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Analysis of metal accidents of main components of steam turbine and boiler 2020-03-26


First, the accident analysis of the heating surface of the boiler tube

 

The heating surface of the boiler tube is long-term work under the action of high temperature, stress and corrosive medium. When the steel of the tube cannot bear the load of its working state, different forms of damage will occur and cause accidents. The common accidents of the heating surface tubes of boilers in thermal power plants are mainly the following types: long-term over-temperature burst tubes, short-term over-temperature burst tubes, poor-quality tubes and corrosion thermal fatigue damage.

 

一 (1) Long time overtemperature burst

 

Over-temperature refers to the operation of metal materials at temperatures exceeding the rated temperature. Rated temperature refers to the maximum allowable temperature of the steel during its design life, and it can also refer to the rated temperature during work. As long as one of the above temperatures is exceeded, it is over temperature operation.

 

Due to the increased atomic diffusion of long-term over-temperature pipe steel, the microstructure of the steel changes, which accelerates the creep rate and reduces the durable strength. Therefore, the pipe cannot be blasted before it reaches the design life. Most of the tube explosions occur on the fire side and the pipe elbow of the outlet section of the high temperature superheater. Water-cooled wall tubes, condensate tubes and economizers also occur from time to time.

 

 In the process of long-term over-temperature tube explosion, corrosive media such as steam and fume played an accelerating role. When the temperature of the tube wall exceeds its critical oxidation temperature, steam and smoke will cause a thick layer of iron oxide on the tube wall; when the tube is swollen, this layer of iron oxide will crack in the direction perpendicular to the stress; Bare metal undergoes stress corrosion under the action of tensile stress and steam or smoke, which accelerates crack growth and eventually causes bursting. Therefore, the fracture has brittle fracture characteristics, and corrosion products often exist in the crack.

 

(Two) short-time overtemperature burst

 

During the operation, the heating conditions of the boiler tubes deteriorated due to cooling conditions and dry burning, which caused the tube wall temperature to rise suddenly in a short period of time, and the temperature reached above the critical point (Ac1). The tube bursts when it breaks. This type of tube is called a short-time overtemperature tube.

 

Most short-term overtemperature tube explosions occur near the fire wall and condensate tubes near the combustion zone of the cold-wall tube and near the burner. Occasionally, economizers and screen superheaters of some high-pressure boilers also occur.

 

Because the temperature of the tube wall is higher than Ac1, and sometimes even higher than Ac3, the steam and water spray during tube bursting is like quenching at different degrees. Therefore, the structure at the break is generally low martensite or bainite. ; Superheater tube break may also be pearlite and ferrite. Obviously, the hardness of the pipe around the breach will increase significantly. In addition to improper structural design of the over-temperature burst tube, it is mainly caused by overload operation, improper operation, or clogging of dirt in the tube. Overload operation will generally increase the outlet temperature of the convection superheater, exacerbating the over-temperature phenomenon, causing the pipe creep to accelerate; abnormal startup, which will cause drastic changes in combustion, fast boosting speed, or fire extinguishing and firing in the furnace will cause pipe overheating Warm; clogged with dirt or salt scale inside the tube, it will cause poor circulation of steam and water, causing local overheating of the tube and quickly causing tube explosion.

 

三 (3) Tube burst caused by bad material

 

 Blasting of poor material refers to the early damage to the pipe caused by the wrong steel or the use of defective steel. Due to the wrong material, it is actually an over-temperature operation. According to the Larson-Miller equation, over-temperature operation will greatly shorten the life of the steel pipe, and some even burst for thousands of hours. If the material itself has defects such as cracks, severe decarburization, or inclusions, or when steel pipes with folds, scars, and cracks are used during installation and maintenance, the strength of the pipe will be severely weakened, and the defect parts are prone to stress during high temperature operation. Concentration, which causes cracks to expand and defects to expand, leading to tube bursts.

 

(4) Corrosive thermal fatigue crack damage

 

The vapor layer of the boiler heating surface tube, the economizer tube steam plug, the superheater with water, and the temperature-reducing and pressure-reducing valve gap opening will cause temperature fluctuations, cause alternating thermal stress, and generate thermal fatigue cracks. In addition, under the action of corrosive media, fatigue cracks on these pipes are particularly easy to occur in notched areas with high corrosion rates such as rough surfaces, scratches, and corrosion pits, so they are called corrosive thermal fatigue cracks. Corrosive thermal fatigue cracks are generally distributed in clusters and are perpendicular to the direction of stress. The inner wall of the pipe is a transverse annular crack with short cracks and a fractured brittle fracture with fatigue characteristics. During the operation of the boiler heating surface, the pipe wall is in direct contact with high temperature smoke, water and steam, and other corrosion phenomena will also occur. , Causing premature rupture of the pipe. If the air preheater works in the open air, there will be low temperature corrosion damage due to the presence of SO2 in the flue gas.

 

Analysis of common accidents of main parts of steam turbine

 

(I) Steam turbine blade accident analysis

 

The damage form of steam turbine blades is mainly fatigue fracture. Due to the harsh working conditions of the blades, the complicated stress conditions, the frequent occurrence of fracture accidents, and the serious consequences, the analysis and research of blade fracture accidents has always received special attention. According to the nature of blade fracture, it can be divided into short-term overload fatigue damage, long-term fatigue damage, high-temperature fatigue damage, stress fatigue damage, corrosion fatigue damage, and contact fatigue damage.

 

1, Overload fatigue damage

 

This kind of damage refers to mechanical fatigue damage that occurs when the blade is subjected to a large external stress or a large excitation force, and the number of vibrations is less than 107 times. If the blade is subject to large stress due to water hammer, or large low-frequency excitation forces such as vibration caused by uneven rotors and cyclic forces due to poor installation, when these forces cause the blade to resonate, the blade will quickly break.

 

The macro characteristics of short-term overload fatigue damage of the blade are: the section is rough, the fatigue front line (that is, the shell pattern) is not obvious, and the area of ??the fatigue area on the section is smaller than the final static tear area; Zigzag pattern.

 

1. The main method to prevent short-term overload fatigue damage is to prevent water hammer, make frequency modulation to eliminate low-frequency resonance, and run under normal cycle.

 

2, long-term fatigue failure

 

Long-term fatigue damage refers to a kind of mechanical fatigue damage that occurs when the blade is subjected to stress below the fatigue strength limit and the number of stress cycles is much higher than 107 times.

 

The causes of long-term fatigue damage are: resonance damage caused by blades or blade groups under high-frequency excitation force; fatigue damage caused by local stress concentration at blade surface defects; low-frequency operation and overload operation cause certain levels Increased blade stress leads to premature damage and more. Long-term fatigue damage is most common in power plant blade fracture accidents.

 

The method to prevent long-term fatigue damage is to avoid the high-frequency excitation force resonance range according to regulations, improve the blade processing quality and improve the operating conditions. Such as to prevent low frequency and overload operation, to prevent corrosion and water hammer.

 

3.High temperature fatigue damage

 

High-temperature fatigue damage refers to a form of damage between creep caused by static stress and fatigue caused by dynamic stress, which is formed by the combined effect of creep and fatigue. The source of the crack is a creep phenomenon. The fracture property is a combination of persistent fracture and fatigue fracture, and it is often accompanied by changes in the material structure.

 

High-temperature fatigue damage cracks are basically transgranular, with a shell pattern on the macroscopic fracture surface and a thick oxide scale on the microscopic fracture surface.

 

High-temperature fatigue damage occurs in the first stage blades of high-pressure cylinders, the first stage blades of intermediate-pressure cylinders in reheating steam turbines, and the governor-stage blades of medium-pressure steam turbines.

 

 The main measures to prevent high-temperature fatigue damage are: choose a metal with good high-temperature performance to manufacture blades that work at high temperature, prevent blade resonance, and prevent radial and axial phase friction of the blade.

 

4. Stress corrosion damage

 

The main causes of stress corrosion are: first, the metal grain boundaries segregate, carbides precipitate, and chromium-depleted areas appear, causing the grain boundaries to corrode; second, stress effects; and then, high concentration salt corrosion. Stress corrosion mainly occurs on the final blades made of 2Cr13 steel. The fracture morphology is granular, the micro-morphology is a boundary crack, there are sliding steps on the cross section, and there are small corrosion pits.

 

The only measures to prevent stress corrosion damage of the blade are: improve the quality of soda, improve the material of the blade, reduce the dynamic stress of the blade, etc.

 

5. Corrosion fatigue damage

 

Corrosion fatigue damage is the fatigue damage caused by the alternating stress in the blade in the corrosive medium. If the damage is dominated by mechanical fatigue, the crack develops rapidly and the crack is of the transcrystalline form; if the damage is dominated by stress corrosion, the crack develops slowly and the crack is mainly along the crystal form.

 

 The main measures to prevent corrosion fatigue damage are: improving the corrosion resistance of the blade material; reducing the level of alternating stress; and improving the quality of soda.

 

6. Contact fatigue damage

 

Contact fatigue damage is a kind of mechanical damage caused by the loose roots of the blades, and the roots of the blades participate in vibration, causing reciprocating traces of relative frictional movement between the blade roots or the contact surfaces of the blades and the turbine. As the friction surface material crystal slips and hardens, many parallel microcracks are generated in the hardened area, and they continue to expand, which causes fatigue fracture. The coexistence of friction crack and friction hardening is the main basic feature of contact fatigue damage. Friction hardening and friction cracks only exist on the contact surface.

 

 The main measures to prevent contact fatigue are: improve the closeness of the blade contact surface, increase the contact area to prevent the stress concentration of the contact point contact, and eliminate or weaken the vibration force of the FM blade.

 

(II) Metal Accident Analysis of Steam Turbine Rotor

 

The rotor must withstand bending stress caused by torque and self-weight, thermal stress of temperature gradient and temperature change, centrifugal force, thermal jacket force, vibration force and short-circuit force distance of the generator during operation. Its working conditions are very harsh.

 

Metal accidents of steam turbine rotors are mainly deformation and cracking of impeller and main shaft (rotor).

 

1, Plastic deformation of main shaft (rotor)

 

When the turbine leaves the factory with excessive residual stress, improper transportation, installation, and insufficient warming up during operation, friction between dynamic and static parts, water hammer, and full water, etc., may cause permanent deformation of the large shaft. Speed ??cannot be forced, but should be straight after stopping.

 

Straight shaft method: Local heating reverse deformation alignment can be used for small carbon steel rotors; the "relaxation method" is mostly used for high-power alloy steel rotors.

 

2, the fracture of the rotor

 

Rupture of the rotor will cause serious accidents, which should be taken seriously. The cause of the fracture was the appearance of cracks. The first macroscopic crack found on the rotor is often used as a sign of the end of the turbine's working life in large steam turbines. The main causes of cracks in the rotor are as follows:

 

(1) Cracks appear under the combined action of thermal alternating stress (low-frequency thermal stress) and creep;

 

(2) Reasons such as a small transition fillet at the cross-section boundary and the presence of knife marks will cause mechanical stress or thermal stress to concentrate, and cracks will occur under the action of alternating stress;

 

3 (3) Poor material and severe metallurgical defects cause cracks;

 

(4) Damage caused by improper operation. Such as starting, stopping, changing load, etc., the temperature change rate and temperature change are too large, causing excessive thermal stress.

 

3. Cracking of impeller

 

Cracking of the impeller occurs mainly in the low-pressure stage. Due to the large diameter of the impeller and the large centrifugal force, cracks are prone to occur in the keyway due to stress concentration during long-term operation. If the crack develops to a certain depth, the entire impeller will fly apart. Impeller cracking is related to the following factors:

 

(1) The machining quality at the keyway is poor, and cracks tend to develop and develop at the stress concentration points;

 

(2) Impeller material has poor performance, low toughness and plasticity, large brittleness, and accelerated crack growth;

 

(3) Improper maintenance after shutdown, or stress corrosion caused by water corrosion.

 

 Measures to prevent impeller cracking: Pay attention to maintenance after shutdown to prevent corrosion; improve metallurgical and processing quality; strengthen inspection and inspection.

 

(3) Deformation and cracks of the cylinder

 

The thickness of the cylinder cross section varies greatly, and the shape of the inlet end is complicated, especially the thickness at the flange is very large. Therefore, during the operation, the temperature difference between the inner and outer walls of the cylinder and the flange is very different, and the thermal stress generated is very large. Due to thermal alternating stress, at the same time, the cylinder is also subjected to steam pressure and the gravity of the stationary part, the working conditions are very harsh, and due to the complex shape, wide thickness difference, large size and other reasons, casting defects inevitably exist, so, Cylinders are prone to metal accidents of deformation and cracking.

 

 The main causes of cylinder deformation are as follows:

 

1 (1) During operation, the temperature difference between the inside and outside of the cylinder wall and inside and outside of the flange is large, causing air leakage at the flange joint surface; the excessive temperature difference between the upper and lower cylinders causes friction or vibration between the dynamic and static parts;

 

(2) Improper stress relief annealing of the cylinder, or full water and water hammer during operation, will cause deformation;

 

(3) The cylinder works at high temperature, the temperature of each part is different, and the creep speed is different, which causes deformation.

 

 The main reasons for the cracking of the cylinder are as follows:

 

1 (1) Cylinders run at high temperature for a long time, creep occurs, and brittleness increases;

 

(2) Cracks, white spots, and slag inclusions in the casting during metallurgical processes are the source of creep and thermal fatigue cracks;

 

(3) Improper heat treatment results in non-uniform material structure, which reduces the lasting strength and lasting plasticity;

 

(4) The long-term high-temperature operation causes changes in the cylinder material structure; low-frequency thermal stress during operation; the combined action of force and creep is more prone to cracks.

 

For the cylinders with cracks, the cracks can be completely removed and repaired to eliminate them.


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