NDT Boilermaker Structural Integrity Assessments 4 — Questions and Answers
Question 1: A time-of-flight diffraction (TOFD) scan of a pressure vessel weld shows a lateral wave interruption paired with a backwall echo interruption at the same position. This combination MOST likely indicates:
- A through-wall or very deep crack (Correct answer)
- A small surface-breaking pit
- Normal weld crown geometry
- Porosity cluster near mid-wall
Correct answer: A through-wall or very deep crack
In TOFD, simultaneous disruption of the lateral wave and backwall echo at the same location suggests a discontinuity that extends from the near surface deep into or through the wall.
Question 2: Which failure mode is MOST associated with boiler components operating above the creep threshold temperature for extended periods?
- Gradual plastic deformation and wall thinning under sustained load (Correct answer)
- Brittle fracture at ambient temperature
- Immediate catastrophic rupture upon startup
- Hydrogen embrittlement
Correct answer: Gradual plastic deformation and wall thinning under sustained load
Creep is the time-dependent plastic deformation that occurs at elevated temperatures under sustained stress, causing progressive thinning and dimensional change over time.
Question 3: During a magnetic flux leakage (MFL) inspection of boiler tubes, a strong localized flux leakage signal with a sharp, high-amplitude response MOST likely corresponds to:
- Pitting or wall-loss defect (Correct answer)
- A clean, defect-free tube section
- Scale deposits on the tube ID
- Tube expander marks at the tube sheet
Correct answer: Pitting or wall-loss defect
Pitting and wall-loss create concentrated disruptions in the magnetic flux that leak outside the tube wall, producing the high-amplitude localized signals characteristic of metal loss.
Question 4: When a boiler undergoes alterations, who is responsible under NBIC (National Board Inspection Code) for certifying that the alteration was performed correctly?
- An authorized 'R' stamp holder (Correct answer)
- The original boiler manufacturer
- The jurisdictional authority only
- The plant maintenance supervisor
Correct answer: An authorized 'R' stamp holder
The National Board 'R' stamp is required for repair and alteration organizations, certifying that work was performed in accordance with the applicable code.
Question 5: In a boiler structural integrity assessment, 'remaining life' calculations using corrosion rate data require knowledge of:
- Current minimum measured thickness and allowable minimum thickness (Correct answer)
- Original fabrication date and heat number
- Water chemistry pH only
- Combustion gas temperature
Correct answer: Current minimum measured thickness and allowable minimum thickness
Remaining life is calculated by dividing the difference between current measured thickness and allowable minimum thickness by the measured corrosion rate.
Question 6: What is the PRIMARY reason that visual examination of welds should be performed BEFORE other NDT methods during a boiler integrity assessment?
- Surface conditions and geometry must be confirmed acceptable before applying other methods (Correct answer)
- Visual testing is the most sensitive method for subsurface defects
- Code requires visual testing to be the final acceptance step
- Other methods cannot function without prior visual clearance
Correct answer: Surface conditions and geometry must be confirmed acceptable before applying other methods
Visual examination identifies surface conditions, dimensional compliance, and obvious discontinuities that could affect the validity or interpretation of subsequent NDT methods.
Question 7: A boilermaker performing a structural integrity assessment finds weld cracks in the heat-affected zone of a P91 chrome-moly steel weld. Which cracking mechanism is MOST likely responsible?
- Reheat cracking during post-weld heat treatment (Correct answer)
- Cold cracking due to hydrogen
- Hot cracking during solidification
- Stress corrosion cracking in service
Correct answer: Reheat cracking during post-weld heat treatment
P91 (9Cr-1Mo-V) steel is particularly susceptible to reheat (stress relief) cracking in the HAZ due to precipitation strengthening that impedes stress relaxation during PWHT.
NDT Boilermaker Structural Integrity Assessments 4
A time-of-flight diffraction (TOFD) scan of a pressure vessel weld shows a lateral wave interruption paired with a backwall echo interruption at the same position.
This combination MOST likely indicates: