API 653 Fitness-for-Service & Repair 4 — Questions and Answers
Question 1: API 653 defines the 'critical zone' of a tank bottom as the area within what distance from the shell?
- 6 inches
- 12 inches
- 18 inches
- 3 feet (Correct answer)
Correct answer: 3 feet
API 653 defines the critical zone as the annular bottom plate area within 3 feet of the inside edge of the shell, where stresses are highest.
Question 2: When a tank inspector finds a roof-to-shell weld that is cracked, the FIRST action should be to:
- Immediately begin weld repair without further evaluation
- Assess whether the crack affects structural integrity and containment (Correct answer)
- Fill the crack with a chemical sealant and monitor
- Apply an external clamp over the cracked weld
Correct answer: Assess whether the crack affects structural integrity and containment
The first step is to assess the severity and extent of the crack to determine if the tank must be taken out of service or if temporary measures are adequate pending repair.
Question 3: Under API 653, which party must approve a fitness-for-service assessment for a tank with damage exceeding the code's simplified criteria?
- The tank owner/operator alone
- A certified API 653 inspector
- An engineer experienced in storage tank design (Correct answer)
- The local fire marshal
Correct answer: An engineer experienced in storage tank design
API 653 requires that complex fitness-for-service assessments beyond the simplified level be reviewed and approved by an engineer experienced in storage tank design.
Question 4: What is the maximum allowable out-of-plumb (tilt) for a tank shell per API 653 before it becomes a fitness-for-service concern?
- 1/100 of the tank height (Correct answer)
- 1/200 of the tank height
- 1/500 of the tank height
- Any measurable tilt requires immediate action
Correct answer: 1/100 of the tank height
API 653 indicates that shell out-of-plumb exceeding 1% (1/100) of the tank height is a concern that may require engineering evaluation.
Question 5: For hot-tap repairs on tanks in service, API 653 requires that the minimum remaining shell thickness at the hot-tap location be:
- At least 0.10 in
- At least 0.25 in
- Sufficient to prevent burn-through based on an engineering analysis (Correct answer)
- Greater than the nominal design thickness
Correct answer: Sufficient to prevent burn-through based on an engineering analysis
Hot-tap repairs require an engineering analysis to confirm sufficient remaining wall thickness to prevent burn-through during welding with the tank in service.
Question 6: API 653 requires that records for tank repairs and alterations be maintained for the life of the tank and include which of the following?
- Only the final inspection report
- Welding procedures, welder qualifications, NDE results, and as-built drawings (Correct answer)
- Contractor invoices and material costs only
- Owner's insurance policy covering the repair work
Correct answer: Welding procedures, welder qualifications, NDE results, and as-built drawings
Complete repair records including WPS, welder qualifications, NDE reports, and as-built documentation must be retained to support future inspections and FFS assessments.
Question 7: When evaluating shell-to-bottom weld corrosion for fitness-for-service, the inspector should measure thickness:
- Only on the outside surface of the shell above grade
- On the bottom plate within the critical zone and the lowest shell course (Correct answer)
- Only at the weld toe using magnetic particle testing
- At the highest shell course where corrosion is most visible
Correct answer: On the bottom plate within the critical zone and the lowest shell course
The shell-to-bottom weld area fitness-for-service requires UT thickness measurements on both the annular bottom plate in the critical zone and the lowest shell course.
API 653 Fitness-for-Service & Repair 4
API 653 defines the 'critical zone' of a tank bottom as the area within what distance from the shell?