We can almost always see expansion joints in the flue gas emission systems of power plant boilers, steel sintering machines, cement kilns and chemical heating furnaces. For many non-professionals, why should a seemingly "complete" metal flue be deliberately added to a "flexible" structure? Why add expansion joints to the flue? This involves multiple considerations of thermodynamics, material mechanics and equipment safety. Simply put, the core function of the expansion joint is to absorb the thermal displacement of the flue due to temperature changes, isolate equipment vibrations, and compensate for installation errors. If there is no expansion joint, the rigid connected flue will twist, crack or even collapse under the action of thermal stress, vibration and external force, which seriously threatens production safety and environmental protection standards. This paper will systematically answer this question from three dimensions: thermal displacement compensation, vibration isolation and equipment protection.
Reason 1: Absorb heat displacement and prevent pipeline stress damage
The most fundamental reason why flue should add expansion joints is thermal expansion and contraction. The flue is installed at normal temperature, and the internal flue gas temperature can reach hundreds or even thousands of degrees Celsius during operation (such as 350-400℃ in the inlet section of the boiler flue). Taking a section of carbon steel flue with a length of 100 meters as an example, when the temperature rises from 20 DEG C to 400 DEG C, its thermal elongation can reach:
Δ L = α × L × Δ T =12×10⁻⁶ ×100×380 ≈ 456mm
That is, the flue will be elongated by nearly half a meter. If the flue employs a rigid fixed connection without expansion joints, the huge thermal elongation can be converted by constraints into compressive stresses up to hundreds of tons — enough to buckle and deform the flue siding, crack the welds, and push down the fixed brackets. The expansion joint absorbs this part of the displacement through its flexible structure (bellows or non-metallic skin), reducing the thermal stress to a safe range. This is the most direct engineering logic for adding expansion joints to the flue.
Reason 2: Isolate equipment vibration and protect downstream components
In the flue gas system, the rotating equipment such as induced draft fan and booster fan will generate continuous mechanical vibration when it runs. If the flue inlet and outlet of the fan are rigid connected, the vibration energy will be transmitted along the flue to the downstream desulfurization tower, dust collector, chimney and other equipment, causing:
- Fatigue cracking of anti-corrosion layer lining of desulfurization tower
- Resonance damage of chimney barrel wall
- Flue support and connecting bolt loose
- Noise pollution exceeds the standard
The multilayer composite skin structure of expansion joints (especially non-metallic expansion joints) has good elasticity and damping characteristics, which can effectively absorb more than 80% of vibration energy. The second answer to why flues have expansion joints is to cut off the vibration propagation path and protect expensive downstream equipment from premature damage.
Reason 3: Compensate for installation and settlement errors and simplify construction
Manufacturing and installation errors are inevitable when large flue systems are prefabricated in sections and assembled on site. At the same time, a certain amount of settlement will occur in the operation of the flue support foundation. If there is no expansion joint, the construction personnel may need to repeatedly cut, grind and re-weld the flange opening with excessive alignment error, which is time-consuming and laborious and affects the construction period.
The expansion joint has some flexibility and can compensate for ±10mm installation deviation and ±5mm foundation settlement displacement. Therefore, the third reason why the flue should be added with expansion joints is valid: it not only reduces the installation difficulty and shortens the construction period, but also reserves a safety margin for uneven foundation settlement in the future.
Types and Applicable Scenarios of Expansion Joints
After understanding why expansion joints are added to the flue, it is also necessary to understand the application scenarios of different types of expansion joints:
| Expansion joint type | Applicable working conditions | Key advantages | Main Limitations |
|---|---|---|---|
| Metal axial type | High temperature clean flue gas, straight section of pipeline | High temperature resistance (up to 800 ℃), strong pressure bearing capacity | Small compensation amount and blind plate force |
| Metal universal hinge type | Space-constrained, multi-directional displacement | Absorbable combined displacement | Complicated structure and high price |
| Non-metallic (fabric) type | Wet desulfurization clean flue gas, strong corrosive medium | Corrosion resistance, good vibration isolation, no blind plate force | Temperature resistance ≤200℃, low pressure capacity |
| rubber-lined expansion joint | Clean flue acidic condensation environment | Anti-corrosion + pressure | Temperature resistance ≤120℃ |
Serious consequences of not adding expansion joints
In engineering practice, accidents caused by failing to install or choose the wrong expansion joint are not uncommon. Typical cases are as follows:
- A thermal power plant: There is no expansion joint in the flue at the outlet of the induced draft fan. After 6 months of operation, the weld at the connection between the flue and the desulfurization tower is torn, and the flue gas leakage leads to an environmental fine of 1.2 million yuan.
- A steel plant: The large flue of the sintering machine uses low-grade expansion joints, and the bellows corroded and cracked only 8 months after it was put into operation, and the loss of unplanned shutdown for maintenance exceeded 3 million yuan.
- A chemical plant: Without considering the thermal displacement of the flue, the fixed bracket was pushed askew, which damaged the adjacent reactor nozzle.
These cases prove from the negative side why the flue should be added with expansion joints-it is not an indispensable "optional accessory", but a "necessary accessory" to ensure the safe operation of the system.
Selection and maintenance recommendations
It is only the first step to correctly answer why expansion joints should be added to the flue. Reasonable selection is equally important as standard maintenance:
- Selection stage: Type and specification are comprehensively determined according to flue gas temperature, medium corrosiveness, compensation requirement and installation space. Metal expansion joints are selected for high-temperature clean flue gas, and non-metal or rubber-lined expansion joints are preferred for low-temperature corrosive flue gas.
- Installation stage: Position strictly according to the drawings, reserve the cold tightness value, symmetrically tighten the flange bolts, and remove the transportation fixture after installation.
- Operation stage: Regularly inspect the surface of the expansion joint for leakage, deformation and corrosion, and replace the non-metallic skin when it is aged or the metal bellows is cracked in time.
epilogue
Why add expansion joints to the flue? The answer can be summarized in three sentences: add to absorb thermal displacement to prevent stress damage; Added to isolate vibration and protect downstream equipment; It is added to compensate for errors and simplify installation and construction. Although the expansion joint is small, it bears the great responsibility of safe, stable and environmentally friendly operation of the flue gas system. If you choose the right thing, install it well and maintain it in place, it can silently guard it for ten or eight years; Ignoring its existence comes at the cost of leaks, downtime and high maintenance costs.