API 579 Fitness-for-Service Assessment for Local Metal Loss in an Industrial Storage Tank
TES Canada’s team supported an API 579 fitness-for-service assessment for an industrial storage tank with metal loss. The work helped evaluate remaining integrity, inspection findings, and practical engineering options for continued safe operation.
Operational Context
An industrial facility operator in British Columbia — specifically a pulp and paper facility — operated a large black liquor storage tank that had been inspected internally. The inspection identified internal coating degradation, localised erosion/corrosion, minor pitting, and areas of metal loss described as vertical grooves and rippled surface damage on the tank shell.
The operator needed a defensible engineering assessment to determine whether the tank remained fit for continued service based on the available inspection data, whether any immediate repair was required, and what future inspection actions would be needed to maintain a valid fitness-for-service basis going forward.
Engineering & Integrity Challenge
The central challenge was making a defensible continued-service determination based on available 2020 inspection data, without the benefit of corrosion trending data or repeated thickness measurement sets. Without a corrosion rate, future corrosion allowance could not be included in the minimum required thickness calculation — requiring the assessment to be conducted on a point-in-time basis with clearly stated limitations.
The damage pattern — vertical grooves and rippled surface texture — was consistent with internal erosion/corrosion rather than uniform general metal loss, requiring careful damage mechanism review to ensure the correct API 579 assessment procedure was selected and applied appropriately. The tank design was governed by API 620, which required design-basis calculations to be used as the foundation for the minimum required thickness assessment rather than a more common API 650 approach.
Why the Situation Was Complex
Black liquor is a chemically aggressive process fluid with erosive characteristics that produce non-uniform internal damage patterns. The groove and ripple morphology of the observed metal loss required interpretation to determine whether it was appropriately characterised as general metal loss under API 579 Part 4, or whether a localised metal loss or pitting assessment would be more representative.
The absence of corrosion trending data meant that the assessment could only address the point-in-time condition. The lack of a validated corrosion rate precluded inclusion of future corrosion allowance, meaning the assessment conservatively evaluated current measured thicknesses without assuming any remaining corrosion margin. This limitation had to be clearly communicated to the operator alongside the assessment conclusion.
The tank was designed to API 620 rather than the more commonly encountered API 650, requiring specific attention to design-basis loading conditions, fill height assumptions, internal pressure, and weight loading in the minimum required thickness calculations.
TES Engineering Thinking
TES Canada reviewed the available inspection report, tank design drawings, shell course thicknesses, design fill height, internal pressure parameters, and weight loading conditions. A damage mechanism review was conducted using API 571 and ASME PCC-3 principles to identify the governing degradation mechanism and confirm the appropriate assessment pathway.
Internal erosion/corrosion metal loss was identified as the governing mechanism. TES Canada performed an API 620-based minimum required thickness calculation for the affected shell courses, establishing the minimum acceptable shell thickness at each assessment location based on the tank's specific design parameters.
An API 579-1 / ASME FFS-1 Part 4 General Metal Loss Level 1 assessment was then performed, comparing the measured remaining thickness values from the 2020 inspection report against the calculated minimum required thicknesses. Conservative assumptions were applied where inspection data was incomplete or where the damage pattern created uncertainty in the representative measured thickness.
Where future inspection requirements were concerned, TES Canada recommended establishment of repeatable matrix thickness measurement locations with datum references to support corrosion trending and Critical Thickness Profile development in future inspection cycles. Inspection of weldments near damaged areas using shear wave ultrasonic testing or radiography was recommended where the damage pattern approached weld heat-affected zones. Clear pathways for future Level 2, Level 3, or repair decision-making were identified for scenarios where additional metal loss is found in future inspections.
Technical Approach
Practical Outcome
TES Canada completed a standards-based fitness-for-service assessment for localised internal metal loss in the black liquor storage tank. Minimum required shell thickness values were calculated using the tank's API 620 design-basis parameters, and the measured metal loss data from the 2020 inspection was assessed under the API 579-1 / ASME FFS-1 Part 4 Level 1 procedure.
The assessment confirmed that the available measured metal loss data was acceptable under the Level 1 procedure based on the 2020 inspection findings and stated assumptions — meaning that repair was not required on the basis of the available data. The assessment clearly documented that future corrosion allowance could not be included due to the absence of validated corrosion rate or trending data, and that this limitation defined the conditions under which the fitness-for-service conclusion remained valid.
Practical recommendations were provided for future inspection — including repeatable thickness measurement matrix locations with datum references for corrosion trending, weld inspection near damaged areas, and defined decision pathways for Level 2 or Level 3 assessment if additional metal loss is found in future inspection cycles.
Lessons Learned
A fitness-for-service assessment is only as defensible as the clarity of its stated assumptions and limitations. Where corrosion trending data is absent, the assessment conclusion must explicitly define the conditions under which it remains valid — not imply broader coverage than the available data supports.
Damage mechanism review is the mandatory first step in FFS assessment selection. The groove and ripple pattern in this case required deliberate assessment to confirm that API 579 Part 4 general metal loss was the appropriate procedure — not a default assumed to apply to any thickness reduction finding.
API 620 design-basis calculations are not interchangeable with API 650 approaches. For tanks outside the more common API 650 scope, the minimum required thickness foundation must be derived from the correct design standard, or the FFS assessment loses its technical credibility.
Establishing repeatable thickness measurement locations and datum references at the time of the first FFS assessment is high-value, low-cost work. It is the difference between a point-in-time assessment and a trending program — and the investment pays compounding returns in every subsequent inspection cycle.
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