Inspection Intervals and Remaining Life Calculations
The arithmetic
Remaining life (RL) = (t measured − t required) ÷ corrosion rate, where t required comes from the original code of construction (e.g., ASME VIII UG-27) at the current MAWP and temperature, and the corrosion rate comes from thickness data at the same locations over a known time: CR = Δt ÷ years. Always use the minimum measured thickness, never the average.
The interval rules
- Internal inspection interval ≤ the lesser of one-half the remaining life or 10 years.
- External inspection interval ≤ 5 years.
- On-stream inspection may replace an internal inspection when corrosion behavior, service history, and access justify it and the inspector concurs.
- Risk-based inspection (API 580/581) may set extent and frequency where the owner/user has a program.
Traps
- Mixing units (mils per year versus inches per year: 1 ipy = 1000 mpy).
- Using design thickness instead of required thickness - design thickness includes corrosion allowance and yields a falsely long life.
- Taking a corrosion rate from a non-comparable location or a period with different service.
- Forgetting that when short-term and long-term rates disagree, the more conservative (higher) rate governs.
Worked example
A shell course read 0.500 in. four years ago and reads 0.470 in. now: CR = 0.030 ÷ 4 = 0.0075 ipy. Required thickness at MAWP is 0.450 in. RL = (0.470 − 0.450) ÷ 0.0075 = 2.7 years. Half of that is about 1.3 years, so the next internal inspection is due in roughly 1.3 years - far inside the 10-year cap - and the vessel becomes a candidate for repair or rerate planning.