Boiler heating surface 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 on heating surface pipes 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.
Pipe steel that has been overheated for a long time will intensify atomic diffusion, causing changes in the steel structure, accelerating creep, and reducing lasting strength. Therefore, the pipe will burst and be damaged before it reaches the design life. Tube explosions mostly occur on the fire-facing side of the outlet section of high-temperature superheater tubes and at 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 temperature of the pipe wall exceeds its critical oxidation temperature, steam, and flue gas will produce a thicker layer of iron oxide on the pipe wall; when the 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
The cooling conditions of the boiler heating surface tubes deteriorate and dry burning occurs during operation, causing the tube wall temperature to rise suddenly in a short period. The temperature reaches above the critical point (Ac1), the tensile strength of the steel drops sharply, and the tube stress 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 on the fire side near the burner and on the slag condensation 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. 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 blockage 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 tube; abnormal starting, drastic changes in combustion, rapid pressure increase, or fire extinguishing and blasting in the furnace will cause the tube to overheat. Warmth; dirt or salt scale in the pipe will cause poor circulation of steam and water, causing local overheating of the pipe and quickly leading to pipe explosion.
(3) Pipe explosion caused by poor material
A burst pipe made of poor material refers to the incorrect use of steel or the use of defective steel that causes early damage to the pipe.
Due to the use of the wrong materials, it is 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 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 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 defective part has a plastic fracture.
(4) Corrosive thermal fatigue crack damage
Steam-water stratification in boiler heating surface tubes, steam plugs in economizer tubes, water in superheaters, the intermittent opening of desuperheating and pressure-reducing valves, etc., will all cause temperature fluctuations, causing alternating thermal stress and thermal fatigue cracks. Moreover, under the action of corrosive media, fatigue cracks on these 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 tube, the tube wall is in direct contact with high-temperature flue gas, water, and steam, which will also cause other corrosion phenomena, causing premature rupture and damage of the tube. If an air preheater is operated in the open air, low-temperature corrosion damage will occur due to SO2 in the flue gas.
Post time: Jan-19-2024