Boiler tube burst refers to the phenomenon where the water-cooled wall tubes, convective tubes, and economizer tubes in the heat exchange surface of a boiler burst due to various factors such as overheating, erosion, and corrosion during operation, leading to high-temperature boiler water leakage and rendering the boiler unable to operate normally. Through years of theoretical accumulation and on-site practice, it has been found that boiler tube bursts are mainly caused by the following reasons:

Prolonged Overheating

Failure Mechanism: Prolonged overheating refers to the situation where the tube wall temperature remains above the design temperature but below the material's lower critical temperature for an extended period. Although the temperature deviation is not significant, the boiler tubes experience carbide spheroidization, thinning of the tube wall due to oxidation, a decrease in long-term strength, accelerated creep rate, uniform expansion of the tube diameter, ultimately leading to brittle fracture and tube burst at the weakest part of the tube. Consequently, the service life of the tubes is shorter than the designed service life. The higher the degree of overheating, the shorter the lifespan. Under normal conditions, prolonged overheating and tube burst mainly occur on the outer circumference of high-temperature superheaters and the fire-facing side of high-temperature reheaters. In abnormal operating conditions, prolonged overheating and tube burst may occur on the fire-facing side of low-temperature superheaters and reheaters. Based on the working stress level, prolonged overheating and tube burst can be categorized into three types: high-temperature creep, stress oxidation crack, and oxidation thinning.

1.Causes of Failure:

Uneven distribution of steam-water flow inside the tubes;

Localized high thermal load inside the furnace;

Internal scaling of tubes;

Foreign object blockage in the tubes;

Misuse of materials;

Initial design flaws.

2. Failure Locations:

High-temperature creep and stress oxidation crack primarily occur on the outer circumference and fire-facing side of high-temperature superheaters; under abnormal conditions, low-temperature superheaters may also experience prolonged overheating and tube burst;

Oxidation thinning mainly occurs in reheaters.

3. Characteristics of Tube Rupture:

The rupture morphology of tubes under prolonged overheating exhibits typical characteristics of creep fracture. The tube rupture shows brittle fracture features, with rough and irregular blunt edges at the rupture, and minimal thinning of the tube wall at the rupture site. The tube wall experiences creep expansion, and the degree of diameter enlargement depends on the material of the tubes. Carbon steel tubes show significant diameter enlargement. For instance, ruptured tubes of 20G steel in high-pressure boilers may expand up to 15% of the tube diameter, while tubes of 12CrMoV steel in high-temperature superheaters only show about 5% diameter enlargement.

(1) High-temperature Creep:

The creep expansion of the tubes significantly exceeds the specified value of metal supervision, and the edges of the rupture are relatively blunt;

There are dense longitudinal cracks around the rupture, and the oxide scale around the rupture is thicker than that in short-term overheating tube burst. The lower the overheating temperature and the longer the duration, the thicker the oxide scale and the wider the distribution range of longitudinal cracks;

Creep voids and micro-cracks exist over a large area around the rupture;

The surface of the tube on the fire-facing side has completely spheroidized;

Recrystallization may occur in the structure of the tube at bends;

There is a significant difference in the degree of carbide spheroidization between the fire-facing side and the backfire side, with carbides on the fire-facing side being fully spheroidized.


(2) Stress Oxidation Crack Type:

The creep expansion of the tubes is close to or lower than the specified value of metal supervision, and the edges of the rupture are relatively blunt, presenting a typical thick-lipped appearance;

Multiple longitudinal cracks exist on the outer wall oxide layer near the rupture, with the distribution range extending throughout the fire-facing side. The oxide scale on the inner and outer walls is thicker than that in short-term overheating tube bursts;

c. Longitudinal stress oxidation cracks propagate from the outer wall to the inner wall, with the possibility of minor voids at the crack tips;

Severe spheroidization occurs on both the fire-facing side and the backfire side, accompanied by a decrease in the strength and hardness of the tube material;

The oxide scale on the inner and outer walls of the tubes undergoes stratification;

Elements such as S, Cl, Mn, and Ca in the combustion products deposit and accumulate in the outer wall oxide layer.

(3) Oxidation Thinning Type:

Both the inner and outer walls of the tubes on the fire-facing side and the backfire side develop oxide scales with a thickness of 1.0 to 1.5 mm;

The tube wall undergoes severe thinning, reaching only 1/3 to 1/8 of the original wall thickness;

Both the inner and outer wall oxide scales exhibit stratification, with uniform oxidation. The inner layer of the inner wall oxide scale shows a ring-shaped pattern;

Complete spheroidization occurs on the fire-facing side, while severe spheroidization occurs on the backfire side, accompanied by a decrease in strength and hardness;

Elements such as S, Cl, Mn, and Ca in the combustion products deposit and accumulate in the outer wall oxide layer, promoting outer wall oxidation.

4. Preventive Measures:

For the high-temperature creep type, improvement of the heating surface, rational distribution of medium flow, improvement of combustion inside the furnace, prevention of high combustion center, and chemical cleaning to remove foreign objects and deposits can be employed for prevention. For the stress oxidation crack type, damaged tubes nearing their design life should be replaced. For the oxidation thinning type, enhancement of the protection measures for the superheater is necessary.


Short-Term Overheating

1. Causes of Failure:

(1) Uneven distribution of working fluid inside the superheater tubes. In tubes with lower flow rates, the cooling capacity of the working fluid on the tube wall is poor, leading to an increase in tube wall temperature and resulting in overheating.

(2) Excessive local heat load (or deviation of the combustion center) inside the furnace, causing the temperature of the nearby tube wall to exceed the allowable design value.

(3) Severe scaling inside the superheater tubes, resulting in overheating of the tube wall.

(4) Foreign objects blocking the tubes, preventing effective cooling of the superheater tubes.

(5) Misuse of steel materials. The misuse of inferior-grade steel can also cause short-term overheating. With increasing temperature, the allowable stress of inferior-grade steel decreases rapidly, resulting in insufficient strength and tube rupture.

(6) Oxide scale peeling off from the inner wall of the tubes, causing blockage at the downstream bend.

(7) Improper injection of cooling water during low-load operation, leading to water blockage inside the tubes and causing local overheating.

(8) Abnormal furnace gas temperature.

2. Fault Location: Commonly occurs on the fire-facing surfaces of the superheater tubes directly in contact with flames and directly receiving radiant heat.

3. Rupture Shape:

(1) Large plastic deformation at the rupture, with significant tube diameter expansion and blade-like thinning of the tube wall.

(2) Generally, the rupture is larger and trumpet-shaped.

(3) The rupture exhibits a typical thin-lipped explosive rupture.

(4) Microscopically, the rupture surface consists of numerous dimples.

(5) The hardness of the tube material around the rupture significantly increases.

(6) The thickness of the oxide scale on the inner and outer walls around the rupture depends on the severity of the long-term overheating before the short-term overheating burst. The more severe the long-term overheating, the thicker the oxide scale.

4. Prevention Measures: 

Methods to prevent short-term overheating include improving the heating surface to ensure rational distribution of medium flow, stabilizing operating conditions, improving combustion inside the furnace to prevent deviation of the combustion center, performing chemical cleaning, removing foreign objects and deposits, and preventing misuse of steel materials by promptly taking corrective measures when misuse is detected.