Explain the types of tube failure that may occur innthe initial stage of boiler operation till stabilization?

During the initial stage of boiler operation, until the system reaches a stable operational phase, several types of tube failures can occur. These failures are often related to start-up conditions, improper handling, design flaws, or material weaknesses. The initial period of boiler operation is critical, as it involves high thermal stress, pressure fluctuations, and exposure to new operating conditions. Below are the common types of tube failures during this phase:

1. Short-Term Overheating (Dry-out Failure)

Cause: Insufficient water circulation or improper firing rates leading to localized overheating of the boiler tubes.

Symptoms: Tube bulging, swelling, or rupture due to exposure to excessive temperatures.

Failure Mechanism: When the heat input is too high and water flow is inadequate, the tubes may not be able to transfer heat effectively. This causes rapid temperature rise, softening, and eventually rupturing of the tubes. This type of failure is common in water-wall tubes and high-heat-flux zones of the furnace.

Prevention: Ensure proper water flow and circulation in the early stages of start-up, along with gradual heat-up rates.


2. Weld Failure or Manufacturing Defects

Cause: Weaknesses or defects in weld joints or tube materials due to improper fabrication, installation, or pre-operation testing.

Symptoms: Cracking or rupture of the tubes at or near weld seams.

Failure Mechanism: Residual stresses from welding and improper heat treatment may become apparent during the initial heating. Poor weld quality, misalignment, or inadequate post-weld heat treatment can cause early failures under pressure and temperature changes.

Prevention: Ensure high-quality welds, proper heat treatment, and thorough inspection before start-up.


3. Thermal Fatigue

Cause: Frequent temperature changes during start-up and shutdown cycles, leading to stress on the boiler tubes.

Symptoms: Cracks in the tubes, especially at welds, bends, or other areas where stress concentration is higher.

Failure Mechanism: During start-up, repeated heating and cooling of the tubes cause expansion and contraction, leading to thermal fatigue. This induces micro-cracks, which propagate over time and result in eventual tube failure.

Prevention: Implement controlled heating and cooling cycles, and avoid rapid temperature changes.


4. Steam Blanketing

Cause: Poor circulation or uneven water distribution causing steam bubbles to form around certain sections of the tubes (typically water-wall tubes).

Symptoms: Overheating, blistering, and cracking of the affected tubes.

Failure Mechanism: Steam blanketing occurs when steam forms a layer between the tube and water, insulating the metal and preventing effective heat transfer. This leads to localized overheating, which weakens the tube material and causes ruptures.

Prevention: Ensure proper circulation patterns and avoid air pockets in the tubes.


5. Caustic Embrittlement

Cause: Poor water chemistry during initial operation, leading to the concentration of caustic substances near high-stress areas, such as tube joints and welds.

Symptoms: Cracks in the tubes, especially in high-stress regions like bends or weld seams, often with brittle fracture surfaces.

Failure Mechanism: Accumulation of caustic substances (e.g., sodium hydroxide) weakens the metal’s structure, causing cracks and embrittlement. This is more likely to occur in areas where metal experiences stress concentrations.

Prevention: Proper water treatment and control of water chemistry from the start-up phase.


6. Hydrogen Damage

Cause: Poor water chemistry control, particularly during initial chemical cleaning or water conditioning, allowing hydrogen to penetrate into the tube material.

Symptoms: Internal blistering or cracking of the tubes, often accompanied by internal scaling.

Failure Mechanism: Hydrogen ions diffuse into the steel during improper cleaning or when acidic conditions are present. Once inside, hydrogen reacts with carbon in the steel, forming methane, which creates internal pressure and leads to cracking and blistering.

Prevention: Maintain correct water chemistry during start-up, and avoid the use of overly aggressive cleaning chemicals.


7. Fabrication Stresses

Cause: Residual stresses left in the tubes during fabrication, such as bending or welding, that were not adequately relieved by heat treatment.

Symptoms: Cracking or splitting of tubes, especially near bends or weld joints.

Failure Mechanism: When the tubes are first exposed to high operating temperatures, residual stresses can be relieved unevenly, leading to cracking or warping of the tube material.

Prevention: Proper stress-relief procedures during tube manufacturing and installation, as well as careful inspection before commissioning.


8. Erosion

Cause: Abrasive particles (such as slag, fly ash, or other debris) in the steam or water mixture impacting the tubes, especially in areas of high flow velocity.

Symptoms: Localized thinning, pitting, or grooving of the tube walls.

Failure Mechanism: High-velocity particles erode the tube surfaces, removing the protective oxide layer and causing material loss. Erosion is common in tubes exposed to areas of high flow velocity or turbulent zones.

Prevention: Proper cleaning of the system during initial operation and regular maintenance to remove debris and prevent erosion.


9. Water Hammer Damage

Cause: Sudden surges of water or steam during start-up, often due to improper venting or filling procedures.

Symptoms: Dents or cracks in tubes, especially in areas close to valves or elbows.

Failure Mechanism: Water hammer occurs when a sudden pressure surge causes a shockwave to travel through the system, creating high mechanical stresses on the tubes. These shockwaves can cause deformation or cracking, particularly in thinner or weaker areas.

Prevention: Careful filling and venting procedures, gradual opening of valves, and avoiding sudden pressure changes during start-up.


10. Chemical Attack from Initial Cleaning

Cause: Improper chemical cleaning procedures during commissioning, which may leave residual cleaning agents or improper passivation.

Symptoms: Pitting, thinning, or cracking of the tube surfaces.

Failure Mechanism: Aggressive cleaning chemicals used to remove scale or other deposits from the system can lead to localized chemical attacks on the tube material, particularly if the chemicals are not properly flushed or neutralized after cleaning.

Prevention: Follow proper chemical cleaning protocols, ensuring complete removal of cleaning agents and passivation of the system before operation.


11. Steam Impingement

Cause: Direct impact of high-velocity steam jets on the tube surfaces, especially during start-up when steam lines may not yet be fully purged or aligned.

Symptoms: Erosion, localized wear, or cracking on the tube surfaces.

Failure Mechanism: High-velocity steam impinges on tube surfaces, stripping away the protective oxide layer and causing localized wear and eventual tube failure.

Prevention: Ensure proper steam line alignment, adequate purging, and control of steam velocity during start-up.


Each of these failure types can be minimized through careful monitoring and control during the initial stages of boiler operation, including ensuring proper water chemistry, gradual heating and cooling cycles, and regular inspection of critical components.