API 579 Part 5 Local Metal Loss Assessment Screening

API 579 Part 5 Local Metal Loss Assessment is used when a localized thin area (LTA) or groove-like feature controls the integrity decision, even if surrounding thickness appears acceptable. Part 5 focuses on defining the local metal loss footprint correctly and evaluating whether the remaining section can safely sustain the required loads at the evaluated operating conditions.

Use this screening workflow to confirm Part 5 applicability and whether you have the inspection basis needed to support a defensible decision. In many cases, the assessment outcome is tied to acceptability checks and Remaining Strength Factor (RSF), and when required, rerated limits such as MAWPr for continued operation until the next planned outage.

Use the screening questions below to determine whether a formal Part 5 evaluation is recommended.

API 579 Part 5 — Local Metal Loss Assessment Screening (Workflow)

Instruction: Answer all questions, then click “Check if FFS is needed”.

1) Has inspection identified metal loss that is best characterized as local metal loss (LTA or groove-like flaw), rather than uniform/general wall thinning?
For example; a 6 in × 8 in thin patch has one minimum UT reading at 0.23 in while surrounding thickness is 0.45–0.55 in (localized, not uniform).
2) Is the flaw type confirmed as either: (a) Local Thin Area (LTA) (length ~ same order as width), or (b) Groove-like flaw (length ≫ width) for Part 5 treatment?
For example; an erosion groove is 30 in long and 2 in wide (groove-like), versus a 7 in × 9 in patch (LTA).
3) If the damage is a gouge (mechanical removal/relocation with possible cold work), have you redirected evaluation to the proper procedure (Part 12), rather than continuing under Part 5?
For example; an excavator bucket scraped a pipe leaving a long mechanical gouge; that case is treated as a gouge and handled per Part 12.
4) Is pitting corrosion the primary damage mechanism (or dominant contributor) requiring a dedicated pitting evaluation (Part 6), rather than treating it as local metal loss only?
For example; widespread deep pits drive the minimum thickness, not a smooth LTA profile—this points toward a Part 6 assessment (and sometimes combined methods).
5) Is the measured metal loss less than the specified corrosion/erosion allowance, and is adequate thickness available for the future corrosion allowance?
For example; nominal 0.50 in, allowance 0.10 in → expected minimum allowed is 0.40 in; the thinnest reading is 0.43 in and future loss to next inspection is 0.02 in—record data and no further action.
6) Was the component originally designed/fabricated to a recognized code/standard (so the Part 5 Level 1/2 applicability condition is satisfied)?
For example; an ASME VIII Div. 1 vessel or B31.3 piping system with known design basis supports Level 1/2 applicability.
7) Is the equipment not operating in the creep range (i.e., design temperature is within the limits used for Part 5 applicability)?
For example; a carbon steel vessel at 650°F is a creep-range concern—this would push evaluation toward creep procedures (Part 10), not Part 5 Level 1/2.
8) Is the material toughness considered sufficient, or has brittle fracture susceptibility uncertainty been addressed (e.g., by performing Part 3 when needed)?
For example; a low-temperature start-up drum with unknown toughness triggers a Part 3 check before relying on Part 5 Level 1/2 conclusions.
9) Is the local metal loss expected to exceed the corrosion allowance (or is predicted to exceed it) before the next scheduled inspection, making a Part 5 assessment necessary?
For example; current minimum thickness is 0.31 in, required with allowance is 0.30 in, and projected corrosion to next outage is 0.03 in—assessment is needed now because it will drop below required before the next inspection.
10) Do you have a thickness profile (UT grid, scan, or C-scan) sufficient to define the local metal loss footprint and the thickness variation (not just a few point readings)?
For example; a 1 in grid scan over and beyond the thin area defines the boundary and the thickness profile; two spot readings do not.
11) Have you established the thickness values needed for assessment (e.g., nominal thickness, corrosion allowance, future corrosion allowance, and the minimum measured thickness in the region)?
For example; you know t_nom = 0.50 in, FCA to next inspection = 0.02 in, corrosion allowance = 0.10 in, and minimum measured thickness t_mm = 0.23 in.
12) For an LTA, have you defined the local metal loss dimensions s (longitudinal) and c (circumferential), including projected growth to the future inspection date?
For example; the thin region is 10 in long × 6 in wide today, but you evaluate 12 in × 8 in after adding predicted growth before the next inspection.
13) For a groove-like flaw, have you measured the groove parameters (length, width, base radius, and orientation angle), and—if angled—projected to equivalent longitudinal/circumferential dimensions when needed?
For example; a helical erosion groove at 35° to the axis is projected into equivalent longitudinal and circumferential extents for evaluation (or treated as an equivalent LTA if appropriate).
14) If the flaw is groove-like, do you have a method to establish the base radius (profile gauge or mold), rather than guessing?
For example; a profile gauge shows a small root radius at the bottom of the groove that could control acceptability for Level 1 groove checks.
15) Have you determined the distance from the flaw to the nearest major structural discontinuity (MSD) (e.g., nozzle, support, attachment, geometric transition) as required for screening checks?
For example; the thin area edge is 3 in from a nozzle reinforcement pad weld—MSD proximity must be treated correctly before accepting Level 1 results.
16) Have all weld seams within the “2s × 2c box” around the LTA (and the entire flaw surface) been examined using MT/PT/UT as recommended (or required when brittle fracture susceptible)?
For example; you MT/PT the nearby seam welds and scan the thin patch surface to confirm there are no crack-like indications adjacent to the thinned region.
17) If any portion of a weld seam inside the 2s × 2c box is thinner than the required thickness, has that weld portion been volumetrically examined (RT or UT) as recommended?
For example; UT mapping shows a seam segment inside the box below required thickness—RT/UT volumetric exam is then used to check for embedded defects.
18) If crack-like flaws or porosity not meeting the original construction code acceptance are found, have you either repaired them or redirected to a crack-like flaw assessment (Part 9)?
For example; MT finds a linear indication at a seam weld near the thin area—repair the indication or perform a Part 9 assessment rather than continuing as “metal loss only.”
19) Is the component a Type A component with the flaw evaluated under internal pressure only (Level 1 boundary condition intent)?
For example; straight shell or straight pipe under internal pressure only (no significant external pressure, bending, wind, or thermal gradient driving the local stress state).
20) Can the local metal loss be reasonably characterized for Level 1 by a single representative thickness with one or two surface dimensions (s and/or c), without large thickness variations or a dense network of closely spaced thin spots?
For example; one thin patch with a consistent thickness band can be represented by a single t_mm; a highly irregular profile with spikes and valleys should move to Level 2 thickness-profile methods.
21) If the region shows significant thickness variation along its length or multiple closely spaced thin regions, are you prepared to perform a Level 2 assessment using thickness profiles (instead of relying on Level 1)?
For example; long patches with thickness changing from 0.22 in to 0.33 in over the footprint (or multiple thin “islands”) typically justify Level 2 to avoid overly conservative Level 1 results.
22) Are external pressure and/or supplemental loads (weight, wind, thermal, nozzle loads, etc.) significant enough that you should not rely on Level 1 “internal pressure only” assumptions?
For example; a large horizontal drum with local thinning near a saddle where weight bending dominates—this is not “internal pressure only,” so Level 2/3 is typically needed.
23) Does the component geometry / flaw complexity suggest Level 3 (e.g., complex shape, complex loading, or failure to pass Level 1/2), where numerical stress analysis is expected?
For example; local thinning at a complicated transition with multiple reinforcements and significant nozzle loads—finite element limit-load methods may be required.
24) For Level 1, do you have the required dimensions (s and c), minimum measured thickness (tmm), and the distances needed for limiting flaw size / MSD spacing checks?
For example; you can state s = 10 in, c = 8 in, t_mm = 0.23 in, and MSD spacing = 4 in, all referenced to the assessed footprint at the future inspection date.
25) If the damage is a groove, do you have the groove base radius data needed to check whether the groove satisfies the Level 1 groove-radius screening requirement?
For example; a sharp-root groove with a very small radius can fail a Level 1 screening check and drive escalation (or alternative methods).
26) Is the local metal loss region the result of blend grinding performed to remove a crack-like flaw, and is it being evaluated as local metal loss per Part 5?
For example; after grinding out a surface crack at a weld toe, the area is now locally thinned and must be qualified as an LTA/groove-like region.
27) If blend grinding was used, has the area been rechecked using appropriate surface NDE (e.g., MT or PT) to confirm no crack-like indications remain before treating it only as metal loss?
For example; after grinding and blending, PT is performed and shows no linear indications; then thickness profiling is used to evaluate remaining wall as local metal loss.
28) Is your intent to determine acceptability and/or rerating (e.g., establish RSF acceptability, or set reduced MAWP / reduced maximum fill height for tanks if needed)?
For example; the unit needs to keep running until turnaround, so you may accept as-is, rerate pressure, or define an operating limit based on the assessment outcome.
29) Do you have corrosion/erosion rate estimates (and growth of flaw footprint size) sufficient to support a remaining life estimate using thickness-based or MAWP-based approaches?
For example; corrosion rate is 8 mpy and the thin area is expanding ~1 in/year in length—this supports a remaining life estimate rather than guessing an interval.
30) If Level 2 indicates the component is not acceptable as-is, have you considered the available paths: rerate/reduce loads, repair/replace, improve weld efficiency via additional examination, or proceed to Level 3?
For example; you may reduce MAWP temporarily, perform a local repair, add additional weld examination to justify a better joint efficiency, or run a Level 3 FE limit-load assessment.
31) Have you defined an in-service monitoring / inspection plan that matches the predicted remaining life and uncertainty in corrosion rate and sizing data?
For example; you shorten the next inspection interval and require repeat thickness mapping on the same grid to confirm progression rate and footprint growth.
32) Will documentation capture: flaw identification, applicability decisions, inspection maps/profiles, dimensions, NDE results near welds, assessment level selection, conclusions, and any operating limits or rerates?
For example; you store the UT scan files, the grid layout, photos, MT/PT reports, calculated limits (if any), and the final operating decision in the permanent equipment record.
Answer all questions, then click “Check if FFS is needed”.

When to Use API 579 Part 5

API 579 Part 5 is typically used when metal loss is localized and the integrity decision cannot be made using broad thickness statistics alone. Common triggers include:

  • A localized thin area (LTA) identified by UT mapping, scans/C-scan, or thickness profiling
  • Groove-like corrosion/erosion features that behave as local metal loss rather than general thinning
  • Local thinning near stress raisers such as welds, nozzles, attachments, or discontinuities where local stresses can govern
  • A need to determine acceptability and, when required, rerated limits (MAWPr) based on the controlling local region
  • A case where data quality and damage definition (length/width/depth/remaining thickness profile) control the credibility of the result

If metal loss is broad and widespread rather than localized, route the evaluation to API 579 Part 4 (General Metal Loss).

What to Gather if Screening Indicates FFS Is Needed

If this workflow indicates that a formal API 579 Part 5 assessment is recommended, prepare the following to speed up the evaluation and strengthen the engineering basis:

  • Inspection dataset that clearly defines the LTA (UT grid, scan/C-scan, or thickness profile)
  • LTA geometry and location details (length, width, depth/remaining thickness, orientation, and proximity to welds/nozzles)
  • Design basis and operating conditions needed to define tmin and evaluate acceptability (pressure, temperature, material)
  • Any local loads or conditions that may increase local stress (attachments, supports, thermal gradients, or cyclic operation)
  • Confirmation of the intended run length to the next outage and inspection/monitoring expectations

Request an API 579 Part 5 Local Metal Loss Assessment (FFS)

If this workflow indicates that an API 579 Part 5 Local Metal Loss Fitness-for-Service (FFS) assessment is needed, the next step is a decision-ready engineering evaluation using your LTA thickness mapping, component details, design basis, and operating conditions.

Inspection 4 Industry LLC (I4I) performs API 579-1 / ASME FFS-1 Part 5 assessments of existing equipment for local metal loss and delivers a complete report stating fit-for-service or not fit-for-service, calculated acceptability/RSF basis when applicable, rerated limits when required (including MAWPr), and practical integrity actions—repair now, repair at turnaround, or monitor and run with a defined inspection plan aligned to the controlling local region.

To proceed, send your thickness dataset (UT grid or scan/C-scan), LTA dimensions/location (including distance to welds/nozzles), and your operating basis and request an API 579 Part 5 Local Metal Loss Assessment (FFS).

 

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