The three levels are not three difficulties of the same calculation. They are three different trades of conservatism against effort, and knowing which one you are in tells you what a pass or a fail actually means.
Every part of API 579 offers up to three assessment levels for the same flaw. They are not sequential steps you must climb, and a Level 1 pass is not a lesser answer than a Level 3 pass. They are three points on a single trade-off.
Higher level = less conservatism, in exchange for more data, more analysis and more competence. The equipment does not change. What changes is how much margin you are holding in reserve because you did not measure something.
What it is: a screening assessment. Tables, charts and simple closed-form equations. Deliberately conservative.
What it needs: minimal data. For local metal loss, roughly: the minimum remaining thickness, the flaw length, the diameter and the design conditions.
Who does it: an inspector or an engineer. No specialist analysis skill required.
What a result means:
What it is: a more detailed calculation that uses the real shape of the damage rather than a bounding assumption.
What it needs: considerably more data. For local metal loss, a full critical thickness profile — how the remaining wall varies along and around the flaw — instead of a single minimum. In practice that means corrosion mapping rather than spot readings, and this is the most common reason a Level 2 cannot be done: the data was never collected.
Who does it: an engineer competent in the method.
Why it recovers margin: Level 1 typically assumes the minimum thickness applies over the whole flaw length. Real damage is rarely that shape — it is usually deepest in the middle and shallower at the edges. Level 2 accounts for the true profile, and the recovered margin can be substantial.
What it is: analysis on the specific component — finite element analysis, fracture mechanics, sometimes testing.
What it needs: everything. Geometry, real material properties, an accurate load history, and a specialist who can defend the modelling choices.
Who does it: a specialist engineer. This is not routine work.
When it earns its cost: when the component is unusual and the simpler methods' assumptions genuinely do not fit; when the consequence of replacement is very large; when the flaw is in a geometry the closed-form methods do not cover; or when combined loading matters and the simplified methods cannot represent it.
| Level 1 | Level 2 | Level 3 | |
|---|---|---|---|
| Conservatism | High | Moderate | Lowest |
| Inspection data | Minimum thickness | Full thickness profile | Detailed geometry and properties |
| Method | Charts and screening curves | Closed-form with real profile | Numerical analysis |
| Performed by | Inspector or engineer | Competent engineer | Specialist |
| Effort | Minutes | Hours | Days to weeks |
| Typical use | Screening everything | The ones Level 1 failed | The few Level 2 also failed, where replacement is costly |
Treating a Level 1 failure as a verdict on the equipment.
Level 1 is designed to be quick and safe, which means it declines to pass things it cannot prove with the little data it was given. When it fails a component, the correct next question is not "what do we replace it with" but "what data would a Level 2 need, and can we still get it?"
That question often has to be asked while the equipment is still open, because the answer is usually a corrosion map, and the scaffolding comes down at the end of the outage. Escalating to Level 2 three weeks later, with only spot readings on file, is a common and expensive dead end.
All three levels sit inside step four of the eight-step procedure that every assessment follows. Whichever level you use, you still have to identify the mechanism, confirm applicability, establish remaining life, decide on remediation and monitoring, and document it. A Level 3 finite element analysis with no remaining-life calculation behind it is not a completed assessment.
The quick check runs both levels on your numbers — minimum thickness, remaining strength factor, MAWP and remaining life, on screen, free.
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