In industrial flue gas treatment system, the choice of material of flue gas inlet and outlet expansion joint is directly related to the service life of equipment and the safe operation of the system. Whether it is the desulfurization system of coal-fired power plant, the energy recovery system of catalytic cracking unit, or the flue gas pipeline in chemical production, the working conditions and environments of expansion joints are different-high temperature, corrosion, dust, vibration and other factors are intertwined, which puts forward differentiated requirements for materials. Correct selection of flue gas inlet and outlet expansion joint material is the key prerequisite to ensure the long-term stable operation of equipment. This paper will systematically analyze the performance characteristics, applicable scenarios and selection principles of various materials, and provide professional reference for engineers and technicians.
1. Classification and performance comparison of expansion joint materials
The materials of flue gas inlet and outlet expansion joints are mainly divided into metal materials and non-metal materials. Different materials have their own advantages and disadvantages in temperature resistance, corrosion resistance and compensation ability.
1. Metal bellows material
The core component of metal expansion joint is corrugated pipe, and its material choice directly affects the temperature resistance and corrosion resistance.
| Material grade | Temperature resistance range | Corrosion resistance characteristics | Applicable working conditions |
|---|---|---|---|
| 304/304L | ≤500℃ | Suitable for general oxidizing environment, not resistant to chloride ion | Hot air, dry smoke |
| 316/316L | ≤550℃ | Contains molybdenum element, and its pitting corrosion resistance is better than 304 | SO₂, steam, weak acid environment |
| 321 | ≤600℃ | Contains titanium stabilizing element, resistant to intergranular corrosion | High temperature flue gas, no heat treatment after welding |
| 310S | ≤1000℃ | High chromium and nickel, excellent oxidation resistance at high temperature | Boiler outlet, high temperature flue |
| 2205 duplex steel | ≤250℃ (wet) | Cl⁻pitting resistance, stress corrosion resistance | Desulfurization wet flue |
| Pure titanium TA2 | ≤250℃ | Hardly corrosive in wet chloride ion environment | Wet desulfurization flue |
According to JB/T 6171-2013 "Multi-layer metal bellows expansion joint" and GB/T 12777-2019 national standard, the design and manufacture of metal expansion joint shall follow the corresponding specifications. For high temperature conditions, nickel-based alloys such as 321 or INCONEL are recommended materials for bellows; For media containing sulfur dioxide, SUS316, 316L have good corrosion resistance.
2. Non-metallic skin material
Non-metallic expansion joints are composed of multiple layers of composite materials, and different levels of materials perform their respective duties.
| Material | Temperature resistance upper limit | Corrosion resistance characteristics | Applicable working conditions |
|---|---|---|---|
| Neoprene rubber | 120℃ | Oil and aging resistance | Low temperature drying flue gas |
| Silicone rubber | 205℃ | Heat and weather resistance | Medium temperature flue gas |
| Fluorine rubber (FKM) | 250℃ | Acid, oil and oxidation resistant | Desulfurization wet flue gas |
| Ethylene propylene monomer (EPDM) | 150℃ | Ozone, acid and alkali resistance | General corrosive environment |
| PTFE/glass fiber composite | 250℃ | Strong acid resistance, non-adhesion | Highly corrosive environment |
2. Guide for selection of zones according to working conditions
The flue gas characteristics of different sections are significantly different, and the material selection of flue gas inlet and outlet expansion joints must be "one area, one policy".
1. High temperature original flue (boiler outlet to dust collector)
The flue gas temperature in this section is high (300-600℃) and the dust content is large. The main challenges are high temperature resistance and erosion resistance.
Material recommendation:
- Metal expansion joint: 321 or 310S stainless steel for bellows
- Guide tube: same material or more wear-resistant material, thickness ≥3mm
- Note: Intergranular corrosion may occur in the normal 304 stainless steel in this temperature range, so it is not suitable for use
In the actual project case, the expansion joint of the flue gas inlet and outlet of the air preheater of a power plant is upgraded with 304 stainless steel metal expansion joint. For higher temperature catalytic cracking flue gas, the bellows need to use nickel-based alloys such as INCONEL600 and 625.
2. Low temperature wet flue (inlet and outlet of desulfurization tower to chimney)
This section is the most corroded area with low flue gas temperature (45-80℃), saturated humidity, strong acidity (pH 2-4) and high Cl⁻¹ concentration.
Material recommendation:
- Non-metal expansion joint: the skin adopts fluororubber + PTFE multilayer composite structure
- Metal frame: make glass flake heavy anti-corrosion coating (thickness ≥2mm)
Typical case: The desulfurization wet flue expansion joint of Huadian Luohe Power Generation Co., Ltd. originally adopted a non-metallic structure. Two sets of skins and one set of frames were replaced successively, and the pressure plate was upgraded to 316L material, all of which failed to effectively solve the water leakage problem. In 2016, pure titanium TA2 expansion joint (thickness 2mm, designed as a split single corrugated structure) was innovatively adopted, which achieved "zero leakage" after continuous operation for one year. During maintenance, the inner surface was as clean as new, and the weld was intact.
This case fully shows that in the high Cl⁻¹ environment of wet desulfurization, the average life of 316L does not exceed two years, while titanium can achieve "zero corrosion and zero leakage".
3. Medium temperature flue (dust collector outlet to desulfurization tower inlet)
The flue gas temperature in this section is about 120-180℃, and the dust content is reduced, but it still contains corrosive components such as SO₂.
Material recommendation:
- Non-metallic expansion joints: fluorine or silicone rubber skin
- Metal expansion joint: 316L stainless steel
The metal expansion joint at the outlet of induced draft fan of a power plant is upgraded with 316L stainless steel. In chemical flue gas pipelines, material 321 (equivalent to 0Cr18Ni10Ti) is also a common choice.
4. Catalytic cracking flue gas (energy recovery system)
The flue inlet and outlet of the flue gas turbine in the catalytic cracking unit has high temperature, large pipe diameter and sensitive force, so the material requirements are extremely strict.
Material recommendation:
- High temperature segment: INCONEL600, 625 or INCOLOY800, 825 nickel-based alloy
- Medium temperature section: SUS321 stainless steel
- Bellows material is selected according to the working temperature: 321/INCONEL is selected for 450-600℃, and INCOLOY is selected for> 600℃
III. Material configuration of typical application scenarios
The following table summarizes the recommended configuration of flue gas inlet and outlet expansion joint materials under different working conditions:
| Application Scenario | Expansion joint type | Core Material | Special requirements |
|---|---|---|---|
| High temperature flue at boiler outlet | Metal | 321/310S/INCONEL | Wear resistance of guide tube |
| SCR denitrification import and export | Metal | 316L/321 | NH₃ corrosion resistance |
| Dust collector inlet and outlet | Nonmetallic | Silicone rubber + glass fiber | Dust bag structure |
| Raw flue gas inlet of desulfurization tower | Nonmetallic | Fluororubber + PTFE | Acid-resistant, drainage hole |
| Clean flue gas outlet of desulfurization tower | Non-metallic/pure titanium | Fluororubber multilayer/TA2 | Heavy anti-corrosion coating |
| Wet flue at chimney entrance | Pure titanium | TA2 | Split design |
| Catalytic cracking hood inlet | Metal | INCONEL600/625 | Planar three-hinge type |
| Catalytic Cracking Hood Outlet | Metal + constant force spring | 321/INCOLOY | Design without blind plate force |
Selection decision-making process
The correct material selection of flue gas inlet and outlet expansion joint should follow the following steps:
Step 1: Clarify the working condition parameters
- Maximum/Minimum Operating Temperature
- Media composition (SO₂, SO₃, Cl⁻¹, dust concentration)
- Pressure range (positive/negative)
- Three-way displacement
- Start-stop frequency (affects fatigue life)
Step 2: Primary Type by Temperature
- > 400 °C: metal expansion joint (321/310S/nickel-based alloy)
- 120-400℃: metal or non-metal can be used
Step 3: Precisely determine the material according to corrosiveness
- Dry smoke, low corrosion: 304/316L or silicone rubber
- Sulfur-containing wet flue gas: fluororubber non-metallic or 316L (note Cl⁻Restriction)
- High Cl⁻wet flue gas (after desulfurization): pure titanium TA2 or 2205 duplex steel
Step 4: Comprehensive economic assessment
- Non-metallic expansion joints have low initial cost, but short replacement cycle (3-5 years)
- The initial cost of titanium expansion joint is the same as that of non-metal (due to the low density and thin thickness of titanium), but the service life can reach more than 10 years, with "zero maintenance" cost
V. Material failure cases and lessons
Case 1: Ordinary stainless steel used in high-temperature sulfur-containing environment
The technical agreement requires that the material of the ignition air duct expansion joint of a 410t/h boiler is not less than 0Cr25Ni20Si2 (310S), but the actual use of ordinary stainless steel leads to rapid corrosion and damage of the expansion joint in high-temperature flue gas. Lesson: High-temperature stainless steel must be used in high-temperature sulfur-containing environment, and the material should not be degraded.
Case 2: 316L for desulfurization wet flue
The survey shows that the average life of 316L material expansion joint in the actual use of desulfurization wet flue is no more than two years. Lesson: 2205 duplex steel or pure titanium should be used in high Cl⁻wet flue gas environment.
Case 3: Repeated water leakage of non-metallic expansion joint
The non-metallic expansion joint of Huadian Luohe Company has successively replaced two sets of skins and one set of frames, but the water leakage problem has not been solved. Finally, the titanium expansion joint has been completely cured. Lesson: Repeated failure should be considered for material upgrading, not repeated replacement.
sum up
The material selection of flue gas inlet and outlet expansion joint is a systematic project, which must be comprehensively decided according to flue gas temperature, medium corrosiveness, displacement requirements and other working conditions parameters:
- High temperature drying section (> 400℃): 321, 310S stainless steel or INCONEL nickel-based alloy is selected
- Intermediate temperature sulfur section (120-400℃): 316L stainless steel or fluororubber non-metallic expansion joint
- Low temperature wet flue (
- Catalytic cracking high temperature section: Nickel-based alloy such as INCONEL600/625
Correct material selection can not only prolong the service life of the equipment, reduce the maintenance cost, but also ensure the long-term safe operation of the flue gas system. It is suggested to entrust professional institutions to conduct working condition analysis and material selection evaluation in the design stage, so as to avoid the vicious circle of "improper material selection and repeated failure" from the source.