Xia Jiesheng

　　Top-fired hot blast stoves have been introduced into my country in 2003 and have been developed. Because of their low furnace body, high air temperature, small floor space, low investment, and quick production, they have developed rapidly in the steel market in my country. Almost 100% of the newly built and capacity replacement blast furnaces in the industry are equipped with various top-fired hot blast furnaces. However, the technology of the top-combustion hot blast stove is not perfect. Frequent occurrences of high-temperature damage to the hot blast outlet, damage to the vault, and serious cracking of the furnace shell seriously affect the blast temperature and corporate benefits. Especially in recent years, cracks in the shells of several thousand-level blast furnaces supporting hot blast furnaces in our country have occurred from time to time. For example, the fifteenth generation shell and surrounding welds of the top-fired hot blast stove of a steel company's No. 2 blast furnace that have been in operation for 4 years have frequently cracked. In May last year, a steel company's No. 1 blast furnace top-fired hot blast stove, which was put into operation for less than two years, had serious cracks around the shell welds of the 14th, 15th, 16th, and 17th zones, which caused high-pressure conveying and high air temperature. There is a huge threat to the safe use of hot blast stoves.

Found the main cause of cracking

　　In the first half of this year, at a top-combustion hot blast stove shell cracking analysis and communication meeting organized by a steel company, experts from Henan Yuxing Hot Blast Stove Engineering Technology Co., Ltd. (hereinafter referred to as Yuxing Company) combined with a steel company No. 2 The top-combustion hot blast stove that has been in use for 4 years and the structural characteristics of the company’s No. 1 blast furnace top-combustion hot blast stove and the company’s research results have made a detailed technical analysis of the current situation of the furnace shell cracking, and proposed the top-combustion hot blast stove The optimization and improvement suggestions were recognized by the participating experts.

　　The experts of Yuxing Company announced at the meeting that the thickness of the steel plate of the top-fired blast furnace shell under pressure for high blast temperature was selected in accordance with the national hot blast stove design code, the corresponding pressure vessel design code, and the refractory structure design code. The blind plate force and thermal stress are analyzed and calculated, and the comprehensive factors that lead to the cracking of the furnace shell are found as follows:

　　One is that the hot blast stoves of several blast furnaces have achieved high blast temperature in the range of 1250℃~1300℃. Due to the huge blind plate force and heat stress concentration of refractory materials, the release is restricted by the conical steel shell structure of the hot blast stove and the furnace shell is designed. The furnace shell welding seam at the part of insufficient thickness appears to be cracked; secondly, the furnace shell welding seam itself is weak due to the presence of stress, which causes the furnace shell to crack under the concentrated action of stress and blind plate force.

　　Liu Shiju, chairman of Yuxing Company, explained in detail the process of calculating the thickness of the top-combustion hot blast stove shell with cracked shell: the calculation is based on the design specification for ironmaking and the pressure vessel design specification for the relationship between pressure and the shell, which is ignored by the design industry. . The No. 1 blast furnace of an iron and steel enterprise realizes a top-combustion hot blast stove with a high air temperature of 1250℃~1300℃, the air supply pressure is 0.45Mpa, the diameter of the combustion chamber shell is 13.5m, and the calculated blind plate force is 6437 tons (R²*pressure*π ), the hot blast stove shell is divided into 25 sections. After comprehensive calculation and comparison with the original furnace shell design thickness, the 14th, 15th, 16th, and 17th zones of the original furnace shell and the calculated deviation value are too large (see design value and Yuxing Calculated value comparison table), the furnace shells are 19%~39.3% thinner respectively. The calculation results are consistent with the calculation results for the furnace shells from 1 to 13 zones, and the furnace shells from 18 to 24 are super thick by 5%~53%. At present, the design specification for hot blast stoves uses the shell diameter multiplied by the empirical modulus to determine the shell thickness of each corresponding part, without considering the air supply pressure. When the furnace shell is ultra-thin, there is still residual stress at the welding seam, which reduces the blind plate resistance and thermal stress resistance of refractory materials. The thermal stress caused by the expansion of refractory materials is limited by the upper and lower furnace shells and the design absorbs thermal stress. When the capacity is insufficient, the huge thermal stress will frequently move the weak furnace shell under the combined action of the blind plate force, resulting in damage to the furnace shell weld. If the furnace shell has sufficient thickness and the weld stress relief is in place, the furnace shell will not crack, and the hot air outlet and the refractory material of the dome will be damaged. Therefore, the ultra-thin design of the hot blast stove shell is the main factor in the shell cracking, not caused by insufficient weld stress relief.

　　Because the conical vault of the top-fired hot blast stove is built on the straight section of the hot blast outlet, and the conical vault is an inverted bell mouth structure with a small upper and a large lower, the straight section and the conical vault are all made of High-expansion silica brick refractories, the expansion rate of silica bricks is 1.25%-1.3%, which forms a superimposed thermal stress accumulation of refractory materials, such as the expansion of 120mm in the straight section of the hot blast stove near the furnace shell of the No. 1 large blast furnace Use cotton to adjust the uncontrollable furnace shell ellipse, masonry ellipse, and non-standard spraying to absorb expansion, so as to avoid swelling stress damaging the furnace shell, but follow the straight section to review the design drawing up to the conical vault and the pre-combustion chamber labyrinth The absorption and expansion of the joint, the adjustment of the ellipse of the furnace shell, the adjustment of the expansion cotton with irregular masonry and irregular spraying are still 120mm, and the height from the labyrinth under the straight section of the hot air outlet to the junction of the conical vault and the pre-combustion chamber is 13.8m Calculated based on the expansion rate of 1.25% and 1.3% of the refractory material, the expansion amount is 172.5mm and 179.4mm respectively. If this set of data is added to the furnace shell welding, masonry, spraying, and the expansion amount of the refractory material is not standardized, its absorption The stress and expansion of the fiber cotton should be set aside 180mm to 230mm. At the same time, due to the thickness deviation of the furnace shell, the blind plate resistance and thermal stress strength are weak, which causes the furnace shell to crack. If the thickness of the furnace shell is sufficient, and the ability to resist the blind plate force and thermal stress is strong enough, the furnace shell can be intact and will not be damaged and the furnace shell will be cracked. In this setting state, the hot air outlet will be damaged, and the hot air outlet will be damaged in severe cases. The collapse and the collapse of the cone-shaped vault, the occurrence rate of top-fired hot blast stoves in my country is as high as 90%.

Put forward a scientific method to solve the problem of cracking

　　Experts of Yuxing Company proposed that the best way to solve the above problems is to eliminate the effect of stress, carefully review and calculate the thermal stress and the actual design thickness of the furnace shell under the corresponding pressure, and reserve enough expansion cotton to adjust and absorb the expansion space. The specific solutions are as follows:

　　One is that the newly-built hot blast stove adopts the tapered vault and the straight section of the hot blast outlet to be divided into two to reduce the accumulation of superimposed thermal stress, so that the thermal stress of the refractory material limited by the upper small and the lower large conical furnace shell is transferred to the furnace shell. . The upper and lower connection of the furnace shell adopts the method of gradually expanding and increasing the pallets and ribs to improve the strength of the anti-blind plate, which doubles the ability of the anti-blind plate, and at the same time can increase the thermal stress of the masonry above the hot air outlet. It can be released freely and is not affected by the comprehensive factors of construction, installation, materials and performance. It eliminates stress, weakens the blind plate force, prevents damage to the combustor structure of the hot air outlet and the vault, and makes the furnace shell red and tapered. The risk of roof collapse is reduced to zero.

　　The second is to increase the circumferential strengthening hoop on the periphery of the furnace shell, which greatly improves the blind plate resistance of the furnace shell.

　　The third is that the hot blast stove is a high-pressure vessel. The design must be based on the design specifications of the pressure vessel, and the geometric dimensions of each part of the furnace shell must be accurately calculated to avoid damage to the furnace shell by the pressure movement at high temperatures.

　　The fourth is to design the groove at the welding seam and adopt layered welding technology to prevent slag from being trapped between the welding seams. After welding, the weld seam should be treated with stress relief to avoid the existence of stress.

　　Fifth, for the No. 1 large blast furnace top-fired hot blast stove of a steel company, the 14-17th furnace shell solution in the high temperature and high pressure area of ​​the combustion chamber; a new layer of reinforcement is added to the corresponding furnace shell belt without stopping the furnace The explosion-proof steel plate welding cover or the furnace shell is replaced with a new furnace shell capable of resisting 6,437 tons of blind plate force (according to the 0.45Mpa wind pressure and pressure vessel calculation formula), and the method of repairing the welding there is adopted. There is no fundamental solution to the hidden safety hazards implied by weak defects. It can also reduce the supply air pressure, lower the air temperature, and avoid major safety accidents. For the argument that the furnace shell weld stress is not eliminated and cracking occurs, scientific calculations are required to verify the relationship between furnace shell weld cracking and pressure. With reference to the thickness of the furnace shell above the 18-25 zone of the hot blast stove, the calculated value is thicker than 5%-53%. There is no corrosion in the weld seam of the furnace shell, and there is no sign of paint fading, and there is no cracking or cracking of the weld seam. The comparison proves that the blind plate force and thermal stress are related to the insufficient thickness of the furnace shell. Therefore, adding a layer of steel plate or replacing the steel plate, reducing the wind temperature, lowering the wind pressure, and improving the safety factor of explosion-proof and compressive resistance are vital safety issues.

Reasonable transformation plan achieves good transformation effect

　　According to the analysis and solution of the cause of cracking in the furnace shell of the top-fired hot blast stove, Yuxing Company adopted the above-mentioned one, two, three and four technical measures in the design (see the schematic diagram of stress relief and anti-blind plate force design), which completely avoided The thermal stress damage and the comprehensive damage to the furnace shell and furnace body caused by thousands of tons of blind plate force have achieved good results. The following are classic design cases:

　　Since 2017, HBIS Laoting 3X3000m³ blast furnace, China Railway Equipment Manufacturing 3X2500m³ blast furnace, Tangshan Jinxi Iron and Steel Company 2×2000m³ blast furnace, Qinhuangdao Anfeng Iron and Steel Company 1080m³ blast furnace, Inner Mongolia Chifeng Yuanlian Iron and Steel Company 1350m³ blast furnace, Liaoning Jiachen Iron and Steel Company 1350m³ blast furnace Other iron and steel companies have adopted the four-stage top-combustion hot blast stove technology that expands the section to transfer thermal stress and strengthen the technical know-how of the furnace shell anti-blind plate force. It is worth mentioning that the 1860m³ blast furnace catenary top-fired hot blast stove of Shanxi Jianbang Group Generalist Company was put into operation on July 10, 2012, and the burners, vaults, and hot blast outlet checker bricks are intact. In October and November 2012, the two-month average high wind temperature was 1314.7℃, the difference between the vault temperature and the air supply temperature was less than 94℃, and the nitrogen oxide emission was less than 50 mg. Various indicators have not been surpassed in my country’s industry so far. The air supply pressure of the furnace is 0.45Mpa, the maximum furnace shell diameter of the hot blast furnace is 10m, the steel plate material of the furnace shell is selected as Q345R, and the blind plate force of the combustion chamber is calculated to be 3,532 tons. It has been used for nearly 8 years, and there is no cracking of the furnace shell weld seam. Like other top-fired hot blast stoves, the furnace body is damaged. Practice has verified that Yuxing hot blast stove technology is mature, indicating that Yuxing hot blast stove’s thermal calculation and furnace shell design specification calculations are accurate, reaching the safety factor under pressure and high temperature conditions and the structure is stable under high blast temperature long-term use environment of.

　　According to the suggestion made by Yuxing Company, a steel company’s No. 3 blast furnace top-fired hot blast stove re-designed part of the optimization design. Although these detailed technical treatment suggestions are trivial, they can actually solve the furnace shell cracking and hot blast outlet. Damage problems can also solve the problems of air leakage in the labyrinth and instability of the support base. This shows that Yuxing Company continues to optimize and improve the outstanding contribution of top-combustion hot blast stoves in terms of stability, longevity, energy saving, high efficiency, high production and high air temperature.

　　Therefore, as long as the combustion chamber structure of the top-combustion hot blast stove is designed reasonably, the stress concentration defects of the combustion chamber structure are solved, and the design accuracy of the anti-blind plate strength steel plate is mastered, the hot blast stove structure is stable, and the furnace shell cracking and hot blast can be avoided. The outlet damage and the collapse of the vault make the top-fired hot blast stove play a greater role in achieving energy saving, longevity, high production, high efficiency and high air temperature.