Two parts of the standard cover metal loss and the difference is not the amount of metal gone. It is the shape — and choosing the wrong one gives you an answer that is either wrong or needlessly conservative.
API 579 splits metal loss into two parts:
The distinction is about how the damage is distributed, not how severe it is.
Part 4 asks a simple question: is the average remaining thickness enough? If the wall is uniformly thinner, the component behaves like a thinner component. Compare the remaining thickness against the required minimum and you have your answer.
Part 5 cannot do that, because a local patch does not behave like a thinner vessel. The sound wall around the flaw carries load and supports the thin area — but the thin area also bulges locally under pressure, which raises stress there. Part 5 accounts for both effects through the remaining strength factor and the bulging factor.
Neither, universally — and that is why it matters. Treating a local patch as general metal loss is usually over-conservative, because you throw away the support the surrounding wall provides. Treating genuinely general thinning with a Part 5 approach is meaningless, because there is no sound wall to do the supporting. Use the wrong one and you either fail something that is fine, or apply a method whose assumptions do not hold.
The standard provides a criterion based on the extent of thinning relative to the shell's characteristic length, √(D·t). In practice the question you are answering is:
If the thinning extends well beyond about √(D·t) in every direction, there is no meaningful support and it behaves as general loss. If it is a patch with full-thickness wall around it within that distance, it is local.
For a 576 mm bore, 12 mm wall vessel, √(D·t) is about 83 mm. A thin patch 100 mm across in an otherwise sound shell is clearly local. Thinning across an entire course of the shell is clearly general. The awkward cases sit in between.
You can usually predict which part applies from the damage mechanism alone, and it is worth doing so before the inspection, because it determines what data you need to collect.
| Mechanism | Usually | Why |
|---|---|---|
| General corrosion, sulphidation, oxidation | Part 4 | Attacks the whole wetted surface at a similar rate |
| Corrosion under insulation | Part 5 | Happens where water sits, which is a patch |
| Flow-accelerated corrosion | Part 5 | Concentrates at fittings and downstream of them |
| Erosion | Part 5 | Where the flow impinges |
| Acid dewpoint corrosion | Part 5 | At the condensing zone |
| Soil-side tank floor corrosion | Either | Depends whether the ground is uniformly wet or locally so |
| Pitting | Part 6 | Discrete pits — a different part again |
This is the practical consequence and it has to be decided before the scaffolding comes down.
Part 4 needs thickness readings across the thinned area — enough to establish an average and a minimum, and to show the loss really is uniform. A grid does this.
Part 5 needs a critical thickness profile: how the remaining wall varies along and around the flaw, in both directions. A grid cannot produce that. You need corrosion mapping.
Take spot readings, find a low one, close up the equipment, and only then discover the flaw is local and a Part 5 assessment is required. The data needed no longer exists and cannot be obtained until the next shutdown. Decide which part applies while you can still scan.
When it genuinely is not clear, two reasonable approaches:
Assess it both ways. If it passes as general metal loss, you are done — that is the conservative treatment and it passed. If it fails as general but passes as local, you need to be able to justify that the local treatment applies, and the thickness profile is the justification.
Look at what caused it. The mechanism usually settles it. Damage from a mechanism that attacks the whole wetted surface is general even if your readings are patchy; patchiness in that case is more likely measurement scatter than real localisation.
Both parts sit inside step four of the eight-step procedure, and neither finishes the job. Whichever applies, you still owe a remaining life calculation from the corrosion rate, a decision on remediation, a monitoring plan, and documentation. An RSF above 0.90 with no remaining life behind it is not a completed assessment — it is a snapshot of a component that is still corroding.
The quick check computes minimum thickness, remaining strength factor and MAWP so you can see what the same damage gives under each treatment.
Open the quick checkWho writes this. A mechanical engineer with twelve years in oil and gas — in-line inspection, fired heater and furnace inspection, and pipeline integrity. What is here comes from the published standards and from what those years in the field actually looked like. It is not written by an API-certified inspector.
This is not an assessment. Nothing on this site may be used to justify a decision about real equipment. Assessing plant requires the current editions of the applicable codes, data from a licensed source, and a competent engineer who signs for the answer. · Integrity Field Guide