visual-inspection

name	visual-inspection
description	Visual inspection (VT) — direct and remote visual inspection (RVI), borescope types (rigid/flexible/videoscope), resolution and illumination requirements (ASME V Article 9), minimum detectable defect size calculation, illumination ≥ 500 lux (direct) / ≥ 50 lux (remote reference), viewing angle, acceptance criteria development, API 510 (pressure vessels) and API 570 (piping) visual requirements, API 653 (storage tanks), weld visual acceptance (AWS D1.1 Table 6.1), surface preparation requirements, fitness-for-service assessment linkage (API 579).
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Visual Inspection — Complete Skill

Fundamentals of Visual Testing

Why Visual Inspection is First

VT is the most fundamental NDT method:

• Performed before all other NDT methods (surface must be visually acceptable first)
• Identifies: cracks, corrosion, erosion, deformation, misalignment, coating defects, weld surface defects
• Often the only method required for non-critical or accessible components
• All inspectors (even UT, MT, PT) should perform VT first

VT scope: Pre-service: acceptance after fabrication (welds, castings, machined surfaces) In-service: corrosion monitoring, crack detection, coating assessment Post-repair: verification of weld geometry, surface finish

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Direct Visual Inspection

Requirements (ASME Section V Article 9)

Access for direct VT: Eye within 600 mm (24 in) of surface; angle ≥ 30° to surface; direct or via mirror/magnifier Illumination: ≥ 500 lux (50 foot-candles) at inspection surface (ASME V T-952.1) Verify illumination with calibrated light meter at beginning of inspection

Resolution: Human eye resolves ≈ 0.1 mm at optimal distance and illumination Typical VT detection capability: cracks ≥ 0.5 mm wide, ≥ 1 mm long (surface, clean) Contrast and surface condition critical: dark, rough surfaces reduce detectability

Magnification: Magnifying glass (2×–4×): increases effective resolution; used for weld toes, small features Mirror: extend viewing angle; minimum 30° viewing angle maintained

Inspector requirements: ASNT SNT-TC-1A Level II or equivalent for code work Near vision check: Jaeger J2 at 300 mm or equivalent; annual re-examination Color vision (Ishihara or Farnsworth D-15 test): for code applications

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Remote Visual Inspection (RVI)

Borescope Types

Rigid borescope: Straight tube with relay lenses (Hopkins rod lens system); high image quality; limited to straight access Field of view: 55–90° (wide-angle models); depth of field: 5–200 mm depending on model Outer diameter: 6–10 mm standard; micro-bore: 1.7–4 mm for small passages Resolution: 1000–5000 line pairs; superior to flexible for same diameter Illumination: fiber optic bundle from external light source; light intensity at object: 2000–10,000 lux

Flexible fiberscope: Coherent fiber bundle carries image; incoherent bundle carries light Resolution: 10,000–50,000 fibers → resolution inferior to rigid (pixelated image) Working length: 0.6–10 m; insertion tube OD: 4–13 mm; 2-way or 4-way articulation Use: turbine hot section, heat exchanger tubes, complex routing

Videoscope (videoendoscope): CCD/CMOS camera at distal tip; no fiber bundle → highest image quality for flexible system Resolution: 640×480 to 1920×1080 (HD); depth measurement by stereo or shadow measurement Light: LED array at tip → 5,000–15,000 lux at working distance 3D measurement capability: stereo (two cameras) or single camera + structured light → accurate pit depth measurement Use: primary tool for in-service aircraft engine inspection (GE, Pratt & Whitney accept OEM-approved videoscopes)

RVI Performance Requirements (ASME V Article 9)

ASME V Article 9 — Remote VT: System resolution: must resolve 0.8 mm (1/32 in) at the distance of inspection Reference image: comparison standard (known defect or resolution target) photographed at same conditions Illumination at object: typically 50+ lux (less critical — measured differently from direct) Working distance: documented; system qualified at that working distance

Tip articulation and orientation: 4-way articulation: up/down/left/right (typically ±130°+ in each plane) Retroview adapters: allow inspection of surfaces behind obstacles

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Acceptance Criteria

Weld Visual Acceptance (AWS D1.1 — Structural Steel)

AWS D1.1 Table 6.1 (visual acceptance for statically loaded structures): Cracks: not permitted (any crack in weld → reject) Weld size: ≥ specified size; underrun ≤ 1.5 mm for 10% of weld length; no underrun at weld ends Underfill / insufficient throat: not permitted (affects design strength) Undercut: ≤ 1 mm (standard); ≤ 0.8 mm in fatigue-critical applications Overlap (cold lap): not permitted Porosity: √(max pore diameter) ≤ 1.5 mm; max pore cluster area < 2% per 100 mm of weld Weld profile (convexity): ≤ 3 mm for welds ≤ 15 mm face width; ≤ B + 3 mm for wider Arc strikes: not permitted outside weld area

AWS D1.1 Table 6.1 (dynamically loaded structures — fatigue): Stricter: no porosity > 0.8 mm; undercut ≤ 0.25 mm; all cracks prohibited including those in HAZ Weld toes: smooth transition required; surface irregularities ground smooth

Pressure Vessel (API 510)

API 510 visual inspection during in-service: Corrosion: measure wall thickness; calculate remaining life using corrosion rate Uniform corrosion: T_remaining = t_measured − t_min_required; remaining_life = T_remaining / corrosion_rate [years] Localized: if depth > 50% wall → immediate repair or FFS assessment per API 579 Blistering: hydrogen blisters from wet H₂S → report to engineer; may require PWHT assessment

MAWP recalculation: If t_measured < t_design: recalculate MAWP = 2S×E×t_measured/(D−0.2×t_measured) [per ASME VIII Div. 1] Reduce MAWP or repair if MAWP falls below operating pressure

Piping (API 570)

API 570 inspection intervals: Risk-based inspection (RBI) determines interval; or prescriptive: 10 years for low-risk, 5 years for high-risk piping Short-term corrosion allowance: retirement date = (t_measured − t_min) / corrosion_rate + inspection_date Injection point inspection: 3× normal frequency (turbulent mixing → accelerated corrosion)

Storage Tanks (API 653)

API 653 floor inspection: Internal inspection when flooded; measure plate thickness; minimum thickness check Floor retirement: if t_measured ≤ 0.1 in (2.5 mm) OR ≤ t_min = H×D/(2×S×E) (whichever governs) Annular plate: stricter → t_min = 0.1 in if no secondary bottom

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Surface Preparation for Visual Inspection

Surface cleanliness requirements: Minimum: remove loose scale, dirt, oil, grease (solvent wipe) Weld inspection: remove welding spatter; wire brush or grind if required by code For best VT: blast cleaned or wire brushed to bare metal; SSPC-SP 6 (commercial blast) or SSPC-SP 10 (near-white blast) for corrosion assessment Coating inspection: intact, dry surface; check for blistering, peeling, rust breakthrough

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Fitness-for-Service (FFS) Assessment Link

API 579-1/ASME FFS-1: When VT identifies defect: assess fitness for continued service using Level 1/2/3 analysis Level 1: simplified assessment (tables/charts); most conservative Level 2: detailed calculation (stress intensity, remaining strength factor RSF) Level 3: advanced analysis (FEA, fracture mechanics); most accurate

RSF (Remaining Strength Factor): RSF = (actual burst pressure of damaged component) / (burst pressure of undamaged component) RSF_allowable = 0.90 (API 579); component acceptable if RSF ≥ 0.90

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Standards

Standard	Scope
ASME Section V Article 9	Visual examination requirements for ASME code
ASNT SNT-TC-1A	VT personnel certification (Level I, II, III)
AWS D1.1	Structural steel weld visual acceptance criteria
API 510	Pressure vessel inspection code
API 570	Piping inspection code
API 653	Aboveground storage tank inspection
API 579-1/ASME FFS-1	Fitness-for-service assessment
ASTM E1616	Visual examination (aerospace)
EN 13018	Visual testing — general principles
NAVSEA 0900-LP-001-7000	Naval ship visual inspection

Output

Provide: inspection scope (component: weld/vessel/piping/tank/aircraft; material; geometry; service: pressure/structural/rotating), access assessment (direct VT feasible? distance [mm] < 600 mm; angle ≥ 30°? yes/no; if no: RVI required), equipment (direct: illumination [lux] ≥ 500 at surface; magnifier if needed; mirror; mirror angle; RVI: borescope type — rigid/flexible/videoscope; OD [mm]; working length [m]; resolution qualification: 0.8 mm resolved at working distance?), surface preparation (cleanliness level; coating removed: yes/no; welding spatter removed: yes/no), inspection findings (location; feature: crack/porosity/undercut/corrosion/blistering; dimensions: length × width × depth [mm]; reference: table/clause for acceptance), acceptance assessment (code: AWS D1.1 / ASME V / API 510 / API 570; finding vs. acceptance criteria; accept or reject), corrosion assessment if in-service (t_measured [mm]; t_min [mm]; corrosion rate [mm/year]; remaining life [years]; RSF if localized per API 579), FFS recommendation (Level 1 check if marginal; repair or continued service with monitoring schedule), and applicable standard (ASME V Article 9 for new construction; API 510/570/653 for in-service; AWS D1.1 for weld visual acceptance).