API 653 Tank Inspection, NDE & Testing 4 — Questions and Answers
Question 1: According to API 653, when is an internal inspection required after a tank has undergone a major repair to its shell?
- Within 6 months of returning the tank to service
- At the next scheduled internal inspection interval only
- Before the tank is returned to service following the repair (Correct answer)
- Within 2 years of the repair regardless of scheduled intervals
Correct answer: Before the tank is returned to service following the repair
API 653 requires that the adequacy of major repairs be verified by inspection before the tank is returned to service, ensuring the repair meets all applicable requirements.
Question 2: What does a 'tee-pee' or 'cone' deformation pattern in a tank floor plate most likely indicate?
- Differential settlement causing upward buckling of floor plates (Correct answer)
- Excessive internal pressure during a previous hydrostatic test
- Severe localized pitting that has caused plate perforation from below
- Thermal expansion of floor plates due to hot product service
Correct answer: Differential settlement causing upward buckling of floor plates
A cone or tee-pee deformation in tank floor plates is typically caused by differential foundation settlement, which creates localized upward buckling.
Question 3: During tank floor inspection using MFL, a 'C-scan' display showing widespread low-amplitude signals across the entire floor most likely indicates:
- The MFL equipment is not properly calibrated
- General uniform corrosion or mill scale throughout the floor (Correct answer)
- The floor plate is too thick for MFL to be effective
- Catastrophic through-wall penetrations requiring immediate action
Correct answer: General uniform corrosion or mill scale throughout the floor
Widespread low-amplitude MFL signals distributed across the floor are consistent with general uniform corrosion or surface mill scale rather than isolated pitting.
Question 4: API 653 requires that all welded repairs on a tank shell in HF (hydrofluoric acid) service must be subject to what additional NDE requirement?
- Post-weld heat treatment (PWHT) followed by hardness verification (Correct answer)
- 100% radiographic examination of all repair welds
- Magnetic particle testing of all weld heat-affected zones
- Acoustic emission testing within 30 days of repair
Correct answer: Post-weld heat treatment (PWHT) followed by hardness verification
HF service imposes strict hardness limitations to prevent stress corrosion cracking; PWHT is required and hardness must be verified to confirm compliance.
Question 5: When calculating the maximum allowable fill height (MAFH) for a tank with an identified corroded shell course, which variable directly determines the controlling calculation?
- The height of the corroded area above the tank bottom
- The total surface area of corrosion on the course
- The minimum measured thickness within the corroded area (Correct answer)
- The distance from the corroded area to the nearest shell nozzle
Correct answer: The minimum measured thickness within the corroded area
The MAFH calculation uses the minimum measured thickness of the corroded shell to determine the maximum liquid head the shell can safely sustain.
Question 6: What is the significance of finding a 'star-shaped' crack pattern at a floor plate lap weld during internal inspection?
- It indicates lamellar tearing from through-thickness stress during original construction
- It is a classic indicator of hydrogen-induced cracking (HIC) caused by wet H₂S service (Correct answer)
- It suggests the plate was improperly preheated before welding
- It confirms that the floor plate has reached end-of-life and must be replaced
Correct answer: It is a classic indicator of hydrogen-induced cracking (HIC) caused by wet H₂S service
Star-shaped or stepwise cracking at welds in wet H₂S environments is a classic morphology of hydrogen-induced cracking (HIC) or sulfide stress cracking (SSC).
Question 7: Under API 653, what is the required minimum preheat temperature for welding repairs on carbon steel shell plate with carbon equivalent (CE) exceeding 0.45?
- No preheat required if ambient temperature is above 60°F
- 150°F minimum (Correct answer)
- 200°F minimum
- 250°F minimum
Correct answer: 150°F minimum
API 653 references AWS D1.1 and API 650 preheat requirements, which specify a minimum preheat of 150°F for carbon steel with CE > 0.45 to prevent hydrogen cracking.
API 653 Tank Inspection, NDE & Testing 4
According to API 653, when is an internal inspection required after a tank has undergone a major repair to its shell?