Boiler tubes fail often enough that the failures have a taxonomy of their own — and usefully, the tube usually names its own killer. The shape of the rupture, the scale on its surfaces and the microstructure at its edge tell an experienced investigator most of the story before any laboratory work begins.
Thin-lipped, fish-mouth rupture: rapid overheating
A wide-open burst with knife-edge lips means the tube overheated quickly and severely — the metal lost its strength within minutes and ballooned until it tore. The cause is almost always a loss of internal cooling: blocked tube, lost circulation, steam blanketing. The investigation's question becomes operational: what starved this tube of flow, and are its neighbours at risk of the same event?
Thick-lipped, narrow rupture: long-term overheating and creep
A tight, blunt-edged split with little ballooning tells the opposite story: the tube ran moderately hot for a long time, accumulating creep damage — voids and cracking that grow over months or years at temperatures the material was never meant to hold continuously. Microstructural examination confirms it: degraded microstructure and creep voiding at the rupture edge. The driver is often internal deposits insulating the steel from its cooling water, which turns the investigation toward water chemistry.
The scale is a thermometer and a diary
Internal oxide thickness records the tube's real operating temperature over time — thick scale means the tube ran hotter than design, and the measurement supports remaining-life estimates for the rest of the bank. Internal deposits record the water-treatment history: hardness salts, iron transport, chemical excursions. External scale and thinning point instead to fireside problems — flue-gas corrosion, sootblower erosion, flame impingement.
Not every tube failure is thermal
- Waterside pitting and oxygen attack — shutdown and lay-up practice writing itself into the tube wall.
- Caustic gouging and hydrogen damage — aggressive under-deposit chemistry in high-pressure units.
- Fatigue at attachments — cyclic stress at welded supports and headers. Fatigue signatures →
- Dew-point corrosion — acid condensation on the cold end of the gas path.
Each has its own signature, and several can act together — which is why the verdict should rest on evidence, not on the first plausible label.
The decision the investigation serves
One tube has failed; the real question is the other several thousand. A correct mechanism identifies which tubes share the exposure — same flow path, same deposit loading, same temperature excursion — and turns one failure into a targeted inspection plan instead of a recurring surprise.
When to contact MTIS
Ideally with the failed tube section uncleaned, both fracture/rupture faces protected, a metre of adjacent tube for reference, and the recent water-chemistry and operating logs (preservation guide →). Request a project quote → Related: Process Industries →