In flue system design, the selection of expansion joint is a common controversy point: How to choose between non-metallic fabric expansion joint and metal bellows expansion joint? Many engineers will ask: Can metal expansion joints be used on the flue? The answer is yes-metal expansion joints can not only be used, but also have irreplaceable advantages in scenarios such as high temperature, high pressure, circular flue, etc. However, it also has strict applicable conditions and selection contraindications. This paper will comprehensively answer this question from four dimensions: adaptability of working conditions, structural characteristics, economy and typical failure cases.
1. Applicable scenarios of metal expansion joints on flue
Can you use a metal expansion joint on the flue depends on the specific operating parameters. Metal bellows expansion joints are preferentially recommended for the following working conditions:
1.1 High temperature flue gas (> 400℃)
The continuous operating temperature of the inner material (fluororubber, silicone rubber, PTFE) of the non-metallic fabric expansion joint usually does not exceed 250 ℃. When the flue gas temperature exceeds 400℃ (such as boiler outlet and kiln flue), the non-metallic skin will quickly carbonize and burn through. At this time, the metal bellows expansion joint is made of stainless steel (304, 316L, Inconel625, etc.), which can withstand high temperatures above 800℃ (need to be lined with heat insulation layer).
1.2 High pressure or high negative pressure flue
Non-metallic expansion joints have limited pressure resistance and are usually only suitable for within ±30kPa. And the metal bellows expansion joint can withstand the pressure of 0.1MPa ~2.5MPa. Metal expansion joints are a safer option for positive pressure dust collection systems, flue gas recirculation pipes, or deep negative pressure (
1.3 Circular flue with limited space
The metal bellows expansion joint has a compact structure, and its radial size is 30% ~50% smaller than that of the non-metal expansion joint. Metal expansion joints are easier to arrange in pipe galleries or equipment-intensive areas with narrow laying space.
1.4 Places with strict fire and explosion protection requirements
Non-metallic skin belongs to combustible materials (although flame retardant is added, the flame retardant performance decreases after long-term aging). All-metal expansion joints must be used on flues that convey flammable gases (e.g. gas, VOCs) or where there is a risk of deflagration.
2. Limitation of metal expansion joint on flue
Although the metal expansion joint excels in the above scenario, it is not a panacea. Can a metal expansion joint be used on a flue-it does, but the following questions must be addressed:
2.1 Higher risk of corrosion than non-metals
The desulfurization flue gas contains acidic gases such as SO₂, SO₃, HCl and HF. When the flue temperature is lower than the acid dew point (110℃ ~140℃), the condensed acid will strongly corrode the stainless steel. 304 stainless steel is very prone to pitting and stress corrosion cracking in chloride ion (Cl⁻¹) -containing condensate. Even if it is upgraded to 316L, it may perforate within 2~3 years in the wet desulfurization net flue. While the fluororubber inner layer of the non-metallic expansion joint is almost immune to condensate acid.
2.2 Limited displacement compensation capability
The axial compensation of single metal bellows expansion joint is usually only 20~40mm, while the non-metal expansion joint can reach 50~80mm. For long-distance rectangular flues with large displacements, metal expansion joints need to be arranged in greater numbers or expensive duplex structures.
2.3 Failure to isolate vibration
Although metal bellows has certain elasticity, its vibration damping effect is far inferior to that of non-metal composite materials. In strong vibration areas such as the outlet of the induced draft fan, the metal expansion joint will transmit the vibration to the downstream equipment, while the non-metal expansion energy saving absorbs 60% ~80% of the vibration energy.
2.4 Higher design requirements for the guide tube
The metal expansion joint must be equipped with accurately designed guide tube, otherwise the high-speed dusty flue gas will directly wash the bellows trough, resulting in rapid abrasion and thinning. However, the guide tube of non-metallic expansion joint requires relatively loose.
III. Detailed Comparison of Metallic and Non-Metallic Expansion Joints
To help with decision-making, the following table provides a comprehensive comparison of the performance differences between the two in flue applications:
| Comparative Item | Metal bellows expansion joint | Non-metallic fabric expansion joint |
|---|---|---|
| Applicable temperature | -200℃ ~800℃ (with thermal insulation layer) | -50℃~250℃ |
| Pressure resistance | ≤2.5MPa | ≤ ± 30kPa |
| Axial compensation amount | 20~40mm (single type) | 50~80mm |
| Lateral compensation amount | ±10~ ±20mm | ±30~ ±50mm |
| Vibration damping performance | General (damping ratio | Excellent (damping ratio 0.1~0.2) |
| Acid resistant condensate | Poor (prone to stress corrosion) | Excellent (fluororubber/PTFE) |
| Resistant to high temperature flue gas scour | Excellent (deflector required) | Poor (inner layer is prone to ablation) |
| Service Life (Typical) | 6~10 years | 5~8 years |
| Cost per piece (same size) | High (about 1.5~2 times) | Low |
| Weight | heavy | Light (about 1/5 of metal) |
| Installation difficulty | Higher (precise alignment required) | lower |
Core conclusion: Can metal expansion joint be used in flue? Metal expansion joint is a better choice under the working conditions of high temperature (> 250℃), high pressure (> 30kPa), circular flue and no strong corrosive condensate; Non-metallic expansion joints are more suitable at low temperatures (
4. Suggestions on the selection of typical flue segments
According to the actual working conditions at different locations of the flue, the following recommendations are made:
| Flue location | Flue gas temperature | Corrosivity | Recommended expansion joint types | reason |
|---|---|---|---|---|
| Boiler outlet to economizer | 350~420℃ | Dry flue gas, low corrosion | Metal | Temperature exceeds non-metal limit |
| SCR denitrification inlet | 300~380℃ | Contains NH₃, a small amount of SO₃ | Metallic (316L) | High temperature and the presence of ammonium salt, the non-metal is easy to age |
| Air preheater outlet | 120~150℃ | Contains SO₃, dust | Non-metallic (fluororubber) | Below acid dew point, acid corrosion resistance is required |
| Dust collector inlet and outlet | 120~180℃ | High dust concentration | Non-metal or metal + wear-resistant guide tube | Low non-metallic costs; If the temperature fluctuates greatly, select the metal |
| Absorption tower inlet | 90~150℃ | Strong acidic, wet smoke | Non-metallic (fluororubber) | Severe condensation acid, intolerable by metals |
| Clean flue (in front of chimney) | 50~80℃ | Low concentration acid mist | Non-metallic or metal | Both are acceptable, but non-metals are more cost effective |
| Bypass flue (accidental discharge) | Instantaneous> 400℃ | dry flue gas | Metal (Inconel) | Possible overtemperature burning of non-metals |
5. Failure case and avoidance of metal expansion joint on flue
Even if the answer to confirm can metal expansion joints be used on flues is yes, wrong design or installation can still lead to premature failure. The following are typical problems and avoidance measures:
5.1 Stress Corrosion Cracking
Case: 316L metal expansion joint was adopted in the desulfurization clean flue of a sintering machine. After 18 months of operation, there were many cracks in the bellows trough.
Reason: Although there is a demister in the clean flue, the residual chloride ions (from the process water) combine with the condensed water, which causes the stress corrosion cracking of chloride ions under the action of tensile stress.
Avoidance: Upgraded to C-276 Hastelloy or 254SMO Super Stainless Steel; Or switch to non-metallic expansion joints.
5.2 Smoke Abrasive Perforations
Case: The metal expansion joint of the front flue of the dust collector was in operation for 3 years, and the bellows was worn out at the crest.
Reason: The guide tube is not installed, and the dusty flue gas directly impacts the windward surface of the bellows.
Avoidance: A wear-resistant guide tube with a thickness of ≥4mm must be installed, and the gap between the guide tube and the bellows shall not be less than 15mm.
5.3 Varus instability
Case: The metal expansion joint of the net flue behind the induced draft fan was inverted when the negative pressure dropped suddenly.
Reason: The design negative pressure is only-8kPa, but it actually reaches-13kPa. Over-limit leads to the instability of bellows.
Avoidance: The design negative pressure is checked according to-15kPa; Install an external limit pull rod; Or replace it with a non-metallic expansion joint (no risk of varus).
6. How to select the metal expansion joint correctly
If after comprehensive evaluation, it is confirmed that the metal expansion joint can be used on the flue and is suitable for this working condition, the following selection steps should be followed:
- Specify the parameters: maximum/minimum temperature, pressure (positive or negative pressure), flue gas composition (especially Cl⁻,F⁻¹ concentration), dust content, required compensation (axial, transverse, angular).
- Select material:
- Dry flue gas, chlorine free: 304 stainless steel
- Contains trace chlorine (
- High chlorine (> 100ppm), high sulfur: Inconel625, C-276 Hastelloy
- High temperature (> 600°C): Incoloy800H
- Determine the structure:
- Single type (no tie rod): used for axial compensation-based and low pressure situations
- Duplex (with intermediate tube): for large axial or transverse compensation required
- Hinged/universal hinged: for angular compensation
- Check the stability: Calculate the column instability and plane instability critical loads of the bellows to ensure that the actual loads do not exceed 80% of the critical value.
- Configure the guide tube and heat insulation layer: the guide tube must be added in dust-containing working conditions; High temperature working condition lined with ceramic fiber thermal insulation.
VII. SUMMARY
Can metal expansion joints be used in the flue? Yes, but subject to certain working conditions. Metal expansion joints have the advantages that non-metals cannot replace in scenarios with high temperature (> 250℃), high pressure (> 30kPa), circular flue and high fire protection requirements. However, non-metallic expansion joints are more reliable in the presence of acidic condensate, chloride ion corrosion, large compensation amounts or strong vibration damping are required.
The correct selection strategy is: According to the actual temperature, pressure, corrosion and displacement requirements of flue sections, the targeted design of "one section, one policy" is carried out. For example, a metal is used for a high-temperature section, and a non-metal is used for a low-temperature wet section. It is not recommended to adopt the same expansion joint type uniformly for the entire flue. When the working conditions are between the two (e.g. temperature 220℃, acid dew point existence), acid-resistant non-metallic expansion joints (fluororubber or PTFE) should be preferred, because the stress corrosion risk of metal expansion joints under such working conditions is much higher than the aging risk of non-metallic materials.
Through scientific comparison and accurate selection, engineers can give full play to the advantages of metal expansion joints, while avoiding their limitations, and realize the optimal reliability and economy of flue systems.