API 579 Part 4 General Metal Loss Assessment Screening

API 579 Part 4 General Metal Loss Assessment is used when metal loss is broad and widespread and the integrity decision is controlled by remaining thickness under pressure—not by a single local thin area (LTA). This assessment method is commonly applied to vessels, piping, and tanks experiencing long-term corrosion or erosion where the key question is whether the component can continue operating at current conditions or needs rerating.

Use this screening workflow to confirm Part 4 applicability and whether you have the right thickness basis to support a defensible decision, including minimum measured thickness (tmm), average measured thickness (tam), and comparison to the required minimum thickness (tmin). If screening indicates concern, the next step is a formal Part 4 Fitness-for-Service (FFS) evaluation to determine acceptability and, when required, rerated limits such as MAWPr and coincident temperature.

Use the screening questions below to determine whether a formal Part 4 evaluation is recommended.

API 579 Part 4 — General Metal Loss Assessment Screening (Detailed Workflow)

Instruction: Answer the questions (Yes/No/N/A), then click “Check if FFS is needed”.

1) Is the identified condition strictly metal loss (corrosion/erosion), and not another flaw type that must be evaluated under another Part?
For example; a crack-like indication at a weld toe is present — route that portion to Part 9 instead of treating it as metal loss only.
2) Does the metal loss exceed (or is it predicted to exceed before the next scheduled inspection) the specified corrosion/erosion allowance, or is there not adequate thickness available for the future corrosion allowance?
For example; measured thinning is still within the specified allowance and enough thickness remains for the planned future allowance → No (record the data; no Part 4 assessment required). If thinning is beyond the allowance or will reach it before next inspection → Yes (assessment required).
3) Is this case appropriate to evaluate under Part 4 (General Metal Loss) — recognizing Part 4 may be used for uniform or local wall loss treated as “general metal loss,” and if thickness profiles show clearly localized behavior, Part 5 may be used as an evaluation option?
For example; broad thinning is clearly general → proceed under Part 4. If later thickness profiles show a localized patch controlling, Part 4 may be conservative and Part 5 may be the better fit.
4) Is the metal loss confirmed to be on the correct surface(s) for your records (internal, external, or both), with the affected region clearly defined?
For example; internal corrosion thinning only (ID) with no evidence of external corrosion (OD), and the thinned band limits are documented.
5) Is the component not operating in the creep range (per the limitation referenced to Table 4.1)?
For example; this is low-temperature/ambient service equipment rather than high-temperature heater outlet service where creep may control.
6) Was the original design criteria in accordance with a recognized code/standard (traceable design basis)?
For example; ASME VIII / B31.3 basis is known so tmin and allowable stresses are defensible.
7) Does the metal loss region have relatively smooth contours (no notch-like geometry / negligible local stress concentrations) for Level 1/2 applicability?
For example; gradual transitions over inches, not a sharp groove or undercut-like feature.
8) Is the component not in cyclic service (or does it satisfy the cyclic service screening procedure referenced to Part 14 for applicability of Part 4 Level 1/Level 2 procedures)?
For example; steady operation with infrequent cycling rather than frequent start-stop or large pressure swings.
9) Have you identified the Part 4 component type (Type A / Type B Class 1 / Type B Class 2 / Type C) using Table 4.2 (or equivalent classification in your workflow)?
For example; straight shell course (Type A) vs nozzle region (Type B) vs discontinuity/junction (Type C).
10) Is the component within the Part 4 applicability limitations for component type and loads (including internal pressure and, where applicable, external pressure and supplemental loads for the selected assessment level)?
For example; internal pressure with typical sustained loads (weight/nozzle loads) rather than complex external loads controlling the case.
11) If external pressure is applicable, is it included appropriately per the selected assessment level and component type limitations?
For example; a column under vacuum is checked for external pressure requirements; a purely internal-pressure-only case may be N/A.
12) If supplemental loads are present (nozzle loads, weight, wind/seismic, piping reactions), are they within the scope/limitations of the selected Part 4 assessment level?
For example; typical sustained loads are considered; if complex load combinations govern, Level 3 may be required.
13) Is the metal loss located sufficiently away from major structural discontinuities (or otherwise handled with the appropriate discontinuity guidance) for Level 1/2 screening applicability?
For example; thinning is not within the influence region of a head-to-shell junction or attachment where discontinuity effects can dominate.
14) Do you have sufficient thickness readings to establish tmm (minimum measured thickness) and tam (average measured thickness) for the affected region?
For example; a UT grid/scan or adequate distributed readings exist to compute tmm and tam.
15) Is the measured thickness everywhere greater than or equal to tmin when corrosion allowance and design basis are considered?
For example; tmin = 0.50 in, but a measured point is 0.47 in → at least one location is below tmin.
16) Have you selected the correct characterization path: Point Thickness Readings (PTR) vs. Thickness Profiles (grid/scan), based on variability and damage appearance?
For example; stable uniform readings support PTR; wide scatter suggests a profile/grid is needed.
17) If using PTR, have you confirmed the metal loss is uniform wall loss (PTR shall not be used when there is an indication of localized metal loss)?
For example; there is no evidence of a localized pit band/groove — the condition is broadly uniform.
18) If using PTR, do you have sufficient readings (recommended distribution; minimum threshold such as ≥15 readings unless other NDE confirms general metal loss)?
For example; 20+ readings distributed across the area, not just 3–5 spot checks.
19) If using PTR, does the minimum measured thickness satisfy the Part 4 averaging protection requirement: tmm ≥ 0.9 × tam?
For example; tam = 0.60 in → 0.9×tam = 0.54 in. If tmm = 0.44 in, then 0.44 < 0.54 → No (PTR averaging is not acceptable; use thickness profiles / consider Level 3 / Part 5 as appropriate).
20) If using thickness profiles, have you taken grid/scan data in the correct inspection plane(s) and density to characterize the metal loss pattern?
For example; a UT grid across the thinned band shows the pattern and confirms it is general metal loss rather than a localized patch.
21) If metal loss is near a major structural discontinuity (nozzle/branch, conical transition, flange connection, cylinder-to-flat head, integral tubesheet, stiffening/support ring), have you taken the added inspection coverage required by the applicable Part 4 figures/zones?
For example; extra UT grid density within the nozzle reinforcement/discontinuity influence zone rather than only away from the junction.
22) Have you checked for and resolved data discrepancies that require additional readings (e.g., a new reading is thicker than the previous inspection reading at the same location)?
For example; latest UT says 0.62 in but last outage reading was 0.58 in — additional readings confirm true thickness and repeatability.
23) Were thickness readings obtained using appropriate NDE and with suitable surface/access considerations (UT point readings vs scans; RT if useful to confirm general vs localized)?
For example; surface prep was done where required and technique selection matches the needed characterization.
24) Were UT measurements taken after appropriate calibration/controls (wall thickness standards, sound velocity, temperature effects/couplant), and with probe selection appropriate for the geometry?
For example; calibrated for the component temperature; small-diameter probe used when needed to reduce the chance of missing localized features.
25) If brittle fracture or crack-like damage could be a concern, have you completed supplemental inspection to confirm no crack-like flaws accompany the thinning (as applicable for your service)?
For example; MT/PT/UT of weld seams and adjacent areas in the region to confirm no crack-like flaws are present.
26) If any measured thickness readings are extremely thin (e.g., ≤ 0.100 in), have you applied the required validation precautions before using the values for decisions?
For example; the thinnest points were re-verified with repeat UT passes and/or alternate technique to confirm reliability.
27) Do your conditions and data support a Level 1 or Level 2 assessment (rather than requiring Level 3 due to limitations/applicability)?
For example; smooth general thinning with allowed loads and adequate thickness data supports Level 1/2; otherwise consider Level 3.
28) If remaining life is required, do you have corrosion rate and the thickness quantities needed to apply the appropriate remaining life method (thickness-based vs MAWP-based, as applicable)?
For example; corrosion rate from trending plus tmm/tam or rerating basis exists to compute remaining life until next inspection.
29) Have you captured the required Part 4 documentation (inspection summary + thickness data organization) and do you have an in-service monitoring plan suitable for general metal loss?
For example; thickness locations/orientation are recorded with sketches, datasets are stored consistently, and repeat monitoring points are defined for trending.
Answer the questions, then click “Check if FFS is needed”.

When to Use API 579 Part 4

API 579 Part 4 is typically used when inspection shows general thinning over an area and the decision can be made using thickness statistics (tmm and tam) compared to tmin. Common triggers include:

  • Widespread corrosion/erosion over a shell course, pipe run, or large region showing general metal loss
  • The need to confirm acceptability at current operating conditions using tmm/tam versus tmin
  • The need to determine rerated limits (such as MAWPr and coincident temperature) to safely run until the next outage
  • A case where the thinning is smooth (no sharp grooves/notches) and the geometry is appropriate for Part 4 Level 1/Level 2 screening
  • A case that is not creep-controlled and is not dominated by severe cyclic service concerns

If the damage is controlled by one localized thin area or groove-like feature, route the evaluation to API 579 Part 5 (Local Metal Loss).

What to Gather if Screening Indicates FFS Is Needed

If this workflow indicates that a formal API 579 Part 4 assessment is recommended, prepare the following to speed up the evaluation:

  • Equipment/component identification and service description
  • Design basis needed to define tmin (pressure, temperature, material, corrosion allowance, and design conditions)
  • Thickness dataset sufficient to define tmm and tam (UT grid, scan/C-scan results, or equivalent)
  • Location and extent of the thinning region and any supporting inspection documentation
  • Confirmation of operating mode (steady vs cyclic) and whether supplemental loads are significant
  • Operating basis for rerating decisions (current and required operating pressure/temperature, planned run length to next outage)

Request an API 579 Part 4 General Metal Loss Assessment (FFS)

If this workflow indicates that an API 579 Part 4 General Metal Loss Fitness-for-Service (FFS) assessment is needed, the next step is a decision-ready engineering evaluation using your inspection thickness data, design basis, and operating conditions.

Inspection 4 Industry LLC (I4I) performs API 579-1 / ASME FFS-1 Part 4 assessments of existing equipment for general metal loss and delivers a complete report stating fit-for-service or not fit-for-service, rerated limits when required (including MAWPr and coincident temperature), and practical integrity actions—repair now, repair at turnaround, or monitor and run with a defined inspection plan.

To proceed, send your available thickness dataset (UT grid or scan/C-scan), equipment details, and operating basis and request an API 579 Part 4 General Metal Loss Assessment (FFS).

 

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