API 579 Part 14 Fatigue Damage Assessment Screening

API 579 Part 14 Fatigue Damage Assessment is used when integrity is controlled by cyclic loading rather than corrosion alone. Fatigue damage and ratcheting are driven by repeated pressure and temperature cycles such as startups/shutdowns, thermal transients, vibration-related cycling, and operational upsets, which can initiate or grow cracks at welds, attachments, or other stress-concentrated locations.

Use this screening workflow to confirm Part 14 applicability and whether your available cycle history and inspection data are sufficient to support a defensible evaluation. If the screening indicates concern, the next step is a formal Part 14 Fitness-for-Service (FFS) assessment to determine acceptability, define cycle or operating limits when required, and set practical integrity actions for continued operation.

Use the screening questions below to determine whether a formal Part 14 evaluation is recommended.

API 579 Part 14 Fatigue Damage Assessment Screening (Workflow)

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

1) Is fatigue damage a potential concern based on cyclic loading (pressure, temperature, or other repeated operating cycles) for this equipment/component?
For example; a compressor discharge circuit or overhead system sees frequent start-ups/shutdowns and pressure/temperature swings, such as 2–4 start-stop cycles per week plus repeated operational transients.
2) Has inspection identified cracking or crack-like indications at locations where fatigue is a credible damage mechanism (e.g., weld toes, attachments, nozzles, transitions, high-vibration points)?
For example; MT/PT finds toe cracking at an attachment weld or a small crack-like indication at a nozzle reinforcement area where repeated thermal swings occur during cycling operation.
3) Do you need to determine whether the component is in cyclic service using the Part 14 cyclic service screening procedure for applicability of other Parts (e.g., Part 4/Part 5/Part 6/Part 12)?
For example; general or local metal loss is present, and the site must confirm whether the equipment is “in cyclic service” before using Level 1/Level 2 procedures in the metal loss Parts.
4) Is the fatigue concern associated with the pressure boundary (or another load-carrying structural part that affects pressure boundary integrity), rather than a non-pressure part with no integrity consequence?
For example; cyclic stress is concentrated at a nozzle-to-shell junction, manway neck, or shell-to-head weld that is part of the pressure boundary.
5) Is operating history available (past + planned future) to define a loading time history that includes all significant cyclic operating loads and events applied to the component?
For example; historian/operations logs show startups/shutdowns, pressure excursions, temperature ramps, and notable transient events over the evaluation period plus the planned future pattern.
6) Can you estimate the historical and future number of full-range pressure cycles including startup and shutdown (design full-range pressure cycles)?
For example; the unit records 220 startups/shutdowns historically and projects another 120 over the next run length, giving an estimated full-range cycle count for evaluation.
7) Can you estimate the number of significant operating pressure cycles where the pressure variation range exceeds the screening threshold (based on integral vs non-integral construction)?
For example; a vessel with integral construction experiences repeated operating pressure swings exceeding ~20% of design pressure; a non-integral detail (e.g., pad/fillet attachment) uses the lower threshold (e.g., ~15%).
8) Do thermal gradients during startup/shutdown produce cyclic metal temperature differences between adjacent points that should be counted as fatigue-driving cycles?
For example; rapid warm-up causes a repeating shell-to-nozzle temperature difference during startups, producing high secondary/peak stresses at the junction.
9) Do thermal gradients during normal operation (excluding startup/shutdown) produce significant cyclic metal temperature differences between adjacent points that should be counted?
For example; repeated feed changes cause cyclic temperature stratification across a header/nozzle region while the unit remains online.
10) Are there welds between materials with different coefficients of thermal expansion where cyclic temperature differences during normal operation could drive fatigue damage?
For example; a dissimilar metal weld between an alloy spool and carbon steel shell sees repeated temperature swings, producing cyclic mismatch strain at the joint.
11) Are there significant cyclic mechanical loads excluding pressure (e.g., piping reactions, vibration loads, rotating equipment pulsation, external bending) that should be included in fatigue screening/assessment?
For example; a nozzle experiences repeated pipe strain cycles during temperature changes and unit rate swings, adding cyclic bending at the nozzle-to-shell weld.
12) Are the identified pressure/temperature cycles primarily driven by operations (not merely atmospheric changes), such that they should be considered in the Part 14 screening/assessment?
For example; pressure changes are driven by control actions and unit transients rather than daily barometric pressure changes.
13) Does the component include fatigue-sensitive features that warrant special attention under cyclic service (e.g., non-integral construction, threaded connections, stud attachments, partial penetration welds, major thickness changes, or nozzles/attachments in knuckle regions)?
For example; a reinforcing pad with fillet welds (non-integral), a threaded drain connection, a stud-mounted attachment, a partial-penetration weld detail, a large thickness transition, or a nozzle located in a formed head knuckle region.
14) Is there existing damage/flaw in the region (e.g., metal loss, hydrogen damage, misalignment/distortion, dents/gouges, laminations) such that fatigue screening/assessment must account for reduced margin (e.g., RSF-based adjustment per the applicable Part)?
For example; local metal loss around a nozzle reduces section capacity, and the fatigue screening must account for the reduced remaining strength per the applicable assessment Part.
15) If using the Level 1 screening approach that is limited by material strength, is the component a steel component with specified minimum tensile strength within the method’s limit?
For example; the component is carbon/low-alloy steel with SMTS ≤ 80 ksi, and the intended screening method is permitted for that material class/strength range.
16) Can the stress/strain range at the location of interest be determined for the cyclic loading conditions (pressure + thermal + mechanical, as applicable) for the evaluation?
For example; pressure ranges are known and either a code-based approach or FEA can determine the resulting equivalent stress range at a nozzle-to-shell weld for the evaluated cycle(s).
17) Do you need to account for mean stress effects (non-zero mean) in fatigue evaluation for the identified stress/strain cycles?
For example; a cycle has a high steady membrane stress with a superimposed cyclic range, so the fatigue method must address mean stress rather than assuming fully reversed loading.
18) Is ratcheting (progressive plastic strain accumulation) a concern due to combined steady-state (primary) stress and cyclic secondary stress from thermal/mechanical loading?
For example; a hot vessel nozzle sees steady membrane stress plus cyclic thermal bending, and ratcheting screening is needed to confirm shakedown vs cyclic plasticity vs ratcheting behavior.
19) Can the cyclic load type be identified for ratcheting screening (based on the combination of cyclic primary and/or secondary stresses), so the appropriate ratcheting decision logic can be applied?
For example; the cycle is dominated by cyclic primary membrane stress, or by cyclic secondary thermal stress, or by a combination that requires a Bree-diagram-based determination of the predicted region.
20) Is there an existing crack-like flaw that must be evaluated using crack-like flaw assessment procedures (with fatigue used for crack growth/remaining life as applicable)?
For example; PAUT/TOFD confirms a planar flaw with known length/depth; acceptability is handled per crack-like flaw procedures, and fatigue is used for growth/remaining life where applicable.
21) Is the fatigue hotspot at a welded joint/detail where a welded-joint fatigue approach (welded joint curve / structural stress method) is appropriate or required?
For example; the controlling location is a weld toe at a nozzle attachment or a fillet-welded support; the evaluation should follow a welded-joint fatigue basis rather than only smooth-bar curves.
22) If a detailed evaluation is needed, can a linear elastic stress analysis (or equivalent code-based stress determination) be performed to obtain the primary + secondary + peak equivalent stress range for each significant cycle?
For example; a linear elastic FEA model provides through-thickness equivalent stress results at the hotspot for each operating case used to form the stress range(s).
23) If cyclic plasticity is expected or required, can an elastic-plastic stress analysis be performed using a material model with cyclic plasticity (including kinematic hardening) to obtain effective strain ranges for cycle counting?
For example; an elastic-plastic analysis is set up to capture stabilized hysteresis behavior and compute effective strain ranges at the hotspot for the counted cycles.
24) Can the cyclic loading history be reduced to counted cycles (including variable amplitude, if applicable) using an appropriate cycle counting method for the selected fatigue approach?
For example; a variable amplitude temperature/pressure history is processed into a set of counted stress/strain ranges for use in cumulative fatigue damage calculations.
25) Are the required fatigue curve inputs available for the material/joint type (e.g., smooth-bar curve for the base material or the appropriate welded-joint curve for welded details) for the intended evaluation method?
For example; the base material curve is selected for equivalent stress/strain evaluation, or the welded joint curve is selected for a weld-detail structural stress approach.
26) Do you need a documented acceptability decision for continued operation under cyclic loading, including remaining life determination and/or an inspection interval basis?
For example; the site must decide whether the component can continue under the current operating pattern, whether cycle limits/operational changes are needed, and what inspection interval supports continued operation.
27) Are you assessing fatigue at a defined future date using current condition information plus a cyclic loading basis (cycle counts and stress/strain ranges) to determine remaining life or required actions?
For example; the unit plans the next turnaround in 18 months and needs confirmation the component can reach that date under projected cycles, or to define actions if it cannot.
28) If screening indicates fatigue evaluation is needed or screening requirements cannot be satisfied, are you prepared to proceed with a higher-level fatigue/ratcheting assessment, rerate/repair with monitoring, or replacement as applicable?
For example; screening is not met, so the site plans a Level 2/3 fatigue evaluation, or implements rerating/repair and tracks accumulated fatigue damage via monitoring/inspection, or replaces the component.
Answer all questions, then click “Check if FFS is needed”.

When to Use API 579 Part 14

API 579 Part 14 is typically used when the controlling concern is fatigue or ratcheting from cyclic service and the decision depends on cycle history and stress concentration effects. Common triggers include:

  • Frequent startups/shutdowns, pressure cycles, thermal cycling, or operational transients
  • Known vibration, thermal gradients, or repeated upset conditions that create repeated stress ranges
  • Cracking indications at welds, nozzles, attachments, supports, or other high-stress locations
  • A need to determine whether the equipment is acceptable now and whether limits are required to safely run to the next outage
  • A need to establish practical integrity actions—monitor with defined re-inspection intervals, limit cycles, repair, or replace

If a crack-like flaw is already identified and needs acceptability evaluation, API 579 Part 9 may also be required as the controlling flaw acceptability method.

What to Gather if Screening Indicates FFS Is Needed

If this workflow indicates that a formal API 579 Part 14 assessment is recommended, prepare the following to support a defensible evaluation:

  • Operating and cycle history (startups/shutdowns, cycle counts, and any known abnormal events)
  • Pressure and temperature ranges for the cycles (normal and upset ranges, including thermal transients)
  • Equipment/component geometry and details (thickness, nozzle/attachment configurations, drawings if available)
  • Inspection results (crack locations, NDE reports, and any history of repeat indications)
  • Service conditions relevant to cyclic damage (vibration sources, thermal gradients, or process instability)
  • Planned run length to next outage and desired inspection interval basis

Request an API 579 Part 14 Fatigue Damage Assessment

If this workflow indicates that an API 579 Part 14 Fatigue Damage Fitness-for-Service (FFS) assessment is needed, the next step is a decision-ready engineering evaluation using your cycle history, inspection findings, equipment details, and operating basis.

Inspection 4 Industry LLC (I4I) performs API 579-1 / ASME FFS-1 Part 14 assessments and delivers a complete report package stating fit-for-service or not fit-for-service, remaining life or cycle limits when applicable, any required operating restrictions, and practical integrity actions—operate with defined cycle/operating limits, monitor with targeted inspection intervals, or repair/replace at a planned outage.

To proceed, send your cycle history (startups/shutdowns, pressure/temperature swings, upset events), inspection findings (crack locations if present), and operating basis, and request an API 579 Part 14 Fatigue Damage Assessment (FFS

 

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