General metal loss (GML) is the most frequently encountered damage mode in piping and pressure vessel inspection programmes. API 579-1 Part 4 provides the formal assessment path. The distinction between Level 1 and Level 2 is not simply about precision - it is about what inspection data you have and how much conservatism you can afford.
Level 1 Screening
Level 1 applies the minimum remaining thickness check based on original design code equations (B31.3, ASME Section VIII Div 1, or equivalent). The check compares measured minimum thickness (tmm) against minimum required thickness for design pressure (tmin). For Level 1, tmm must be from UT measurement, and inspection coverage is assumed representative of worst-case location. Level 1 will reject equipment that Level 2 would accept.
Level 2 Screening - Critical Thickness Profile (CTP)
Level 2 uses a more structured inspection approach. Multiple thickness readings taken along defined inspection paths. A Critical Thickness Profile (CTP) is constructed the minimum thickness profile along each path. The assessment computes average thickness along the path and compares it against the Level 2 acceptance criterion, which accounts for the statistical reality that the true minimum thickness is unlikely to be at a single inspection point. Less conservative for distributed corrosion and more appropriate for piping systems with general surface corrosion.
When to Go to Level 3
Level 3 FEA is warranted whe the damage zone extends through complex geometry (nozzle junction, reducer, tee); GML is combined with local thinning requiring combined Part 4 + Part 5 assessment; the equipment has documented process excursion history changing load assumptions.
Full Level 2 GML assessment methodology on carbon steel piping document.
API 579 Part 4 GML Assessment - Level 1 vs Level 2: When to Move Up
