API 510 Safety & Risk Management 5 — Questions and Answers
Question 1: API 510 defines the 'corrosion allowance' as the material provided beyond the minimum required thickness to account for:
- Metal loss due to corrosion and erosion over the service life (Correct answer)
- Weld reinforcement requirements only
- Thermal expansion stresses
- Hydrostatic test overpressure
Correct answer: Metal loss due to corrosion and erosion over the service life
Corrosion allowance is the additional material thickness provided above the calculated minimum to accommodate expected metal loss from corrosion and erosion during the vessel's service life.
Question 2: When performing a thickness measurement on a vessel shell, the inspector finds a localized thin area. Under API 579 Level 1 assessment for general metal loss, the critical thickness profile (CTP) requires measurements that cover:
- The most corroded region with a grid pattern to establish the extent of thinning (Correct answer)
- Only the single thinnest point reading
- The nominal thickness from the manufacturer's data sheet
- Any area at random that is convenient to access
Correct answer: The most corroded region with a grid pattern to establish the extent of thinning
API 579 Level 1 general metal loss assessment requires a CTP established by systematic grid measurements over the affected region to accurately characterize the damage.
Question 3: An inspector notes high-temperature hydrogen attack (HTHA) damage concern on a vessel operating above 300°F with hydrogen partial pressure. The primary reference document for assessing HTHA susceptibility is:
- API 941 (Nelson Curves) (Correct answer)
- API 571 Section on Galvanic Corrosion
- ASME Section VIII Div. 2
- API 579 Level 3 FFS
Correct answer: API 941 (Nelson Curves)
API 941 contains the Nelson Curves, which define the operating limits of carbon and alloy steels for HTHA based on hydrogen partial pressure and temperature.
Question 4: In a risk-based inspection plan, equipment with HIGH consequence of failure but LOW probability of failure should be prioritized by:
- Focusing inspection on verifying damage mechanism controls and barrier integrity (Correct answer)
- Reducing inspection frequency since probability is low
- Transferring the equipment to a lower-risk service immediately
- Applying only external visual inspection annually
Correct answer: Focusing inspection on verifying damage mechanism controls and barrier integrity
High-COF, low-POF equipment warrants ensuring damage mechanisms are controlled and protection layers are intact, even though current failure probability is low.
Question 5: The Maximum Allowable Working Pressure (MAWP) of a pressure vessel after metal loss is recalculated using the:
- Actual measured minimum thickness, not the original design thickness (Correct answer)
- Original design thickness from the manufacturer's drawings
- Nominal pipe schedule thickness from ASME tables
- Corrosion allowance remaining, not total measured thickness
Correct answer: Actual measured minimum thickness, not the original design thickness
After metal loss, MAWP must be recalculated using the actual minimum measured thickness to reflect the vessel's current condition, not the original design values.
Question 6: During an RBI re-assessment, an inspector discovers that the actual corrosion rate is significantly higher than the rate assumed in the original RBI study. The required response is to:
- Update the RBI analysis with the new rate and revise inspection intervals accordingly (Correct answer)
- Continue using the original intervals until the next scheduled RBI review date
- Increase operating pressure to reduce the corrosion rate
- Remove the vessel from RBI and revert to fixed API 510 maximum intervals only
Correct answer: Update the RBI analysis with the new rate and revise inspection intervals accordingly
RBI is a living program; when actual rates deviate significantly from assumed rates, the analysis must be updated and intervals revised to maintain acceptable risk levels.
Question 7: According to API 510 and process safety principles, a 'near miss' incident involving a pressure vessel should be:
- Investigated and documented to identify root causes and prevent recurrence (Correct answer)
- Recorded only if property damage occurred
- Reported to OSHA within 24 hours in all cases
- Handled informally since no injury or release occurred
Correct answer: Investigated and documented to identify root causes and prevent recurrence
Near misses are leading indicators of potential catastrophic events; API 510 and PSM principles require investigation and documentation to identify and correct root causes before a real incident occurs.
API 510 Safety & Risk Management 5
API 510 defines the 'corrosion allowance' as the material provided beyond the minimum required thickness to account for: