2004.

4. ASME Boiler & Pressure Vessel Code, Section V, Article

4, Mandatory Appendices VI-VIII.

5. ASME Boiler & Pressure Vessel Code, Section V, Article 4, Mandatory Appendices IV-V.

6. ASME Code Case 2638, "Alternative Piping Calibration Blocks: Section V," January 20, 2010.

7. ASME Code Case 2235-9, "Use of Ultrasonic Examination in Lieu of Radiography." Section I; Section VIII, Divisions 1 and 2; and Section XII", October 11, 2005.

 

Additional References

1. ASME Section V, Article 4, Non Mandatory Appendices N and O, "Time of Flight Diffraction (TOFD) Interpretation" and "Time-of-Flight Diffraction (TOFD) Technique: General Examination Configurations."

2. F. Jacques, F. Moreau and E. Ginzel, "Ultrasonic Backscatter Sizing Using Phased Array – Developments in Tip Diffraction Flaw Sizing," Insight, Vol. 45, No. 11, November 2003, p. 724.

3. J. Mark Davis and M. Moles, "Resolving Capabilities of Phased Array Sectorial Scans (S-Scans) on Diffracted Tip Signals," Insight, Vol. 48, No. 4, April 2006, p. 1.

 

Source: nationalboard .org                                                                            FALL 2012 NATIONAL BOARD BULLETIN