2022年6月20日星期一

Cause Analysis of Breakage of Blast Furnace Air Nozzle

Blast Furnace Air Nozzle

Cause Analysis of Breakage of Blast Furnace Air Nozzle


Preface

Blast furnace operation is a very continuous operation process. Generally, the small air outlet sleeves of the blast furnace are managed for life and replaced regularly. If abnormal damage occurs to the small nozzle sleeve of the blast furnace, it will seriously affect the normal production of the blast furnace. Besides the effect of air break on the production, it also includes the increase of fuel ratio caused by water leakage, the cooling of the blast furnace, and the damage to refractories of the hearth. Frequent air breaks can also cause changes in the position of the melting zone, resulting in wall thickness, nodulation, or inactivation of the hearth until the hearth accumulates. Therefore, the study of the mechanism and cause of air outlet breakage and the effective reduction of air outlet breakage in production is the focus of daily attention of furnace operators.


Causes of Breakage of Blast Furnace Air Nozzles

1) Objective Factors of Blast Furnace Air Nozzles Breakage

There are many reasons for air outlet breakage, many of which are caused by objective conditions, such as air outlet structure, manufacturing defects, the water quality of cooling water, pressure, flow rate, etc. These objective factors may cause air outlet breakage.

Firstly, with the development of technology, the structure of air outlets used in blast furnaces is also constantly improved, such as changing from cavity type to tubular type, at the same time, the manufacturing quality and processing technology are also continuously improved. The air outlet breakage caused by air outlet structure and manufacturing quality is no longer the main cause of air outlet breakage.

The air outlet is damaged due to quality reasons. Water leakage is treated. Pores such as pinholes are usually visible and the leakage holes are small outside and large inside. Crack water leakage caused by air quality, usually occurs at the welding seam. Of course, affected by blast furnace condition, cooling, and other factors, the air vent can also cause cracks and water leakage under the action of thermal gradient under momentary huge thermal load, which is not a quality problem of air vent.

Secondly, in terms of cooling, the formation of scale can generally be found at the air outlet or pipeline due to water quality problems such as air outlet breakage caused by excessive water quality. As for water pressure and water speed, the design standards for each blast furnace have been in place at the beginning of the construction of the furnace. Some fluctuations in production should also be able to meet the needs of air outlet cooling, so water pressure and water speed should not be the main cause of air outlet breakage.


2) Breakage of Blast Furnace Air Nozzle Caused by Operation


For general blast furnaces, the breakage of the air outlet caused by the operation is the main cause at present. The breakage of the air outlet caused by the improper operation is mainly manifested in the following aspects:

The first, is excessive development of gas flow at the edge of the blast furnace. Due to excessive airflow at the edge, the reaction at the edge of the blast furnace increases, resulting in a large amount of slag iron. In contrast to normal conditions, when slag iron enters the hearth along the surface of the swing zone of the air outlet, a small amount of slag iron will drip down the furnace wall. When a small amount of slag iron drips down the upper end of the air outlet, the air outlet will be damaged.

The burned parts of the air outlet caused by this reason are mostly on the upper part of the air outlet. Most of the burned holes are large outside and small inside, similar to the phenomenon of droplets passing through. When the edges are over-developed, frequent rises and falls in front of the air vents can also be seen through the air vent mirrors.

In addition, excessive development of edge airflow often results in unstable slag skin at the furnace belly, frequent shedding, and falling slag skin running down the furnace wall. On the one hand, it causes wear and water leakage at the front of the air outlet. On the other hand, a large number of slag blocks that are not fully preheated melt in front of the air outlet, which makes the temperature of the molten slag iron droplets decrease rapidly, resulting in poor fluidity and can not rapidly penetrate into the lower part of the furnace cylinder and accumulate in front of the air outlet, thus causing the air outlet to burn out.

Second, the BF hearth is inactive and accumulated. The volume of the hearth decreases as a result of central or edge accumulation. As the number and time of blast furnaces are usually fixed, under the same circumstances, the slag iron surface will be higher than the original one after the hearth is accumulated, and the pressure relationship will be tense outside the blast furnaces, slow charging and other phenomena will appear, so the slag iron in the furnace is easy to burn out the air vent. Sometimes, due to peripheral accidents, the iron tapping time is delayed and the air vent may be burned out.

However, the main reason for the damage to the air outlet caused by the accumulation of hearth is that after the accumulation of hearth, the permeability of the dead coke reactor of the blast furnace is poor, resulting in the accumulation of slag iron in front of the air outlet, which burns out the air outlet.

If the condition of the blast furnace is restored after it is cooled, a large number of airports will be damaged. The main reason is the poor liquid permeability of dead coke in the hearth, as well as the low physical heat and poor fluidity of the newly formed slag iron, which can not penetrate the hearth in time and the slag iron accumulates in front of the airports. The burned area of the air outlet caused by this kind of reason is usually the lower part of the air outlet.

Third, the kinetic energy of the blast furnace is insufficient. For example, for long-term air reduction of the blast furnace, the area of the air outlet is not adjusted in time. Due to insufficient dynamic energy of the blast, the swing area of the air outlet becomes smaller and slag iron may burn the front end of the air outlet.

When the wind speed of the fourth blast furnace is too high and the kinetic energy of the blast is too high, when the wind speed is too high, a negative pressure zone will be formed around the air outlet. The higher the wind speed, the higher the negative pressure around the air outlet. When the negative pressure is high to a certain extent and has a certain attraction to the slag iron dripped from the upper part, the slag iron dripped from the upper part will be absorbed on the surface of the air outlet and burned out. At the same time, the high air speed will also cause turbulence in the air outlet area. This changes the dripping direction of slag iron and burns the air vent. There are a number of cases where blast furnaces were forced to blow because they wanted to penetrate the center, resulting in a large number of burned air outlets.

Fifth, BF is not smooth and the reason for the suspended charge. After suspension, air-reducing and even air-resting materials may be filled with slag from the air outlet, which will burn the air outlet. It is also possible that the material column suddenly falls from the upper part, causing the air vent to break, especially if the suspension is stubborn for a long time, which is even more dangerous. There have been incidents in which enterprises smashed air vents while sitting down due to long-term vicious suspension.

Improper operation of the sixth flushing furnace results in slag peel falling off, burnout air vent, long-term addition of fluorite, and development of edge airflow, which is also liable to burnout. The burnout mechanism can be seen in << Hazard of fluorite to furnace smelting >>

Seventh, the cause of coal powder scouring in the coal injection process. After pulverized coal is injected into the blast furnace, the air vent can be worn out in a short time due to the improper position of the lance. Even if the gun position is correct, the friction of pulverized coal on the air vent is very serious. Statistics from enterprises show that the wear of pulverized coal can enlarge the inner diameter of the air vent by 0.5-0.8 mm every month. Therefore, when coal is relatively high, the influence of coal powder scouring can not be neglected.

The reason for the damage to the air outlet is summarized in the hope that in our daily operation, it can be prevented slightly and gradually, and the operation with the possibility of burning the air outlet can be avoided as far as possible so as to reduce the loss in production. The unnecessary wind break in the village will create conditions for stable production and the long life of the blast furnace.

Of course, there are many reasons why air vents are damaged, but they are usually shown by the above forms, such as harmful elements in raw materials and fuels which cause slag to stick, lead to cylinder accumulation, and damage air vents. In production, it is necessary to make specific analyses according to actual conditions so as to avoid a large number of burns of air outlets affecting the smooth running of the blast furnace.


Conclusion

(1) A large number of air outlets are damaged, which is very disadvantageous to the continuous production of blast furnace technology. Effective measures must be taken to reduce the breakage rate of air outlets.

(2) Damage to the air vent is closely related to the operation of the blast furnace, but only by changing the original and fuel conditions, can the blast furnace adopt the most economical operation system and fundamentally eliminate the cause of air vent breakage.

(3) It is also an effective way to reduce the burning of air nozzles by properly increasing the water pressure and flow rate of cooling water, strengthening the cooling, and improving the cooling conditions from the outside.


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