API 579 Part 8 Weld Misalignment Assessment Screening

API 579 Part 8 Weld Misalignment Assessment is used when geometry—rather than wall loss—controls integrity. Part 8 applies to conditions such as weld misalignment, peaking, out-of-roundness (ovality), local bulging, and shell distortion that can create high local stresses even when thickness readings appear acceptable.

Use this screening workflow to confirm Part 8 applicability and whether your dimensional data is sufficient to support a defensible evaluation. In many cases, the assessment depends on accurate measurement of the distortion shape and magnitude, the affected length/extent, and the operating basis for continued pressure service.

Use the screening questions below to determine whether a formal Part 8 evaluation is recommended.

API 579 Part 8 — Weld Misalignment & Shell Distortions Assessment Screening (Workflow)

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

Tip: Use N/A only when the question truly does not apply to your component/service.

1) Has inspection identified a weld misalignment and/or a shell distortion that could create a geometric stress concentration (not just cosmetic)?
For example: a butt-weld joint shows measurable “hi-lo” (centerline offset) and/or a peaked profile across the weld; or a shell course shows ovality (out-of-roundness) or a localized outward bulge.
2) Is the issue you’re trying to disposition primarily one of these Part 8 categories: (a) weld misalignment, (b) general shell distortion, (c) out-of-roundness, or (d) bulge?
For example: oval cross-section at a vessel elevation (out-of-roundness), a “flat spot” on shell (general shell distortion), or a local outward bump with definable radius/extent (bulge).
3) If other flaw types are present, have you identified which other API 579 Part(s) must also be used (e.g., crack-like flaws, metal loss, blistering/HIC, creep)?
For example: a linear MT/UT indication at the weld toe suggests a crack-like flaw → Part 9 may control; significant thinning near the weld → Part 4/5 may control.
4) For weld misalignment: can you classify it as centerline offset, angular misalignment (peaking), or a combination of offset + peaking at a butt-weld joint?
For example: internal surfaces are flush but the outside shows a step (offset), plus a “kink” in shell profile across the weld (peaking).
5) For shell distortion: can you classify it as general shell distortion (multiple curvatures / flat spot), out-of-roundness (oval/arbitrary cross-section), or a localized bulge (local radii + angular extent)?
For example: a local outward bump with definable radius/extent = bulge; a flat spot = general shell distortion (not a bulge).
6) Is the “bulge” actually associated with a blister (i.e., damage mechanism consistent with blistering/HIC/SOHIC)?
For example: bulge is at a known blistered area in wet H2S service → Part 7 should be used for blister assessment (Part 8 bulge procedure is not the right primary path).
7) Can the condition be dispositioned as non-impacting to pressure boundary integrity at current operating conditions (i.e., it is truly non-structural/cosmetic)?
For example: a cosmetic dent on a non-pressure boundary attachment might be non-structural; a peaked butt weld in a pressure shell is structural until proven otherwise.
8) Do you know the original construction code/standard used for fabrication tolerances (e.g., ASME VIII-1/VIII-2, B31.3, API 620/650/653, etc.)?
For example: API 650 tank shell tolerances for out-of-roundness and peaking; ASME VIII tolerances for vessel butt joints.
9) Based on measured geometry, does the component still meet original fabrication tolerances for the applicable code/standard?
For example: measured hi-lo and peaking are within the construction tolerance table for that code; measured ovality is within the code’s allowable radius/diameter deviation.
10) Even if fabrication tolerances are met, is the component in cyclic service (fatigue-sensitive) or does it have a localized bulge that may still require more than Level 1?
For example: frequent start/stop or pressure/thermal cycles; or a localized bulge (bulges typically push you toward Level 3).
11) Is the distortion type one that typically requires Level 3 because it is a general shell distortion (e.g., flat spot / multiple curvatures) or complex geometry/loading?
For example: a broad “flat spot” that changes curvature along height, or multiple local curvatures around the shell → usually needs numerical (Level 3) analysis.
12) Is the component operating in the creep range such that creep + geometric nonlinearity may be required (often outside a simple Level 2 check)?
For example: hot piping/vessel section at sustained high temperature for long durations; creep assessment methods (Part 10) may be needed in addition to Part 8.
13) Do you have the basic equipment data required for Part 8 (component type, dimensions/geometry, material properties needed for stress analysis, and design/operating conditions)?
For example: shell diameter/radius, thicknesses each side of weld, design/operating P & T, and material yield strength / elastic modulus as needed for analysis.
14) Do you have relevant maintenance/operational history that could explain cause and whether the condition is stable (e.g., settlement, thermal bowing, external impact, jacking, nozzle loads)?
For example: foundation movement created ovality; a prior hydrotest or repair introduced peaking; recurring nozzle loads may keep distortion “active.”
15) For weld misalignment: have you measured the radial/centerline offset (hi-lo) at the weld, and identified which surfaces are flush (ID flush vs OD flush) and the thicknesses on each side?
For example: UT/fit-up data show plates t1 and t2, ID is flush, and max centerline offset at weld is recorded.
16) For weld misalignment: have you measured angular misalignment (peaking) using an appropriate method that avoids weld cap interference and captures maximum deviation?
For example: a notched template spanning beyond the locally deformed region is used so it does not sit on weld reinforcement; maximum deviation is captured across the joint.
17) For out-of-roundness: do you have the required cross-section measurements (global ovality: max/min/mean diameters; arbitrary shape: enough radius points around circumference to define the profile), and if using Level 2 do you confirm the out-of-roundness is constant along the cylinder axis?
For example: global: Dmax/Dmin recorded at the same elevation; arbitrary: radius is measured at many evenly spaced points around the shell to define the shape; and survey confirms the ovality is not a short “local dent” that varies along height.
18) For arbitrary out-of-roundness: did you take an even number of equally spaced radius measurements, with a density sufficient to define the profile (recommended minimum is often high, e.g., dozens of points)?
For example: 24 points around the circumference at the evaluation elevation to define the cross-section profile (more points if shape is irregular).
19) For a bulge: have you measured geometry to characterize it (location, local radius/radii, angular extent, height/deviation, and whether it is spherical vs cylindrical)?
For example: bulge centered near a seam weld, with measured max outward deviation and mapped extent around the shell to define its shape for analysis.
20) Is the distortion/misalignment location defined relative to weld joints, discontinuities, attachments, thickness changes, stiffeners, nozzles, or other stress raisers?
For example: ovality peak is aligned with a longitudinal weld; bulge is 8 in from a nozzle reinforcement; distortion is between stiffening rings.
21) Have you defined the loading that coincides with the distortion region: internal/external pressure and any supplemental loads (weight, wind, seismic, thermal gradients, nozzle loads, support loads), and for Level 2 do these loads result in a membrane state of stress in the component (excluding bending except that caused by the misalignment/distortion itself)?
For example: pressure + allowable supplemental loads that produce mainly membrane stress; note that Level 2 external-pressure stability rules are not intended for cases where external pressure combines with supplemental loads that create significant longitudinal compressive stress.
22) Is the component in cyclic service or does it experience meaningful pressure/thermal cycles such that a fatigue check may be required (even if static stress acceptability looks OK)?
For example: frequent start-ups, batch cycling, on/off loading, or daily temperature swings causing repeating membrane stress ranges.
23) If fabrication tolerances are met and the component is not fatigue-sensitive (and no bulge concern), is a Level 1 disposition sufficient for your required documentation?
For example: measured hi-lo and peaking are within original code tolerance; service is steady-state with low cycling; no localized bulge present.
24) If Level 1 is not satisfied, do you have enough inputs to run a Level 2 check under Part 8 applicability (recognized code/standard basis, eligible component geometry, and pressure and/or supplemental loads that produce a membrane state of stress, plus distortion parameters)?
For example: you can compute membrane stress, confirm the component type is one covered by Level 2 applicability (e.g., shell/pipe/eligible geometry), define the weld misalignment parameters (offset/peaking) or ovality parameters, and use the applicable Part 8 equations/tables.
25) Does your case involve a bulge that will likely require a Level 3 assessment (or a general shell distortion that typically requires numerical analysis)?
For example: localized outward bulge in a vessel shell course; complex distortion shape not representable by simple idealized ovality parameters.
26) Is there any crack-like or groove-like feature at/near the misalignment/distortion location that must be evaluated separately (e.g., Part 9 for crack-like flaws)?
For example: linear indication at weld toe; sharp notch-like feature; or a groove that behaves like a crack root under cyclic loading.
27) Is the controlling concern primarily insufficient wall thickness (general/local metal loss) rather than geometric stress concentration from distortion?
For example: t is near/under tmin at the same location; even perfect geometry would still fail thickness requirements → Part 4/5 may control.
28) Do you need a documented decision for continued operation at current conditions (or to define operating limits) with this misalignment/distortion present?
For example: operations needs to know if the equipment can run to next turnaround without restrictions, or if pressure/temperature/cycle limits must be applied.
29) If a Level 2 check does not pass, are you prepared to consider the allowed outcomes: rerate, repair, replace, move to Level 3, or retire?
For example: reduce allowable pressure, add reinforcement/stiffening, mechanically correct ovality, or perform a detailed nonlinear stress analysis.
30) Do you need a remaining life basis / inspection interval basis tied to this condition (metal loss evaluation, cyclic fatigue life, or high-temperature/creep life)?
For example: use corrosion allowance for future thinning; use fatigue evaluation when cycling is significant; use creep methods if high temperature operation applies.
31) Are remediation options feasible and acceptable for your case (e.g., reinforcement, stiffening/lap patch concepts, or mechanical correction such as controlled jacking for out-of-roundness)?
For example: install stiffening plates designed to original code allowables; or use hydraulic jacks to bring a stiffened shell closer to tolerance while monitoring loads to avoid new damage.
32) If the condition is complex or highly localized stress is expected, are you prepared for a detailed Level 3 analysis (potentially nonlinear, with material toughness considerations when appropriate)?
For example: severe localized misalignment causing high bending stresses; engineering decision is to perform detailed stress analysis or proceed to repair/replacement.
33) Is in-service monitoring needed to manage uncertainty (e.g., periodic dimensional checks, monitoring of crack-like indications if present, monitoring of distortion growth/stability)?
For example: repeat ovality survey at the same elevation each inspection cycle; monitor peaking at weld after settlement stabilization; add NDE monitoring if crack-like features are a concern.
34) Can you document the assessment inputs and decisions (damage characterization, measurements, code tolerances used, selected Level, acceptance/rerating/remediation decisions, and any monitoring plan)?
For example: keep the dimensional survey, calculation sheets/model outputs, assumptions (loads, material props), and the final disposition/operating limits as part of the FFS record.
Answer all questions, then click “Check if FFS is needed”.

When to Use API 579 Part 8

API 579 Part 8 is typically used when the controlling condition is measured distortion or misalignment and the decision cannot be made using thickness checks alone. Common triggers include:

  • Weld misalignment or offsets at seams, shell-to-head welds, or nozzle regions
  • Peaking, ovality (out-of-roundness), bulging, or shell distortion identified by dimensional survey
  • Settlement-related distortion on towers, drums, or large shells that creates local stress concentration
  • A need to determine whether the distortion is acceptable for continued operation or whether correction is required
  • A need to establish rerated limits or operating restrictions when distortion governs acceptability

If the primary concern is cracking or a crack-like flaw, route the evaluation to API 579 Part 9. If the primary concern is widespread thinning, route to Part 4, and for localized thinning route to Part 5.

What to Gather if Screening Indicates FFS Is Needed

If this workflow indicates that a formal API 579 Part 8 assessment is recommended, prepare the following to support a defensible evaluation:

  • Dimensional survey data defining the distortion (ovality/peaking values, offsets, bulge dimensions, affected length)
  • Location details (shell course, weld type, nozzle region, distance from discontinuities)
  • Component geometry and drawings if available (diameter, thickness, joint configuration)
  • Operating basis (pressure, temperature, and any significant external loads or constraints)
  • Inspection documentation (photos, measurement methods, repeatability/accuracy)
  • History of the distortion (new vs longstanding, settlement progression, and any changes over time)

Request an API 579 Part 8 Weld Misalignment Assessment (FFS)

If this workflow indicates that an API 579 Part 8 Weld Misalignment Fitness-for-Service (FFS) assessment is needed, the next step is a decision-ready engineering evaluation using your dimensional measurements, equipment details, and operating conditions.

Inspection 4 Industry LLC (I4I) performs API 579-1 / ASME FFS-1 Part 8 assessments of existing equipment for weld misalignment and shell distortions and delivers a complete report stating fit-for-service or not fit-for-service, any required operating restrictions or rerated limits, and practical integrity actions—correct now, correct at turnaround, reinforce when required, or monitor with defined inspection scope aligned to the controlling distortion mechanism.

To proceed, send your dimensional survey data (misalignment offset, peaking/ovality values, affected length/area, and location details) and your operating basis and request an API 579 Part 8 Weld Misalignment Assessment (FFS).

 

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