Inspection Practices, Methods, and Referenced Code Requirements
Inspection types and where to look
API 570 recognizes external inspection, internal (intrusive) inspection, and on-stream inspection that may substitute for internal inspection when justified. CMLs must be placed where damage is most likely and most severe: injection points and downstream mixing zones, low points, deadlegs, turbulent areas, downstream of valves and orifices, and soil-to-air interfaces. Every CML needs a unique identifier marked on the pipe and tied to a drawing. A thickness survey needs enough points to find localized loss, not just a nominal average.
Methods and their limits
UT thickness is the workhorse, but it measures remaining wall, not cracks, and misses tight planar defects unless the grid is fine. Radiography, including profile and double-wall techniques, suits small-bore and insulated lines and is used for CUI screening. PT and MT find surface-breaking flaws; shear-wave UT and phased array find planar flaws; guided wave screens long buried or inaccessible runs; acoustic emission, corrosion probes, and coupons monitor activity. NDE personnel must be qualified under a recognized written practice such as ASNT SNT-TC-1A or an equivalent.
Referenced documents worth recognizing: ASME B31.3, API 571 (damage mechanisms), API 574 (inspection practices), API 577 (welding inspection), API 578 (weld flaw evaluation), API 580 (risk-based inspection), API 583 (CUI), and API 579-1/ASME FFS-1 (fitness-for-service).
Rule of thumb: match method to mechanism. Profile radiography and insulation windows for CUI, shear-wave UT for cracking, fine-grid UT for pitting. A method that cannot detect the expected mechanism does not count as inspection of that mechanism.