13.3 NDE Examination Requirements for Shell, Bottom, Roof & Nozzle Welds (API 653 Section 12.1)

API 653 Section 12.1 NDE Examination Architecture

Following any repair, alteration, or reconstruction of an aboveground atmospheric storage tank, the integrity of every newly deposited weld bead, plate cut, and structural attachment must be verified. API Standard 653 Section 12.1 establishes the governing quality assurance and NDE framework, invoking the examination procedures of ASME Section V (Nondestructive Examination) and the acceptance standards of API Standard 650 (Section 8) and ASME Section VIII, Division 1.

The philosophy of API 653 post-repair examination distinguishes between three distinct failure prevention tiers:

1. Surface Flaw Detection (VT, MT, PT): Targeting surface-breaking fatigue cracks, toe undercutting, lack of sidewall fusion, and stress concentrations that initiate brittle or cyclic failure.
2. Volumetric Flaw Interrogation (RT, UT): Interrogating the full depth of full-penetration butt seams to detect embedded planar cracks, lack of penetration, slag inclusions, and porosity.
3. Containment Verification (LT): Vacuum box, pneumatic, tracer gas, and diesel wicking tests to confirm zero through-wall leakage.

Shell Butt Weld Radiography: API 653 Section 12.2

Shell butt welds carry the highest primary membrane stresses in the tank. Radiographic coverage for repairs and alterations is set out in API 653 Section 12.2, Radiographs — not in Section 12.1 — and the opening sentence of 12.2.1 frames everything that follows: the number and location of radiographs shall be in accordance with API Std 650 and the following additional requirements. Every rule below is therefore a supplement stacked on top of the API 650 new-construction radiography, never a replacement for it.

+-------------------------------------------------------------------------+
|            API 653 12.2.1 ADDITIONAL RADIOGRAPHS - DECISION GRID        |
|                                                                         |
|                       | new plate to | new plate to | repaired joint in |
|                       | NEW plate    | EXISTING     | EXISTING plate    |
|   --------------------+--------------+--------------+-------------------|
|   Vertical joints     | none beyond  | ONE added RT | ONE added RT in   |
|   (12.2.1.1)          | API 650      | in EACH joint| EACH joint        |
|   --------------------+--------------+--------------+-------------------|
|   Horizontal joints   | none beyond  | ONE added RT | ONE added RT per  |
|   (12.2.1.2)          | API 650      | per 50 ft of | 50 ft of repaired |
|                       |              | repaired weld| horizontal weld   |
|   --------------------+--------------+--------------+-------------------|
|   Vert/horiz          | none beyond  | EACH         | ALL repaired      |
|   intersections       | API 650      | intersection | intersections     |
|   (12.2.1.3)          |              | radiographed | radiographed      |
+-------------------------------------------------------------------------+

1. Vertical Joints (API 653, 12.2.1.1)

2. Horizontal Joints (API 653, 12.2.1.2)

3. Intersections of Vertical and Horizontal Joints (API 653, 12.2.1.3)

There is no "2 inches into each leg" rule in API 653. The coverage requirement the standard actually states is 12.2.1.7: the minimum diagnostic length of each radiograph shall be 6 in. If a question hands you a number for how far a T-junction radiograph must extend, 6 in. of diagnostic length is the defensible answer.

4. Replacement Plates and Door Sheets (API 653, 12.2.1.6)

New and replaced shell plate and door sheet welds shall be radiographed, and all junctions between repair welds and existing welds shall be radiographed. If defects are found, 100 % radiography shall be performed on the repaired weld. Two plate geometries get their own rules:

• Circular replacement plates (12.2.1.6.1): a minimum of one radiograph shall be taken regardless of thickness. When the circular replacement plate sits in a shell plate thicker than 1 in., the weld shall be fully radiographed.
• Square and rectangular replacement plates (12.2.1.6.2): at least one radiograph in a vertical joint, at least one in a horizontal joint, and one in each corner. When the plate sits in a shell plate thicker than 1 in., the vertical joints shall be fully radiographed.

5. Penetrations Installed Using Insert Plates (API 653, 12.2.1.8)

For penetrations installed using insert plates as described in API 653 9.8, the completed butt welds between the insert plate and the shell plate shall be fully radiographed — a 100 % requirement that has no thickness qualifier.

6. Reconstructed Tanks (API 653, 12.2.1.4 and 12.2.1.5)

When a tank is dismantled and re-erected under API 653 Section 10, the base radiography is the API 650 Section 8.1 spot regime for new construction, plus two API 653 additions:

• 12.2.1.4: each butt-welded annular plate joint shall be radiographed in accordance with API 650.
• 12.2.1.5: radiographic inspection is required for 25 percent of all junctions of new welds over existing seams. The owner/operator, with the consent of the contractor, determines the extent of any further inspection and repair; further inspection or repair of existing welds is handled by contractual agreement.

7. Marking and Identification of Radiographs (API 653, 12.2.3)

• Each film shall show an identification of the welder(s) who made the weld. A weld map showing weld locations, weld number, radiograph number, welder identification, and the grading of each weld is an acceptable alternative.
• Radiographs and radiograph records of all repaired welds shall be marked with the letter "R".

Critical Joint Examination: Shell-to-Bottom, Nozzles, Bottom & Roof

Specific structural joints require targeted multi-method examination sequences:

1. Shell-to-Bottom Weld (API 653 Section 12.1.6)

The corner fillet weld connecting the bottom plate to the lowest shell course is subjected to intense secondary bending and plastic rotation. API 653 gives a primary method and one explicitly permitted alternative — know both.

12.1.6.1 — the primary method. New welding on the shell-to-bottom joint shall be inspected for its entire length by:

• a right-angle (corner) vacuum box and a solution film, or by applying light diesel oil; and
• the first weld pass shall additionally be inspected by applying light diesel oil to the side opposite the first weld pass. The oil shall be allowed to stand at least 4 hours — preferably overnight — and the weld then inspected for wicking action. The oil shall be removed before the weld is completed.

12.1.6.2 — the permitted alternative. In place of 12.1.6.1, the initial weld passes inside and outside the shell may have all slag and non-metals removed and be examined visually; after completion of the inside and outside fillet or partial-penetration welds, the welds are tested by pressurizing the volume between the inside and outside welds with air to 15 psig and applying a solution film to both welds. To make sure the air reaches all parts of the welds, a sealed blockage must be welded into the annular passage between the inside and outside welds, with a small pipe coupling welded on each side of and adjacent to the blockage — air supply on one end, pressure gauge on the other.

12.1.6.3 — existing weld under a welded-on patch plate. The existing shell-to-bottom weld shall be examined visually and by MT or PT for its full length under a welded-on patch plate, plus an additional 6 in. of the joint on each side of the patch plate, before the repair plate is placed.

Where the "root pass and final pass, MT or PT" requirement really comes from. That rule is not the general 12.1.6 shell-to-bottom rule. It appears in 12.1.7.2 for a welded-on patch plate weld in the critical zone, and again in 12.3.2.3 as a condition of taking a hydrostatic test exemption for a repair in the critical zone. Attaching it to every shell-to-bottom weld is a common and costly error.

2. Shell Penetrations and Reinforcing Plate Welds (API 653 Section 12.1.2)

• Ultrasonic lamination check (12.1.2.1): UT of the shell plate for laminations shall be made in the immediate affected area when adding a reinforcing plate to an existing unreinforced penetration or adding a hot tap connection. It is not a blanket requirement for every nozzle.
• Gouged repad attachment welds (12.1.2.2): cavities resulting from gouging or grinding out the attachment welds of existing reinforcing plates shall be examined by MT or PT.
• Completed nozzle welds (12.1.2.3): completed welds attaching the nozzle neck to shell, and the reinforcing plate to shell and to nozzle neck, shall be examined by MT or PT. Additional examination (for example fluorescent MT and/or UT) should be considered for hot tap connections to shell plates of unknown toughness thicker than 1/2 in., or of unknown toughness with a minimum shell design metal temperature below the API 653 Figure 5.2 curve.
• Stress-relieved assemblies (12.1.2.4): completed welds of stress-relieved assemblies shall be examined by MT or PT after stress relief but before hydrostatic testing.
• Pneumatic pad test: the repad cavity is pressurized to 15 psig through the telltale hole with a solution film applied, per API 650.

3. Shell Plate to Shell Plate Welds (API 653 Section 12.1.5)

• 12.1.5.1 — new to existing: new welds attaching existing shell plate to existing or new shell plate shall be examined by radiographic methods (see 12.2). In addition, for plate thicknesses greater than 1 in., the back-gouged surface of the root pass and the final pass (each side) shall be examined for its complete length by MT or PT.
• 12.1.5.2 — new to new: new welds joining new shell plate material to new shell plate material (partial or full shell course replacement or addition) need only be examined radiographically in accordance with API 650.

4. Shell Plate Repairs (API 653 Section 12.1.8)

• 12.1.8.1 — weld metal deposit: areas of shell plate repaired by weld metal deposit shall be examined visually and by the magnetic particle method (or liquid penetrant method).
• 12.1.8.2 — lap-welded patches: the attachment welds of new lap-welded shell patches shall be examined visually and by MT or PT.

5. Bottom Plate Welds (API 653 Section 12.1.7)

• 12.1.7.1: on completion of welding on a tank bottom, the plates and the entire length of new welds shall be examined visually for defects and leaks, with particular attention to sumps, dents, gouges, three-plate laps, bottom plate breakdowns, arc strikes, temporary attachment removal areas, and welding lead arc burns. All new welds — including the weld attaching a patch plate to the bottom, areas restored by welding, and restored defective welds found during internal inspection — shall be inspected by one of the methods specified in API 650. Leaking areas shall be repaired by grinding and rewelding, and retested.
• 12.1.7.2: the root and final pass of a welded-on patch plate weld in the critical zone shall be examined visually and by MT or PT over its full length.
• 12.1.7.3: areas of bottom plate repaired by welding shall be examined by MT or PT, and the repaired area shall also be tested using a vacuum box and solution, or a tracer gas and detector.

6. Roof Welds (API 653 Section 12.1.9)

Newly welded roof joints and repairs shall be examined in accordance with API 650 (visual examination of the roof joints, with a vacuum box or light-oil test of the roof seams where gastightness is required). Floating-roof pontoon compartments are separately proof-tested with low-pressure air and a bubble solution — typically 1 to 2 psig — as part of the roof repair package.

Examiner Qualification & Code Acceptance Standards (ASNT SNT-TC-1A & ASME VIII)

Under API 653 Section 12.1.1.2 — which states simply that personnel performing nondestructive examinations shall be qualified in accordance with API Std 650 and any supplemental requirements of API 653 — the reliability of NDE data depends on strict personnel qualification and calibrated procedural execution:

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|                    NDE QUALIFICATION & GOVERNANCE HIERARCHY             |
|                                                                         |
|                         Storage Tank Owner / User                       |
|                                    |                                    |
|                                    v                                    |
|                        API 653 Authorized Inspector                     |
|            [Audits NDE procedures, verifies examiner certs,             |
|             independently reviews RT films and UT data]                 |
|                                    |                                    |
|                                    v                                    |
|                             ASNT Level III                              |
|             [Approves employer written practice, qualifies              |
|              procedures per ASME Section V, certifies tech]             |
|                                    |                                    |
|                                    v                                    |
|                     Certified NDE Technicians (Level II)                |
|             [Performs calibrations, examinations, and code              |
|              evaluations per ASME V and API 650/653 rules]              |
+-------------------------------------------------------------------------+

Personnel Qualification per ASNT SNT-TC-1A / CP-189

1. Written Practice: NDE examiners must be qualified and certified in accordance with an Employer's Written Practice complying with ASNT SNT-TC-1A or ANSI/ASNT CP-189.
2. Certification Levels: Examinations, calibrations, and formal evaluations must be executed by certified Level II technicians or Level III personnel. Level I personnel may assist but cannot independently interpret or sign off on inspection results.
3. Vision Examinations: Examiners must pass annual near-vision visual acuity tests (Jaeger J-1 or J-2 at 12 inches / 300 mm) and color vision examinations every three years.
4. Authorized Inspector Role: The API 653 Authorized Inspector is responsible for auditing examiner certifications, verifying that NDE procedures comply with ASME Section V, witnessing examinations, and reviewing radiographic films and UT scan records.

Acceptance Standards Summary

• Radiography (RT): Evaluated per ASME Section VIII, Division 1, Paragraph UW-51 (for full RT) or Paragraph UW-52 (for spot RT). Under UW-51, planar flaws (cracks, lack of fusion, incomplete penetration) have zero tolerance.
• Magnetic Particle (MT) & Liquid Penetrant (PT): Evaluated per ASME Section VIII, Division 1, Appendix 6 (MT) and Appendix 8 (PT). All linear indications ($L > 3W$) are rejectable. Rounded indications $> 3/16\text{ in.}$ are rejectable.
• Ultrasonic Testing (UT): Shear-wave angle-beam examination evaluated per API 650 Annex U or ASME Section VIII, Division 1, Appendix 12.

Master NDE Examination Matrix for Tank Repairs & Alterations

The following master table consolidates the examination methods, extent of coverage, and governing clauses across the primary storage tank components. Read the "Governing Code Section" column carefully: examination method rules live in 12.1, radiograph counts live in 12.2.