Study guide
13.3 NDE Examination Requirements for Shell, Bottom, Roof & Nozzle Welds (API 653 Section 12.1)
API 653 Aboveground Storage Tank Inspector Exam
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1 Chapter 1: API 653 Scope, Inspector Certification & Roles
1.1 Scope of API 653, Tank Types & Construction Standards 1.2 Inspector Roles, Responsibilities & Certification Requirements 1.3 Inspection Planning, Safety, Confined Space Entry & Preparation
2 Chapter 2: Tank Shell & Roof Components Inspection
2.1 Fixed Roof Types, Structural Supports & Inspection Practices 2.2 Floating Roofs, Seals, Drainage & Operational Issues 2.3 Shell Courses, Welds, Attachments & Wind Girders 2.4 Tank Appurtenances, Nozzles, Manways & Piping Connections
3 Chapter 3: Tank Bottom & Foundation Inspection
3.1 Tank Bottom Types, Annular Plates & Internal Bottom Inspection 3.2 Foundation Types (Concrete Ringwall, Earth, Asphalt) & Inspection 3.3 Leak Detection Systems, Secondary Containment & Under-Tank Monitoring
4 Chapter 4: Corrosion & Damage Mechanisms in Storage Tanks
4.1 Atmospheric Corrosion, Condensation & External Deterioration 4.2 Corrosion Under Insulation (CUI) & Piping/Nozzle Interfaces 4.3 Soil-Side Corrosion, Under-Bottom Pitting & Microbiologically Induced Corrosion (MIC) 4.4 Product-Side Deterioration: Chemical Attack, Ethanol SCC, Concentration Cell & Sulfuric Acid Corrosion
5 Chapter 5: Materials, Toughness & Brittle Fracture Prevention
5.1 Tank Steels, Material Specifications & Historic Standards 5.2 Brittle Fracture Assessment: Figure 5.1 Decision Tree & Exemption Curves (API 653 Section 5) 5.3 Charpy V-Notch Impact Testing & Minimum Design Metal Temperature (MDMT)
6 Chapter 6: Cathodic Protection & Interior Linings
6.1 Cathodic Protection Principles & Systems for Tank Bottoms (API RP 651) 6.2 CP Criteria, Surveys, Potential Measurements & Interference (API RP 651) 6.3 Tank Bottom Interior Linings, Surface Prep & Inspection (API RP 652)
7 Chapter 7: Corrosion Rates, Remaining Life & Inspection Intervals
7.1 Determining Corrosion Rates (Short-Term vs. Long-Term) 7.2 Remaining Life Calculations & Limiting Components 7.3 Establishing In-Service & Out-of-Service Inspection Intervals (API 653 Section 6)
8 Chapter 8: Tank Shell Minimum Thickness Evaluation
8.1 Shell Minimum Thickness Calculations (tmin for Hydrostatic & Operating Levels) 8.2 One-Foot Method, Variable Design Point & Reconstructed-Tank Shell Thickness 8.3 Joint Efficiency (E) Selection, Missing Data & Radiography Factors 8.4 Evaluation of Pitting, Localized Thin Areas (LTA) & Critical Length (L)
9 Chapter 9: Bottom Plate & Annular Ring Thickness Evaluation
9.1 Minimum Bottom Thickness Formulations (MRT, MRTbottom, MRTlinear) 9.2 Annular Plate Evaluation, Stress Criteria & Critical Zones 9.3 Bottom Pitting Evaluation & Remaining Thickness Before Next Inspection
10 Chapter 10: Tank Repairs & Alterations Standards
10.1 Classification: Repair vs. Alteration vs. Reconstruction (API 653 Section 9) 10.2 Shell Plate Replacement, Lapped Patch Plates & Insert Plates 10.3 Shell Penetrations, Door Sheets & Nozzle Additions 10.4 Hot Tapping Requirements, Limitations & Pre-Engineering (API 653 Section 9.15 & API Publ 2201)
11 Chapter 11: Welding Procedures, Qualifications & Quality Control
11.1 Welding Procedure Specification (WPS) & Procedure Qualification Record (PQR) per ASME IX 11.2 Welder Performance Qualification (WPQ) & Continuity per ASME IX 11.3 Essential, Non-Essential & Supplementary Essential Variables 11.4 Preheating, Post-Weld Heat Treatment (PWHT) & Weld Flaw Remediation
12 Chapter 12: Nondestructive Examination (NDE) Methods
12.1 Visual Examination (VT) & Magnetic Particle Testing (MT) per ASME V 12.2 Liquid Penetrant (PT) & Ultrasonic Thickness Measurement (ASME V Article 6 & SE-797) 12.3 Radiographic Testing (RT): Acceptance Criteria, Film Interpretation & ASME V/API 650
13 Chapter 13: Hydrostatic Testing & Post-Repair Examination
13.1 Hydrostatic Test Triggers, Exemption Criteria & Major Alterations (API 653 Section 12.3) 13.2 Leak Testing: Vacuum Box, Tracer Gas, Diesel & Air Pressure Tests 13.3 NDE Examination Requirements for Shell, Bottom, Roof & Nozzle Welds (API 653 Section 12.1)
14 Chapter 14: Relief Devices, Tank Settlement & Nameplate Records
14.1 Pressure & Vacuum Relief Devices, Venting Principles (API RP 576 & API 2000) 14.2 Tank Settlement Types: Uniform, Tilt, Out-of-Plane & Edge Settlement (API 653 Annex B) 14.3 Evaluating Settlement Limits, Cosine Curve Analysis & Remedial Action 14.4 Tank Marking, Nameplates, Inspection Reports & Recordkeeping (API 653 Section 13)
13.3 NDE Examination Requirements for Shell, Bottom, Roof & Nozzle Welds (API 653 Section 12.1)
Key Takeaways Explain This Topic AI
API 653 Section 12.1 sets the nondestructive examination requirements for shell penetrations, shell-plate-to-shell-plate welds, the shell-to-bottom weld, bottoms, shell plate repairs, and roofs, while API 653 Section 12.2 separately sets the number and location of radiographs - and 12.1.1.1 makes both additional to API 650 rather than a substitute for it.
API 653 12.2.1 adds radiographs on top of API 650: one additional radiograph in each vertical joint joining a new replacement plate to an existing plate, one additional radiograph per 50 ft of repaired horizontal weld, and radiography of each new-to-existing or repaired vertical/horizontal intersection - with a minimum diagnostic length of 6 in. per radiograph under 12.2.1.7.
New welding on the shell-to-bottom joint is inspected for its entire length under 12.1.6.1 by a right-angle vacuum box with solution film or by light diesel oil, with the first weld pass additionally checked by applying light diesel oil to the opposite side and allowing it to stand at least 4 hours before looking for wicking; 12.1.6.2 permits an alternative in which the volume between the inside and outside welds is pressurized to 15 psig behind a welded sealed blockage.
Bottom welds are visually examined over their entire length under 12.1.7.1, the root and final pass of a welded-on patch plate weld inside the critical zone additionally require full-length MT or PT under 12.1.7.2, and areas of bottom plate repaired by welding require MT or PT plus a vacuum box or tracer gas test under 12.1.7.3.
API 653 12.1.1.2 qualifies NDE personnel by reference to API 650, which requires certification under an employer written practice meeting ASNT SNT-TC-1A or ANSI/ASNT CP-189, subject to review and verification by the API 653 Authorized Inspector.
Last updated: September 2026
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:
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.
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.
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)
- Joint configuration Additional radiographs required
- New replacement shell plate to new shell plate None beyond what API 650 requires for new construction
- New replacement shell plate to existing shell plate One additional radiograph in each joint
- Repaired joints in existing shell plates One additional radiograph in each joint
- 2. Horizontal Joints (API 653, 12.2.1.2)
- Joint configuration Additional radiographs required
- New replacement shell plate to new shell plate None beyond API 650
New replacement shell plate to existing shell plate One additional radiograph for each 50 ft of repaired horizontal weld
- Repaired joints in existing shell plates One additional radiograph for each 50 ft of repaired horizontal weld
- 3. Intersections of Vertical and Horizontal Joints (API 653, 12.2.1.3)
- Joint configuration Additional radiographs required
- New to new None beyond API 650
- New to existing Each intersection shall be radiographed
- Repaired intersections in existing shell plates All repaired intersections shall be radiographed
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: +-------------------------------------------------------------------------+
- | 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
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 .
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.
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.
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 > 3 W L > 3W L > 3 W ) are rejectable. Rounded indications > 3 / 16 in. > 3/16\text{ in.} > 3/16 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 .
Tank Component / Weld Type Governing Code Section Mandatory NDE Methods Extent of Coverage Acceptance Standard
New shell plate to existing shell plate API 653 12.1.5.1 Radiography per 12.2; plus MT or PT when t > 1 t > 1 t > 1 in. RT per the 12.2.1 grid; for t > 1 t > 1 t > 1 in., MT/PT of the back-gouged root pass and final pass, each side, full length API 650 Section 8 / ASME Section VIII UW-51
New shell plate to new shell plate API 653 12.1.5.2 Radiography only Per API 650 new-construction rules; no API 653 additions API 650 Section 8
Vertical joints — additional radiographs API 653 12.2.1.1 Radiography One additional RT in each joint (new-to-existing, and repaired joints in existing plate) API 650 Section 8
Horizontal joints — additional radiographs API 653 12.2.1.2 Radiography One additional RT per 50 ft of repaired horizontal weld API 650 Section 8
Vertical/horizontal intersections API 653 12.2.1.3 Radiography Each new-to-existing intersection, and all repaired intersections API 650 Section 8
Any radiograph API 653 12.2.1.7 Radiography Minimum diagnostic length 6 in. API 650 Section 8
Circular replacement plate API 653 12.2.1.6.1 Radiography Minimum one RT regardless of thickness; fully radiographed if in shell plate > 1 > 1 > 1 in. API 650 Section 8
Square / rectangular replacement plate API 653 12.2.1.6.2 Radiography One RT in a vertical joint, one in a horizontal joint, one in each corner; vertical joints fully radiographed if in shell plate > 1 > 1 > 1 in. API 650 Section 8
Insert-plate penetration butt welds API 653 12.2.1.8 Radiography Fully radiographed , no thickness qualifier API 650 Section 8
Shell-to-bottom weld (new welding) API 653 12.1.6.1 Right-angle vacuum box + solution film, or light diesel oil; plus diesel-oil wicking check of the first pass Entire length; oil stands at least 4 hours (preferably overnight) Zero wicking, zero continuous bubbles
Shell-to-bottom weld (alternative) API 653 12.1.6.2 Visual of initial passes, then 15 psig air between inside and outside welds with solution film Entire length, using a welded sealed blockage and pipe couplings Zero bubbles at 15 psig
Existing shell-to-bottom weld under a welded-on patch API 653 12.1.6.3 Visual + MT or PT Full length beneath the patch plus 6 in. each side, before placing the plate Zero cracks
Shell penetrations (lamination check) API 653 12.1.2.1 Straight-beam UT Immediate affected area, when adding a repad to an unreinforced penetration or a hot tap No rejectable laminations
Nozzle neck-to-shell and repad welds API 653 12.1.2.3 MT or PT Completed welds ASME VIII App. 6 (MT) / App. 8 (PT)
Stress-relieved assemblies API 653 12.1.2.4 MT or PT After stress relief, before hydrostatic testing ASME VIII App. 6 / App. 8
Bottom welds (general) API 653 12.1.7.1 Visual Entire length of all new welds; retest after any leak repair API 650
Welded-on patch plate weld in the critical zone API 653 12.1.7.2 Visual + MT or PT Root pass and final pass, full length ASME VIII App. 6 / App. 8
Bottom plate areas repaired by welding API 653 12.1.7.3 MT or PT plus vacuum box or tracer gas 100 % of the repaired area Zero continuous bubble stream
Shell plate repaired by weld metal deposit API 653 12.1.8.1 Visual + MT (or PT) Repaired area ASME VIII App. 6 / App. 8
Lap-welded shell patch attachment welds API 653 12.1.8.2 Visual + MT or PT 100 % of attachment welds ASME VIII App. 6 / App. 8
Roof joints and repairs API 653 12.1.9 Per API 650 Visual; vacuum box or light-oil test of seams where gastightness is required API 650