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Boiler Tube Failure Analysis
A tube failure is a symptom. The mechanism is what decides whether the unit comes back up or fails again in the same header.
Paresh Haribhakti, Managing Director of TCR Advanced Engineering, is a co-author of Failure Investigation of Boiler Tubes: A Comprehensive Approach, published by ASM International, Ohio, in 2018.
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TCR Engineering Services Pvt. Ltd. performs boiler tube failure analysis at its Navi Mumbai, India laboratory, NABL ISO/IEC 17025:2017 accredited (certificate NABLT0726MH18640) and an IBR Well-Known Material Testing Laboratory since 2014. It investigates boiler, superheater, reheater, economiser, waterwall and fired heater tube failures, drawing on 1,500+ documented tube failure investigations and 400+ boiler remaining-life assessments.
Types of Boiler Tube Failure, and Why the Mechanism Decides the Repair
Every boiler tube failure looks broadly the same from the walkway: a hole, a split, a burst. The mechanisms behind them are entirely different, and so are the corrective actions. Replacing the tube without naming the mechanism buys the interval to the next failure and nothing more.
- Short-term overheating gives a thin-lipped fish-mouth rupture with heavy plastic deformation and a transformed microstructure. The cause is a loss of flow, not a loss of material.
- Long-term overheating and creep gives a thick-lipped rupture, longitudinal cracking, external oxide scale and a microstructure that has spheroidised, coarsened its carbides and, at the end, cavitated on the grain boundaries.
- Waterside corrosion fatigue gives transverse, oxide-filled cracks initiating on the inside surface at points of restraint, and it is a cycling and chemistry problem together.
- Caustic gouging and acid phosphate corrosion give irregular internal wall loss under a deposit, and both are settled by deposit analysis, not by looking at the metal.
- Hydrogen damage gives a thick-lipped, window-type failure with decarburisation and intergranular microfissuring, and the surrounding tubes are usually affected before they leak.
- Fireside and coal-ash corrosion gives external wastage under a molten or semi-molten deposit, with a characteristic layered scale.
- Erosion from fly ash, soot blowers, falling slag or coal particles gives directional external thinning with a polished or scalloped surface.
- Dissimilar metal weld failure gives a fracture at or just off the ferritic side of the transition, and it is a design and thermal-expansion problem, not a welding-quality problem.
- Graphitisation, thermal fatigue, stress corrosion cracking and maintenance damage complete the set, and each has its own signature.
TCR reports against the full named taxonomy rather than against a general description, because a plant that is told "overheating" cannot act, and a plant that is told "long-term overheating with an estimated tube metal temperature above design and a spheroidised microstructure" can.
What Causes Boiler Tube Failure Due to Overheating?
Boiler tube failure due to overheating happens when tube metal runs hotter than its material allows, either briefly and severely or slightly for months. TCR Engineering, an IBR Well-Known Material Testing Laboratory in Navi Mumbai, India, separates the two, because short-term and long-term overheating have different causes and the plant fix for one does nothing for the other.
| Overheating pattern | What starves or insulates the tube | Where it usually appears | What the plant corrects |
|---|---|---|---|
| Short-term overheating | A sudden loss of cooling flow: a tube blocked by debris or a foreign object left after maintenance, low drum level, or firing raised faster than circulation was established | Waterwall and superheater tubes, within hours of the upset | The flow path and the start-up practice, not the tube material |
| Long-term overheating and creep | Steam-side oxide scale or water-side deposit that insulates the wall, so metal temperature climbs over years at unchanged firing | Superheater and reheater outlet legs and the hottest elevations | Scale and deposit control, the tube grade, and the remaining life of the circuit |
| Flue-gas imbalance | Uneven gas temperature across the furnace: burner imbalance, slagging that diverts gas, or soot-blower coverage gaps | The same panels or lanes, failing repeatedly in the same position | Combustion tuning and soot-blower coverage |
| Material or layout shortfall | A lower-grade tube installed in a hot location, or a circuit change that altered the flow distribution | At replacement tubes, transitions and modified circuits | Material verification at procurement and the circuit design |
Reading Tube Metal Temperature from the Tube Itself
The most useful number in a superheater or reheater investigation is rarely on any instrument. It is the temperature the metal actually saw, and the tube carries two independent records of it.
| Evidence | What is measured | What it establishes |
|---|---|---|
| Steam-side oxide scale thickness | Scale thickness measured metallographically on a transverse section | An estimate of the accumulated tube metal temperature history, read through the standard oxide-growth correlations |
| Microstructural degradation stage | Pearlite spheroidisation, carbide coarsening, carbide precipitation at grain boundaries, creep cavitation | How far through its creep life the material has run, independently of any temperature record |
| Hardness | Hardness traverse across the wall | Softening consistent with the degradation stage, and the presence of any unintended transformed layer |
| Dimensional change | Outside diameter swell and wall thinning against nominal | Creep strain accumulated, and whether the tube was already at the replacement criterion |
| Deposit and scale chemistry | Internal deposit and external scale analysed by EDS and wet chemistry | Whether the driver was water chemistry, fuel chemistry or both |
Where the failure raises a question about the rest of the circuit, the tube evidence feeds directly into a remaining life assessment. TCR has completed 400+ boiler remaining-life assessments and, within the TCR group, the Central Boilers Board's remnant life assessment recognition is held by TCR Advanced Engineering, Vadodara.
What to Send, and How to Cut It
A tube failure investigation is only as good as the section that reaches the laboratory. The single most common avoidable problem is a sample cut too close to the damage, which removes the material the comparison depends on.
- Length: at least 300 mm either side of the failure where the outage allows it, so that undamaged parent material from the same tube is available as the control.
- Orientation: mark the fireside, the crown and the flow direction on the tube before it is cut. Once the section is off the wall that information cannot be recovered.
- Cutting: cut cold where possible. A torch cut through the region of interest destroys the microstructure that the investigation is there to read.
- Deposits: do not clean, flush or wire-brush the internal surface. The deposit is evidence, and in a caustic gouging or acid phosphate case it is the primary evidence.
- Identification: the tube specification, the location in the circuit, the elevation, the hours in service and the operating conditions, including any recent excursion.
Where the unit is down and a decision is needed inside the outage, say so at the point of sampling. The sequence is ordered to give the operational answer first and the full documented report after.
The Fleet Question, Answered Without Cutting
One failed tube raises the question of every tube on the same header and every unit of the same design. Cutting the rest out to answer it is not an option, which is what in-situ metallographic replication is for.
- In-situ metallography to ASTM E1351: the microstructure is read in place, on the live component, with no cutting and no sample removal. TCR has taken more than 100,000 in-situ replicas and fields 12 in-situ metallographic teams across the group.
- Hardness in place: portable hardness on the same locations, so degradation is scored against two independent measurements.
- Replica interpretation against an archive: a replica is only as good as the reference base it is read against, and TCR reads them against its own.
- Where it leads: a boiler audit, a remaining life assessment under Regulation 391A of the Indian Boiler Regulations, or a knowledge-based risk assessment that ranks the circuit rather than the single tube.
Adani Power Mundra had its 4,620 MW boiler and piping fleet assessed this way, at 20 days per unit. Reliance Jamnagar had 1,200 in-situ replicas taken and interpreted in 15 days inside a live turnaround.
The book on boiler tube failure, written here
Paresh Haribhakti, Managing Director of TCR Advanced Engineering, wrote Failure Investigation of Boiler Tubes: A Comprehensive Approach with P.B. Joshi and Rajendra Kumar. ASM International, the American materials society, published it on 1 December 2018. The investigations behind this service page and the investigations behind that book are the same body of work.
The book is a comprehensive treatment of boiler tube failure: the recognised damage mechanisms, from short-term and long-term overheating through waterside and fireside corrosion, caustic gouging, hydrogen damage, stress corrosion cracking, creep, corrosion fatigue, erosion and graphitisation; the field and laboratory evidence that identifies each one, including in-situ metallography, oxide-scale measurement, dimensional survey, replication and fractography; and the corrective and preventive actions that follow from the diagnosis. It draws on more than 1,500 boiler tube failure investigations carried out by the TCR group.
ASM's own description: “Coal-fired power plants rely on a network of boiler tubes that would stretch several kilometers if placed end to end. These tubes operate under extreme conditions and, despite due care, fail from time to time, leading to forced outages. Failure Investigation of Boiler Tubes: A Comprehensive Approach is a detailed guide to understanding damage mechanisms and how to identify and control them. It covers operating principles and interactions, material properties and selection, characterization tools and techniques, and application challenges. It also includes information on water chemistry and life assessment, and presents several well-illustrated case histories.”
| Field | Value |
|---|---|
| Title | Failure Investigation of Boiler Tubes: A Comprehensive Approach |
| Authors | Paresh Haribhakti, P.B. Joshi, Rajendra Kumar |
| Publisher | ASM International |
| Publication date | 1 December 2018 |
| Print ISBN | 978-1-62708-156-6 |
| Electronic ISBN | 978-1-62708-253-2 |
| DOI | 10.31399/asm.tb.fibtca.9781627082532 |
| Publisher's page | dl.asminternational.org |
Both the book extract and TCR's own six-page investigation primer are in the Documents section below.
The book, in the author's words
Paresh Haribhakti on Failure Investigation of Boiler Tubes: A Comprehensive Approach, the book he wrote with P.B. Joshi and Rajendra Kumar and ASM International published in 2018.
Documents
Download the reference documents for this page. Every file is hosted on this domain and is also listed in the site document library.
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Frequently asked questions
How are boiler tube failure mechanisms classified?
They fall into six families: stress rupture, water-side corrosion, fire-side corrosion, erosion, fatigue, and lack-of-quality or maintenance damage. Each family carries its own named mechanisms, and a TCR report names the specific mechanism rather than the family, drawing on 1,500+ documented tube failure investigations.
How do you estimate the tube metal temperature after the failure?
From the tube. Steam-side oxide scale thickness measured metallographically gives an estimate of the accumulated temperature history, and the microstructural degradation stage, spheroidisation through carbide coarsening to creep cavitation, gives an independent read of how far through creep life the material has run. The two are reported together.
How long a tube section should be sent?
At least 300 mm either side of the failure where the outage allows it, so undamaged parent material from the same tube is available as a control. Mark the fireside, the crown and the flow direction before cutting, cut cold where possible, and do not clean the internal surface.
Can the internal deposit be analysed as well as the metal?
Yes, and in a caustic gouging, acid phosphate corrosion or hydrogen damage case the deposit is the primary evidence. It is analysed by EDS and by wet chemical methods. Flushing or wire-brushing the tube before it is sent removes the answer.
Does a tube failure investigation lead to a remaining life assessment?
It does where the evidence points beyond the one tube. The same metallurgical evidence feeds a remaining life assessment of the circuit, and TCR has completed 400+ boiler remaining-life assessments and, within the TCR group, the Central Boilers Board's remnant life assessment recognition is held by TCR Advanced Engineering, Vadodara.
Is TCR recognised under the Indian Boiler Regulations?
Yes. TCR Engineering has been an IBR Well-Known Material Testing Laboratory since 2014, Form XVI-H certificate 38, valid to 2 April 2029.
Why does the same boiler tube fail again after it is replaced?
Because replacing the tube removes the symptom and leaves the cause in place. If the failure was long-term overheating under internal scale, a water chemistry upset or a flue-gas imbalance, the new tube sees the same conditions and follows the old one. Naming the mechanism is what lets the plant correct the circuit rather than the tube.
Does TCR investigate fired heater and industrial boiler tubes as well as power station boiler tubes?
Yes. Refinery and petrochemical fired heater tubes and captive and industrial boiler tubes are investigated alongside utility superheater, reheater, economiser and waterwall tubes. The same Navi Mumbai laboratory reads the microstructure, the scale and the deposit, and the damage taxonomy changes only where the fuel and the process chemistry change.