API 653 Corrosion Assessment 4 — Questions and Answers
Question 1: According to API 653, when should an owner/operator consider using an API 579 Level 2 or 3 fitness-for-service assessment for corroded tank components?
- When corrosion is detected during any inspection
- When measured thickness falls below the minimum required by the original construction standard but continued service is desired (Correct answer)
- Only when preparing for tank demolition
- When the tank is less than 5 years old
Correct answer: When measured thickness falls below the minimum required by the original construction standard but continued service is desired
A Level 2 or 3 FFS assessment per API 579 is appropriate when thickness has fallen below the construction standard minimum but the owner wants to justify continued operation.
Question 2: Which parameter is NOT typically included in a corrosion rate calculation per API 653?
- Measured thickness at previous inspection
- Current measured thickness
- Time elapsed between inspections
- Operating temperature during inspection (Correct answer)
Correct answer: Operating temperature during inspection
Corrosion rate is calculated from thickness measurements and elapsed time between inspections; operating temperature at the time of inspection is not part of the rate formula itself.
Question 3: What is the primary corrosion concern for tanks storing crude oil with high water content (BS&W)?
- External atmospheric corrosion of the roof
- Underdeposit and microbiologically influenced corrosion at the water/oil interface and bottom (Correct answer)
- Stress corrosion cracking in the vapor space
- Erosion corrosion at the inlet nozzle only
Correct answer: Underdeposit and microbiologically influenced corrosion at the water/oil interface and bottom
Free water and sediment settling to the tank bottom creates ideal conditions for MIC and underdeposit corrosion, making the bottom the primary concern for BS&W crude tanks.
Question 4: During an API 653 internal inspection, pit depth measurements are taken. A plate has a nominal thickness of 0.375 inches and the deepest pit measures 0.280 inches. The remaining thickness is 0.095 inches. What action is required if the calculated minimum thickness is 0.100 inches?
- The tank can continue in service without restriction
- The pitted area must be repaired or the tank must be removed from service (Correct answer)
- Only increased monitoring is required
- The tank can continue until the next scheduled inspection
Correct answer: The pitted area must be repaired or the tank must be removed from service
Since the remaining pit depth (0.095 in) is below the minimum required thickness (0.100 in), the pitted area must be repaired or the tank must be taken out of service per API 653.
Question 5: What is the purpose of a holiday detector (spark tester) during tank inspection?
- To measure coating thickness on the tank shell
- To locate discontinuities (pinholes) in protective coatings (Correct answer)
- To detect cracks in weld seams
- To measure cathodic protection current
Correct answer: To locate discontinuities (pinholes) in protective coatings
A holiday detector applies a voltage across a coating and detects current flow at discontinuities (holidays/pinholes) where the coating has failed.
Question 6: Which type of corrosion is characterized by preferential attack along grain boundaries and is associated with improper heat treatment of stainless steels?
- Pitting corrosion
- Galvanic corrosion
- Intergranular corrosion (Correct answer)
- Uniform corrosion
Correct answer: Intergranular corrosion
Intergranular corrosion attacks along grain boundaries, commonly caused by sensitization of stainless steel where chromium carbides precipitate, depleting chromium from the grain boundary region.
Question 7: When assessing a tank with unknown construction records, API 653 requires determining the minimum thickness based on what primary source?
- Manufacturer's published standard thicknesses
- A fitness-for-service assessment using current measurements (Correct answer)
- Visual inspection of plate condition
- The owner's estimated original thickness
Correct answer: A fitness-for-service assessment using current measurements
Without original construction records, API 653 requires a fitness-for-service assessment based on current actual measurements to establish safe operating limits.
API 653 Corrosion Assessment 4
According to API 653, when should an owner/operator consider using an API 579 Level 2 or 3 fitness-for-service assessment for corroded tank components?