In the sintering process of iron and steel metallurgy industry, the high-temperature dusty flue gas generated by the sintering machine is collected through a large flue and sent to the electrostatic precipitator and the main exhaust fan. Sintering large flues are usually large in diameter (up to 3-5 meters), flue gas temperature fluctuates between 100-200℃, and contain a large amount of dust, SO₂ and other corrosive gases. Because of the long flue length and frequent start-and-stop machines, the problem of thermal displacement is very prominent. The sintered large flue expansion joint is the key component to solve this thermal displacement and prevent flue deformation and equipment damage. This paper will introduce the common types of expansion joints, selection points, installation specifications and daily maintenance from the working characteristics of sintering large flue, which will provide practical reference for sintering system equipment management and engineers and technicians.

1. Characteristics of working conditions of sintering large flue and requirements for expansion joint

The large sintering flue is connected with the sintering machine bellows, dust collector and main exhaust fan, and its working conditions are more severe than those of ordinary industrial flues. To understand the selection logic of sintered large flue expansion joint, we must first master the following four core characteristics:

1. Frequent temperature fluctuation: During the sintering machine starting and stopping process, the flue gas temperature can rise from normal temperature to 200℃ and then drop to normal temperature in a short time, and the number of heat cycles is large
2. High dust content: The sintering flue gas carries a large amount of mineral powder and dust, which causes strong erosion wear on the inner wall of the expansion joint and the bellows
3. Corrosive medium: The flue gas contains SO₂, NO₂, water vapor, etc., and it is easy to form acidic condensate in low temperature zone to corrode metal parts
4. Negative pressure operation: The large flue is under negative pressure (usually-10 to-20kPa) under the action of the main exhaust fan, and the expansion joint must have sufficient rigidity and tightness to prevent deflation or leakage

Therefore, ordinary expansion joints are difficult to meet the sintering large flue conditions, so special expansion joints with wear resistance, corrosion resistance, negative pressure resistance and sufficient compensation ability must be selected.

2. Main types of sintered large flue expansion joints

According to the differences of structural forms and application parts, sintered large flue expansion joints are mainly divided into the following types:

1. Metal bellows expansion joint

Metal expansion joint is the most commonly used type of sintered large flue, which is mainly composed of bellows, end pipe, guide tube and tie rod. Bellows are hydraulically or roll-formed in stainless steel (304, 316L or weathered steel) and typically have thicker wall thicknesses than regular expansion joints to withstand dust wear.

• Axial type: absorbs the heat expansion and contraction of the straight pipe section of the flue and is installed in the middle section of the large flue
• Transverse: Absorbs lateral displacement at elbows or reducers, often used at flue corners
• Hinged Type: Used in pairs to absorb in-plane angular displacement

In large flue systems, axial metal expansion joints are the most widely used. The guide tube is a standard component, and its function is to guide the flue gas to flow along the axial direction to avoid dust directly washing the inner wall of the bellows.

2. Non-metallic fabric expansion joint

The non-metallic expansion joint is made of multi-layer composite materials (silicone cloth, fluorine adhesive cloth, glass fiber cloth and heat insulation layer), which is suitable for rectangular flue or parts that need multi-directional compensation. Its advantages include:

• Can simultaneously absorb axial, transverse and angular displacement
• The vibration isolation effect is good, and the vibration transmission of the fan can be reduced
• Lower cost than metal expansion joints
• Excellent resistance to acid gas corrosion

In the sintering large flue, non-metallic expansion joints are often used in the inlet and outlet of electrostatic precipitator, the inlet and outlet of main exhaust fan and the rectangular flue with larger diameter. However, it should be noted that its temperature resistance generally does not exceed 350℃, and its pressure bearing capacity is limited (

3. Corrosion-resistant composite expansion joint

For sintering flue gas desulfurization system or flue in low temperature section, the problem of acid condensation corrosion is prominent. At this time, you can choose:

• PTFE-lined metal expansion joints: PTFE lining insulates acidic media from contacting metal surfaces
• High nickel alloy expansion joint: Inconel 625 or Hastelloy, resistant to chloride and sulfuric acid corrosion
• All non-metallic expansion joints: fluorine tape skin, suitable for strong corrosion but low pressure applications

3. Key points of selection of key parts

A complete sintered large flue system usually contains multiple expansion joints, and the selection strategy of different parts is different.

1. Sintering machine bellows outlet to large flue collection pipe

The flue gas temperature in this area is high (150-200℃), and the dust concentration is the largest. It is recommended to choose the metal axial expansion joint with thickened guide tube. It is recommended to use wear-resistant steel plate (such as 16Mn) or wear-resistant ceramic lining for the guide tube, and 316L stainless steel for the bellows. The fatigue life of the expansion joint should be designed to be no less than 2000 cycles.

2. Large flue straight pipe section

The length of the straight pipe section of large flue can reach tens of meters, and the accumulated thermal displacement is large. An axial type metal expansion joint is usually set every 20-30 meters. Pre-stretching or pre-compression is required according to the calculated value during installation, and the pre-deformation amount is generally 50% of the total compensation amount. Flue bearings should be provided with sliding bearings or rolling bearings to reduce frictional resistance.

3. Electrostatic precipitator inlet and outlet

The flue at the inlet and outlet of the electrostatic precipitator is usually rectangular in section, and there are vibration and sealing requirements. The non-metallic fabric expansion joint is the best choice here because it can absorb multi-directional displacement, effectively isolate vibration, and at the same time have good sealing. The frame of non-metallic expansion joint should be made of Q235B or weather-resistant steel, and the skin should have anti-aging and acid resistance.

4. Main exhaust fan inlet and outlet

The front and back of the main exhaust fan are the parts with the most severe vibration, and the fan inlet is in a high negative pressure state. Non-metallic expansion joints with internal support rings or reinforced metallic expansion joints should be used here. The negative pressure stiffness of the expansion joint should be checked emphatically when selecting the model to prevent deformation by suction. At the same time, the connection between the expansion joint and the fan interface should be provided with a flexible sealing gasket.

5. End of large flue to chimney

The flue gas temperature in this area has dropped below 100°C, but the acid condensate corrosion risk is highest. It is recommended to choose corrosion resistant metal expansion joints (316L or higher) or all non-metal expansion joints with drainage holes at low points to prevent condensate accumulation.

Material selection and applicable working conditions

Installation and maintenance specifications

1. Installation precautions

• It is strictly prohibited to knock the bellows: Do not contact the bellows surface directly with a crow bar or hammer during installation
• Correct direction of guide tube: The open end of guide tube must be directed toward the direction of flue gas flow
• Pre-offset setting: The metal expansion joint shall be cold tightened according to the design value, and the transport fixture shall be removed after installation
• Bracket setting: guide brackets shall be set on both sides of the expansion joint, and the distance between the first guide bracket and the expansion joint shall be ≤4 times the flue diameter
• Welding protection: Wet cloth should be used to cover the bellows during on-site welding to prevent burning by welding slag

2. Key points of daily inspection

• Check the surface of metal bellows for corrosion, crack, wear and thinning
• Inspect non-metallic skin for damage, pinhole leakage, or aging cracking
• Check the expansion joint for abnormal deformation, bending of tie rod or loose connecting bolt
• Measure the surface temperature of the expansion joint to determine whether the heat insulation layer has failed
• Check whether the drainage hole is unobstructed to prevent condensate accumulation and corrosion

3. Replacement cycle recommendations

• Metal expansion joint: design life is 8-10 years, it is recommended to conduct sampling inspection of wall thickness every 2 years
• Non-metallic expansion joint: the skin life is generally 3-5 years, and should be replaced as a whole when due
• The following conditions should be replaced immediately: the bellows has penetrating cracks, extensive skin damage, and severe deformation that cannot be reset

VI. Common Faults and Treatment Measures

conclusion

Sintering large flue expansion joint is the core component to ensure the safe, stable and environmentally friendly operation of flue system of sintering machine. This paper systematically combs the working characteristics of sintering large flue and the requirements of expansion joints, and introduces in detail the structure and applicable scenarios of three main expansion joints: metal bellows type, non-metal fabric type and corrosion-resistant composite type. The key selection principles can be summarized as follows: the metal axial type with guide tube is selected for the outlet and straight pipe section of the bellows, the non-metal type is preferred for the inlet and outlet of the electrostatic precipitator and the fan port, and the acid corrosion resistance is focused on the low-temperature section and drainage holes are set. At the installation end, the pre-offset setting, the direction confirmation of the guide tube and the arrangement of the guide bracket must be strictly implemented; At the operation and maintenance end, a regular inspection and wall thickness sampling inspection system should be established, and the non-metallic skin should be replaced in time when it expires. Strictly following the above-mentioned selection, installation and maintenance specifications can significantly reduce the failure probability of sintered large flue expansion joint, prolong the overall life of flue system, and provide a strong guarantee for continuous and efficient production of sintering process. It is hoped that this paper can provide clear and practical technical reference for equipment management and engineering technicians in the field of iron and steel metallurgy.