API 579 Part 7 Hydrogen Damage Assessment Screening

API 579 Part 7 Hydrogen Damage Assessment is used when hydrogen-related damage indications are present and integrity cannot be judged by thickness averages alone. Part 7 addresses conditions such as hydrogen blistering, HIC (Hydrogen-Induced Cracking), and SOHIC (Stress-Oriented Hydrogen-Induced Cracking), where damage morphology, orientation, and proximity to welds or discontinuities can control the integrity decision.

Use this screening workflow to confirm Part 7 applicability and whether your inspection/NDE dataset is sufficient to support a defensible evaluation. In many cases, the next step requires properly characterizing the indications (size, depth/through-wall position, spacing, and location) and determining whether the condition should be evaluated as hydrogen damage alone or linked to other API 579 Parts when controlling conditions apply.

Use the screening questions below to determine whether a formal Part 7 evaluation is recommended.

API 579 Part 7 — Hydrogen Damage Assessment Screening (Workflow)

Instruction: Answer all questions (use N/A when truly not applicable), then click “Check if FFS is needed”.

Note: Part 7 addresses hydrogen blisters, HIC, and SOHIC as distinct damage types and provides different assessment levels depending on damage type and component class.

1) Has inspection identified hydrogen blistering, HIC, and/or SOHIC in the pressure boundary?
Example: Straight-beam UT mapping on a sour-water drum reports blister-like laminar indications and planar HIC-type reflectors in a defined shell plate area.
2) Is the damage type clearly distinguished as one (or more) of: (a) blistering, (b) HIC, (c) SOHIC (instead of “generic lamination”)?
Example: The report differentiates a bulged blister cavity vs. an array of planar HIC indications at different depths vs. a crack-like SOHIC feature near weld HAZ.
3) Is there evidence that the indication is crack-like (surface-breaking, oriented feature) rather than only a contained laminar reflector?
Example: MT/PT finds surface-breaking linear indications, or UT/angle-beam suggests a crack-like feature consistent with SOHIC.
4) Is the component class understood well enough to decide which assessment levels apply (e.g., Type A vs Type B Class 1, etc.)?
Example: You confirm the damaged item is a Type A cylindrical shell under internal pressure (or identify it as a Type B Class 1 component requiring the correct pathway).
5) Are you trying to use Level 1 or Level 2 for SOHIC?
Example: The only reported damage is SOHIC near a weld — Part 7 does not provide Level 1/2 for SOHIC, so this drives toward Level 3 crack-like evaluation.
6) Is the damage located close to a weld seam or major structural discontinuity (attachments/nozzles/major geometry changes)?
Example: Blister/HIC edge is near a longitudinal seam weld or near a nozzle reinforcement — this is an explicit driver toward Level 3 per Part 7 level applicability.
7) Does the component contain a multitude of closely spaced blisters (clustered blisters where interaction matters)?
Example: Multiple blisters are packed into one band around the shell; interaction/combining rules are needed and may drive Level 3.
8) Is the assessment objective to establish MAWPr and/or MFHr at a coincident temperature for continued operation?
Example: You need a reduced MAWP for the damaged zone (or reduced max fill height) to operate safely until the next outage.
9) Do you have an inspection sketch / map that shows locations of each blister/HIC/SOHIC and identifies which plate/course/zone is affected?
Example: A shell course map marks each damage zone with coordinates (e.g., clock position/elevation) and unique IDs tied to UT screenshots.
10) For HIC: are the required spacing measurements captured (edge-to-edge spacing to nearest HIC/blister and longitudinal spacing)?
Example: The report gives HIC-to-HIC edge spacing in circumferential direction and separate longitudinal spacing values, all shown on the inspection sketch.
11) For HIC: is spacing to the nearest weld joint documented?
Example: You record the shortest distance from the HIC zone edge to the seam weld toe/fusion line and note weld orientation.
12) For HIC: is spacing to the nearest major structural discontinuity documented?
Example: You measure distance from HIC edge to a nozzle reinforcement edge / attachment / major geometry change and record it on the sketch.
13) For HIC: is the through-thickness extent of damage defined, and can you report remaining undamaged metal thickness on both sides when subsurface?
Example: UT sizing gives maximum HIC depth extent plus minimum remaining ligament to ID and OD surfaces (with future corrosion allowance addressed if required).
14) For HIC: is the damage classified as subsurface vs surface-breaking (needed for later crack-like triggers)?
Example: Based on remaining ligament criteria, you document “subsurface HIC” or “surface-breaking HIC” for each zone.
15) For SOHIC: are the required spacing measurements captured (to nearest HIC/blister and to nearest major discontinuity), and are crack-like dimensions defined?
Example: SOHIC is sized as a crack-like flaw with defined length/depth parameters, and distances to nearby damage and discontinuities are shown on a sketch.
16) For blisters: is the blister size recorded in the form needed for the intended level (largest diameter for Level 1; longitudinal & circumferential dimensions for Level 2-style local metal loss equivalency)?
Example: You record “blister diameter = max(longitudinal, circumferential)” for screening, and also capture both axes if you may need Level 2.
17) For blisters: is blister spacing to the nearest blister/HIC documented, and have you considered whether neighboring blisters must be combined/treated as interacting?
Example: Two blisters are close enough that you evaluate them as a combined feature rather than isolated (interaction/combining logic is applied).
18) For blisters: are bulge direction (ID/OD bulge) and projection above the surface recorded?
Example: OD bulge projection is measured with a straightedge (or profile) and recorded, along with whether the cavity is internal vs external.
19) For blisters: is the minimum remaining wall thickness to the blister cavity determined (ligament from the opposite surface)?
Example: For an internal blister, you measure remaining ligament from OD to the cavity; for an external blister, you measure remaining ligament from ID to the cavity.
20) For blisters: have you checked and documented periphery cracking and whether it extends in-plane and/or through-thickness?
Example: You record “no periphery cracks found” or map periphery crack lengths and whether they track the weld fusion line direction.
21) For blisters: have you checked for crown cracking and/or existing vent holes, and recorded crack/vent dimensions if present?
Example: The blister crown has a crack or vent opening; you record its length/diameter because it affects strength calculations.
22) For blisters: is spacing to nearest weld joint and to nearest major discontinuity measured and shown on the sketch?
Example: The closest distance from blister edge to seam weld is documented and compared against “near weld” requirements that drive additional assessment.
23) Is thickness data available to define the “undamaged thickness away from the damage area” and the “minimum measured thickness at the damage” (as needed for remaining strength logic)?
Example: You have a UT grid away from the damage to establish a representative remote thickness plus local readings that define the damage ligament.
24) Were appropriate NDE methods used for the damage type (e.g., UT for subsurface HIC/blisters; MT/PT for surface-breaking HIC as applicable; angle-beam UT for HIC/SOHIC monitoring)?
Example: The plan uses straight-beam UT for HIC/blister sizing and angle-beam UT to monitor crack-like SOHIC near welds; surface exams are used only when surface-breaking is possible.
25) For subsurface HIC: did you avoid relying on methods that cannot size/define subsurface HIC (i.e., do you have UT or equivalent volumetric characterization rather than only RT or only surface NDE)?
Example: You do not accept “RT clean” as proof of no HIC; instead you use UT mapping to define the array dimensions and remaining ligament.
26) For HIC Level 2-style decision making: does the equipment remain in hydrogen charging service with no effective halt (barrier coating/overlay/process change)?
Example: The unit still runs in a sour/hydrogen charging environment and no barrier coating or process change has been implemented — crack-like evaluation triggers become more likely.
27) Is any HIC classified as surface-breaking (which triggers crack-like considerations in the workflow)?
Example: UT + surface exam indicates the HIC array has broken through to the surface — treat as crack-like rather than only metal-loss equivalency.
28) Does the through-wall extent of HIC appear significant enough that a fracture (crack-like) assessment may be required rather than only local metal loss equivalency?
Example: HIC depth is a large fraction of the available ligament and meets a “proceed to fracture assessment” trigger in the Level 2 workflow.
29) If a crack-like assessment is needed: are you prepared to evaluate HIC/SOHIC as a crack-like flaw (e.g., longitudinal and circumferential cases) using the appropriate Part 9-style pathway when directed?
Example: You can define crack length as the HIC extent in the evaluated direction and crack depth as the max through-thickness extent for the crack-like case.
30) Do you have a basis for remaining life planning (e.g., evidence damage is stable or progressing at a known/verifiable rate, or you can set monitoring intervals accordingly)?
Example: Prior turnarounds show similar HIC zone size (stable), or you implement a defined UT monitoring interval with trendable sizing.
31) Is there a plan to eliminate or control hydrogen charging (coating/overlay/process change/inhibitors) if continued operation is desired?
Example: You plan a barrier coating or process chemistry adjustment, and you document how it reduces hydrogen charging severity.
32) If blister venting is being considered: are venting prerequisites and safety constraints addressed (e.g., blister orientation confirmed, not already vented by cracking, avoid prohibited services, safe drilling approach)?
Example: You plan to vent via a small hole from the bulged side using an air-driven drill with ignition controls/purging, and you confirm the blister isn’t already vented by crown/periphery cracking.
33) If repair is being considered (blend grinding / weld repair): is there awareness that hydrogen-charged steel may require controls (e.g., bake-out consideration) and re-examination after repair?
Example: You schedule hydrogen bake-out prior to welding and require MT/PT re-check of the repaired area before returning to service.
34) Is there a defined in-service monitoring approach (UT methods and/or hydrogen probes) when hydrogen charging conditions persist or damage is near welds/discontinuities?
Example: Periodic straight-beam UT for blister/HIC plus angle-beam UT for SOHIC near welds, combined with hydrogen probes to trend charging severity and adjust intervals.
35) Are you prepared to document the assessment inputs/decisions in a structured way (tables/formats + detailed sketch), including measurements listed above?
Example: You compile a table of HIC zones and blister features (dimensions, spacing, ligament thickness, cracking/vents) and attach an annotated sketch that matches those IDs.
Answer all questions, then click “Check if FFS is needed”.

When to Use API 579 Part 7

API 579 Part 7 is typically used when inspection indicates hydrogen damage features and the decision requires more than a simple thickness check. Common triggers include:

  • UT/advanced UT scanning identifies blistering, HIC-type indications, or SOHIC-type arrays
  • Indications are near welds, seam welds, nozzles, attachments, or other stress-concentrated locations
  • The concern includes potential interaction between indications (spacing, alignment, and clustering)
  • A need to determine whether the component can safely continue operating at the evaluated conditions or requires repair/replacement
  • A need to establish practical integrity actions and inspection planning aligned to the controlling hydrogen damage mechanism

If the primary concern is a crack-like flaw acceptability case (planar flaw-focused), the controlling evaluation may be routed to API 579 Part 9 rather than treating the condition only as hydrogen damage.

What to Gather if Screening Indicates FFS Is Needed

If this workflow indicates that a formal API 579 Part 7 assessment is recommended, prepare the following to support a defensible evaluation:

  • Inspection/NDE outputs that characterize indications (UT scan maps, sizing results, depth/through-wall position, and locations)
  • Description of damage morphology (blistering vs HIC vs SOHIC, and whether indications form arrays)
  • Component and location details (shell course/nozzle region/weld proximity, internal vs external surface)
  • Design basis and operating conditions (pressure, temperature, material, and service environment)
  • Prior inspection history and any evidence of growth or active damage progression
  • Planned run length to next outage and inspection/monitoring expectations

Request an API 579 Part 7 Hydrogen Damage Assessment (FFS)

If this workflow indicates that an API 579 Part 7 Hydrogen Damage Fitness-for-Service (FFS) assessment is needed, the next step is a decision-ready engineering evaluation using your inspection/NDE results, equipment details, and operating basis.

Inspection 4 Industry LLC (I4I) performs API 579-1 / ASME FFS-1 Part 7 assessments of existing equipment for hydrogen blistering, HIC, and SOHIC and delivers a complete report stating fit-for-service or not fit-for-service, any required operating restrictions or rerated limits when applicable, and practical integrity actions—repair now, repair at turnaround, replace affected components when required, or monitor and run with a defined inspection plan aligned to the controlling hydrogen damage mechanism.

To proceed, send your available inspection findings (UT scan maps, indication sizing/locations, weld proximity details, and supporting records) and your operating basis and request an API 579 Part 7 Hydrogen Damage Assessment (FFS).

 

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