API 653 Tank Inspection, NDE & Testing 5 — Questions and Answers
Question 1: According to API 653, a tank that has experienced a significant leak must undergo what type of inspection before being returned to service?
- External visual inspection only
- Internal inspection with floor scanning and full NDE of affected areas (Correct answer)
- Hydrostatic test to 1.5 times the maximum operating pressure
- Radiographic examination of all shell welds
Correct answer: Internal inspection with floor scanning and full NDE of affected areas
A tank that has leaked must undergo an internal inspection including floor scanning and NDE of affected areas to identify the cause and extent of damage before returning to service.
Question 2: When performing vacuum box testing on a tank floor weld, the vacuum pressure typically applied is:
- 2-3 psi
- 3-4 psi
- 5-6 psi (Correct answer)
- 8-10 psi
Correct answer: 5-6 psi
API 650 and API 653 specify that vacuum box tests are performed at a minimum vacuum of 3-4 psi (approximately 21-28 kPa), but 5-6 psi is the typical working range used in practice.
Question 3: What is the primary limitation of using eddy current testing (ECT) for inspecting aboveground storage tank floors?
- ECT cannot detect corrosion on the underside (soil side) of floor plates (Correct answer)
- ECT requires the tank to be completely free of all hydrocarbons
- ECT can only inspect floor plates thicker than 3/8 inch
- ECT is ineffective at temperatures above 100°F
Correct answer: ECT cannot detect corrosion on the underside (soil side) of floor plates
Conventional eddy current testing is limited to near-surface detection on the side being scanned and has poor sensitivity for soil-side corrosion beneath the floor plate.
Question 4: An API 653 inspector is evaluating a tank that shows uniform shell corrosion. The corrosion rate is calculated as 0.010 in/yr and the current measured thickness is 0.280 in with a minimum required thickness of 0.210 in. What is the remaining life?
- 5 years
- 7 years (Correct answer)
- 10 years
- 14 years
Correct answer: 7 years
Remaining life = (current thickness – minimum required thickness) / corrosion rate = (0.280 – 0.210) / 0.010 = 7.0 years.
Question 5: Which of the following is a key advantage of phased array ultrasonic testing (PAUT) over conventional single-element UT for tank weld examination?
- PAUT equipment is less expensive and more portable than conventional UT
- PAUT can electronically steer and focus the beam, providing greater coverage with fewer probe positions (Correct answer)
- PAUT eliminates the need for couplant and can inspect through coatings
- PAUT automatically provides ASME-accepted permanent records without any post-processing
Correct answer: PAUT can electronically steer and focus the beam, providing greater coverage with fewer probe positions
PAUT uses multiple elements and electronic beam steering/focusing, allowing comprehensive weld coverage with fewer scan passes and improved defect characterization compared to conventional UT.
Question 6: API 653 requires that inspection records for a tank include which of the following as a minimum?
- Only the date of inspection and the inspector's name
- Thickness measurements, observed defects, inspection method, date, and inspector qualifications (Correct answer)
- A complete set of original construction radiographs and mill certifications only
- The insurance carrier's risk assessment and the owner's maintenance budget
Correct answer: Thickness measurements, observed defects, inspection method, date, and inspector qualifications
API 653 requires inspection records to contain thickness measurements, defect observations, NDE methods used, inspection date, and the inspector's name and qualifications.
Question 7: When evaluating a dent in a tank shell, API 653 considers the dent acceptable without repair if it meets which primary criterion?
- The dent depth does not exceed 2% of the tank diameter
- There are no stress concentrations at the dent and the remaining thickness meets minimum requirements (Correct answer)
- The dent is located above the maximum liquid level
- The dent depth does not exceed 1 inch regardless of shell thickness
Correct answer: There are no stress concentrations at the dent and the remaining thickness meets minimum requirements
API 653 evaluates dents based on the absence of stress risers (sharp edges, gouges) within the dent and whether the remaining wall thickness still meets the minimum required thickness for the service condition.
API 653 Tank Inspection, NDE & Testing 5
According to API 653, a tank that has experienced a significant leak must undergo what type of inspection before being returned to service?