API 653 Material Evaluation 5 — Questions and Answers
Question 1: When performing a fitness-for-service assessment of a shell with general corrosion, the corrosion rate used for projecting future thickness should be the:
- Average of long-term and short-term rates, always
- Lower of long-term and short-term rates
- Higher of long-term and short-term rates (Correct answer)
- Short-term rate only, regardless of long-term data
Correct answer: Higher of long-term and short-term rates
API 653 requires using the higher (more conservative) of the long-term or short-term corrosion rate to ensure the inspection interval and retirement date are safe.
Question 2: API 653 defines the 'remaining corrosion life' of a shell course as:
- The time until the next scheduled inspection
- The number of years before the next internal inspection is required
- (Actual thickness − tmin) divided by the corrosion rate (Correct answer)
- (Nominal thickness − tmin) divided by the original design corrosion allowance
Correct answer: (Actual thickness − tmin) divided by the corrosion rate
Remaining corrosion life equals the difference between actual measured thickness and tmin, divided by the applicable corrosion rate.
Question 3: A shell course on an API 653 tank has a measured thickness of 0.300 inches, a tmin of 0.220 inches, and a corrosion rate of 0.010 in/yr. What is the remaining corrosion life?
- 3 years
- 5 years
- 8 years (Correct answer)
- 22 years
Correct answer: 8 years
(0.300 − 0.220) / 0.010 = 0.080 / 0.010 = 8 years remaining corrosion life.
Question 4: Which material defect is typically planar, parallel to the plate surface, and caused by non-metallic inclusions rolled flat during steel manufacturing?
- Pitting
- Lamination (Correct answer)
- Seam crack
- Hot tear
Correct answer: Lamination
Laminations are internal planar defects created when elongated non-metallic inclusions become flattened parallel to the plate surface during rolling.
Question 5: The allowable stress value used in the API 653 tmin formula for a shell course is based on:
- The original design code allowable stress for the plate material (Correct answer)
- The current measured yield strength from field hardness testing
- A fixed value of 15,000 psi regardless of material grade
- The inspector's engineering judgment
Correct answer: The original design code allowable stress for the plate material
API 653 uses the allowable stress from the original construction code (typically API 650) for the identified shell plate material.
Question 6: Sulfide stress cracking (SSC) in high-strength steel tank components is most likely to occur when the material hardness exceeds:
- HRC 22 (approximately 237 HB) (Correct answer)
- HRC 10 (approximately 100 HB)
- HV 150
- HB 130
Correct answer: HRC 22 (approximately 237 HB)
NACE MR0175/ISO 15156 specifies that hardness must not exceed HRC 22 (237 HB) to prevent SSC in H₂S-containing environments.
Question 7: During material evaluation, an inspector finds a shell plate with extensive mill scale covering a suspicious area. The BEST next step before UT thickness readings is to:
- Proceed with UT through the mill scale without any surface preparation
- Remove the mill scale with wire brushing or grinding to ensure good UT coupling (Correct answer)
- Apply liquid penetrant directly on the mill scale
- Mark the area and defer evaluation to the next inspection cycle
Correct answer: Remove the mill scale with wire brushing or grinding to ensure good UT coupling
Mill scale must be removed to ensure good acoustic coupling and accurate UT readings; measuring through scale can give false or artificially high thickness values.
API 653 Material Evaluation 5
When performing a fitness-for-service assessment of a shell with general corrosion, the corrosion rate used for projecting future thickness should be the: