A fired heater tube fails by creep, and creep is the one mechanism where the damage accumulates invisibly for years and then goes quickly. Everything in a heater inspection is aimed at catching it while there is still margin.
Heater tubes operate closer to their limits than almost anything else in a refinery. They run hot, they carry pressure, and they are expected to do it for years. The mechanism that ends them is creep: slow deformation under stress at temperature, accumulating damage that never reverses.
What makes creep difficult is the shape of the curve. Damage accumulates slowly through most of the life, then accelerates sharply at the end. A tube can look acceptable at one outage and be near rupture at the next.
A tube under internal pressure at creep temperature grows. That diametral strain is the most direct evidence of creep damage available without cutting the tube out, and retirement criteria are commonly written around a percentage of original diameter.
Measured by laser profilometry from inside — a tool pulled through recording internal diameter continuously along the whole length. What matters is not a single value but the profile: where along the tube the swelling is, which tells you where the hot spot is and therefore what is causing it.
Creep strain thins the wall as it stretches, and oxidation, sulphidation and carburisation remove metal from both surfaces. Measured ultrasonically by the same internal tool, so diameter and wall come from one run at the same positions.
The combination matters. A tube that has swelled and thinned has less section carrying more stress at the same temperature — which accelerates the creep that caused it. That is a runaway, and it is why the last part of tube life is short.
Coke or scale on the inside is an insulating layer. Heat flux that used to reach the process now has to push through it, so the metal behind it runs hotter. Creep rate is exquisitely sensitive to temperature — a rise of 15 °C can halve remaining life.
The profilometry tool sees the deposit as an apparent reduction in bore, which is why a clean decoke before inspection matters: otherwise you cannot distinguish coke from metal.
Coke builds where flow is slowest. A tube with slightly restricted flow runs hotter, which lays down coke faster, which restricts flow further. The feedback is positive and it concentrates damage into a small number of tubes rather than spreading it evenly. This is why heater inspection reports rank individual tubes rather than reporting an average.
Infrared through the peepholes while the heater fires. This is the only measurement on this page that happens before the damage, and it is the cheapest thing in the whole programme.
It finds flame impingement, hot tubes, and restricted flow while there is still time to change the firing, clean a burner or re-balance the passes. A tube caught at 40 °C over its neighbours and corrected has years of life restored.
The catch: absolute temperature by infrared through a hot atmosphere is genuinely hard — emissivity, viewing angle and flame radiation all interfere. The pattern across tubes is far more reliable than any single reading, and that is enough to act on.
Fixed skin thermocouples on selected tubes give a continuous record. Their value is not the instantaneous reading but the accumulated history — which is what a Larson-Miller or life fraction calculation needs.
Creep damage begins as cavities on grain boundaries long before there is any measurable strain. Field replication — polish, etch, lift the microstructure onto a film — can classify that damage from isolated cavities through to oriented cavities, microcracks and macrocracks.
This is the only method that sees creep before it has a shape. It examines the surface at the point polished, so it is used to confirm a condition suspected from the other measurements rather than to survey.
Long exposure at temperature spheroidises carbides and softens the steel. A hardness survey can show that a tube has run hotter than intended for a long time, which is corroboration rather than proof.
Creep is assessed under API 579 Part 10, and the approach is different from every other part of the standard. There is no flaw to size. Instead life is consumed:
Two consequences follow, and both are worth internalising:
Consumed life never goes down. Cooling the tube stops further accumulation; it does not recover what is gone. A tube at 0.7 life fraction is at 0.7 forever.
Temperature dominates everything. The relationship is exponential. Small temperature errors produce large life errors, which is why skin thermocouples and thermography earn their keep, and why "we think it runs at about..." is not an acceptable input to the calculation.
Tube inspection is scheduled around outages, and outages are expensive, so the interval stretches. But creep accelerates at the end of life, which means the interval that was comfortable for the first fifteen years is not comfortable for the last two — and nothing in the data tells you which phase you are in except the life fraction you have been keeping. The tubes that fail are usually the ones whose inspection was deferred one cycle.
The creep demonstration in the guide shows what happens to remaining life when metal temperature moves — move the temperature and watch the life collapse.
Open the demonstrationsWho 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