API 510 Code Compliance & Safety Regulations 4 — Questions and Answers
Question 1: API 510 requires that pressure vessels operating at temperatures above 750°F (400°C) in hydrogen service be assessed for:
- Sulfide stress cracking (SSC)
- High-temperature hydrogen attack (HTHA) per API 941 (Correct answer)
- Crevice corrosion per NACE SP0472
- Galvanic corrosion per ASTM G71
Correct answer: High-temperature hydrogen attack (HTHA) per API 941
API 941 (Nelson Curves) guides assessment of High-Temperature Hydrogen Attack risk for vessels operating in hydrogen service at elevated temperatures.
Question 2: When applying an RBI program under API 510, the inspection interval derived from RBI must not exceed the maximum interval defined by:
- The plant turnaround schedule
- API 510 Section 6 maximum intervals (Correct answer)
- The original construction code edition
- The vessel manufacturer's recommendation
Correct answer: API 510 Section 6 maximum intervals
Even with RBI justification, API 510 Section 6 establishes absolute maximum inspection intervals that cannot be exceeded regardless of low calculated risk.
Question 3: Under API 510, pressure relief devices (PRDs) protecting pressure vessels must be set at or below:
- 90% of MAWP
- The MAWP of the protected vessel (Correct answer)
- 110% of operating pressure
- The design pressure minus corrosion allowance
Correct answer: The MAWP of the protected vessel
PRDs must be set to open at or below the vessel's MAWP to prevent operation above the rated pressure.
Question 4: According to API 510, a pressure vessel inspector's primary obligation when safety and production schedules conflict is to:
- Balance both equally to minimize plant downtime
- Prioritize safety and code compliance over production schedules (Correct answer)
- Defer to the plant manager's final decision
- Consult OSHA before making a determination
Correct answer: Prioritize safety and code compliance over production schedules
API 510 is explicit that the inspector's primary duty is to ensure safe and code-compliant operation, which takes precedence over production considerations.
Question 5: Which of the following conditions would require a pressure vessel to be immediately removed from service under API 510?
- Minor surface rust on the exterior shell
- A crack in the pressure boundary that cannot be monitored safely (Correct answer)
- A corrosion rate slightly higher than predicted
- Missing insulation on a non-pressurized support leg
Correct answer: A crack in the pressure boundary that cannot be monitored safely
An uninspectable or unmonitorable crack in the pressure boundary represents an imminent safety hazard requiring immediate shutdown.
Question 6: Under API 510, which entity is responsible for maintaining inspection records throughout a pressure vessel's service life?
- The Authorized Inspector
- The original equipment manufacturer
- The owner/operator (Correct answer)
- The jurisdiction's inspection department
Correct answer: The owner/operator
API 510 assigns responsibility for maintaining complete and current inspection records to the owner/operator of the pressure vessel.
Question 7: A 'cold-wall' vessel in hydrogen service where the insulation maintains the shell below 400°F — what Nelson Curve classification governs material selection?
- The curves apply based on process-side fluid temperature
- The curves apply based on the actual metal temperature (shell temperature) (Correct answer)
- The curves apply based on the average of shell and process temperatures
- Nelson Curves do not apply to cold-wall vessels
Correct answer: The curves apply based on the actual metal temperature (shell temperature)
For HTHA assessment per API 941, the relevant temperature is the actual metal (shell) temperature, so cold-wall vessels where the shell stays cool have reduced HTHA risk.
API 510 Code Compliance & Safety Regulations 4
API 510 requires that pressure vessels operating at temperatures above 750°F (400°C) in hydrogen service be assessed for: