API 653 Fitness-for-Service & Repair 3 — Questions and Answers
Question 1: Under API 653, what is the minimum shell plate thickness for a tank to remain in service if no other defects are present?
- 0.05 in regardless of tank size
- 0.10 in for all shell courses
- Calculated minimum thickness based on shell design formula (Correct answer)
- Half the original nominal thickness
Correct answer: Calculated minimum thickness based on shell design formula
API 653 requires the measured thickness to meet or exceed the calculated minimum required thickness from the shell design equation, not a fixed universal value.
Question 2: Which of the following is classified as an alteration rather than a repair under API 653?
- Replacing a corroded shell course with the same thickness plate
- Adding a new nozzle to an existing shell (Correct answer)
- Re-welding a leaking seam weld
- Installing a flush patch over a thinned area
Correct answer: Adding a new nozzle to an existing shell
Adding a new nozzle changes the original design configuration and is classified as an alteration, which requires engineering review and potentially a hydrostatic test.
Question 3: API 653 requires that weld repairs on tank shells be performed by welders qualified per which standard?
- AWS D1.1
- ASME Section IX (Correct answer)
- API 1104
- ASME B31.3
Correct answer: ASME Section IX
API 653 mandates that welders and welding procedures for tank repairs be qualified in accordance with ASME Section IX.
Question 4: When using the API 653 settlement fitness-for-service criteria, what type of settlement is most damaging to tank shell-to-bottom welds?
- Uniform settlement across the entire tank base
- Edge settlement causing differential tilt around the perimeter (Correct answer)
- Center settlement creating a dome in the bottom
- Settlement outside the tank footprint
Correct answer: Edge settlement causing differential tilt around the perimeter
Edge (perimeter) differential settlement causes bending stresses at the shell-to-bottom weld, which is the most structurally critical area for tank integrity.
Question 5: A lap-welded repair patch on a tank shell must have a minimum distance from any existing weld seam of:
- 1 inch
- 2 inches
- 6 inches (Correct answer)
- 12 inches
Correct answer: 6 inches
API 653 requires lap-welded repair patches to be located at least 6 inches from any existing weld seam to avoid heat-affected zone interactions.
Question 6: For a tank bottom with pitting corrosion, API 653 considers a pit acceptable for continued service if the remaining thickness is at least:
- 0.05 in
- 0.10 in (Correct answer)
- 50% of the original plate thickness
- Equal to the calculated minimum required thickness
Correct answer: 0.10 in
API 653 requires a minimum remaining bottom plate thickness of 0.10 in (including any corrosion coating) for the tank to remain in service.
Question 7: Which NDE method is most commonly used to verify the quality of a repair weld on a tank shell before returning to service?
- Visual inspection only
- Radiographic testing (RT) or ultrasonic testing (UT) of the weld (Correct answer)
- Hydrostatic pressure testing to 1.5× operating pressure
- Dye penetrant testing of the weld face only
Correct answer: Radiographic testing (RT) or ultrasonic testing (UT) of the weld
RT or UT of repair welds provides volumetric examination to detect internal discontinuities that visual or surface methods cannot find.
API 653 Fitness-for-Service & Repair 3
Under API 653, what is the minimum shell plate thickness for a tank to remain in service if no other defects are present?