Analysis of the causes of the leakage of the four tubes of the boiler and countermeasures

Causes and precautions for leakage of burst pipes in superheaters and reheaters

The root causes affecting the superheater and reheater burst pipe are: overheating, wear, corrosion, welding quality, etc., combined with the boiler overheating and reheater burst pipe of each power plant, it can be seen that the burst pipe due to metal overheating in the superheating and reheater burst pipe accounts for about 30%, the wear accounts for about 15%, the corrosion accounts for about 10%, the welding quality accounts for about 30%, and other reasons account for 15%, so the heating surface is super-warm and the difference in welding quality is the main reason for the overheating and reheater burst Pipe Below we mainly analyze the cause of the burst pipe from these two aspects.

1． 1 Poor pipe quality or poor welding quality caused by overheating and reheater burst pipe 1. 1． 1 Poor pipe quality If the pipe itself has defects such as delamination and slagging, the operation is affected by temperature and pressure, and the expansion of the defect will lead to the burst pipe of the superheater tube. For example, the low temperature superheater of the No. 1 furnace of a factory has a burst pipe in use, and the burst pipe crack is peach-shaped, and the opening has been significantly thinned due to bursting. From the metallographic analysis of the burst site, it can be seen that the bursting of the superheater tube is due to the poor material of the furnace tube, the uneven organization, the incomplete normalizing tissue, and there are a large number of microscopic holes in the base of the base material, which affect the strength of the material and eventually lead to the bursting of the superheater tube. 1． 1． 2 Poor welding quality. In the manufacture or maintenance, due to the poor quality of welding, the presence of porosity, slag, weld tumors, etc. in the weld seam will lead to frequent bursting of pipes. A factory No. 1 furnace low temperature reheater in the operation of the leakage, from the burst pipe macroscopic inspection shows that the weld quality of the weld is poor, there are a large number of weld tumors at the root of the weld, the leakage point is mostly distributed in the weld fusion line and the heat affected zone, the metallographic inspection results show that the metallographic structure of the pipe bundle is normal, the weld seam, the fusion line, the metallographic organization of the heat affected area is a coarse Wei's tissue ferrite, the tissue is extremely coarse, it is not normal organization, after the above analysis, the burst pipe is due to the poor welding quality of the pipe weld, In the weld fusion zone, there are coarse Wei's structure, a large number of non-metallic inclusions and other defects.

1． 2 Over-temperature of the heating surface caused by overheating and reheating the tube burst the metal exceeds its rated temperature, there are short-term ultra-temperature and long-term over-temperature two situations, so the overheating of the heating surface of the burst pipe has short-term overheating and long-term overheating two types of phenomena, after the heating surface overheating, the pipe metal exceeds the allowable limit temperature, the internal organization changes, reducing the permissible stress, the pipe produces plastic deformation under the action of internal stress, and finally leads to the over-temperature burst pipe.

1． 2． 1 Short-term overheating of the heated surface. When the heat transfer situation inside the heating surface of the boiler deteriorates seriously in a short period of time, the wall temperature rises sharply, which greatly reduces the strength of the steel, which will cause the metal to overheat and cause the pipe to burst in a short period of time. The causes of short-term overheating are: serious uneven distribution of soda flow in the tube, excessive local heat load in the furnace, serious scaling inside the pipe, serious blockage of foreign matter, and misuse of steel.

1． 2． 2 The heating surface is overheated for a long time. Boiler heating surface pipe due to thermal deviation, hydrodynamic deviation or fouling, blockage, misuse of steel and other reasons, the working fluid in the tube heat exchange is poor, the metal is not very large for a long time in the over-temperature state of operation, the pipe metal under the action of stress creep (pipe swelling) until rupture. Long-term overheating mainly occurs in the outer ring of the high-temperature superheater to the fire surface, low temperature superheater may also occur For example: when the #3 boiler of a factory is overhauled, the boiler inspection center conducts spot checks on the heating surface of the boiler, and does metallographic analysis of the high-temperature superheater tube, and finds that its organization is ferritic carbide, the pearlite has been spheroidized, the degree of spheroidization is 3 to 4 grades, the carbide is distributed into small spheres on the grain boundary of the ferrite, the tensile strength has been significantly reduced, and it is recommended to replace the high-temperature superheater tube. The main reason for the analysis is that the steam load of the whole plant is low in recent years, the No. 3 furnace has been in a low-load operating state for a long time, the steam flow rate in the superheater tube is low, the heat transfer effect is poor, and the pipe wall is in an over-temperature state for a long time, resulting in a long-term overheating of the superheater tube and changes in the internal organization.

1． 3 Causes of overheating of the superheater tube and measures that should be taken In the design, if there is a low height of the boiler furnace, the flame center is behind, the hydrodynamic conditions are poor, the steam flow is low and the structure of the heating surface is unreasonable, etc., factors will cause the superheater to be generally overheated or there is a large thermal deviation of the local overtemperature; in the manufacturing, installation and maintenance, if there is a blockage of foreign matter in the tube, the screen over-connecting box partition and other defects, it will cause poor flow of the working fluid, causing the heating surface to be over-temperature; if there is improper combustion control, the flame moves up, Flame deflection, insufficient exhaustion of smoke temperature at the furnace outlet, incomplete combustion caused by secondary combustion of the flue, insufficient steam flow, improper stop of cooling water, low input rate of high-pressure heater, etc., will also cause overheating of the superheater tube; in addition, the poor quality of the feed water causes scaling salt in the tube, affecting heat transfer, and will also cause the superheater tube to overheat in operation. In order to prevent over-temperature of the superheater tube, in operation, it should be operated in strict accordance with the provisions of the operating procedures, the boiler should be started and stopped in strict accordance with the start-stop curve, the boiler parameters and the temperature of the superheater tube wall should be controlled within the allowable range; the main indicators such as boiler steam parameters, evaporation and water level should be closely monitored to prevent over-temperature and overpressure, full water, and water shortage accidents; do a good job in boiler combustion adjustment, prevent flame deflection, pay attention to controlling the fineness of pulverized coal, rational use of wind, prevent coking, reduce thermal deviation, and prevent the tail of the boiler from re-burning Strengthen the management of soot blowing and sootblowing to prevent serious ash accumulation on the heating surface; ensure the normal quality of boiler feed water and the quality of soda in operation.

2 Causes and preventive measures of coal-saving apparatus burst pipes

There are the following reasons for the burst pipe of the economizer: poor water quality, oxygen content in the water, resulting in corrosion of the inner wall of the pipe; low temperature corrosion of flue gas; poor quality of pipe quality and welding; large changes in the temperature of the feed water and the flow of the feed water, resulting in excessive thermal stress of the pipe; serious fly ash wear, etc. Among the above reasons, the main cause of the explosion of the economizer is the two aspects of fly ash wear and welding quality.

2． 1 Poor welding quality Similar to the reason for the burst pipe of the superheater tube, the manufacture or maintenance will also lead to frequent bursting of the economizer pipe due to the poor quality of the welding. A plant #2 furnace low temperature economizer in the same part of the two consecutive leakage, after macroscopic inspection and metallographic analysis showed that the first burst pipe is due to poor installation weld quality, the presence of porosity and slag caused, and the second burst pipe occurred in the last replacement of the pipe section of the weld seam, due to poor weld quality caused by secondary leakage.

2． 2 Fly ash wear Operation practice shows that among the three main convection heating surfaces of the boiler superheater, economizer and air preheater, the wear of the economizer is the most serious. Because the economizer is usually staggered arrangement, the washout of the flue gas to the staggered pipe bundle is relatively strong, the wear is much more serious than the sequential column, and the flue gas temperature at the economizer is relatively low, and the ash particles in the flue gas are relatively hard, these two factors make the economizer wear much more serious than the superheater and the air predeterminer, and the economizer wear burst pipe mainly occurs in the following parts:

2． 2． 1 The second and third rows of pipes of the economizer. Because the economizer is usually arranged in a staggered row, the first row of the drainage pipe is washed away at a lower smoke speed, that is, the smoke in the empty flue before entering the economizer, and after entering the second row of pipes, due to the reduction of the flue gas circulation section, the speed suddenly increases, and the impact of ash particles in the flue gas is larger, so the wear of the second and third rows of pipes is more serious than that of the following rows of pipes. 2． 2． 2 Economizer tube elbow. The gap between the elbow of the economizer pipe and the wall on both sides of the shaft flue pipe forms a flue gas corridor, which is more severely worn due to the small resistance and high flue gas flow rate. 2． 2． 3 Rows of serpentine tubes against the back wall. For π boilers, the flue gas turns from the horizontal flue into the shaft flue, and due to the centrifugal force, most of the ash particles in the flue gas are concentrated in the shaft flue

The back wall, so the rows of snake-shaped pipes against the back wall are more severely worn. #3号炉低温省煤器靠后墙处蛇形管发生泄漏, macroscopic inspection showed that the fracture cracked from the weld, and there were obvious thinning marks on the outer wall of the tube. From the above analysis, it can be seen that this economizer pipe is due to the fact that the strength of the outer wall of the pipe is significantly reduced after being diluted by flue gas erosion.

2． 3 Causes and preventive measures for the wear of the economizer The reasons for the wear are: high ash content of coal combustion, hard particles entrained in fly ash; too high or local smoke velocity; uneven distribution of ash concentration of flue gas, and excessive concentration of local ash. Fly ash wear is often prevented by reducing the number of fly ash impact pipes, reducing the flue gas flow rate, or increasing the wear resistance of the pipes. Such as: (1) control the flue gas flow rate, especially the flue gas flow rate at the flue gas corridor, so in the installation and maintenance, should minimize the distance between the economizer pipe and the wall, while making the distance between the snake-shaped pipes as equal as possible; (2) for the local flue gas flow rate is too high, the pipe should be easy to wear parts such as: elbow, the second and third row of pipes, several rows of pipes on the back wall and other places to install anti-wear devices; (3) the economizer pipe cold spray high temperature wear-resistant paint to increase the wear resistance of the pipe.

3 Causes and preventive measures of water-cooled wall burst pipes

3． 1 The water-cooled wall burst pipe mainly causes the pipe to be blocked by debris locally, resulting in poor water circulation, causing the pipe wall to overheat and produce bulges or cracks. This kind of burst pipe belongs to the short-term overheating burst pipe; the water quality is unqualified, resulting in scaling in the pipe; the furnace is improperly operated during the ignition and boosting process; when the boiler is seriously short of water, a large amount of water suddenly produces a huge stress to damage the pipe; the pipe material is unqualified or the welding quality is poor.

3． 2 Measures to prevent water-cooled wall burst pipes ensure that the quality of the feed water and furnace water is qualified, reduce the scaling and corrosion in the water-cooled wall pipes; adjust the combustion, make the flame evenly unbiased, prevent the water-cooled wall pipes from being heated unevenly, and the water circulation is destroyed; prevent the external wear of the water-cooled wall pipes; strictly follow the regulations when heating up and boosting, and control the boosting speed.

In order to effectively prevent the leakage of four-pipe blasting, in addition to taking the above measures in terms of installation, maintenance and operation adjustment, it is also necessary to do a good job of metal supervision of the heating surface of the boiler, mainly including the following aspects: (1) regular inspection of the heating surface for expansion, deformation and wear; (2) supervision and control of the heating surface that has long existed in the heating problem; (3) regular pipe cutting inspection, metallographic inspection of high temperature superheater and low temperature superheater pipe, and scaling of the inner wall of the water-cooled wall pipe in the highest area of the furnace heat load. Inspection of corrosion conditions. As long as the root cause of the three-pipe blasting is found and effective preventive measures are taken, the problem of boiler bursting pipe can be fundamentally solved and the occurrence of four-pipe blasting leakage accidents can be effectively prevented.