API 510 Inspection Intervals & Remaining Life Calculations — Questions and Answers
Question 1: A pressure vessel had a measured thickness of 0.625 in. at its last internal inspection 5 years ago. The current measured thickness is 0.575 in. What is the short-term corrosion rate?
- 0.005 in/yr
- 0.010 in/yr (Correct answer)
- 0.050 in/yr
- 0.025 in/yr
Correct answer: 0.010 in/yr
Short-term corrosion rate = (previous thickness − current thickness) ÷ years between readings = (0.625 − 0.575) ÷ 5 = 0.050 ÷ 5 = 0.010 in/yr. The short-term rate uses only the two most recent readings and reflects current conditions, whereas the long-term rate uses the original construction thickness.
Question 2: Using the API 510 formula, what is the remaining life of a vessel with a current measured thickness of 0.500 in., a minimum required thickness (t_min) of 0.300 in., and a corrosion rate of 0.020 in/yr?
- 15 years
- 10 years (Correct answer)
- 25 years
- 16.7 years
Correct answer: 10 years
Remaining life = (actual thickness − t_min) ÷ corrosion rate = (0.500 − 0.300) ÷ 0.020 = 0.200 ÷ 0.020 = 10 years. This is the fundamental remaining life formula in API 510. It tells you how long until the vessel wall reaches the minimum acceptable thickness at the current rate of corrosion.
Question 3: According to API 510, the maximum interval between internal or on-stream inspections for a pressure vessel that is not on an RBI program shall not exceed:
- 5 years or one-quarter of the remaining corrosion life, whichever is less
- 10 years or one-half of the remaining corrosion life, whichever is less (Correct answer)
- 15 years or three-quarters of the remaining corrosion life, whichever is less
- 20 years, provided the corrosion rate is less than 0.005 in/yr
Correct answer: 10 years or one-half of the remaining corrosion life, whichever is less
API 510 Section 6.4 states that the maximum interval between internal or on-stream inspections shall not exceed 10 years or one-half of the remaining corrosion life, whichever is less. This 'half-life rule' ensures that action can be taken before the vessel reaches its minimum thickness. RBI programs may justify longer intervals under specific conditions.
Question 4: API 510 specifies that the maximum interval for external inspections of pressure vessels shall not exceed:
- 1 year
- 3 years
- 5 years (Correct answer)
- 10 years
Correct answer: 5 years
API 510 Section 6.6 establishes a maximum external inspection interval of 5 years. External inspections check for corrosion under insulation (CUI), surface corrosion, support integrity, and condition of external coatings. External inspections are less intrusive than internal inspections and can often be performed on-stream.
Question 5: When both a short-term and a long-term corrosion rate are available for a vessel, API 510 requires the inspector to use which rate for determining the inspection interval and remaining life?
- Always the long-term rate, because it accounts for the full history of the vessel
- Always the short-term rate, because it reflects the most recent operating conditions
- The more conservative (higher) of the two rates (Correct answer)
- The average of the short-term and long-term rates
Correct answer: The more conservative (higher) of the two rates
API 510 Section 7.1.1 requires use of the more conservative (higher) corrosion rate when both short-term and long-term rates are available. This conservative approach ensures that unexpected acceleration in corrosion does not result in under-inspection. The inspector may use engineering judgment to justify a different rate with appropriate documentation.
Question 6: A vessel's long-term corrosion rate is calculated using which two thickness values?
- The thickness at the most recent inspection and the thickness at the inspection just before it
- The original fabrication (nominal) thickness and the current measured thickness (Correct answer)
- The minimum required thickness and the current measured thickness
- The thickness at the first post-construction inspection and the thickness at the most recent inspection
Correct answer: The original fabrication (nominal) thickness and the current measured thickness
The long-term corrosion rate = (original thickness − current thickness) ÷ total years in service. It uses the original nominal or fabrication thickness as the starting point, divided by the total time the vessel has been in service. This contrasts with the short-term rate, which uses only the last two consecutive inspection readings.
API 510 Inspection Intervals & Remaining Life Calculations
A pressure vessel had a measured thickness of 0.625 in. at its last internal inspection 5 years ago.
The current measured thickness is 0.575 in.
What is the short-term corrosion rate?