In the process of iron and steel smelting, converters, electric furnaces and refining furnaces will produce a large amount of high-temperature dusty flue gas. These flue gases need to be discharged after waste heat recovery and dust removal through the vaporization cooling flue system. Due to the intermittent and high temperature characteristics of the steelmaking process, the flue system is subjected to severe temperature alternating and mechanical vibration. The function of flue expansion joint in steelmaking is this-as the key component of flexible compensation in flue system, it can ensure the safe, stable and long-term operation of the whole flue gas treatment system. This paper will systematically analyze the three core functions of flue expansion joint in steelmaking and the key points of engineering practice.

1. Absorb thermal expansion to realize flexible connection

Steelmaking production is a cyclical process. Taking converter steelmaking as an example, the flue gas temperature in the flue is as high as 800~1000℃ in the blowing period, and it quickly cools to normal temperature in the non-blowing period. This severe temperature alternating causes significant thermal expansion and contraction of the vaporization cooling flue.

In the vaporization cooling flue for expansion structure type steelmaking converter, the movable hood, the stove mouth fixed section flue, the middle section flue and the final section flue are sequentially connected, and the stove mouth fixed section flue and the middle section flue are connected by an expansion joint。 The function of expansion joint of steelmaking flue is firstly reflected in: on the one hand, the expansion joint can absorb the expansion and contraction amount of flue during thermal expansion and cold contraction, and realize flexible connection; on the other hand, it can also eliminate the accumulated error during installation of flue。

If the expansion joint is not set, the huge stress caused by thermal expansion will lead to flue weld cracking, bracket deformation, and even equipment damage. This is the fundamental reason why expansion joints must be set in steelmaking flues.

2. Isolate vibration shock and protect flue structure

The steelmaking flue is connected to power equipment such as fans and dust collectors, which generate continuous mechanical vibration when operating. At the same time, the converter tilting and feeding operation will also produce impact load on the flue system.

The role of flue expansion joint in steelmaking is equally critical in vibration reduction. As a flexible connecting element, the expansion joint can effectively absorb the vibration generated by the operation of the equipment and reduce the influence of the vibration of the equipment on the pipeline。 The metal corrugated expansion joint has the characteristics of good flexibility, corrosion resistance, wear resistance and long fatigue life, and can absorb the axial, transverse and three-way displacement of the pipeline。

For the hot blast furnace system of steel plant, special high temperature resistant expansion joint can greatly improve the stress corrosion resistance and high temperature resistance of expansion joint, and stabilize the operation for a long time。 The fiber fabric of the non-metallic expansion joint and the thermal insulation cotton body have the functions of sound absorption and vibration transmission isolation, which can effectively reduce the noise and vibration of boiler, fan and other systems。

3. Withstand severe working conditions and ensure the system seal

The working environment of steelmaking flue is extremely harsh: high temperature (up to over 1000℃), strong corrosion (containing acidic gases such as SO₂ and SO₃), high dust content and intermittent operation. The role of steelmaking flue expansion joint is also reflected in the ability to adapt to these extreme working conditions, while ensuring the tightness of the system.

3.1 High Temperature Resistance

Steelmaking flue expansion joints need to withstand extremely high temperatures. Select different materials according to temperature level:

• When the flue gas temperature exceeds 400℃, the metal expansion joint can be upgraded to stainless steel (304/316L/321), and the ring belt is made of high temperature composite material
• For higher temperatures (such as hot blast furnace systems), special high temperature resistant expansion joints can withstand media temperatures up to 1200°C.

3.2 Anti-corrosion Design

Steelmaking flue gas contains SO₂, SO₃ and other corrosive components. When the flue temperature is lower than the acid dew point, condensate acid will be produced, which will cause serious corrosion to the expansion joint. The non-metallic expansion joint of flue gas pipeline of metallurgical furnace has obviously improved the service life of the expansion joint by adopting corrosion-resistant inner layer material and anti-corrosion layer of deflector。

3.3 Sealing Performance

The non-metallic expansion joint has good sealing performance, and the composite materials such as fluorine rubber and silicone rubber are used as the isolation layer to ensure no leakage。 This is essential to meeting increasingly stringent environmentally friendly emission standards.

4. Common Types and Applications of Flue Expansion Joints in Steelmaking

According to the working conditions of different processes of steelmaking, the appropriate expansion joint type should be selected:

In the practical application of steel mills, the large flue of sintering flue gas desulfurization and denitrification system often needs to replace the expansion joint online. These on-site pipeline media are high temperature, negative pressure or slightly positive pressure, dusty flue gas, and the working surface is at high altitude and confined space, which requires extremely high reliability and replacement convenience of expansion joints。

V. Key points of installation and maintenance

In order to ensure the full play of the function of flue expansion joint in steelmaking, attention should be paid to the following during installation and maintenance:

5.1 Installation Points

• Check whether the model, specification and pipe configuration of the expansion joint meet the design requirements before installation
• The direction of the guide tube should be consistent with the direction of the medium flow to ensure smooth air flow
• It is strictly prohibited to adjust the pipeline installation excess tolerance by the method of expansion joint deformation
• The welding slag is not allowed to splash onto the bellows surface during the welding process to prevent mechanical damage

5.2 Daily Inspection

• Check the surface of the expansion joint regularly for damage, deformation and leakage
• Monitor whether the actual displacement of the expansion joint is within the design range
• Check the tightening state of flange bolts to prevent loose leakage
• For non-metallic expansion joints, check the skin for aging and bulging

Practical cases show that the damage of the flue expansion joint of converter will affect the steelmaking process, and at the same time, there are hidden safety risks。 Therefore, it is crucial to establish a regular inspection and replacement system.

VI. SUMMARY

The function of flue expansion joint in steelmaking can be summarized as three core values:

• Absorb thermal expansion to realize flexible connection: Absorb the amount of expansion and contraction during thermal expansion and cold contraction of the flue to ensure the integrity of the flue structure
• Isolate vibration impact, protect flue structure: absorb equipment vibration, reduce noise transmission, and prolong equipment life
• Withstand severe working conditions to ensure system sealing: high temperature resistance, corrosion resistance, reliable sealing, meeting environmental protection emission requirements

In engineering practice, the types of expansion joints should be selected scientifically according to the specific working conditions (temperature, pressure, media corrosion, displacement) of each steelmaking process, and the installation specifications should be strictly followed. Metal corrugated expansion joints are preferred for high-temperature areas, while non-metallic expansion joints are preferred for low-temperature acid-containing areas. Correct selection and maintenance can effectively avoid equipment failures such as flue leakage and weld cracking, and ensure the continuity of steelmaking production and environmental protection emission standards.