When Phased Array Is Allowed to Replace Radiography

The Decision Is Made by the Construction Code, Not the Technology

A fabricator who has just bought a 32:128 phased array unit and a scanner still cannot substitute it for radiography on the strength of image quality. The construction code names which volumetric method is required, and swapping it requires a clause that authorises the swap. Section I calls radiography in PW-11. Section VIII Division 1 calls it in UW-11. AWS D1.1 puts radiography in Clause 8 Part E and ultrasonics in Part F. Until you cite the substitution clause, the drawing still says RT.

This is the reason the parent question of RT versus UT does not resolve into a technical comparison. Whether phased array finds a lack-of-fusion plane better than film is not in dispute, and the physics is set out in the companion piece on RT versus UT weld inspection. What is in dispute on a live job is whether the authorised inspector, the jurisdiction and the owner's specification will accept the data package you hand over.

So the workflow runs backwards from the code. Identify the construction code and edition. Find the substitution clause it offers. Read what that clause demands of acquisition, procedure demonstration, acceptance criteria and personnel. Then decide whether the economics still work. Doing that reading before mobilising is what an ASNT Level III consulting engagement exists to shorten, and it is far cheaper than discovering the gap during a first article.

What ASME Permits, and the Code Case History Behind It

ASME approved Code Case 2235 in 1996, permitting ultrasonic examination of welds in lieu of radiography for Section VIII Division 1 and Division 2 vessels, and it was significant because it introduced fracture-mechanics accept-reject criteria rather than workmanship criteria. That single change made defect allowance considerably more tolerant of an indication a fracture-mechanics analysis showed to be harmless. The Case was revised repeatedly; revision 2235-9 was issued in October 2005 covering Sections I, VIII and XII.

Phased array techniques arrived through their own Code Cases: 2558 for manual phased array E-scan examination per Article 4 in December 2006, 2559 for linear phased array E-scan in January 2008, and 2600 for linear phased array S-scan in January 2008. Those were folded into Section V Article 4 as Mandatory Appendices in July 2010, alongside the TOFD appendix that had been published in 2004. Today Mandatory Appendix IV covers phased array manual raster techniques using linear arrays and Mandatory Appendix V covers phased array E-scan and S-scan linear scanning.

The current picture is a split. Code Case 2235-13, approved July 9, 2014, addresses Section I and Section XII, because Section VIII Division 1 absorbed the substitution into the Code itself at UW-51(a)(4). Section I offers three routes with two different acceptance philosophies: Code Cases 2235 and 2816 use fracture mechanics, while PW-52.1 routes to Section V Article 4 Mandatory Appendix VII and workmanship-based criteria. Choosing the wrong one changes which indications are rejectable.

Manual PAUT Does Not Count

This is the single most common misunderstanding on site. Across Sections I, VIII Division 1 and VIII Division 2, permitted ultrasonic examinations in lieu of radiography are restricted to TOFD and phased array with computer-based data acquisition and analysis abilities, using automatic or semi-automatic equipment that is mechanically mounted and guided on the examination surface. Manual straight beam, manual angle beam and manual phased array are all excluded as substitute techniques.

Section V Article 1 Mandatory Appendix I defines the terms precisely. Semi-automated ultrasonic examination uses equipment and search units that are mechanically mounted and guided and manually driven, recording response data including scanning positions through integral encoders so the acquired data can be imaged. Automated ultrasonic examination is the same with remote, motor-driven operation and no technician adjustment. The common requirement in both definitions is the encoder and the mechanical guide, not the beam-forming electronics.

A technician sweeping a phased array wedge by hand and reading a live sectorial image is doing genuine, useful ultrasonic testing — and is not doing anything that substitutes for required radiography. Manual UT retains specific allowances that are frequently confused with the substitution rules, such as Section VIII Division 1 Mandatory Appendix 12 for a final closure seam, and Section VIII Division 2 paragraph 7.5.4 for Type 7 and Type 8 joints in some instances. Those are separate permissions with separate scopes.

Why PAUT Wins on Shutdown Avoidance

The economic case for phased array is almost never about finding more flaws. It is about not stopping the plant. Radiography needs a controlled boundary surveyed against the 10 CFR 20 dose limits, physically barriered, posted and kept under continuous direct visual surveillance. Everyone inside that boundary stops working. On a congested unit, one exposure clears a lot more real estate than the weld it examines.

Phased array carries none of that. A crew scans while pipefitters work three metres away, welders continue on the next joint, and scaffolding stays occupied. On a turnaround where the critical path passes through a small number of welds, examining them without emptying the surrounding area moves the whole schedule. That is why owners rather than inspection contractors usually drive the switch — the saving lands on the production account, not the inspection account.

The second win is immediacy. Encoded data is analysed on the spot, so a rejectable indication is known before the crew demobilises and the repair happens in the same window. Radiography introduces a processing and reading cycle between exposure and verdict. On tank and vessel work where access scaffolding is the dominant cost, that cycle time is money, which is why it shows up in API 653 tank inspector services planning as often as in weld procedure discussions.

Where PAUT Loses: Thin Sections and Awkward Geometry

The thin-wall floor is real and the codes state it. Code Case 2235-13 opens with welds in material 1/2 in. (13 mm) or greater in thickness — below that, the Case offers nothing. AWS D1.1 Annex H is reported to cover encoded PAUT from 3/16 in. to 8 in., a lower floor than ASME's because structural acceptance criteria differ, but still a floor. Physics sets both: near-field length and wedge dead zone consume the very wall you are trying to interrogate.

Geometry removes more. Nozzle-to-shell welds with compound curvature, set-through configurations, dissimilar-metal welds where the clad interface reflects, austenitic welds where grain structure scatters and steers the beam, and small-bore socket welds where there is no scanning surface all degrade coverage. A scan plan must demonstrate coverage of the required volume, and Code Case 2235-13(a) defines that volume explicitly: weld plus the lesser of 1 in. (25 mm) or t on each side for material 8 in. (200 mm) or under, or 2 in. (50 mm) each side above that.

Sectorial scanning has its own limitation, stated in the Code Case footnote. S-scans demonstrate good detectability from side-drilled holes because those are omnidirectional reflectors, but the beams can be misoriented for planar reflectors such as lack of fusion and cracks, particularly in thicker sections. The Case recommends multiple linear passes with S-scans for components greater than 1 in. (25 mm) thick. A single sectorial sweep sold as full coverage is the classic audit finding.

The Procedure Demonstration Burden Is the Real Barrier

Every substitution route ends at a demonstrated procedure, and the demonstration is where budgets break. Code Case 2235-13(c) requires the written procedure to conform to Section V Article 4 and to have been demonstrated on qualification blocks prepared by welding or the hot isostatic process, containing a minimum of three flaws oriented to simulate flaws parallel to the production weld's fusion line: one at the OD surface, one at the ID surface, one subsurface. If the block can be flipped during examination, two flaws suffice.

Acceptable performance is defined, not asserted. For amplitude-based techniques, response from the maximum allowable flaw and other flaws of interest must exceed the reference level. For techniques that do not use amplitude recording levels, every imaged flaw including the maximum allowable flaws must show an indicated length equal to or greater than the actual flaw length in the block. Flaw size in the block cannot exceed the applicable Table 1, 2 or 3 value for the thickness being examined.

Then comes documentation. Paragraph (b) requires a scan plan showing transducer placement, movement and component coverage, plus beam angles, beam directions relative to weld centreline and the volume examined for each weld, made available to the Owner/User on request. Paragraph (e) requires the complete unprocessed data set with no gating, filtering or thresholding. Building that package once, properly, is a written practice and procedure development exercise rather than a field improvisation.

Personnel: Who Is Allowed to Touch the Data

Code Case 2235-13(f) sets the rule plainly: personnel performing and evaluating ultrasonic examinations shall be qualified and certified in accordance with their employer's written practice, with ASNT SNT-TC-1A or CP-189 used as a guideline, and only Level II or III personnel shall analyse the data or interpret the results. That is a certification requirement and an interpretation restriction in one sentence, and it is checked in audits far more often than the scan plan.

Paragraph (h) tightens it further. Personnel who acquire and analyse ultrasonic data must be trained using the specific equipment named in paragraph (d) and must participate in the demonstration under paragraph (c). A phased array analyst who joined after the procedure was qualified is not covered by that qualification. Paragraph (g) then requires contractor qualification records of certified personnel to be approved by the Certificate Holder and maintained by their employer.

There is a parallel prerequisite on the ISO 9712 side worth knowing if you work internationally: a valid UT Level 2 certification is a prerequisite for a PAUT or TOFD Level 2, and UT Level 3 for the corresponding Level 3. Either way, keeping demonstration participation, method certifications and expiry dates auditable is exactly the problem NDT personnel certification tracking solves, because the record an inspector asks for is the one nobody filed.

The Equipment and Training Step Change

Moving from conventional UT to code-compliant encoded PAUT is a capital step, not an upgrade. The instrument is the smallest part. You add encoders, mechanical scanners or crawlers appropriate to the geometry, wedges matched to each configuration, scan plan software, demonstration blocks with embedded flaws representing the production weld, and data storage and review workstations. Every one of those is required by the acquisition definitions, not optional refinement.

Training is the second step and the one most often underfunded. Published third-party rates give a sense of scale: ASNT's own eLearning catalogue lists a Phased Array Level II course at 80 contact hours priced at $1,365 for members and $1,465 for non-members, and that covers the theory rather than procedure-specific qualification on your blocks. AWS D1.1 Annex H work is reported to require a currently certified Level II UT inspector to add 320 hours of PAUT experience plus a practical examination on two flawed samples.

Multiply that across a crew and the number stops looking like a training line item. It looks like a capability decision with a payback period, which is why fabricators tend to run it as a structured programme against their own mockups through corporate NDT training rather than sending individuals to open-enrolment courses. If you want the transition mapped against your codes and crew before committing, request a scoping conversation.

AWS D1.1 Annex H: The Structural Steel Route

Structural steel got its own answer in the 2020 edition of AWS D1.1 with the addition of Annex H (Normative) — Phased Array Ultrasonic Testing (PAUT), retained in D1.1/D1.1M:2025, the 25th edition. It sits alongside Annex G on qualification and calibration of UT units with approved reference blocks, and Annex N, the informative guidance on UT of welds by alternative techniques. The key word is normative: Annex H is a requirement set the code owns, not commentary.

Annex H brings its own tables. Table H.1 defines essential variables for PAUT, Table H.2 gives PAUT acceptance criteria, and Table H.3 covers discontinuity classification. That replaces the amplitude-and-length judgment of Table 8.2 and Table 8.3 in Clause 8 Part F with criteria written for imaged, encoded data. Annex H is reported to apply from 3/16 in. to 8 in. using encoded linear scanning with a maximum of 1% data loss during acquisition.

For a fabricator this converts a negotiation into a citation. Before 2020, using PAUT on a D1.1 job meant seeking the Engineer's approval for alternate acceptance criteria under 8.8. After 2020, it means naming Annex H. The remaining work is calibration to Annex G or the supplemental reference blocks Annex H illustrates, and getting inspectors qualified — the same demonstration discipline ASME imposes, expressed in structural language.

How to Get a Client to Accept PAUT in Lieu of RT

Lead with their code, not your equipment. A one-page substitution request that names the construction code and edition, the authorising clause, the acceptance appendix or table, the thickness range and the personnel level restriction answers the questions an authorised inspector will ask before they are asked. A request that leads with instrument specifications and sample images invites scepticism, because it looks like a vendor pitch rather than a compliance argument.

Bring the demonstration evidence with it. Include the qualification block description with flaw locations, the demonstration results against the acceptance definition in the Case, the scan plan showing coverage of the defined examination volume, and the personnel records showing which analysts participated. That package is what makes the difference between conditional approval and a job stopped at first article, and it is reusable across every subsequent weld under the same procedure.

Then be honest about where the substitution stops. Thin wall, small bore and geometries the scan plan cannot cover stay on radiography, and saying so up front builds more credibility than claiming universal coverage. Where an owner is weighing the change across a whole facility rather than one weld, an NDT programme audit and gap assessment puts numbers against both the qualification cost and the shutdown avoided.

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