Inspection Intervals & Remaining Life Calculations Flashcards

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?

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.

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?

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.

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:

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.

API 510 specifies that the maximum interval for external inspections of pressure vessels shall not exceed:

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.

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?

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.

A vessel's long-term corrosion rate is calculated using which two thickness values?

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.