NDT Boilermaker Structural Integrity Assessments 5 — Questions and Answers
Question 1: Which factor MOST significantly influences the minimum required thickness calculation for a cylindrical boiler shell under internal pressure?
- Inside radius, allowable stress, and joint efficiency (Correct answer)
- Outside diameter and wall material color
- Boiler age and number of previous repairs
- Combustion chamber volume
Correct answer: Inside radius, allowable stress, and joint efficiency
The hoop stress formula t = PR/(SE) shows that inside radius (P), allowable stress (S), and joint efficiency (E) directly determine required wall thickness.
Question 2: When using eddy current testing to inspect non-ferromagnetic boiler tubes, a phase angle shift of approximately 90° between two indications MOST likely distinguishes:
- Inside diameter (ID) defects from outside diameter (OD) defects (Correct answer)
- Cracks from corrosion pits
- Ferrous from non-ferrous materials
- Deep defects from shallow defects only
Correct answer: Inside diameter (ID) defects from outside diameter (OD) defects
In eddy current tube inspection, the phase angle of the signal response indicates the depth location of the discontinuity, with ID and OD defects typically separated by approximately 90° on the impedance plane.
Question 3: A fitness-for-service assessment per API 579-1/ASME FFS-1 for a pressure vessel with general metal loss typically uses which Level 1 screening criterion?
- Remaining thickness ratio (RTR) compared to a minimum allowable ratio (Correct answer)
- Comparing the original design thickness to the corroded thickness percentage
- Comparing corrosion rate to industry average rates
- Visual comparison to standard corrosion severity charts
Correct answer: Remaining thickness ratio (RTR) compared to a minimum allowable ratio
API 579 Level 1 FFS assessment for general metal loss uses the remaining thickness ratio (RTR = measured thickness / required thickness) compared to a minimum acceptable ratio.
Question 4: During a boiler inspection, an inspector identifies hydrogen damage characterized by internal fissures and blistering in the tube wall. What operating condition MOST likely caused this damage?
- Hydrogen sulfide attack at high temperature (high-temperature hydrogen attack) (Correct answer)
- Overpressure during hydrostatic testing
- Thermal fatigue from rapid temperature cycling
- Caustic stress corrosion cracking
Correct answer: Hydrogen sulfide attack at high temperature (high-temperature hydrogen attack)
High-temperature hydrogen attack (HTHA) occurs when atomic hydrogen at elevated temperatures reacts with carbides in steel, forming methane bubbles that cause internal fissuring and decarburization.
Question 5: In the context of boiler structural integrity, what does 'minimum design metal temperature' (MDMT) define?
- The lowest temperature at which the component can be pressurized without risk of brittle fracture (Correct answer)
- The minimum temperature required for proper combustion
- The lowest feedwater temperature allowed
- The minimum ambient temperature during installation
Correct answer: The lowest temperature at which the component can be pressurized without risk of brittle fracture
MDMT is the lowest permissible temperature at which a pressure component may be subjected to full design pressure, below which brittle fracture risk becomes unacceptable.
Question 6: When ultrasonic testing reveals a planar indication parallel to the fusion line of a boiler weld, this orientation makes the indication MOST significant because:
- It is oriented perpendicular to tensile hoop stresses, maximizing stress intensity (Correct answer)
- Parallel indications are always rejectable regardless of size
- It indicates the weld was made with improper electrode angle
- Planar indications parallel to welds are always false calls
Correct answer: It is oriented perpendicular to tensile hoop stresses, maximizing stress intensity
A planar flaw oriented perpendicular to the principal tensile stress (hoop stress in a cylinder) maximizes the stress intensity factor, making it most susceptible to fracture or fatigue crack growth.
Question 7: A boiler inspector finds that a longitudinal seam weld in a shell has a measured misalignment (hi-lo) exceeding code limits. What is the PRIMARY structural concern with excessive weld misalignment?
- Secondary bending stresses are induced at the offset, reducing fatigue life (Correct answer)
- The weld will have insufficient penetration
- Misalignment increases corrosion rate in the weld zone
- The boiler pressure rating automatically doubles
Correct answer: Secondary bending stresses are induced at the offset, reducing fatigue life
Weld misalignment introduces secondary bending stresses at the offset location due to eccentricity under internal pressure loading, which can significantly reduce cyclic fatigue life.
NDT Boilermaker Structural Integrity Assessments 5
Which factor MOST significantly influences the minimum required thickness calculation for a cylindrical boiler shell under internal pressure?