Inspection, Examination, and Testing
The inspection modes
API 510 recognizes internal inspection (vessel out of service, opened, cleaned, examined from inside), external inspection (visual and NDE from outside, often while running), and on-stream inspection (external inspection performed during operation, which can substitute for an internal inspection when the code conditions are met). The inspector decides the extent of examination; the owner/user must provide access, cleaning, lighting, and safe working conditions.
Methods and the codes behind them
Typical tools are visual examination, UT thickness gauging, RT, MT, PT, eddy current, PMI, and hardness testing. The method is governed by ASME BPVC Section V; welder and procedure qualification by Section IX; damage-mechanism identification by API 571; vessel inspection practice by API 572; weld inspection by API 577.
What makes it defensible
Thickness surveys must be systematic - reproducible locations, documented, tied to a corrosion-circuit map - because the next interval is calculated from them. Any deterioration that could affect safe operation must be evaluated, and if measured thickness falls below required thickness, the vessel is not fit for its current rating until repaired, rerated, or accepted through a fitness-for-service assessment.
Traps
- Treating a hydrostatic test as an inspection; it is a test, not a substitute for internal examination.
- Substituting on-stream inspection for internal inspection without meeting the code conditions and inspector agreement.
- Reporting an average thickness instead of the minimum reading at a critical location.
Rule of thumb
Every recorded thickness must answer two questions: exactly where was it taken and what was it last time? Without both, no corrosion rate can be defended.