Overheating and Bursting of Boiler Tubes
Compared to mechanical equipment, severe wear, vibration, significant fatigue, and other problems don’t occur in boiler tubes. However, accidents occur due to the quality of steam and water, operating conditions, flue gas flow, and other factors. According to statistics, the 8# furnace of a thermal power plant was shut down 15 times in 2003, with 6 of those shutdowns due to three-tube explosions and leakages, accounting for 40%. In 2004, the 7# furnace was shut down 7 times, with 4 of those shut down due to three-tube explosions and leakage, accounting for 57%. Therefore, preventing leakages of boiler tubes have become key to improving the safe and economic operation of power plants.
1. Overheating and bursting
Overheating is divided into long-term and short-term overheating according to the different temperature ranges.
1.1 Long-term overheating and bursting
When the temperature range of the tube is not too significant, the material's durable strength decreases; the creep rate accelerates, and the tube swells and ruptures, which is called long-term overheating and bursting.
The rupture is generally a rough brittle fracture with a blunt edge. The rupture is generally small and bulge-shaped. There is a thick oxide scale on the outer surface, and many longitudinal cracks form on the inner and outer walls. Some may penetrate the thickness of the tube wall, but the rupture is not obvious. There is usually no significant increase in the pipe circumference around the rupture. The structure often changes. The carbide is clearly spheroidized; the pearlite is graphitized, and sometimes it segregates to the grain boundary. The alloy elements transfer from the solid solution to the carbide, and creep holes generate at the grain boundary. Creep reduces the material's fracture toughness and plasticity.
Causes of failures
The boiler tube has been operating in an over-temperature state for a long time, which is caused by overload, poor steam-water circulation, combustion center deviation, and scaling on the inner wall of the boiler tube.
MeasuresPickling: Perform pickling to remove scaling on the boiler inner wall. Pickling is a commonly used cleaning and descaling method that can help remove impurities such as rust and scale in boiler pipes, keeping inner walls clean and smooth, improving thermal conductivity, reducing thermal stress concentration, and reducing the risk of overheating, thereby extending their service lives.
Lowering the temperature: For long-term overheating, the first solution is to lower the temperature of the boiler pipe by monitoring the operating temperature, adjusting the fuel supply, improving water circulation, and adding a cooling system.
Strengthening maintenance and monitoring: Long-term overheating may cause aging and degradation of the pipe materials, so it is necessary to strengthen the maintenance and monitoring of the boiler pipe. Regularly check the pipe to promptly detect and deal with the abnormal condition.
Checking the pipe materials: Long-term overheating may cause the performance of the pipe materials to deteriorate, including their strength and toughness. Therefore, it is necessary to regularly check the pipe materials to ensure that they meet the design requirements and replace aging or damaged pipes in a timely manner.
Improving the design: For pipe bursts caused by long-term overheating, it is necessary to re-evaluate the design of the boiler system, including the selection of pipe materials, and the design of the cooling system, to improve the heat resistance and safety of the system.
1.2 Short-term overheating and bursting
The edge of the rupture is sharp and significantly thinned. The area near the rupture of the tube is swollen and thickened. The rupture is often in a large, trumpet shape. Oxidation on the outer wall is not prominent. There are no axial cracks parallel to the rupture. The metallographic structure at the rupture varies depending on the degree of overheating. It is often a hardened structure or a hardened structure with ferrite. The hardness at the rupture is significantly higher than in other parts. The tube wall temperature is below Ac1. After the tube bursts, the structure is elongated ferrite and pearlite. The tube wall temperature ranges from Ac1 to Ac3 or exceeds Ac3. The structure is determined by the cooling capacity of the working fluid ejected after the rupture. Low-carbon martensite, bainite, pearlite, and ferrite can be obtained, and the strength of the steel is greatly reduced.
Causes of failures
The reasons for the short-term overheating and bursting of boiler tubes are as follows:
Foreign matter blocking the pipe: The blockage of foreign matter in the pipe reduces the flow cross-sectional area, increasing the water flow rate and partial temperature, resulting in overheating.
Unstable boiler combustion: Unstable boiler combustion: Unstable boiler combustion leads to uneven distribution of heat, causing some pipes to burst due to partial overheating.
Low steam flow rate or steam-water stratification: When the steam flow rate is too low or steam-water stratification occurs, it can cause partial overheating in the pipeline.
Inaccurate temperature regulation: Inaccurate temperature regulation of the boiler system can cause the pipeline temperature to exceed the design range, leading to pipeline overheating.
Scaling: Scaling in the pipeline reduces heat transfer efficiency, increasing thermal resistance and causing partial overheating.
In view of the above reasons, some measures can be taken:
Regularly clean the pipeline: Regularly clean foreign matter and scaling in the pipeline to ensure a smooth flow and reduce the possibility of blockage.
Optimize boiler combustion: Strengthen regulation and monitoring of the boiler combustion system to ensure stable combustion and avoid incomplete and uneven combustion.
Control steam flow rate: Reasonably control the steam flow rate to avoid excessively low
or high flow rate and reduce the possibility of steam stratification and pipeline overheating.
Improve temperature accuracy: For inaccurate temperature regulation, strengthen the maintenance and management of the boiler temperature regulation system to ensure accuracy.
Strengthen pipeline monitoring: Regularly inspect and monitor pipelines, detect abnormal conditions in time, and handle them to prevent pipeline overheating.
1.1 Long-term overheating and bursting
When the temperature range of the tube is not too significant, the material's durable strength decreases; the creep rate accelerates, and the tube swells and ruptures, which is called long-term overheating and bursting.
The rupture is generally a rough brittle fracture with a blunt edge. The rupture is generally small and bulge-shaped. There is a thick oxide scale on the outer surface, and many longitudinal cracks form on the inner and outer walls. Some may penetrate the thickness of the tube wall, but the rupture is not obvious. There is usually no significant increase in the pipe circumference around the rupture. The structure often changes. The carbide is clearly spheroidized; the pearlite is graphitized, and sometimes it segregates to the grain boundary. The alloy elements transfer from the solid solution to the carbide, and creep holes generate at the grain boundary. Creep reduces the material's fracture toughness and plasticity.
Causes of failures
The boiler tube has been operating in an over-temperature state for a long time, which is caused by overload, poor steam-water circulation, combustion center deviation, and scaling on the inner wall of the boiler tube.
MeasuresPickling: Perform pickling to remove scaling on the boiler inner wall. Pickling is a commonly used cleaning and descaling method that can help remove impurities such as rust and scale in boiler pipes, keeping inner walls clean and smooth, improving thermal conductivity, reducing thermal stress concentration, and reducing the risk of overheating, thereby extending their service lives.
Lowering the temperature: For long-term overheating, the first solution is to lower the temperature of the boiler pipe by monitoring the operating temperature, adjusting the fuel supply, improving water circulation, and adding a cooling system.
Strengthening maintenance and monitoring: Long-term overheating may cause aging and degradation of the pipe materials, so it is necessary to strengthen the maintenance and monitoring of the boiler pipe. Regularly check the pipe to promptly detect and deal with the abnormal condition.
Checking the pipe materials: Long-term overheating may cause the performance of the pipe materials to deteriorate, including their strength and toughness. Therefore, it is necessary to regularly check the pipe materials to ensure that they meet the design requirements and replace aging or damaged pipes in a timely manner.
Improving the design: For pipe bursts caused by long-term overheating, it is necessary to re-evaluate the design of the boiler system, including the selection of pipe materials, and the design of the cooling system, to improve the heat resistance and safety of the system.
1.2 Short-term overheating and bursting
The edge of the rupture is sharp and significantly thinned. The area near the rupture of the tube is swollen and thickened. The rupture is often in a large, trumpet shape. Oxidation on the outer wall is not prominent. There are no axial cracks parallel to the rupture. The metallographic structure at the rupture varies depending on the degree of overheating. It is often a hardened structure or a hardened structure with ferrite. The hardness at the rupture is significantly higher than in other parts. The tube wall temperature is below Ac1. After the tube bursts, the structure is elongated ferrite and pearlite. The tube wall temperature ranges from Ac1 to Ac3 or exceeds Ac3. The structure is determined by the cooling capacity of the working fluid ejected after the rupture. Low-carbon martensite, bainite, pearlite, and ferrite can be obtained, and the strength of the steel is greatly reduced.
Causes of failures
The reasons for the short-term overheating and bursting of boiler tubes are as follows:
Foreign matter blocking the pipe: The blockage of foreign matter in the pipe reduces the flow cross-sectional area, increasing the water flow rate and partial temperature, resulting in overheating.
Unstable boiler combustion: Unstable boiler combustion: Unstable boiler combustion leads to uneven distribution of heat, causing some pipes to burst due to partial overheating.
Low steam flow rate or steam-water stratification: When the steam flow rate is too low or steam-water stratification occurs, it can cause partial overheating in the pipeline.
Inaccurate temperature regulation: Inaccurate temperature regulation of the boiler system can cause the pipeline temperature to exceed the design range, leading to pipeline overheating.
Scaling: Scaling in the pipeline reduces heat transfer efficiency, increasing thermal resistance and causing partial overheating.
In view of the above reasons, some measures can be taken:
Regularly clean the pipeline: Regularly clean foreign matter and scaling in the pipeline to ensure a smooth flow and reduce the possibility of blockage.
Optimize boiler combustion: Strengthen regulation and monitoring of the boiler combustion system to ensure stable combustion and avoid incomplete and uneven combustion.
Control steam flow rate: Reasonably control the steam flow rate to avoid excessively low
or high flow rate and reduce the possibility of steam stratification and pipeline overheating.
Improve temperature accuracy: For inaccurate temperature regulation, strengthen the maintenance and management of the boiler temperature regulation system to ensure accuracy.
Strengthen pipeline monitoring: Regularly inspect and monitor pipelines, detect abnormal conditions in time, and handle them to prevent pipeline overheating.
