Journal of
Pressure Vessel
Technology
Technology Review
Keywords: Welds, Ultrasonic Phased Arrays, Beam Steering,
Focusing, Electronic Scanning, Defect Sizing, TOFD, Back
Diffraction, Pressure Vessels, Pipelines
Introduction to PV and Pipeline Inspections
Michael Moles
e-mail: michael.moles@rd-tech.com
R/D Tech, 73 Superior Avenue, Toronto, ON M8V 2M7,
Canada
Noël Dubé
e-mail: noel.dube@rd-tech.com
Simon Labbé
e-mail: simon.labbe@rd-tech.com
R/D Tech, 505 boul. du Parc Technologique, Québec, PQ
G1P 4S9, Canada
Ed Ginzel
e-mail: eginzel@mri.on.ca
Materials Research Institute, 432 Country Squire Road,
Waterloo, Ontario N2K 4G8, Canada
Major improvements in weld inspection are obtained using
Phased Array technology with capability for beam steering, electronic scanning, focusing, and sweeping the ultrasonic beams.
Electronic scanning is much faster than raster scanning, and can
optimize angles and focusing to maximize defect detection. Pressure vessel (PV) inspections typically use “top, side, end” or “top,
side, TOFD” views, though other imaging is possible. Special
inspections can be performed, e.g., for specific defects, or increased coverage. Defects can be sized by pulse-echo as per code,
by time-of-flight Diffraction or by back diffraction. New PV inspection codes, particularly ASME Code Case 2235, permit the
use of advanced ultrasonic inspection techniques. Pipeline girth
weld inspections use a unique inspection approach called “zone
discrimination,” and have their own series of codes. While similar
equipment is used in pipeline as in PV inspections, the pipeline
philosophy is to tailor the inspection to the weld profile and predicted lack of fusion defects. Pipeline displays are specifically
designed for near real-time data analysis. Both ASME CC 2235
and the pipeline codes permit the use of Fitness-For-Purpose,
which reduces construction costs. Overall, phased array systems
meet or exceed all PV and pipeline codes.
关DOI: 10.1115/1.1991881兴
Contributed by the Pressure Vessels and Piping Division for publication in the
JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript receieved: February 3, 2005.
Final manuscript received: March 29, 2005. Review conducted by: Sam Zamrik.
Journal of Pressure Vessel Technology
The petrochemical and other industries use a wide variety of
pressure vessels and piping. These vessels and pipes are typically
made in sections and welded together. Welding often produces
defects, which occasionally propagate with time and fail. During
construction, welds are inspected for defects, usually using the
ASME or similar code. For pressure vessel inspections, ASME
Section V 关1兴 is the relevant code, with NonDestructive Testing
共NDT兲 covered under Articles 4 and 5. The ASME code has been
the mainstay of PV inspections for decades. For pipelines, several
codes are possible: ASTM E-1961-98 关2兴, API 1104 19th Edition
关3兴, DNV OS F101 关4兴, and ISO 13847 关5兴. All use, or implicitly
accept, the alternative zone discrimination approach.
For many years, welds were radiographed. However, radiography has significant technical disadvantages: First, it is hazardous,
and the PV may need moving to a safe location or inspections
performed off-shift; second, radiography often generated production delays; third, radiography is poor at detecting critical lack of
fusion or cracking defects; fourth, radiography cannot size defects
for Fitness-For-Purpose 共FFP兲, 关also called Engineering Critical
Assessment 共ECA兲 or Fracture Mechanics兴; fifth, radiography is
subjective, and the cost of film is high.
The competing technology is ultrasonics. Ultrasonics is safe,
fast, can be performed as soon as the weld is cool, and can size the
vertical height of defects with some accuracy. However, manual
ultrasonics is time-consuming and very operator-dependent.
Mechanized or automated ultrasonics 共AUT兲 has been available
for some years, but was slow and relatively expensive. Multiprobe
scans have been used in pipelines and other applications, but are
inflexible. Speeds have increased significantly, especially with the
advent of phased array equipment for PVs. The arrival of
diffraction-based sizing techniques like Time-Of-Flight Diffraction 共TOFD兲 has significantly increased the potential for ECA
applications.
Inspection Codes for AUT of Welds
There are now many different potential codes for inspecting
vessel welds, but these can be divided into three categories: traditional or standard raster-type codes based on manual ultrasonic
principles 关1兴, the new ASME 2235 code case 关6兴, and “tailored”
codes like ASTM E-1961 关2兴.
Raster Codes. ASME Section V 关1兴 is normally used for
manual ultrasonic testing but has provisions for AUT as well.
These codes typically require ultrasonic scanning 共or rastering兲
over the weld and adjacent heat affected zone at two or more
angles to detect defects. The ASME code has been extensively
used in the nuclear and other industries for decades, with their
many variations. These include Sections I, III, VIII, and XI, as
well as piping codes B31.1 and B31.3. These rastering techniques
Copyright © 2005 by ASME
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Review of Ultrasonic Phased Arrays
for Pressure Vessel and Pipeline
Weld Inspections
Fig. 1 Schematic showing generation of electronic and sectorial scans using phased arrays
ASME Code Case 2235. More recently, ASME revised Code
Case 2235 关6兴, which permits “nonamplitude, computer recorded”
inspection techniques. These are widely interpreted as TOFD,
though pulse-echo rastering techniques are permitted. CC 2235 is
an option for replacing radiography for FFP/ECA acceptance under specific conditions. This code case requires a performance
demonstration, showing detection of three flaws, as a minimum.
“Tailored” Pipeline Codes. Pipeline AUT is at the forefront of
ferritic steel weld inspections. ASTM E-1961 关2兴 was the first
code written specifically for AUT, and clearly defines linear scanning and zone discrimination. E-1961 is very specific on the types
of reflectors, tolerances, scan patterns, displays and analysis for
pipelines. DNV OS F101 关4兴 is similar to E-1961, but is targeted
at offshore use. API 1104 19th Edition 关3兴 permits AUT, but is
much more flexible in requirements. The EU code, ISO 13847 关6兴,
is also a more general code.
Ultrasonic Phased Arrays
Ultrasonic phased arrays 共PAs兲 use multiple ultrasonic elements
and electronic time delays to create beams by constructive interference. The physics, theory, and applications are described in 关7兴
in detail. PAs offer significant technical advantages for weld inspections over conventional ultrasonics by electronically steering,
scanning, sweeping, and focusing the beam 共see Fig. 1兲.
•
•
•
•
Electronic scanning permits very rapid coverage of the components.
Tailored angles can be used for mapping components to
maximize detection of defects.
Sectorial scanning is useful for inspections where only a
minimal footprint is possible, and other applications.
Electronic focusing permits optimizing the beam shape and
size at the expected defect location, and consequently to
optimize defect detection.
Overall, the use of phased arrays permits maximizing defect detection while minimizing inspection time.
Ultrasonic PAs 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 关8–10兴, though industrial uses have been
limited until the last few years.
Phased arrays use an array of elements, all individually wired,
pulsed and time-shifted. These elements are typically pulsed in
groups from 4 to 32, usually16 elements for welds. In order to
make a user-friendly system, a typical setup calculates the timedelays from operator-input, or uses a pre-defined file: Inspection
angle, focal distance, scan pattern, etc. 共see Fig. 1兲. The time delay
352 / Vol. 127, AUGUST 2005
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.
While it can be time-consuming to prepare the first setup, the
information is recorded in a file and only takes seconds to re-load.
Also, modifying a prepared setup is quick in comparison with
physically adjusting conventional transducers.
Types of Scans. Using electronic pulsing and receiving provides significant opportunities for a variety of scan patterns.
Electronic Scans
Electronic scans are performed by multiplexing along an array
共see Fig. 2兲. Typical arrays have up to 128 elements, pulsed in
groups of 8–16. 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. Electronic scans are straightforward to program. For
example, a phased array can be readily programmed to inspect a
weld using 45 deg and 60 deg shear waves, which emulate conventional manual inspections.
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 共see Fig. 3兲.
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 关11兴, to map out the features 共and defects兲. Depending primarily on the array frequency and element spacing, the sweep angles
can vary from ±20 deg up to ±80 deg. S-scans are also used for
weld inspections, either multiple or single passes, though single
S-scans are not necessarily recommended for construction weld
inspections as the incident angles can be inappropriate 关12兴.
Combined Scans
Combining electronic scanning, sectorial scanning and precision focusing leads to a practical combination of displays. Optimum angles can be selected for welds and other components,
while electronic scanning permits fast and functional inspections.
Combined raster scans can be performed, e.g., 45 deg and 60 deg
plus TOFD 共see Fig. 4兲. A related approach applies to tailored
weld inspections, where specific angles are required for given
weld profiles 共see Tailored Inspections below兲; for these applications, specific beam angles are programmed for specific weld fac-
Fig. 3 Schematic showing sectorial scanning used on turbine
rotor
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provide a satisfactory inspection for most defects, but have poor
defect detection capabilities for midwall planar defects like centerline cracking.
Fig. 2 Schematic illustration of electronic scanning
Fig. 4 Schematic showing phased array performing 45 deg
and 60 deg raster scans on a weld
Linear Scanning of Welds
Manual ultrasonic weld inspections are performed using a
single transducer, which the operator “rasters” back and forth to
cover the weld area 关see Fig. 5共a兲兴. Many automated weld inspection systems use a similar approach, with a single transducer
scanned back and forth over the weld area. This is very time
consuming, since the system has dead zones at the start and finish
of the raster.
In contrast, many multitransducer systems and phased arrays
use a “linear scanning” approach 关see Figure 5共b兲兴. Here the probe
pan is scanned linearly along the weld, while each transducer
Fig. 6 Typical “top, side, end” view with waveform. TOFD can
be added, along with other displays.
sweeps out a specific area of the weld. The simplest approach to
linear scanning is found in pipe mills, where a limited number of
transducers inspect ERW pipe welds. AUT as practiced on pipelines uses up to twenty-four transducers, which makes pipeline
AUT much faster than single transducer techniques.
Phased arrays for linear weld inspections operate on the same
principle as the multitransducer approach 关13兴; however, phased
arrays offer considerably greater flexibility than conventional
AUT. Typically, it is much easier to change the setup electronically, either by modifying the setup or reloading another; often it
is possible to use many more beams 共equivalent to conventional
transducers兲 with phased arrays; custom inspections can be implemented simply by loading a setup file.
Phased array applications take advantage of one or more of the
dominant features of PAs:
•
•
•
•
Speed: Scanning with phased arrays is an order of magnitude faster than single transducer conventional mechanical
systems, with better coverage;
Flexibility: Setups can be changed in a few minutes, and
typically a lot more component dimensional flexibility is
available;
Inspection angles: A wide variety of inspection angles can
be used, depending on the requirements and the array;
Small footprint: Small matrix arrays can give infinitely
more flexibility for inspecting restricted areas than conventional transducers.
Applications for Weld Inspections
ASME Raster Inspections. The ASME code 关1兴 requires scanning at a minimum of two angles at least 10 deg apart; this is
shown schematically using phased arrays in Fig. 4. Phased arrays
can readily fulfill this requirement of the ASME code, and can run
a wide variety of inspection angles, TOFD and other scans to
optimize the displays and inspections 共see Fig. 6兲.
Fig. 5 „Top… Conventional raster scanning; „bottom… linear
scanning
Journal of Pressure Vessel Technology
ASME Code Case 2235. This code 关6兴 permits automated
scanning for weld inspections, though data must be collected in
raw, encoded form. Time-Of-Flight Diffraction is a relatively recent arrival in the petrochemical business, and so far has primarily
been used in pipelines. This technique uses the same arrays as
pulse-echo, or can use dedicated TOFD transducers at higher frequency. TOFD essentially gives a through wall image of the weld,
with the inside and outside surfaces and any defects displayed.
TOFD detection capability is very good, though there are dead
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ets at specific locations, which can be performed using an automated set-up program and the appropriate weld profile.
Fig. 10 PV inspection using phased arrays on a rotating
vessel
zones at the outside and inside surface. TOFD is an excellent
sizing technique, with significantly better sizing than amplitude
approaches. Figure 7 below shows a typical TOFD image of a
weld.
These authors strongly recommend using both pulse-echo and
TOFD for improved defect detection and sizing. Back diffraction
offers some sizing advantages over TOFD 共or forward scattering兲:
Smaller dead zones and better sizing for small defects 关14兴.
Tailored Inspections and Zone Discrimination. There are a
number of methods of performing tailored inspections of welds,
but the best known is AUT of pipeline inspections 共2兲. “Zone
discrimination” provides 100% coverage and rapid inspections
primarily for narrow gap welds in pipelines. Gas pipelines have
major inspection requirements: High speed, high quality, high detection capability 关15兴, and rapid data interpretation. Also, the use
of ECA makes major demands on sizing capability. AUT acts as
process control because it is performed soon after welding. ECA
and process control typically reduce the reject rate significantly
over radiography 关16,17兴, and save substantial costs.
AUT for pipelines uses four special features: Zone discrimination, calibration blocks, dual gate output display and defect sizing.
Figure 8 shows the concept of zone discrimination with linear
scanning, and Fig. 9 a typical code-compliant calibration block.
Delivery Systems
Fig. 8 Schematic of zone discrimination. Top: Selection of
zones. Bottom: Position and angles for zone discrimination
inspection
There are many different methods of mechanically scanning a
weld 关18兴. The simplest is to rotate the vessel, as shown in Fig. 10.
Fig. 9 Typical AUT calibration block, as per code ASTM E-1961 „2…
354 / Vol. 127, AUGUST 2005
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Fig. 7 Typical TOFD image showing OD, ID and top and bottom of defects
Other options include magnetic wheel scanners, robots, pipe scanners 共as shown in Fig. 11兲, handscanners, and even a low-cost
encoded array 共see Fig. 12兲.
All the scanners have their advantages and disadvantages, in
terms of cost, convenience, accuracy.
Sample Results
ASME Raster Scans on Thick Section Welds. Figure 13
shows a typical “top, side. TOFD” display with dual dedicated
TOFD pairs for improved detection and sizing. Weld overlays
共more visible in the “top, side, end” view in Fig. 6兲 assist the
operator in interpretation. Full waveform data is collected, and
scanning rates are 10 mm/ s or higher, depending on data transfer
rates, wall thickness, number of waveforms, etc. Table 1 shows a
comparison of some of the key parameters from PV and pipeline
inspections using phased arrays. Pipeline multiprobe systems are
Fig. 13 Above shows a typical “top, side” view at left, combined with a twin TOFD view at right. The “top, side” view is
made by merging both the 45 and 70 deg data.
Fig. 12 Encoded array for semiautomated inspections
Table 1 Comparison
inspections
of
pressure
vessel
and
pipeline
Fig. 14 Typical AUT display, with dual gate strip charts, mapping channels, TOFD, position and coupling. This display
shows multiple boxed defects in red.
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Fig. 11 Belt scanner for pipeline AUT with probe pan attached
lighter, faster, can scan almost any diameter and wall thickness, can provide improved imaging and special setups, and
use automated setups for convenience.
3. For ASME raster inspections of thick-section welds, phased
arrays offer flexibility, imaging, and speed.
4. In all cases, TOFD is strongly recommended as a “no cost”
addition.
Fig. 15 Customized weld inspection using two different size
arrays due to geometric constraints
Tailored Inspections. Figure 14 below shows a typical AUT
strip chart display, with dual gates for each zone 共amplitude and
Time-Of-Flight兲, plus TOFD, B-scans maps for porosity, coupling
charts, and position round the weld.
Defect sizing is performed rapidly by counting the number of
zones that the defect is detected on. The TOF signals change to
red once above-threshold defects are detected 共usually 40% screen
height兲. TOFD 共Time-Of-Flight Diffraction兲 is used to aid sizing,
and also to confirm the presence 共or absence兲 of a defect.
Figure 15 below shows a tailored weld inspection of a component with significant geometric restrictions. One side of the weld
is unobstructed, but the other needs a reduced size array. The
electronic set-ups can generate code-acceptable ultrasonic inspections.
Conclusions
Ultrasonic phased arrays have several advantages over conventional AUT and radiography for pressure vessel weld inspections.
1. In general, phased arrays:
共a兲 Are faster;
共b兲 Are more flexible, both in setups and in component
geometries;
共c兲 Can optimize defect detection by tailoring inspection
angles and focal spot size;
共d兲 Have a smaller footprint.
2. For zone discrimination scans, probe pans are smaller,
356 / Vol. 127, AUGUST 2005
关1兴 ASME Boiler and Pressure Vessel Code Section V, 2001, 2003 rev., American
Society of Mechanical Engineers, New York.
关2兴 ASTM, American Society for Testing and Materials, ASTM E-1961-98, “Standard Practice for Mechanized Ultrasonic Examination of Girth Welds Using
Zonal Discrimination With Focused Search Units,” September 1998.
关3兴 API, American Petroleum Institute Standard 1104, “Welding of Pipelines and
Related Facilities,” Nineteenth Edition, September 1999.
关4兴 DNV, Det Norske Veritas OS-F101 “Submarine Pipeline Systems,” January
2000.
关5兴 ISO 13487, International Standard ISO 13847:2000 Technical Corrigendum 1,
“Petroleum and Natural Gas Industries–Pipeline Transportation Systems–
Welding of Pipelines,” Published 2001-12-15.
关6兴 ASME, Code Case 2235-4, “Use of Ultrasonic Examination in Lieu of Radiography, Section I and VIII, Divisions 1 and 2,” November 30, 2001, American Society of Mechanical Engineers.
关7兴 R/D Tech, Inc., “Introduction to Phased Array Ultrasonic Technology Applications,” Coordinator, N. Dube, © R/D Tech August 2004.
关8兴 Clay A. C., Wooh, S-C., Azar, L., and Wang, J-Y., 1999, “Experimental Study
of Phased Array Beam Characteristics,” J. Nondestruct. Eval., 18共2兲, p. 59.
关9兴 Wüstenberg H, Erhard, A., and Shenk, G., “Some Charateristic Parameters of
Ultrasonic Phased Array Probes and Equipments,” NDT.net, vol. 4, No. 4,
http://www.ndt.net/article/v04n04/wuesten/wsuesten.htm
关10兴 Lafontaine G. and Cancre, F., “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.
关11兴 Ciorau P., MacGillivray, D., Hazelton, T., Gilham, L., Craig, D., and Poguet,
J., “In-Situ Examination of ABB l-0 Blade Roots and Rotor Steeple of LowPressure Steam Turbine, Using Phased Array Technology,” 15th World Conference on NDT, Rome, Italy, October 11–15, 2000.
关12兴 Moles M. D. C., and Zhang, J., 2005, “Construction Weld Testing Procedures
Using Ultrasonic Phased Arrays,” Mater. Eval., 63共1兲, p. 27.
关13兴 Dubé, N., “Electric Resistance Welding Inspection,” 15th WCNDT, Rome,
Italy, October 2000.
关14兴 Jacques F, Moreau, F., and Ginzel, E. A., 2003, “Ultrasonic Backscatter Using
Phased Array–Developments in Tip Diffraction Flaw Sizing,” Insight, 45共11兲,
p. 724.
关15兴 Gross B., O’Beirne, J., and Delanty, B., “Comparison of Radiographic and
Ultrasonic Inspection Methods on Mechanized Girth Welds,” Pipeline Technology Conference, 15–17 October, 1990, Ostend, Belgium.
关16兴 Connelly, T., “Update on the Alliance Pipeline”, International Conference on
Advances in Welding Technology, October 26–28, 1999, Galveston, Texas,
sponsored by EWI and AWS.
关17兴 Morgan L., Nolan,P., Kirkham, A., and Wilkinson, P., “The Use of Automated
Ultrasonic Testing 共AUT兲 in Pipeline Testing,” Insight November 2003.
关18兴 Moles, M. D. C., and Labbé, S., “Automated Ultrasonic Inspection of Pressure
Vessel Welds,” 16 WCNDT, Montreal, Canada, August 30–Sept 3, 2004.
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similar to PA pipeline inspections; single probe raster scans are
significantly slower, but the speed depends on the inspection details.
References