API 653 Fitness-for-Service & Repair 2 — Questions and Answers
Question 1: When performing a fitness-for-service assessment on a tank shell with general corrosion, which thickness value is compared against the minimum required thickness?
- Nominal thickness from the original design drawings
- Average measured thickness over a 1-foot grid
- Minimum measured thickness in the corroded area (Correct answer)
- Thickness at the mid-height of the shell course
Correct answer: Minimum measured thickness in the corroded area
API 653 requires comparing the minimum measured thickness against the calculated minimum required thickness to determine fitness-for-service.
Question 2: A tank has a corrosion rate of 0.010 in/yr and a remaining corrosion allowance of 0.050 in. What is the remaining life?
- 3 years
- 4 years
- 5 years (Correct answer)
- 6 years
Correct answer: 5 years
Remaining life = remaining corrosion allowance ÷ corrosion rate = 0.050 ÷ 0.010 = 5 years.
Question 3: According to API 653, what is the maximum interval between internal inspections for a tank in service with a known corrosion rate?
- 5 years
- 10 years
- RCA ÷ 2 or 20 years, whichever is less (Correct answer)
- RCA ÷ 4 or 20 years, whichever is less
Correct answer: RCA ÷ 2 or 20 years, whichever is less
API 653 sets the maximum internal inspection interval as half the remaining corrosion allowance divided by the corrosion rate, or 20 years, whichever is less.
Question 4: Which API 653 repair method is typically used to restore shell plate that has been corroded below the minimum required thickness?
- Applying a fiberglass overlay
- Installing a flush patch using full-penetration butt welds (Correct answer)
- Peening the thinned area to work-harden it
- Applying a high-build epoxy coating only
Correct answer: Installing a flush patch using full-penetration butt welds
API 653 specifies flush patches with full-penetration butt welds as the standard repair method for shell plate corroded below minimum thickness.
Question 5: What is the primary purpose of a hydrostatic test after a major repair or alteration to an aboveground storage tank?
- To measure the tank's actual capacity
- To verify the integrity of the repaired area under operating pressure (Correct answer)
- To clean the tank interior before returning to service
- To recalibrate the level gauging instruments
Correct answer: To verify the integrity of the repaired area under operating pressure
A hydrostatic test after repair or alteration confirms that the repaired area and the tank structure can safely contain fluid under pressure equivalent to or greater than operating conditions.
Question 6: When evaluating a tank bottom for fitness-for-service, which condition requires immediate removal from service under API 653?
- Any pit deeper than 10% of the original floor plate thickness
- A through-hole or leak in the bottom plate (Correct answer)
- Bottom plate thickness below 0.10 in on a non-critical zone
- Corrosion product (scale) accumulation exceeding 1/8 in
Correct answer: A through-hole or leak in the bottom plate
A through-hole or active leak in the tank bottom requires immediate removal from service because it represents a direct loss of containment.
Question 7: For API 653 fitness-for-service of a shell nozzle weld area showing localized corrosion, which document provides supplemental Level 2 assessment procedures?
- API 510
- API RP 579-1/ASME FFS-1 (Correct answer)
- ASME B31.3
- API 650 Appendix A
Correct answer: API RP 579-1/ASME FFS-1
API RP 579-1/ASME FFS-1 provides detailed Level 2 and Level 3 fitness-for-service methodologies referenced by API 653 for complex damage assessments.
API 653 Fitness-for-Service & Repair 2
When performing a fitness-for-service assessment on a tank shell with general corrosion, which thickness value is compared against the minimum required thickness?