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Analysis of accidents prone to occur on the heating surface of boiler steel tubes

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Boiler heating surface steel pipes work for a long time under the action of high temperature, stress, and corrosive media. When the steel pipes cannot withstand the load of their working conditions, different forms of damage will occur and cause accidents. Common accidents of steel pipes on the heating surface of boilers in thermal power plants mainly include the following types: long-term over-temperature pipe explosion, short-term over-temperature pipe explosion, pipes with poor material and corrosion, and thermal fatigue damage.

(1) Long-term over-temperature pipe explosion
Overtemperature refers to the operation of metal materials above the rated temperature. The rated temperature refers to the maximum allowable temperature of the steel operating under the design life, and can also refer to the rated temperature during operation. As long as one of the above temperatures is exceeded, it is an over-temperature operation. Steel pipes that have been overheated for a long time will intensify atomic diffusion, causing changes in the steel structure, accelerating creep, and reducing lasting strength. Therefore, the steel pipes will burst and be damaged before they can reach the design life. Tube explosions mostly occur on the fire side of the outlet section of high-temperature superheater tubes and at steel pipe elbows. Water-cooled wall tubes, slag condensation tubes, and economizers also occur from time to time. During the long-term over-temperature pipe explosion process, corrosive media such as steam and flue gas play an accelerating role. When the pipe wall temperature exceeds its oxidation critical temperature, steam, and flue gas will produce a thicker layer of iron oxide on the pipe wall; when the steel pipe expands, this layer of iron oxide will crack in the direction perpendicular to the stress; then it will be re-formed. Exposed metal will produce stress corrosion under the action of tensile stress and steam or smoke, accelerating crack expansion and eventually leading to bursting. Therefore, the fracture has brittle fracture characteristics, and corrosion products often exist in the crack.

(2) Short-term over-temperature pipe explosion
During operation, the cooling conditions of the steel pipe on the heating surface of the boiler deteriorate and dry burning causes the pipe wall temperature to rise suddenly in a short period of time. The temperature reaches above the critical point (Ac1), the tensile strength of the steel drops sharply, and the stress of the steel pipe exceeds the yield limit, causing shearing. The pipe bursts due to rupture. This kind of pipe burst is called a short-term over-temperature pipe burst.
Short-term over-temperature tube explosions mostly occur near the combustion zone of cold wall tubes and on the fire side near the burner and the slag tube. They also occasionally occur in economizers and screen superheaters of some high-pressure boilers. Since the pipe wall temperature of short-term overtemperature is higher than Ac1, and sometimes even higher than Ac3, the steam and water injection during pipe explosion is like quenching to varying degrees. Therefore, the structure at the breach at this time is generally low martensite or bainite. ; Superheater tube breaks may also be pearlite and ferrite structures. Obviously, the hardness of the pipe around the break will increase significantly. In addition to improper structural design, over-temperature pipe explosions are mainly caused by overload operation, improper operation, or clogging of dirt in the pipe. Overload operation will generally increase the outlet temperature of the convection superheater, exacerbating the over-temperature phenomenon and accelerating the creep of the steel pipe; abnormal starting will cause drastic changes in combustion, rapid pressure increase, or fire extinguishing and blasting in the furnace will cause the steel pipe to overheat. Temperature; The dirt or salt scale in the pipe will cause poor circulation of steam and water, causing local overheating of the steel pipe and quickly leading to pipe explosion.

(3) Boiler steel pipe explosion caused by poor material
Pipe bursting with poor material refers to the incorrect use of steel or the use of defective steel, causing premature damage to the steel pipe. Due to the use of the wrong materials, it is actually an over-temperature operation. According to the Larson-Miller equation, over-temperature operation will greatly shorten the life of steel pipes, and some may even burst after thousands of hours of operation. If the material itself has defects such as cracks, severe decarburization or inclusions, or steel pipes with folds, scars, or cracks are used during installation and maintenance, the strength of the steel pipe will be severely weakened, and the defective parts are prone to stress during high-temperature operation. Concentration will cause cracks to expand, defects to expand, and lead to pipe bursting. When a defective steel pipe is blasted, the fracture edge can often be divided into two parts: the defective part has a rough fracture edge and a brittle fracture (the fracture defect is open); the non-defective part has a plastic fracture.

(4) Corrosive thermal fatigue crack damage
Steam-water stratification in the boiler heating surface steel pipe, steam plug in the economizer pipe, water in the superheater, intermittent opening of the desuperheating and pressure reducing valve, etc., will cause temperature fluctuations, causing alternating thermal stress and thermal fatigue cracks. Moreover, under the action of corrosive media, fatigue cracks on these steel pipes are particularly likely to occur in notched areas with high corrosion rates such as surface roughness, scratches, corrosion pits, etc., so they are called corrosive thermal fatigue cracks. Corrosive thermal fatigue cracks are generally distributed in clusters in a single row and are perpendicular to the stress direction. There are transverse annular cracks on the inner wall of the pipe, the cracks are short, and the fracture is a brittle fracture with fatigue characteristics. During the operation of the boiler heating surface steel pipe, the pipe wall is in direct contact with high-temperature flue gas, water, and steam, which will also produce other corrosion phenomena, causing premature rupture and damage to the steel pipe. If an air preheater is operated in the open air, low-temperature corrosion damage will occur due to SO2 in the flue gas.

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