In the industrial flue gas pipeline system, when the flue section is rectangular or square, the ordinary circular expansion joint cannot be used directly, at this time, the flue square expansion joint needs to come out. Whether it is the desulfurization flue of power plant, the dust removal air duct of steel plant, or the exhaust gas exhaust pipe of chemical plant, the flue square expansion joint plays a key role in absorbing thermal displacement, isolating vibration and preventing leakage. Because its cross section is non-circular, the stress state and compensation characteristics are significantly different from those of circular expansion joints, so the selection and installation have unique requirements. This paper will systematically explain the structural characteristics, selection points and installation specifications of flue square expansion joint.

1. Classification and structural characteristics of flue square expansion joint

1. Square non-metallic expansion joint

Square non-metallic expansion joint, also known as rectangular non-metallic compensator, is the most commonly used type in smoke and air duct。 It is composed of composite non-metallic band (skin), metal frame and connector/flange, which can meet the requirements of multi-angle and multi-direction displacement compensation in a narrow size。

Core Structure Composition：

The main advantages of square non-metallic expansion joint are: strong multi-directional compensation ability, good vibration isolation and noise reduction effect, no blind plate force transmission, especially suitable for rectangular flue of flue gas desulfurization, dust removal, hot air and other systems。

2. Square metal expansion joint

Square metal corrugated expansion joint (JX type) uses stainless steel corrugated pipe as compensation element, with square flanges or connectors at both ends。 The corrugated pipe is formed by hydraulic pressure or mechanical, and has the characteristics of precise size, compact structure, large compensation, corrosion resistance and long service life。

Metal expansion joints are suitable for higher temperature (> 400℃) and higher pressure, but the compensation direction is mainly axial, and the transverse and angular compensation ability is not as good as that of non-metallic expansion joints。

3. Rectangular fillet expansion joint (CE standard)

Rectangular fillet expansion joint is a flue square expansion joint designed according to the US CE standard. Its distinguishing feature is that the corners adopt large circular arc transition instead of conventional right-angle welding。 This design can effectively reduce the corner stress level, eradicate the dead corner and local high stress damage risk, and prolong the service life of the product.

2. Main points of selection of square expansion joint of flue

1. Implementation standards

The main implementation standards for flue square expansion joints are JB/T 12235-2015 (non-metal compensator) and GB/T 12777-2019 (metal bellows expansion joints)。 The manufacturer shall be required to provide the test report that meets the corresponding standards when selecting the model.

2. Key selection parameters

3. Model Example Interpretation

Take model FXD5000×5000F L =400 as an example：

• FXD: Non-Metallic Square Expansion Joint Code
• 5000×5000: Inner port size 5m ×5m
• F: Flange connection (with matching flanges and bolts)
• L =400: The total length of the expansion joint is 400mm

4. Material selection principle

The material selection of the flue square expansion joint shall be determined according to the working temperature：

• t ≤400℃: Carbon steel (Q235/Q345) for metal structural parts, fluororubber + glass fiber composite for ring belt
• t> 400℃: Metal structural parts are upgraded to stainless steel (304/316L/321), and the ring belt is made of high-temperature composite material (ceramic fiber)

For the clean flue of wet desulfurization system (temperature about 50℃, high humidity, strong acid corrosion), the expansion joint made of pure titanium TA2 should be selected. Its chloride ion pitting resistance is far better than that of stainless steel, which can effectively prevent "running, dripping and leaking"。

3. Installation specification of flue square expansion joint

1. Preparation before installation

• Check whether the model, specification and inner opening size of the flue square expansion joint are consistent with the design
• Inspect skin/bellows for damage and scratches
• Confirm the flow direction identification: The direction of the inner sleeve must be consistent with the flow direction of the medium when the guide tube is included
• Clean flue flange surface to remove old gasket residue

2. Installation Process Control

3. Prebias Setting

For high temperature flue, the flue square expansion joint shall be pre-stretched or pre-compressed (cold tightening) according to the design requirements when installing. The auxiliary components used for pre-deformation shall be removed after the pipeline is installed。

4. Inspection after installation

• Confirm that all transport fixtures have been removed
• Check that all moving elements are not stuck by external components
• Conduct airtightness test to verify no leakage

4. Anti-leakage measures of square non-metallic expansion joints

The flue square expansion joint in the clean flue of wet desulfurization system is faced with the corrosion problem of strong acid condensate, which is a high incidence of leakage。 Leak-proof design should focus on:

1. Drain hole setting

Horizontally mounted square expansion joints shall be provided with drainage holes at the lowest point of the frame. Taking the flue expansion joint of a thermal power plant as an example, the bottom of the expansion joint is provided with an acid drainage port, and the acid drainage pipe is installed vertically to lead the condensed acid to the collection system。 This can prevent the acid from accumulating in the skin groove for a long time, and fundamentally prevent leakage.

2. Material upgrade

For flue square expansion joints with repeated leakage, it is recommended to upgrade the skin inner layer from silicone rubber to fluororubber (FKM), or upgrade the traditional non-metallic expansion joints to pure titanium TA2 expansion joints. The renovation case of Huadian Luohe Power Generation Co., Ltd. shows that the pure titanium expansion joint can achieve "zero corrosion and zero leakage"。

V. Routine maintenance and common fault treatment

1. Check items regularly

• Appearance inspection: Check the skin every week for damage, bulging and aging
• Bolt tightening: Tighten the flange bolts once in 1 month and once in 3 months after commissioning, and then check them quarterly
• Leak detection: Inspect flange joints and skin joints with soapy water or leak detector
• Temperature monitoring: Infrared temperature measurement. If the surface temperature rises abnormally, the insulation layer may be damaged

2. Common faults and handling

VI. Summary of selection decision

The selection of flue square expansion joints should follow the following decision-making process:

Core points:

• Implementation standards: JB/T 12235-2015 (non-metallic) and GB/T 12777-2019 (metallic) are the latest valid standards
• Anti-corrosion: Desulfurization wet flue is made of fluororubber skin or pure titanium material, and drainage holes must be set for horizontal installation
• Structural optimization: Large circular arc transition is adopted at rectangular expansion corner, which can effectively reduce stress risk
• Installation specifications: consistent flow direction marks, strictly prohibited deformation adjustment, timely removal of transportation rod

Although the flue square expansion joint has a special shape, as long as we grasp the four principles of "cross-section matching, suitable temperature material, corner arc and drainage and anti-accumulation", we can select the right products, install the equipment and protect the system, so as to ensure the safe and stable operation of smoke and air ducts for a long time.