API 653 Tank Inspection, NDE & Testing 2 — Questions and Answers
Question 1: When performing magnetic flux leakage (MFL) scanning on a tank floor, what condition most significantly reduces the reliability of results?
- Presence of product residue on the floor surface
- Floor plate thickness greater than 1/2 inch
- Scanning speed exceeding the equipment manufacturer's maximum recommended speed (Correct answer)
- Ambient temperature below 50°F during scanning
Correct answer: Scanning speed exceeding the equipment manufacturer's maximum recommended speed
Exceeding the manufacturer's maximum scanning speed reduces the dwell time of the magnetic field and can cause the equipment to miss or mischaracterize corrosion indications.
Question 2: According to API 653, which type of weld joint is typically used at the connection of a new bottom plate annular ring to the shell?
- Double-welded butt joint (Correct answer)
- Single-welded butt joint with backing strip
- Full-penetration T-joint fillet weld
- Lap joint with double fillet welds
Correct answer: Double-welded butt joint
API 653 requires a double-welded butt joint for the connection of the annular ring to the shell to ensure structural integrity at this critical location.
Question 3: A tank is being inspected and the inspector finds a blister in the shell plate. What is the FIRST action the inspector should take?
- Grind the blister flush and reinspect with UT
- Determine whether the blister is live (contains trapped gas under pressure) (Correct answer)
- Apply an external patch plate over the blister
- Remove the tank from service immediately without further evaluation
Correct answer: Determine whether the blister is live (contains trapped gas under pressure)
Before any repair or continued inspection, the inspector must determine if the blister is live (pressurized with trapped hydrogen or gas) as this poses an immediate safety hazard.
Question 4: Under API 653, what is the minimum number of shell thickness measurements required per course when performing a standard inspection?
- One measurement per course at the most corroded area
- At least four measurements evenly spaced around each course (Correct answer)
- Measurements at every 10 feet around the circumference
- Two measurements per course, 180 degrees apart
Correct answer: At least four measurements evenly spaced around each course
API 653 requires at least four thickness measurements evenly spaced around the circumference of each shell course to adequately characterize general corrosion.
Question 5: Which NDE method is best suited for detecting subsurface laminar defects in tank shell plate?
- Magnetic particle testing (MT)
- Liquid penetrant testing (PT)
- Ultrasonic straight-beam testing (UT) (Correct answer)
- Visual inspection with magnification
Correct answer: Ultrasonic straight-beam testing (UT)
Ultrasonic straight-beam (compression wave) testing is the preferred method for detecting laminar defects because it propagates through the plate thickness and reflects off internal discontinuities.
Question 6: When using radiographic testing (RT) to examine a tank shell weld, what does a dark irregular indication on the radiograph most likely represent?
- Tungsten inclusion from GTAW welding
- Incomplete fusion along the weld root
- Porosity or gas void in the weld (Correct answer)
- Excessive reinforcement height
Correct answer: Porosity or gas void in the weld
Dark irregular indications on a radiograph represent areas of lower density (voids), which are consistent with porosity or gas pockets trapped in the weld.
Question 7: During a hydrostatic test of a repaired tank, a small weep is observed at a previously repaired weld area. What is the correct response according to API 653?
- Mark the location and continue the test to full test pressure
- Reduce the water level to normal operating level and inspect the weld under reduced head
- Immediately drain the tank and re-weld the area (Correct answer)
- Accept the weep if it does not grow within 24 hours
Correct answer: Immediately drain the tank and re-weld the area
Any visible leakage during a hydrostatic test requires the tank to be drained, the defect repaired, and the test repeated to confirm integrity.
API 653 Tank Inspection, NDE & Testing 2
When performing magnetic flux leakage (MFL) scanning on a tank floor, what condition most significantly reduces the reliability of results?