SPED Stress Analysis & Mechanical Integrity 3 — Questions and Answers
Question 1: Which failure mode is addressed by checking the sustained stress ratio (SL/Sh) in ASME B31.3?
- Fatigue cracking from cyclic loading
- Plastic collapse under sustained loads (Correct answer)
- Stress corrosion cracking
- Creep rupture at elevated temperature
Correct answer: Plastic collapse under sustained loads
The sustained stress check ensures the pipe will not undergo plastic collapse or excessive deformation under continuously applied pressure and weight loads.
Question 2: When a piping system has high vibration levels, which type of support is most commonly added to mitigate dynamic response?
- Variable spring hanger
- Constant effort hanger
- Snubber (shock arrestor) (Correct answer)
- Rigid rod hanger
Correct answer: Snubber (shock arrestor)
Snubbers allow slow thermal movements but lock up and resist rapid dynamic loads such as vibration, water hammer, or seismic events.
Question 3: In Caesar II or similar piping analysis software, what does a 'restraint report' typically identify?
- Pipe material stress-strain curves
- Forces, moments, and deflections at each support point (Correct answer)
- Insulation heat loss at each node
- Corrosion allowance at each fitting
Correct answer: Forces, moments, and deflections at each support point
Restraint reports summarize the loads imposed on each support, which are used to design the structural steel and evaluate equipment nozzle loads.
Question 4: Which piping code primarily governs stress analysis of high-pressure steam piping in power plants?
- ASME B31.3 Process Piping
- ASME B31.1 Power Piping (Correct answer)
- ASME B31.4 Pipeline Transportation
- ASME B31.8 Gas Transmission
Correct answer: ASME B31.1 Power Piping
ASME B31.1 applies to power piping including steam, feedwater, and other services in power-generation facilities.
Question 5: A pipe stress engineer discovers that the calculated sustained stress exceeds the allowable. Which corrective action is most effective?
- Increase pipe wall thickness to reduce hoop stress only
- Relocate or add pipe supports to reduce bending moments from weight (Correct answer)
- Increase design pressure allowance in the stress report
- Switch to a lower allowable stress material
Correct answer: Relocate or add pipe supports to reduce bending moments from weight
Adding or repositioning supports reduces the unsupported span and the bending moments from gravity loads, directly lowering sustained bending stress.
Question 6: The stress intensification factor (SIF) for a butt-welded tee is greater than 1.0 because:
- Tees are manufactured from lower-grade materials
- The branch connection creates a local stress concentration (Correct answer)
- Tees have thicker walls than straight pipe
- Flanges attached to tees increase pressure drop
Correct answer: The branch connection creates a local stress concentration
The geometric discontinuity at a branch connection concentrates stresses, requiring the SIF to amplify the calculated moment stresses above the straight-pipe baseline.
Question 7: What is the difference between a 'variable spring hanger' and a 'constant effort hanger'?
- Variable hangers maintain constant load regardless of displacement; constant hangers vary with deflection
- Variable hangers change load with pipe movement; constant hangers maintain the same load throughout travel (Correct answer)
- Variable hangers are used below 200°F only; constant hangers for higher temperatures
- There is no practical difference; both provide identical support behavior
Correct answer: Variable hangers change load with pipe movement; constant hangers maintain the same load throughout travel
Variable spring hangers exert increasing or decreasing load as the pipe deflects, while constant effort hangers maintain the same support load throughout their full travel range.
SPED Stress Analysis & Mechanical Integrity 3
Which failure mode is addressed by checking the sustained stress ratio (SL/Sh) in ASME B31.3?