In flue gas discharge system, waste heat recovery pipeline and desulfurization and dust removal device, expansion joint is the key component to ensure the safe operation of pipeline. However, wrong flue expansion joint selection can directly lead to premature equipment failure, air leakage and even system shutdown. In the face of high temperature, corrosion, vibration and other changing working conditions, how to scientifically complete the selection of flue expansion joint? This paper starts from the five core steps, combining the material, structure and installation limitations, and provides a landing selection guide to help enterprises avoid common pitfalls.
1. Clarify the working condition parameters-the data basis of type selection
Any rigorous flue expansion joint selection must start with complete working condition data acquisition. Be sure to collect the following four categories of parameters:
- Temperature parameters: maximum/minimum continuous operating temperature, instantaneous limit temperature (e.g. accident conditions). For example, the temperature of the flue inlet section of coal-fired boiler can reach above 850℃, while the flue after desulfurization is only 50-80℃.
- Pressure parameters: system design pressure, working pressure and possible positive or negative pressure values (significant difference before and after induced draft fan).
- Media composition: the concentration of corrosive substances such as SO₂, NO₂ and Cl⁻¹ in the flue gas, as well as the dust content and particle size distribution.
- Displacement quantity and frequency: axial, transverse and angular displacements caused by thermal expansion of pipelines, and the number of cycles caused by starting and stopping of equipment.
Tip: Ignoring either item may cause the flue expansion joint selection to fail. For example, by providing only temperature without informing media corrosiveness, selected stainless steel bellows may experience stress corrosion cracking within one week in a chlorine-containing environment.
Second, selection of corrugated pipe material-the determinant of life
Material is the most direct influence on durability in flue expansion joint selection. According to the flue gas characteristics, the following matching schemes are recommended:
| Operating condition characteristics | RECOMMENDED MATERIAL | Limit of tolerance |
|---|---|---|
| Temperature ≤400℃, weak corrosion | 304 stainless steel | High cost performance |
| Temperature ≤650℃, moderate corrosion | 316L/321 stainless steel | Intergranular corrosion resistance |
| Temperature ≤850℃, strong corrosion | Inconel 625/825 | Nickel-based alloy |
| Contains high concentrations of Cl⁻¹ or H₂SO₃ | Composite PTFE liner or C-276 alloy | Pitting resistance |
Note: If the budget is limited but the temperature is high in the selection of flue expansion joint, a double-layer bellows structure can be considered-the inner layer is made of high-temperature resistant alloy and the outer layer is made of ordinary stainless steel, which not only ensures sealing performance but also controls cost.
3. Determine the structural form-the core of compensation ability
The structural form of the expansion joint determines which directions of displacement it can absorb and the amount of compensation. Common types and applicable scenarios are as follows:
- Single axial type
It is suitable for straight flues with only axial displacement and long pipe sections. If the displacement of flue expansion joint exceeds 50mm when selecting the type, it is recommended to adopt a structure with pull rod to prevent the internal pressure thrust from damaging the fixed bracket. - Complex hinge type
It is composed of two bellows plus an intermediate pipe and a hinge plate, which can absorb large lateral displacement. Often used at "L" or "Z" flue turns. - Pressure balance type
With its own balanced bellows, it can eliminate the blind plate force caused by internal pressure on the fixed bracket. When the flue internal pressure exceeds 30kPa and the load-carrying capacity of the fixed bracket is limited, this type of structure is the best choice for flue expansion joint selection. - Universal hinge type
It can absorb the angular displacement in any plane, and is suitable for the arrangement with limited space and complicated pipeline direction.
Selection logic: first calculate the combined displacement in three directions, and then compare the allowable compensation values of each type of single group of bellows. Usually, it is recommended that the actual use value should not exceed 80% of the rated value, and the safety margin should be kept.
4. Configuration of flow guide tube and heat insulation layer-protection design cannot be saved
Many users neglect the internal protection when selecting the flue expansion joint, resulting in the corrugated pipe being eroded or burned out in a short time. The following two configurations should be considered mandatory:
- Guide tube (inner bushing)
Install on the inside of the bellows, guide the flue gas to flow along the center, and avoid the high-speed dusty flue gas directly washing the bellows trough. It is recommended that the thickness of the guide tube is not less than 3mm, and the direction must be consistent with the direction of air flow. - Thermal insulation
When the flue gas temperature exceeds 550 ℃, the corrugated metal material will undergo high temperature creep. A ceramic fiber layer with temperature resistance above 1000℃ should be filled between the guide tube and the bellows, and the thickness should be calculated by temperature drop-usually every 10mm thickness can reduce the outer wall temperature by about 80-100℃.
The qualified flue expansion joint selection scheme must be clearly marked with "whether it contains guide tube + heat insulation layer", otherwise it will be easily returned during the drawing review stage.
V. Interface and installation space verification-the last step of landing
After the theoretical calculation is completed, it is necessary to check the actual constraints on the site:
- Flange connection vs welding: Flange connection should be selected for pipe sections that need frequent maintenance, but attention should be paid to the problem of bolt slack at high temperature. It is recommended to use disc spring preloading. Welded connections have better sealing, but are difficult to remove.
- Minimum installation distance: Sufficient length of straight pipe section should be reserved on both sides of the bellows, which is generally required to be 1.5-2 times the diameter of the flue, so as to avoid direct impact of turbulence.
- Fixed bracket position: Flue expansion joint selection must be coordinated with pipe bracket design. The main fixing brackets shall be arranged at both ends of the expansion joint, and the spacing between the guide brackets shall not exceed 4 times the diameter of the pipe.
Common Selection Error Alert
According to maintenance statistics, the following three types of errors cause more than 80% of expansion joints to fail early:
- Only select model according to normal temperature design, the actual operating temperature is higher than 200℃
- Common carbon steel bellows is selected without considering acid dew point corrosion
- Additional lateral displacement caused by equipment foundation settlement is ignored