In the design and purchase of flue gas treatment system, the model selection of flue gas pipeline expansion joint is the key link to determine whether the equipment can run stably for a long time. Faced with many models such as axial type, transverse type, hinge type and non-metallic type, as well as material options such as 304, 316L, 321 and Inconel, engineers and technicians often feel confused. If the wrong model is selected, the expansion joint will fail in advance at the least, and pipeline tearing, equipment damage and even safety accidents will occur at the worst. This paper will systematically explain the professional method of model selection of flue gas pipe expansion joint from the selection process and model interpretation to actual cases.
1. Model composition and interpretation of flue gas pipe expansion joint
1. The basic components of the model
A complete flue gas duct expansion joint model typically contains the following core information:
| element | Description | example |
|---|---|---|
| Nominal diameter (DN) | Pipe inner diameter, in mm | DN80, DN1800 |
| Structural form code | Axial, transverse, hinge, etc. | YSTA (axial internal pressure type) |
| wavenumber | Number of corrugations of bellows | ×6J/F (6 waves) |
| Pressure rating | Design pressure, MPa | 1.0 MPa, 0.25 MPa |
| Material code | Bellows material | 304, 316L, 321 |
| Connection mode | Flanged or welded | J/F (flanged) |
2. Interpretation of common model examples
| Model Example | Interpretation | Application Scenario |
|---|---|---|
| 1.0YSTA80×8J/F | 1.0MPa axial internal pressure type, DN80, 8 waves, flanged connection | Small diameter sampling tube |
| 1.0YSTA300×6J/F | 1.0MPa axial internal pressure type, DN300, 6 waves, flanged connection | Medium diameter flue |
| TN1800*1-J-546 | Diameter 1800mm, pressure 1MPa, for flue gas pipeline | Flue gas pipeline of conversion section |
| WJL1800*1-J-746 | Diameter 1800mm, pressure 1MPa, for outlet of section II | Conversion stage II outlet |
3. Correspondence between model and working condition
The key to correctly select the model of flue gas pipe expansion joint lies in understanding the corresponding relationship between the parameters in the model and the actual working conditions:
- Diameter: Consistent with pipe inner diameter
- Pressure rating: Not lower than design pressure
- Wave number: Determine the amount of compensation. The larger the thermal displacement, the more wave number is required
- Material: Determined according to flue gas temperature and corrosiveness
2. Select the structure form according to the displacement direction
1. Mainly axial displacement
When the thermal expansion of the pipeline mainly occurs in the axial direction, the axial expansion joint should be selected.
Applicable scenario:
- Long straight pipe section
- Straight line segment between two fixed brackets
Model features: YSTA (axial internal pressure type), YSTB (axial external pressure type)
Extreme use conditions of axial expansion joints:
- Outer diameter of bellows × wave number × wave pitch
- Beyond this condition, duplex or hinge structures should be considered
2. Lateral displacement is dominant
When the pipeline is arranged in L-shape or Z-shape and needs to absorb transverse displacement, transverse or universal expansion joints should be selected.
Applicable scenario:
- pipe bend
- Restricted area of equipment entry and exit space
- SCR denitrification reactor inlet and outlet flue
Model characteristics: transverse type (X type), universal hinge type (WJ type)
3. Mainly angular displacement
When the pipeline needs to absorb bending deformation, the hinge expansion joint should be selected. Hinged expansion joints are usually used in pairs or in combination of three (planar three-hinged type), which can absorb both vertical and horizontal displacements simultaneously without blind plate force.
Applicable scenario:
- L-pipe
- Catalytic Cracking Hood Import and Export
Model characteristics: Single hinge type (J type), double hinge type
4. Multidirectional displacement and space-constrained
When there are axial, transverse and angular displacements in pipelines, and the installation space is limited, non-metallic expansion joints should be selected.
Model characteristics: Non-metallic fabric type (FJ type)
3. Select materials according to flue gas temperature and medium
1. Matching of temperature and material
In the model selection of flue gas pipe expansion joint, the material code is an important part of the model. Different temperature ranges correspond to different bellows materials:
| Temperature range | Recommended Bellows Materials | Domestic brand | Applicable working conditions |
|---|---|---|---|
| ≤350℃ | 304, 316L | 0Cr18Ni9, 00Cr17Ni14Mo2 | Low temperature flue gas |
| 350-450℃ | 304、316、321 | 0Cr18Ni9, 1Cr18Ni9Ti | Medium temperature flue gas |
| 450-600℃ | 321, INCONEL600 | 1Cr18Ni9Ti, nickel-based alloy | High temperature flue gas |
| 600-800℃ | INCONEL625, 310S | Nickel-based alloy, 0Cr25Ni20 | Ultra-high temperature flue gas |
| 800-1000℃ | INCOLOY825, 310S | Nickel-based alloy | Flue gas regeneration by catalytic cracking |
Special Notice: Intergranular corrosion will occur in ordinary 304 stainless steel when used for a long time above 450℃, so it must be upgraded to 321 or titanium-containing stabilized stainless steel.
2. Material selection of corrosive media
| Medium environment | Recommended Material | Description |
|---|---|---|
| SO₂-containing raw flue gas | 316L, 316 | Sulfur dioxide corrosion resistance |
| Net flue gas after desulfurization | Fluororubber non-metallic, 316L, 2205, titanium | High Cl⁻¹ environment, chloride ion concentration |
| Catalytic cracking flue gas | INCONEL, INCOLOY | Stress corrosion + high temperature oxidation |
3. Material upgrade path
When the expansion joint fails repeatedly in the same position, the material should be upgraded according to the following path:
304 → 316L → 321 → 2205 → INCONEL → Titanium
4. Calculate and determine the wavenumber according to the compensation amount
1. Calculation formula of thermal displacement
In the model selection of flue gas pipe expansion joint, the determination of wave number needs to calculate the thermal displacement first:
Δ L = α × L × Δ T
- α: Line expansion coefficient of pipeline (carbon steel 12×10⁻⁶/℃, stainless steel 16×10⁻⁶/℃)
- L: length of pipe section between two fixed brackets (mm)
- Δ T: Difference between operating temperature and installation temperature (℃)
Example: 10m carbon steel pipe, installation temperature 20℃, working temperature 450℃:
Δ L =12×10⁻⁶ ×10000×430=51.6 mm
2. Wavenumber selection
The compensation amount of the axial expansion joint is proportional to the wave number. When selecting the model, ensure that:
Rated compensation ≥1.2× calculated thermal displacement
| Nominal diameter DN | wavenumber | Axial compensation amount (mm) | Applicable tube length (Δ T =400℃) |
|---|---|---|---|
| 80 | 8 waves | 30 | ≤6m |
| 80 | 10 waves | 37 | ≤7.5m |
| 200 | 4 waves | 56 | ≤11.5m |
| 200 | 6 waves | 84 | ≤17.5m |
| 300 | 4 waves | 65 | ≤13.5m |
| 300 | 6 waves | 97 | ≤20m |
5. Select special structure according to the thrust requirements of equipment
1. Thrust-sensitive occasions
In situations where the pipeline thrust is strictly restricted, such as the inlet and outlet of catalytic cracking hood and the inlet and outlet of fan, the model selection of flue gas pipeline expansion joint should give priority to the structure without blind plate force.
| Expansion joint type | Blind plate force | Scope of application |
|---|---|---|
| Axial type | Yes (P × A) | Ordinary pipes, carried by fixed brackets |
| hinge type | without | The inlet and outlet and elbow of the cigarette hood |
| Universal type | without | Multidirectional displacement sensitive equipment |
| Pressure balance type | Complete elimination | Equipment interface thrust is strictly limited |
| Non-metallic type | without | Connection of various fans and equipment |
2. Fixed bracket design
When choosing an axial expansion joint, the bearing capacity of the fixed bracket must be checked:
F = P × A
- F: blind plate force (N)
- P: working pressure (Pa)
- A: Effective area of bellows (m²)
VI. Practical cases of model selection
Case 1: Selection of High Temperature Long Straight Pipe Section
Working conditions: Boiler outlet to economizer flue, DN1200, temperature 450℃, tube length 15 meters, installation temperature 20℃
Calculation: Δ L =12×10⁻⁶ ×15000×430=77.4mm
Selection result: Axial metal expansion joint, DN1200, material 321, wave number determined according to compensation amount (≥93mm), flange connection
Case 2: Selection of inlet of catalytic cracking hood
Working condition: L-shaped pipe at the inlet of the hood, DN1800, temperature 680℃, need to absorb both vertical and horizontal displacement, strictly restrict the thrust of the hood
Selection result: Planar three-hinge expansion joint (balance ring universal type), material INCONEL625, design without blind plate force
Case 3: Selection of Desulfurization Clean Flue
Working conditions: Absorption tower outlet to chimney, DN2000, temperature 50℃, wet saturated flue gas, high Cl⁻¹ concentration
Selection results: Non-metal expansion joint, fluororubber skin, metal frame for glass flake anti-corrosion, set drainage hole
VII. Type selection checklist
After completing the model selection of flue gas pipe expansion joint, please confirm item by item:
- The diameter is consistent with the pipe
- Design pressure ≥ working pressure
- Design temperature ≥ maximum operating Temperature +50℃
- Material suitable for flue gas temperature, humidity and corrosiveness
- Rated compensation ≥1.2× calculated thermal displacement
- The structure without blind plate force is selected for the thrust sensitive equipment
- Matching design of guide bracket and fixed bracket
- Horizontal installation with drainage hole (wet smoke)
- Correct direction of guide tube (small end towards incoming flow)
sum up
The model selection of flue gas pipe expansion joint is a systematic project, and the correct selection should follow the "three-step method":
- Select structure according to displacement direction: axial displacement → axial type, transverse displacement → transverse/universal type, multi-directional displacement → non-metallic type, thrust sensitive → hinge/pressure balance type
- Select materials according to temperature: 304/316L for ≤350℃, 321 for 350-600℃, INCONEL/310S for ≥600℃, and fluorine rubber non-metal or titanium for wet flue gas
- Select the wave number according to the compensation amount: Δ L = α × L × Δ T, and the rated compensation amount is ≥1.2 times the calculated value
A complete model should contain: diameter + structure form + pressure class + wave number + material + connection method. The correct model selection ensures the service life and system safety of the expansion joint from the source. It is suggested that a professional organization should be entrusted to conduct pipeline stress analysis and special selection of expansion joints in the design stage.