API 653 Tank Inspection, NDE & Testing 3 — Questions and Answers
Question 1: API 653 defines the 'critical zone' of a tank floor as the area within what distance from the shell-to-floor weld?
- 3 inches
- 12 inches (Correct answer)
- 18 inches
- 24 inches
Correct answer: 12 inches
API 653 defines the critical zone as the area of the tank floor within 3 inches of the inside of the tank shell, where corrosion is most likely and structurally consequential.
Question 2: What is the primary purpose of performing acoustic emission (AE) testing on an in-service aboveground storage tank?
- Measuring exact wall thickness at specific points
- Detecting active corrosion, cracking, or leakage while the tank remains in service (Correct answer)
- Verifying the integrity of the roof-to-shell weld
- Determining the exact location of soil-side corrosion under the annular plate
Correct answer: Detecting active corrosion, cracking, or leakage while the tank remains in service
Acoustic emission testing is used to screen in-service tanks for active defect growth (corrosion, cracking) or leaks without requiring the tank to be taken out of service.
Question 3: When applying the API 653 formula to calculate minimum acceptable shell thickness, which factor is NOT included in the standard calculation?
- Hydrostatic head (fill height)
- Allowable stress of the shell material
- Corrosion allowance
- Wind load on the shell (Correct answer)
Correct answer: Wind load on the shell
The API 653 minimum shell thickness formula (modified API 650) accounts for hydrostatic head, allowable stress, and a corrosion allowance, but wind load is not part of the minimum thickness calculation for corrosion retirement.
Question 4: A tank inspector identifies pitting on the internal shell surface. Which measurement is most critical for determining whether the shell remains fit for service?
- The diameter of the largest pit
- The remaining wall thickness at the deepest pit location (Correct answer)
- The number of pits per square foot
- The surface roughness profile between pits
Correct answer: The remaining wall thickness at the deepest pit location
The remaining wall thickness at the deepest pit determines whether the shell still meets the minimum acceptable thickness per API 653, making it the critical measurement.
Question 5: Which of the following correctly describes the use of the 'point-of-tangency' method in API 653 shell inspection?
- It locates the transition between elastic and plastic deformation in dented shell plate
- It establishes the datum for measuring tank settlement around the circumference
- It identifies the exact line where the shell plate transitions from vertical to the knuckle radius (Correct answer)
- It determines the precise fill height at which the annular plate begins to flex
Correct answer: It identifies the exact line where the shell plate transitions from vertical to the knuckle radius
The point of tangency is the location where the shell plate transitions from the vertical cylinder to the curved knuckle radius, important for determining weld locations and inspection zones.
Question 6: For a tank that has been in service for 20 years, API 653 recommends that the external inspection interval should NOT exceed:
- 1 year
- 3 years
- 5 years (Correct answer)
- 10 years
Correct answer: 5 years
API 653 specifies that the maximum interval for external inspections is 5 years, regardless of the tank's age or service history.
Question 7: Shear wave ultrasonic testing (angle-beam UT) is preferred over straight-beam UT for tank weld examination primarily because:
- It can measure exact weld reinforcement height more accurately
- It detects planar defects oriented perpendicular to the plate surface, such as cracks in weld fusion zones (Correct answer)
- It requires less surface preparation than straight-beam scanning
- It provides permanent recorded images like radiography
Correct answer: It detects planar defects oriented perpendicular to the plate surface, such as cracks in weld fusion zones
Angle-beam (shear wave) UT directs sound at an angle through the weld, making it sensitive to planar defects like cracks and lack of fusion that straight-beam would not detect effectively.
API 653 Tank Inspection, NDE & Testing 3
API 653 defines the 'critical zone' of a tank floor as the area within what distance from the shell-to-floor weld?