In the industrial ventilation and flue gas delivery system, the square expansion joint of the flue of the main smoke exhaust machine is the key flexible component connecting the inlet and outlet of the smoke exhaust machine with the flue. If the high temperature, vibration and thermal expansion effects generated during the operation of smoke exhaust machine are not reasonably compensated, they will directly threaten the safety of equipment and system stability. This paper will systematically explain the structural characteristics, selection calculation, material selection and installation and maintenance points of the square expansion joint of the flue of the main smoke exhaust machine, and provide professional guidance for the selection and operation and maintenance of engineers and technicians.

1. What is the square expansion joint of the flue of the main smoke extractor?

The square expansion joint of the flue of the main smoke exhaust machine is a compensation device installed at the connection between the inlet and outlet of the smoke exhaust fan and the flue, which is specially used to absorb the thermal displacement, mechanical vibration and thermal expansion of the pipeline generated during the operation of the fan. The square cross-section design is adapted to the shape of the main flue and the fan interface, and can make full use of the installation space and improve the compensation efficiency compared with the circular expansion joint。

The expansion joint can be divided into two categories according to the material: metal corrugated type and non-metal fabric type. The metal type is suitable for high temperature and high pressure working conditions, while the non-metal type has significant advantages in vibration and noise reduction and corrosive medium treatment。

Core Structure Composition

A complete main fume extractor flue square expansion joint typically contains the following core components:

2.1 Metal square expansion joint structure

• Bellows: Thin-walled stainless steel bellows (304/316L) made of hydroforming or mechanical forming, which is the core elastic element that absorbs displacement
• End pipe/flange: Used for connection with flue and fan interface, welding or flange connection can be adopted
• Guide tube: lined inside of bellows to protect corrugation from direct scouring by medium and reduce flow resistance
• Positioning screw: used to fix the length of the expansion joint during installation to prevent damage during transportation and installation

2.2 Non-metallic square expansion joint structure

• Composite belt (skin): Multi-layer composite structure comprising fluororubber/silicone rubber layer, glass fiber reinforced layer and PTFE anti-corrosion layer
• Metal frame: connecting flanges and pressing devices, usually made of carbon steel or weathering steel
• Heat insulation filler: aluminum silicate fiber cotton and other materials, used for heat insulation protection under high temperature working conditions

3. Core parameters of model selection and calculation

The following key technical parameters should be mastered to correctly select the square expansion joint of the flue of the main smoke exhaust machine:

3.1 Calculation of Compensation Amount

The displacement to be absorbed by the expansion joint mainly includes:

• Axial displacement: Changes in length caused by thermal expansion and contraction of flue
• Lateral displacement: Lateral displacement caused by turbine start-stop and foundation settlement
• Angular displacement: pipeline installation deviation and thermal deformation

For rectangular section expansion joints, the reference values of maximum allowable expansion per wave are: ±24mm for full height and ±12mm for half height。

3.2 Temperature applicable range

Select the appropriate type of expansion joint according to the working condition temperature：

• Low temperature type: ≤200℃, suitable for conventional ventilation and low temperature flue gas
• Medium temperature type: 200℃ ~400℃, suitable for boiler flue and hot air duct
• High temperature type: 400℃ ~800℃, stainless steel bellows and high temperature insulation structure are required

3.3 Pressure Class

The smoke duct system is mostly in low pressure or slightly negative pressure conditions:

• Metal expansion joint: pressure bearing capacity usually ≥0.1MPa
• Non-metallic expansion joint: applicable pressure ≤0.05MPa

3.4 Section Size and Wave Number

The cross-sectional dimension of the square expansion joint is determined by the size of the inner port of the flue. The outer dimensions (length × width) of the flue should be provided during type selection, and the waveform (full height 216mm or half height 108mm) and wavenumber (single corrugation not more than 6 waves) should be determined according to the stress and strain requirements。

4. Material Selection Guide

4.1 Material of metal bellows

4.2 Non-metallic skin material

Data Source:

V. Key points of installation and maintenance

5.1 Installation Precautions

1. Cold drawing compensation: In order to reduce the number of bellows nodes, it is recommended to consider 50% cold drawing
2. Guide bracket setting: Guide supports should be set on both sides of the expansion joint to ensure directional movement of the expansion joint
3. Ash baffle setting: For dusty flue, ash baffle should be used to protect the bellows
4. Installation protection: Retain the positioning screw during transportation and installation, and can only be removed after installation is in place

5.2 Maintenance Check Items

• Check the bellows surface regularly for cracks or corrosion
• Nonmetallic expansion joints need to be checked for aging and cracking of skin
• Check the flange connecting bolts for loosening
• Observe for signs of leakage (smoke or moisture)

Application Scenarios and Typical Cases

The square expansion joint of the flue of the main smoke exhaust machine is widely used in：

• Inlet and outlet flue of induced draft fan of boiler in thermal power plant
• Main Fume Exhaust System of Sintering Machine in Iron and Steel Plant
• High temperature fan flue at kiln end of cement plant
• Waste gas treatment system of chemical enterprise

In engineering practice, the catalytic cracking unit of a petrochemical enterprise once faced the risk of unplanned shutdown due to the cracking of the bellows of the expansion joint (the crack length is about 30cm, and the flue gas temperature reaches 680℃). The leakage point was successfully eliminated by emergency pressure plugging and box packing treatment。 This case fully demonstrates the importance of expansion joint selection and maintenance-under high temperature working conditions, expansion joints with suitable materials and compensation ability must be selected, and a regular inspection system must be established.

VII. SUMMARY

The square expansion joint of the flue of the main smoke exhaust machine is the key component to ensure the safe operation of the smoke exhaust system. When selecting, we should focus on the following points:

1. Accurate calculation: Comprehensively determine the required compensation amount based on the thermal expansion of the pipeline, the vibration amplitude of the fan and the installation deviation
2. Material Matching: Select metal or non-metal structure according to flue gas temperature, corrosiveness and dust content, and determine the appropriate corrugation/skin material
3. Structural adaptation: The square section size should be accurately matched with the flue interface, and the wave number and wave height should be reasonably designed according to the stress and strain requirements
4. Standard installation: Strictly implement the installation requirements of cold drawing, guide bracket and ash baffle
5. Regular maintenance: Establish a patrol inspection system to discover and deal with cracks, corrosion and leakage dangers in time

It is recommended that engineers and technicians refer to the national standard GB/T 12777 and the industry standard JB/T 12235-2015 when selecting the model, and give priority to the products of expansion joint manufacturers with qualification and certification. Only by combining the theoretical calculation with the actual working conditions can the main smoke exhaust machine and flue system operate safely, stably and efficiently for a long time.