Explain the other types of tube failure that may occur during long term operation of the boilers.

During long-term operation of boilers, tube failures become more complex and can arise from prolonged exposure to thermal, mechanical, and chemical stresses. These failures often develop gradually and can be due to issues like material degradation, operational inefficiencies, or water chemistry problems. Below are the common types of tube failures that occur during long-term boiler operation:

1. Creep Failure

Cause: Prolonged exposure to high temperatures and pressures beyond the material’s limits.

Symptoms: Swelling or bulging of the tubes, followed by rupture.

Failure Mechanism: Over time, sustained high temperatures and stresses cause the metal to undergo slow plastic deformation, known as creep. This weakens the tube material, leading to a rupture. Creep failure typically occurs in areas of the boiler operating at high temperatures, such as superheaters and reheaters.

Prevention: Proper material selection for high-temperature zones, maintaining operational parameters within design limits, and regular inspections to detect early signs of creep.


2. Corrosion Fatigue

Cause: Repeated thermal cycling (heating and cooling) combined with corrosive water or steam conditions.

Symptoms: Cracking of the tube surfaces, often starting at areas with high stress concentrations such as bends, welds, or supports.

Failure Mechanism: Repeated thermal cycling causes expansion and contraction of the tubes, which, in combination with corrosion, weakens the tube walls. This type of failure commonly occurs in water-wall tubes, economizers, or superheaters.

Prevention: Ensure smooth temperature transitions during start-up and shutdown, maintain proper water chemistry, and avoid operational cycling.


3. Oxidation

Cause: Long-term exposure to high temperatures and oxygen in the flue gases.

Symptoms: Surface scaling and gradual thinning of the tube walls, with a gray or black oxidized layer forming on the external surfaces.

Failure Mechanism: Prolonged oxidation depletes the protective oxide layer on the tube surface, causing metal wastage. This failure is more likely in areas of high-temperature exposure, like superheaters and reheaters.

Prevention: Use oxidation-resistant materials, such as chromium alloys, for high-temperature zones, and monitor for early signs of surface degradation.


4. High-Temperature Corrosion

Cause: Interaction of the tube material with corrosive elements such as sulfur, chlorine, or other compounds in the fuel or flue gases.

Symptoms: Tubes develop a green, reddish, or brownish scale due to the corrosion products, often accompanied by material thinning and pitting.

Failure Mechanism: Corrosive gases attack the metal surfaces at high temperatures, forming sulfates or chlorides that weaken the material. This is more common in boilers using fuels with impurities or in high-heat areas such as superheaters and economizers.

Prevention: Use fuels with low impurity levels, ensure proper flue gas conditioning, and apply protective coatings to the tubes.


5. Erosion

Cause: Continuous impact of particles such as fly ash, slag, or soot on the tube surfaces.

Symptoms: Pitting or grooving on the tube walls, with a smooth or polished appearance in localized areas.

Failure Mechanism: High-velocity particles in the steam or flue gas erode the protective oxide layer on the tubes, resulting in material loss. Erosion is most common in bends, economizers, or tubes directly exposed to flue gas or soot blowers.

Prevention: Proper control of steam and gas velocities, use of erosion-resistant materials in high-flow zones, and regular cleaning to prevent particle buildup.


6. Soot Blower Erosion

Cause: Erosion from high-pressure soot blower jets impinging directly on tube surfaces.

Symptoms: Localized thinning or wear in areas where soot blowers frequently clean the tubes, with shiny or polished areas showing material loss.

Failure Mechanism: Soot blowers, used to remove slag and ash deposits, can cause mechanical wear on the tubes over time if the jets are not properly aligned or controlled. Repeated cleaning can eventually lead to tube thinning and failure.

Prevention: Proper alignment and control of soot blowers, reducing the frequency of soot blowing, and using wear-resistant materials in affected areas.


7. Fly Ash Corrosion (Erosion-Corrosion)

Cause: A combination of corrosion and erosion caused by the abrasive action of fly ash particles and corrosive gases on the tube surfaces.

Symptoms: Localized thinning of the tube walls, especially in economizers or areas with high gas velocities, with evidence of pitting and rough surfaces.

Failure Mechanism: Fly ash particles erode the protective oxide layer on the tubes, while corrosive gases further weaken the metal. This type of failure occurs in regions with high fly ash content and where flue gases are still corrosive.

Prevention: Improve flue gas and ash removal systems, use erosion-resistant materials, and apply protective coatings to vulnerable tubes.


8. Caustic Corrosion (Caustic Embrittlement)

Cause: Accumulation of caustic substances (like sodium hydroxide) in areas of high stress, such as tube bends, welds, or near steam/water separation points.

Symptoms: Brittle cracks or pitting, often localized at points where caustic substances have concentrated.

Failure Mechanism: Caustic embrittlement occurs when caustic substances penetrate the tube metal, causing cracks due to localized stress. This is more likely to occur in stressed areas, especially at joints or where water circulation is inadequate.

Prevention: Proper water chemistry control, avoiding concentration of caustic substances, and using caustic-resistant materials or stress-relief techniques.


9. Hydrogen Damage

Cause: Hydrogen penetration into the tube material due to improper water chemistry, often under acidic conditions.

Symptoms: Internal blistering or cracking, often with a brittle fracture surface.

Failure Mechanism: Hydrogen ions penetrate the tube metal, reacting with carbon to form methane. This results in internal pressure build-up, leading to blistering and cracking of the tubes. Hydrogen damage is more common in low-pressure boilers with improper water treatment.

Prevention: Strict control of water chemistry, particularly avoiding acidic conditions and ensuring proper chemical treatment.


10. Graphitization

Cause: Long-term exposure to temperatures above 425°C, causing the carbon in steel to precipitate as graphite.

Symptoms: Tubes become brittle, and the material may feel soft or powdery when fractured.

Failure Mechanism: Graphitization weakens the steel by replacing carbon atoms with graphite, reducing the metal’s strength and making it prone to cracking. This type of failure occurs in tubes exposed to high temperatures over long periods, such as in superheaters.

Prevention: Use graphitization-resistant materials, such as chromium-molybdenum alloys, and monitor areas exposed to high temperatures.


11. Corrosion Under Deposits (Under-Deposit Corrosion)

Cause: Accumulation of deposits such as scale, sludge, or impurities on the internal surface of the tubes, leading to localized corrosion.

Symptoms: Pitting or localized thinning underneath the deposits, which may appear as crusty or flaky buildup on the tube walls.

Failure Mechanism: Deposits create areas of localized concentration of corrosive agents, leading to accelerated corrosion under the scale. This type of failure is common in water-wall tubes where scale or deposits accumulate due to poor water treatment.

Prevention: Regular chemical cleaning, ensuring proper water chemistry, and avoiding conditions that promote deposit formation.


12. Fatigue Cracking

Cause: Repeated thermal or mechanical cycling during long-term operation.

Symptoms: Cracking, often at points of stress concentration such as welds, supports, or areas subject to vibration.

Failure Mechanism: Over time, thermal cycling or mechanical stress causes cracks to initiate and propagate through the tube material. This type of failure is common in areas subject to operational cycling or vibration, such as in economizers or reheaters.

Prevention: Minimize operational cycling, control vibration, and use materials with high fatigue resistance.


13. Water-Side Corrosion

Cause: Poor water chemistry control, leading to corrosion on the internal surfaces of the tubes.

Symptoms: Tubes exhibit pitting or grooving on the internal surfaces, often accompanied by deposits or discoloration.

Failure Mechanism: Corrosion due to dissolved oxygen, carbon dioxide, or improper pH levels in the boiler water. This type of failure is common in economizers and water-wall tubes.

Prevention: Proper water treatment to control oxygen and pH levels, and regular monitoring of water-side conditions.


By understanding these long-term failure mechanisms, maintenance teams can take preventive actions such as regular inspection, proper water chemistry control, operational