Boiler tube bursts occur when tubes in the heat exchange surface of a boiler burst due to factors like overheating, erosion, and corrosion, resulting in high-temperature water leakage and disrupting normal boiler operation. Research suggests several main causes for these bursts.
Wear
This includes fly ash erosion, slagging erosion, soot blowing erosion, and coal particle erosion. Fly ash erosion is analyzed here as an example. Fly ash erosion refers to the high-speed scouring of the
tube surface by hard particles such as SiO2, Fe2O3, and Al2O3 carried in fly ash, resulting in thinning of the tube wall and tube rupture.
1. Causes of Failure:
(1) Hard particles carried in fly ash from coal-fired boilers.
(2) Excessive flue gas velocity or local flue gas velocity (e.g., when ash accumulates, the flue gas passage becomes smaller, increasing the flue gas flow velocity).
(3) Non-uniform distribution of ash concentration in the flue gas, with locally high ash concentration.
2. Fault Location: Commonly occurs at bends near the entrance of the superheater flue gas, exit tubes, and tubes with uneven lateral pitch.
3. Rupture Characteristics:
(1) Thinning of the tube wall at the fracture, resembling a blade.
(2) Smooth erosion surface, appearing gray.
(3) No change in the metallographic structure, and the tube diameter generally does not expand.
4. Prevention Measures:
Fly ash erosion is typically prevented by reducing the number and velocity of fly ash impacts on the tubes or by increasing the abrasion resistance of the tubes. Measures include changing the flow direction and velocity field by adding screens, installing dust removal devices inside the furnace, preventing local flue gas velocity from becoming too high, and installing anti-wear cover plates on the surfaces of tubes prone to erosion. Additionally, appropriate furnace types for the type of coal used should be selected, coal fineness should be improved, combustion should be adjusted properly, and complete combustion should be ensured.
Corrosion Fatigue
Corrosion fatigue is mainly caused by the chemical properties of water, where the oxygen content and pH value are the primary factors influencing corrosion fatigue. The medium inside the tube undergoes electrochemical reactions due to the depolarization effect of oxygen, leading to pitting corrosion at the rupture of the passivation film inside the tube. Under the combined action of corrosion medium and cyclic stress (including internal stress caused by start-stop cycles and vibration), corrosion fatigue tube rupture occurs.
1. Causes of Failure:
(1) Stress concentration at bends promotes pitting.
(2) Thermal shock at bends induces fatigue cracks in the neutral zone of the bend inner wall.
(3) Accumulation of water at the bottom of bends during shutdown.
(4) Presence of small amounts of alkali or free carbon dioxide in the medium inside the tube.
(5) Excessive device startup and chemical cleaning cycles.
2. Fault Location: Commonly occurs on the water side, then extends to the outer surface. Pitting or crater-like corrosion occurs on the inner wall of superheater bends, mainly during shutdowns due to corrosion fatigue.
3. Rupture Characteristics:
(1) Pitting or crater-like corrosion occurs on the inner wall of superheater bends, with a typical corrosion shape resembling shells.
(2) During operation, the corrosion fatigue product is black magnetic iron oxide, firmly bonded to the metal; during shutdowns, the corrosion fatigue product is brick-red iron oxide.
(3) There is no change in the metallographic structure in the pitting and crater-like corrosion areas.
(4) Corrosion pits develop along the tube axis direction, and cracks are transverse with relatively wide and blunt surfaces, with oxide scales present at the crack locations.
4. Prevention Measures:
To prevent oxygen corrosion, attention should be paid to shutdown protection. During initial boiler commissioning, chemical cleaning should be performed to remove rust and dirt, forming a uniform protective film on the inner wall. During operation, ensure that the water quality meets standards, and appropriately decrease the pH value or increase the chloride and sulfate content in the boiler.
Stress Corrosion Cracking
This refers to the phenomenon of pipe rupture caused by static tensile stress or residual stress under the conditions of chloride ion-containing medium and high temperature.
1. Causes of Failure:
(1) The presence of chloride ions in the medium, high temperature environment, and high tensile stress are the three basic conditions for stress corrosion cracking.
(2) Stress corrosion cracking can also occur under the action of moist air.
(3) During startup and shutdown, water containing chloride and oxygen may enter the steel pipe.
(4) Thermal stress caused by residual stress generated during processing and welding.
2. Fault Location: Stress corrosion cracking commonly occurs in the high-temperature zone pipes and sampling pipes of the superheater.
3. Rupture Characteristics:
(1) The rupture exhibits brittle morphology, generally appearing as transgranular stress corrosion fracture.
(2) Corrosion medium and corrosion products may be present on the rupture surface.
(3) The cracks have branching characteristics, originating from the corrosion pits, and multiple crack sources may exist.
4. Prevention Measures: To prevent stress corrosion cracking, attention should be paid to removing the residual stress of the pipe; strengthen protection during installation, and pay attention to corrosion prevention during shutdowns; prevent leaks in the condenser and reduce the content of chloride ions and oxygen in steam.
Thermal Fatigue
Thermal fatigue refers to the fatigue damage that occurs in boiler tubes due to the repeated occurrence and disappearance of thermal stress caused by boiler start-ups and shutdowns, as well as the alternating stress caused by vibration.
1. Causes of Failure:
(1) Substances such as sulfur (S), sodium (Na), vanadium (V), and chlorine (Cl) in the flue gas promote corrosion fatigue damage.
(2) Water flushing of the furnace chamber results in a rapid change in pipe wall temperature, leading to thermal shock.
(3) Overheating leads to a significant decrease in the fatigue strength of the pipe material.
(4) Units designed for basic load operation are changed to peak load operation.
2. Fault Location: Failure commonly occurs on the outer surface of pipes in the high-heat flow region of the superheater.
3. Prevention Measures: Measures to prevent thermal fatigue include modifying the structural components to reduce the concentration of alternating stress, adjusting operating parameters to minimize the magnitude of pressure and temperature gradients, considering thermal expansion and contraction during intermittent operation in the design phase, avoiding mechanical vibration during operation, adjusting the flow distribution between tubes to reduce thermal deviations and the temperature of adjacent tube walls, and appropriately increasing the temperature of the flushing medium to reduce thermal shock.
High-Temperature Corrosion
Compounds with low melting points such as Na2SO4 destroy the protective oxide layer on the outer surface of pipes. They interact with metal components, generating loose structured oxides at the interface, resulting in pipe thinning and eventual rupture.
1. Causes of Failure:
(1) Fuels containing low melting point compounds such as V, Na, and S.
(2) Excessive local flue gas temperatures, causing corrosive low melting point compounds to adhere to metal surfaces, leading to high-temperature corrosion.
(3) The presence of coverings in the corrosion area, reducing gases in the flue gas, and direct flue gas impingement, all contribute to the generation of high-temperature corrosion.
2. Fault Location: High-temperature corrosion commonly occurs on the outer surface of superheaters, as well as on the fire-side surfaces of hangers and positioning components.
3. Rupture Characteristics:
(1) Cracks originate on the outer surface of the pipe, exhibiting brittle thick-lipped fractures.
(2) Longitudinal cracking with shallow grooves and corrosion pits at approximately 10 o'clock and 2 o'clock positions, resembling mouse bites.
(3) The outer surface shows significant thinning, although unevenly, without noticeable swelling.
(4) Presence of oxide scales on the outer surface, displaying a crocodile skin pattern, with deposits of yellow, white, and brown-colored substances. The scale is loose, with molten-like deposits, and the innermost layer of oxide appears as hard and brittle black-gray.
4. Prevention Measures: Methods to prevent high-temperature corrosion include controlling local flue gas temperatures to prevent the adhesion of corrosive low melting point compounds to metal surfaces, optimizing flue gas flow paths to minimize thermal deviations, adding additives such as CaSO4 and MgSO4 in coal-fired boilers, employing surface protective coatings or installing baffles in areas prone to high-temperature corrosion, and removing deposits from the surface of pipes.
Dissimilar Metal Welding
Failure occurs at the welding joint due to the mismatch in creep strength between two different metals and carbon migration near the weld interface, leading to fracture failure at the interface of dissimilar metal welding. The significant difference in creep strength between the two metals is the primary cause of early failure in dissimilar metal welding.
Fault Location: Failure commonly occurs at the welding joints of two different metals at the outlet of the superheater. When the creep strength of the weld equals that of one metal, fracture occurs at the weld interface of the other metal.
Prevention Measures: Stable operation is the most critical factor in reducing failures in dissimilar metal welding. When welding two different metals, an intermediate section with intermediate creep strength should be added to significantly reduce the difference in creep strength on both sides of the weld interface. Different welding rods with properties matching those of the two metals should be selected for each side of the intermediate section.
Quality Control Errors
Quality control errors refer to damage caused by external errors during manufacturing, installation, and operation. The reasons for quality control errors include: repair damage, chemical cleaning damage, pipe material defects (such as inappropriate or substandard pipe materials), welding defects, etc. Strengthening the management of power plant operation, maintenance, and various systems is an effective way to prevent quality control errors.
