API 579 Part 3 Brittle Fracture Assessment Screening

API 579 Part 3 Brittle Fracture Assessment helps determine whether an in-service pressure vessel, piping component, or tank has a credible risk of brittle fracture at low metal temperatures. Unlike corrosion-based damage, brittle fracture is controlled by toughness, temperature exposure, and local stress conditions that can make a component vulnerable to sudden fracture with little warning.

Use this screening workflow to quickly check key factors—minimum metal temperature exposure, material toughness basis, thickness effects, weld details, and operating conditions—to decide if a Fitness-for-Service (FFS) evaluation is needed. If the screening indicates concern, the next step is a more detailed Part 3 assessment to establish safe operating limits such as minimum allowable temperature (MAT) and critical exposure temperature (CET) for restart and continued operation.

Use the screening questions below to determine whether a formal Part 3 evaluation is recommended.

API 579 Part 3 Brittle Fracture Assessment Screening (Workflow)

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

If a question does not apply to your equipment, select N/A.

1) Is brittle fracture a potential integrity concern for this equipment based on metal temperature exposure and operating conditions?
Example: A cold-side exchanger channel or light-ends receiver sees a winter metal temperature excursion below the site’s expected operating envelope during start-up.
2) Do you need to establish or confirm a pressure–temperature operating basis (pressure–temperature envelope) for continued operation?
Example: Operations needs a documented pressure–temperature envelope for winter running before approving restart.
3) Are you changing operating conditions or rerating in a way that requires reconfirming pressure–temperature limits?
Example: A process change will lower metal temperature at the same pressure, or increase pressure at low temperature, so the P–T basis must be revalidated.
4) Is your decision basis tied to Part 3 temperature criteria such as CET and/or MAT (and you need to establish/confirm acceptability by comparing the controlling CET to MAT or the applicable P–T basis)?
Example: The site needs to confirm acceptability at a specific low metal temperature during start-up using CET/MAT (or the applicable P–T basis).
5) Do you have the required Part 3 Level 1 basis inputs available (e.g., year of fabrication, nominal thickness, design temperature, CET/MAT items as applicable)?
Example: A 1970s vessel has incomplete records; you cannot confirm nominal wall thickness and required input data for Level 1 screening.
6) Are original hydrotest records part of your brittle fracture basis (original hydrotest pressure and temperature during the original hydrotest, as applicable)?
Example: The restart basis relies on confirming the original hydrotest conditions to support low-temperature operation decisions.
7A) Is Charpy V-notch (impact) data available for this equipment/material?
Example: MTRs show Charpy results for base metal plate, or welding records include impact testing relevant to the component.
7B) If Charpy data is NOT available, does your Part 3 path need to explicitly address missing impact data (i.e., you cannot rely on impact test results)?
Example: Older equipment has no impact records, so you must proceed using the screening/assessment route applicable to missing Charpy data.
8) Is your current concern primarily a crack-like flaw acceptability case (flaw-focused) rather than a temperature-based brittle fracture screening decision?
Example: PAUT finds a planar indication at a nozzle-to-shell weld and the question is crack-like flaw acceptability (Part 9), not just temperature screening.
9) Is the pressure boundary material outside the intended Part 3 screening scope (i.e., not carbon or low alloy steel for the main screening route)?
Example: The component is made from a material not addressed by the standard Part 3 screening scope and you need a different approach.
10) Is the equipment type covered by Part 3 (pressure vessels, piping, and atmospheric/low-pressure aboveground tanks per API 650/API 620 as applicable)?
Example: You are screening an API 650 tank shell course for low-temperature pressurization concerns.
11) Is the equipment included in an in-service inspection/maintenance program consistent with API 510 / API 570 / API 653 (or equivalent)?
Example: The vessel is on an API 510 inspection plan with documented intervals and history supporting continued-service decisions.
12) Is environmental cracking credible (e.g., wet H2S) or is there a credible service condition that may reduce material toughness (requiring Level 3 per Part 3 guidance)?
Example: A low alloy steel component has credible toughness degradation risk from service exposure and needs a Level 3 route for brittle fracture resistance.
13) If the component has metal loss (general, local, or pitting), have you qualified the metal loss per Part 4/5/6 AND performed supplemental inspection to confirm no crack-like flaws exist (as required for using Part 3 Level 1/2 with metal loss present)?
Example: A corroded nozzle neck is qualified for thickness per Part 4/5/6, but you still need supplemental crack-detection NDE before using Part 3 Level 1/2 screening.
14) Do you need an explicit load (pressure) vs. temperature envelope (not just a single MAT value), implying a Level 2 approach?
Example: The unit wants a documented operating envelope for multiple pressures during cold start-up, not one single screening temperature.
15) Are you relying on (or planning) a hydrotest / low-temperature pressurization event as part of qualifying continued service?
Example: The component will be hydrotested and you must ensure brittle fracture resistance for the pressurization temperature and pressure basis.
16) Is there potential for shock chilling (rapid metal temperature drop due to sudden contact with much colder liquid/two-phase fluid, meeting Part 3 shock-chilling criteria)?
Example: A hot surface suddenly contacts a much colder liquid, rapidly chilling the metal (shock chilling concern).
17) Is the component constructed from SA-212 Grade B (or do you suspect it), which Part 3 flags for elevated brittle fracture concern and recommends thorough crack-focused NDE?
Example: Older components built with SA-212 Gr B warrant extra crack-detection emphasis before relying on screening decisions.
18) Have in-service inspections detected fabrication/material flaws (non-growing flaws) that could affect current or altered operating limits (often requiring Level 3 per Part 3 guidance)?
Example: An original non-growing fabrication indication becomes relevant after operating limits change, so brittle fracture susceptibility must be reassessed.
19) Do you have (or can you defensibly establish) the design temperature and the controlling metal temperature for brittle fracture screening (CET), to compare against MAT where required?
Example: You cannot confirm the controlling CET during winter start-up, so a defensible brittle fracture screening basis cannot be completed.
20) For storage tanks, do you have tank-specific Level 1 data (e.g., product specific gravity and design liquid height, plus hydrotest temperature as applicable) needed to support the brittle fracture screening basis?
Example: You are screening an API 650 tank shell course but do not have product SG and design liquid height available for the basis inputs.
21) Has a process hazards review / operating review identified credible minimum metal temperatures lower than previously assumed (e.g., lower than design minimum or historical basis)?
Example: A PHA identifies a credible cold-start scenario that drops a vessel’s metal temperature below the minimum temperature used in the current operating envelope.
22) Was the equipment designed/fabricated to an older construction code/edition where brittle fracture prevention rules may not have been comparable to newer practice (or the governing brittle-fracture basis is uncertain)?
Example: A 1960s/1970s pressure vessel has limited documentation of low-temperature impact requirements, so the brittle fracture operating basis needs confirmation.
23) Do you need to establish/confirm a minimum hydrotest temperature (or hydrotest conditions) as part of the brittle fracture operating/qualification basis?
Example: A hydrotest is planned during cool weather, and you must confirm that the test temperature is acceptable from a brittle fracture standpoint.
24) Have repairs/alterations occurred that change stress-relief/PWHT assumptions (e.g., originally PWHT/stress-relieved, but later repairs were performed without PWHT so stress-relief credit may not apply)?
Example: A vessel was originally PWHT’d, but later a nozzle repair was done without PWHT; you should not assume the same stress-relief benefit for screening.
25) Is there an active or credible cracking damage mechanism present (or suspected) that could exist during operation (i.e., not just “metal temperature screening”)?
Example: A sour service circuit has credible cracking concerns; brittle fracture screening should not assume “no crack-like flaws” without crack-focused NDE and/or the appropriate assessment route.
26) (Tankage) At the controlling low-temperature case, is the general primary membrane tensile stress (including net section bending) expected to exceed 8 ksi (considering maximum credible coincident pressure + supplemental loads)?
Example: A tank shell course sees high membrane stress at low temperature when internal liquid head plus loads are combined; the screening basis must reflect the coincident case.
27) Is the lowest one-day mean atmospheric temperature (or equivalent governing environmental minimum) part of the controlling CET scenario for this equipment?
Example: Outdoor equipment must consider the lowest one-day mean ambient temperature unless a higher minimum is enforced by procedures/controls that reliably prevent colder exposure.
28) If you are relying on a minimum temperature higher than the site’s lowest one-day mean ambient, do you have specific procedures/controls that reliably prevent operation/exposure below that enforced minimum?
Example: A written operating procedure prevents start-up below a defined minimum metal temperature, and instrumentation/interlocks verify compliance.
29) Are credible upset/failure scenarios applicable that could drive the CET lower (e.g., utility failure, pump/valve events, colder-than-expected stream conditions, exchanger/reboiler malfunction)?
Example: A control valve failure or pump trip causes unexpected depressurization/cooling that can lower metal temperature beyond the normal operating envelope.
30) Is autorefrigeration due to depressurization (including potential repressurization before warming back up) a credible scenario that could control CET?
Example: A depressurization event causes autorefrigeration cooling, and the system may be repressurized before the metal temperature recovers—this combined case may control CET.

When to Use API 579 Part 3

Part 3 is typically used when the integrity decision depends on low-temperature brittle fracture resistance and you need a defensible pressure–temperature operating basis for continued operation. Common triggers include:

  • Low metal temperature exposure during winter operation, startup, depressuring, or upset conditions
  • Establishing or confirming a pressure–temperature operating envelope for continued operation
  • A decision basis tied to Critical Exposure Temperature (CET) and/or Minimum Allowable Temperature (MAT)
  • Older equipment with limited records (fabrication year, thickness, PWHT history, hydrotest basis, or missing Charpy data)
  • The need to document and defend restart/continued-operation limits in a traceable engineering package

Note: If your primary concern is a crack-like flaw acceptability case (flaw-focused), the controlling evaluation is typically routed to API 579 Part 9 rather than using Part 3 as the main decision path.

What the screening is checking

This screening is intended to identify whether your case likely requires an API 579 Part 3 Brittle Fracture Assessment to establish safe operating limits and support low-temperature operation decisions. In many cases, the final decision is tied to defining allowable low-temperature operation using:

  • Minimum allowable temperature (MAT) for continued operation
  • Critical exposure temperature (CET) for the lowest credible metal temperature exposure
  • A documented pressure–temperature envelope when required to support operations and management decisions
  • Rapid cooling or “shock chilling” exposure conditions (may require Level 3 evaluation for a defensible low-temperature decision)

What to gather if the screening indicates FFS is needed

If the workflow indicates that a formal API 579 Part 3 assessment is recommended, having the following information ready will speed up the evaluation and improve result quality:

  • Equipment/component identification and service description (vessel/piping/tank component)
  • Year of fabrication and material identification (as available)
  • Nominal thickness and governing thickness basis
  • PWHT history (initial construction and repairs)
  • Hydrotest basis (pressure and temperature where applicable)
  • Charpy impact data (if available) or confirmation that it is not available
  • Operating basis needed to define pressure–temperature limits (pressure, temperature, transients, startup conditions)

Request an API 579 Part 3 Brittle Fracture Assessment (FFS)

If this workflow indicates that an API 579 Part 3 Brittle Fracture Fitness-for-Service (FFS) assessment is needed, the next step is a decision-ready engineering evaluation using your equipment details, inspection/NDE results, material data, and operating envelope.

Inspection 4 Industry LLC (I4I) performs API 579-1 / ASME FFS-1 Part 3 assessments of existing equipment for brittle fracture 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 for safe restart and continued operation.

To proceed, send your available inspection findings and operating basis and request an API 579 Part 3 Brittle Fracture Assessment (FFS).


 

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