Five Types of Failures for Boiler Tubes
The five types of boiler tube failures are caused by wear, corrosion, thermal fatigue, leakages at welding parts, and incorrect steel usage.
1. Wear
Wear causes the boiler tube to thin and eventually burst.
When the angle with the fly ash is between 30° and 45°, the wear is greatest.
Causes of failures
Poor coal quality and hard particles in the fly ash scour the surface of the tube.
Measures
Select appropriate coal. Design the economizer structure to prevent excessively high smoke speed. Install flow-averaging baffles and anti-wear covers on the tube surface.
2. Corrosion
2.1 High-temperature corrosion of the tube’s outer wall
The cracks are short and thick, filled with corrosive media and products, with the characteristics of corrosive sulfur, and the crack direction is transcrystalline.
Causes of failures
When the angle with the fly ash is between 30° and 45°, the wear is greatest.
Causes of failures
Poor coal quality and hard particles in the fly ash scour the surface of the tube.
Measures
Select appropriate coal. Design the economizer structure to prevent excessively high smoke speed. Install flow-averaging baffles and anti-wear covers on the tube surface.
2. Corrosion
2.1 High-temperature corrosion of the tube’s outer wall
The cracks are short and thick, filled with corrosive media and products, with the characteristics of corrosive sulfur, and the crack direction is transcrystalline.
Causes of failures
- High-temperature corrosion originates from the sulfur composition in the fuel. After combustion, sulfur oxides are generated, causing corrosion and thinning of the tube’s outer wall, eventually leading to bursting when it cannot withstand the internal pressure.
- The tube wall is subjected to axial alternating stress, which may arise from thermal expansion and contraction, as well as furnace tube vibration during boiler start-up and shutdown.
Measures
Improve the component structure in areas where alternating stress is concentrated to avoid mechanical vibration during operation.
2.2 Low-temperature corrosion of the tube’s outer wall
Low-temperature corrosion often occurs in economizers and air preheaters with low flue gas temperatures, with the latter being the most harmful. It is caused by the combination of gaseous SO in the flue gas and water vapor, forming concentrated sulfuric acid. Accelerated corrosion of the tube’s outer wall is one of the causes of leakage in the low-temperature economizer. At low temperatures, SO increases due to catalytic conversion. This is the main difference from SO generation during high-temperature corrosion. Low-temperature corrosion affect boiler safety and economic output, so it must be strictly prevented.
Measures
Increase the cold acid temperature of the air preheater; use low-oxygen combustion; reduce the flue gas dew point; blow soot regularly; keep the heating surface clean, and use corrosion-resistant materials.
2.3 Oxygen corrosion
Oxygen corrosion occurs on the inner walls of economizers, superheaters, and feed water pipes. The corrosion is in the form of spots or pits. There is no overheating in the corroded area. Corrosion fatigue cracks may occur. Cracks primarily occur in economizers and feed water pipes during operation, while they occur for superheaters when the furnace is shut down.
Causes of failures
- The passivation film on the tube’s inner wall breaks, leading to electrochemical corrosion at the rupture.
- Stress concentration at the elbow promotes pitting corrosion.
- The elbow undergoes thermal shock, causing fatigue cracks on its inner wall.
- There is water accumulation in the tube when the furnace is shut down.
Measures
Strengthen boiler tube protection before use. Maintain water quality; strictly control pH and oxygen content, and ensure protection during shutdown.
3. Thermal fatigue
This type of damage often occurs at transition points where the metal temperature changes significantly, such as the feed water pipe hole, economizer, water-cooled wall, lower header, and desuperheater without a protective sleeve on the drum. Numerous cracks generate, but there is no obvious plastic deformation. The cracks extend along the crystal structure, and corrosion products form around them. The amount of pearlite decreases, usually accompanied by partial overheating and spheroidization. Its structure comprises ferrite, pearlite, and granular carbide.
Causes of failures
The temperature cycle changes, resulting in high alternating pressure and causing damage, while mechanical constraints cause cracks at stress concentrations.
Measures
- Improve the structure of components to adapt to significant changes in heat load.
- Increase the feed water temperature of thick-walled components such as the header during startup.
- Control the desuperheater’s temperature.
- Relax mechanical constraints to reduce stress concentrations.
4. Leakages at welding parts
The welding seam’s quality is too poor, causing leaks during operation. Leakages at boiler tube welding parts may be caused by various reasons, including poor welding quality, unreasonable welding joint design, improper material selection, and excessive pipeline stress. To address leakages at welding parts, appropriate measures must be taken according to the specific situation. The following are some possible treatments:
- Re-welding: In the case of poor welding quality or problems with the welding joint, re-welding is required. Ensure that the welding process meets standards and the welding quality is good during the re-welding process to avoid future leaks.
- Repair welding: In the case of partial leakages at welding parts, consider repairing the leaked part by repair welding. Ensure that repair welding quality and welded joint strength meet design requirements.
- Replace the welded joint: If the welded joint has serious quality problems or is damaged, replace and re-weld it.
- Strengthen pipeline support and fixation: If excessive pipeline stress causes leaks at welding parts, consider strengthening the pipeline's support and fixation to reduce stress and leakage risk.
- Inspection and maintenance: Regular inspection and maintenance of boiler pipeline systems are crucial to promptly discover and address potential problems, preventing leaks at welding parts.
- Strengthen pipeline material and welding process control: When designing and manufacturing new boiler pipelines, strengthen control of pipeline materials and welding processes to ensure the selected materials meet requirements and the welding process is correct, avoiding leaks at welding parts.
5. Incorrect steel usage
In 1994, the aluminum plant burst due to the misuse of steel in the steam pipe. If inappropriate steel is selected for manufacturing boiler tubes, the following problems can occur:
- Corrosion: If the selected steel is not suitable for the corrosive medium in the boiler, it may cause surface corrosion or allow the corrosive medium to penetrate the pipeline wall, causing damage.
- Insufficient temperature resistance: Boiler tubes experience high temperatures and high pressures during operation. If the steel's heat resistance is insufficient, the pipeline may deform, crack, or even melt, affecting the boiler's operation.
- Strength: Under high temperatures and pressures, if the selected steel's strength is insufficient, the pipeline will not withstand the internal pressure, leading to deformation and cracking.
- Welding: If the steel’s welding performance is poor, it may cause cracking at the welding seam, affecting the pipeline’s sealing and pressure resistance.
- Poor chemical stability: If the steel is prone to chemical reactions or corrosion in the working environment, it will accelerate pipeline aging, shorten the service life, and potentially cause accidents.
- If the selected steel cannot withstand the stress during boiler operation, it will cause stress concentration and plastic deformation, accelerating fatigue and failure.
To deal with these problems, the following measures are usually taken:
- Repair or replace the pipeline: Problematic pipelines must be repaired or replaced to ensure safe operation.
- Strengthen monitoring and maintenance: Strengthen the monitoring of boiler pipelines; regularly check pipeline conditions; promptly identify and address problems, minimizing the possibility of accidents.
- Optimize design and select appropriate materials: When designing new boiler pipelines, carefully select appropriate steel to ensure it meets working conditions and avoid problems from using inappropriate steel.
- Strengthen training and management: Strengthen the training of operators and maintenance personnel; improve their awareness of safe boiler pipeline operation; strengthen pipeline management, and reduce risks caused by human errors.
- Strengthen the management of metal parts and conduct spectral analysis of steel before storage and during use to prevent incorrect steel usage.
