Weld quality in carbon steel storage tanks is critical to prevent leaks and structural failures. Here are key international standards and methods for inspecting tank welds:

• 1. International Codes and Standards

  (1) API 653: Tank Inspection, Repair, Alteration, and Reconstruction

  Scope: Applies to aboveground storage tanks (ASTs) for petroleum products.

  Weld Requirements:

  • Mandates visual inspection (VT) of all welds.
  • Radiographic testing (RT) or ultrasonic testing (UT) for critical seams (e.g., bottom-to-shell welds).
  • Magnetic particle testing (MT) or liquid penetrant testing (PT) for surface defects.

  (2) ASME BPVC Section VIII: Pressure Vessels

  Scope: Covers design, fabrication, and inspection of pressure vessels, including storage tanks.

  Weld Requirements:

  • RT/UT for full-penetration welds in high-stress areas.
  • Qualification of welders and welding procedures via ASME Section IX.

  (3) ISO 16938: Welding of Metallic Materials

  Scope: Global standard for welding processes and inspection.

  Key Aspects:

  • Defines acceptance criteria for weld discontinuities (e.g., porosity, cracks).
  • Requires pre-weld and post-weld heat treatment (PWHT) for specific steel grades.

• 2. Non-Destructive Testing (NDT) Methods

  (1) Visual Inspection (VT)

  Standard: ASME BPVC Section V, Article 9.

  Procedure:

  • Examine welds for surface irregularities (e.g., cracks, undercutting).
  • Measure weld dimensions (e.g., throat thickness) against design specs.

  (2) Radiographic Testing (RT)

  Standard: ASTM E94 (American) or ISO 17636 (International).

  Purpose: Detect internal flaws (e.g., lack of fusion, porosity).

  Limitations: Hazardous radiation requires safety protocols.

  (3) Ultrasonic Testing (UT)

  Standard: ASTM E164 (American) or ISO 17640 (International).

  Procedure:

  • Use ultrasonic waves to detect subsurface defects.
  • More sensitive than RT for thin-walled tanks.

  (4) Magnetic Particle Testing (MT)

  Standard: ASTM E709 (American) or ISO 9934 (International).

  Purpose: Detect surface and near-surface cracks in ferromagnetic materials (e.g., carbon steel).

  (5) Liquid Penetrant Testing (PT)

  Standard: ASTM E165 (American) or ISO 3452 (International).

  Procedure:

  • Apply dye penetrant to the weld surface.
  • Reveal surface-breaking defects through capillary action.

• 3. Acceptance Criteria

  Standard	Acceptance Criteria
  API 653	Cracks, incomplete fusion, and linear indications are unacceptable. Porosity and slag inclusions are limited based on size and location.
  ASME BPVC Section VIII	Defines allowable defect sizes based on stress levels and tank service conditions.

• 4. Frequency of Inspections

  • New Construction: 100% of critical welds must pass NDT.
  • In-Service Tanks:
    • API 653 requires periodic inspections (e.g., every 5-10 years) based on risk assessment.
    • Welds in areas prone to corrosion (e.g., bottom plates) are prioritized.

• 5. Special Considerations

  Post-Weld Heat Treatment (PWHT)

  Required by ASME for thick-walled tanks or high-strength steels to reduce residual stress.

  Welder Qualification

  Welders must be certified per ASME Section IX or ISO 14732.