API 653 Material Evaluation 2 — Questions and Answers
Question 1: When evaluating shell plate corrosion, which measurement technique is preferred for determining the minimum thickness in a corroded area?
- Ultrasonic thickness measurement (UT) (Correct answer)
- Visual inspection only
- Magnetic particle testing
- Radiographic testing
Correct answer: Ultrasonic thickness measurement (UT)
Ultrasonic thickness measurement is the preferred method for accurately determining remaining wall thickness in corroded areas without requiring tank entry.
Question 2: According to API 653, what is the term for the minimum shell thickness required for structural integrity at a given liquid level?
- Design thickness
- Retirement thickness
- Minimum acceptable shell thickness (MAST)
- Calculated minimum thickness (tmin) (Correct answer)
Correct answer: Calculated minimum thickness (tmin)
API 653 uses 'calculated minimum thickness' (tmin) to define the lowest acceptable thickness for a given course at a specified liquid level.
Question 3: A tank shell plate has a measured thickness of 0.285 inches. The tmin calculated per API 653 is 0.250 inches. What is the corrosion allowance remaining?
- 0.250 inches
- 0.285 inches
- 0.035 inches (Correct answer)
- 0.535 inches
Correct answer: 0.035 inches
The remaining corrosion allowance is the difference between measured thickness and tmin: 0.285 − 0.250 = 0.035 inches.
Question 4: Which standard is most commonly referenced for the material toughness requirements of shell plates used in low-temperature service tanks?
- ASTM A36
- ASTM A516 Grade 70
- ASTM A283
- API 650 Appendix A (Correct answer)
Correct answer: API 650 Appendix A
API 650 Appendix A addresses low-temperature and cold-service design requirements including material toughness for tanks operating at reduced temperatures.
Question 5: When assessing a pitted area on a tank bottom, the critical dimension used for remaining strength calculations is the:
- Maximum pit depth
- Average pit depth
- Pit diameter at the surface
- Remaining wall thickness at the deepest pit (Correct answer)
Correct answer: Remaining wall thickness at the deepest pit
The remaining wall thickness at the deepest pit location governs the fitness-for-service assessment because that represents the minimum metal available.
Question 6: What does a high hardness reading on a weld HAZ suggest during material evaluation of a tank?
- Excellent ductility
- Potential hydrogen-induced cracking susceptibility (Correct answer)
- Low carbon equivalent
- Proper post-weld heat treatment
Correct answer: Potential hydrogen-induced cracking susceptibility
Elevated hardness in the heat-affected zone indicates a martensitic microstructure that is susceptible to hydrogen-induced cracking.
Question 7: Per API 653, which factor is NOT directly used in the tmin shell thickness formula for a liquid-containing tank?
- Specific gravity of the stored liquid
- Tank diameter
- Allowable stress of the shell material
- Tank height above grade (Correct answer)
Correct answer: Tank height above grade
The tmin formula uses product height (liquid level above the assessment point), not total tank height above grade, along with specific gravity, diameter, and allowable stress.
API 653 Material Evaluation 2
When evaluating shell plate corrosion, which measurement technique is preferred for determining the minimum thickness in a corroded area?