In industrial flue system, expansion joint is the key flexible component connecting pipeline and equipment, and its performance is directly related to the safe operation of the whole desulfurization, denitrification and boiler smoke exhaust system. Many operation and maintenance personnel often only pay attention to the thermal compensation ability of the expansion joint, but neglect its core value in three dimensions: vibration reduction, sealing and corrosion protection. This paper will focus on the efficacy and function of flue expansion joint, and carry out professional analysis from four aspects: vibration and noise reduction, high-efficiency sealing, corrosion protection and equipment protection, so as to help readers comprehensively grasp the deep value of this part.

1. Vibration reduction: the primary barrier to protect the fan and flue structure

During the high-speed flow of flue gas, due to the imbalance of induced draft fan impeller, turbulence of air flow and combustion pulsation, continuous mechanical vibration will occur. This vibration, if transmitted directly through the rigid flue to the adjacent equipment, will result in:

• Bearing wear of induced draft fan is aggravated, and its life is shortened by more than 30%
• Fatigue crack in flue weld due to alternating stress
• Instrument pipe connection is loose, causing measurement error or leakage

The effect and function of flue expansion joint are particularly prominent in vibration reduction. Non-metallic fabric expansion joints use multilayer flexible composite materials (fluororubber, glass fiber cloth, silicone rubber, etc.), and their elastic modulus is only one thousandth of that of metal materials. When the vibration wave propagates along the flue to the expansion joint, the multilayer structure can absorb 60% ~80% of the vibration energy through interlayer friction and material damping effect.

Measured case: After installing non-metallic expansion joint at the outlet of induced draft fan of a 300MW unit, the vibration amplitude of fan side decreased from 0.45mm to 0.08mm, and the vibration acceleration of flue wall decreased by 72%. At the same time, the expansion joint divides the flue into several independent sections, which prevents the vibration in a certain area from propagating to the chimney and absorption tower along the hundreds of meters of flue, and plays a key role as "vibration isolator".

Second, sealing: environmental protection line to prevent smoke leakage

With the increasingly strict environmental emission standards (ultra-low emission requires dust

• Untreated raw flue gas (containing high concentrations of SO₂ and NOx) enters the atmosphere, causing environmental protection penalties
• The abnormal increase of oxygen at the inlet of induced draft fan, diluted flue gas concentration, affected the representativeness of CEMS online monitoring data
• Corrosive gas invades insulation layer and accelerates corrosion of steel structure

The efficacy and function of flue expansion joint in the sealing layer is reflected in its unique laminated structure. Taking a typical non-metallic expansion joint as an example, its airtightness can reach more than 99.99%:

• The inner isolation membrane (fluororubber or PTFE) is in direct contact with flue gas, with high density and leakage rate less than 0.5%
• The intermediate reinforcement layer (glass fiber cloth + stainless steel wire mesh) provides compressive strength and prevents the inner layer from being sucked by negative pressure
• The outer pressure plate and the bolt apply uniform pre-tightening force to ensure that there is no gap between the skin and the flange

Compared with the packing box compensator that needs frequent tightening, the expansion joint achieves "maintenance-free seal after installation", which is especially suitable for severe working conditions such as the inlet of absorption tower (high corrosion) and the outlet of SCR denitrification (high temperature and dust). A comparative study shows that the annual leakage rate of flue with high-quality non-metallic expansion joints is only 0.2%, while the leakage rate of flue with ordinary compensator is as high as 3.5%.

3. Anticorrosion: the key barrier against acidic condensate erosion

The flue gas usually contains acidic gases such as SO₂, SO₃, HCl, HF, etc. When the temperature of the inner wall of the flue is lower than the acid dew point (usually 110℃ ~140℃), strong corrosive condensed acid will be generated. The corrosion rate of ordinary carbon steel flue can reach 3~5mm/year under long-term immersion of condensate acid, and it can be perforated in a few months.

The efficacy and function of flue expansion joint is not passively waiting in anticorrosion, but actively resisting corrosion through material and structural design:

• Fluorine rubber isolation layer: the fluorine content is as high as more than 65%, it has excellent tolerance to sulfuric acid, hydrochloric acid and hydrofluoric acid, and can be used for a long time below 180℃.
• Full PTFE composite structure: For post-denitrification flue with risk of ammonium bisulfate (ABS) deposition, the PTFE material is completely non-adhesive and temperature resistant to 260℃, which is the only reliable anti-corrosion solution.
• Guide tube protection: Built-in 316L stainless steel guide tube, which separates the high-temperature flue gas from the inner layer of the skin. At the same time, the corrosion resistance of the guide tube itself is far better than that of carbon steel.

Furthermore, the flexible feature of the expansion joint avoids cracking of the corrosion protection layer due to thermal stress. When the rigid connection flue is thermally expanded, the glass scale or rubber plate lined with it will peel off due to the deformation of the matrix; However, the displacement at both ends of the expansion joint is absorbed, and the adjacent flue lining layer is hardly stressed, so the service life can be prolonged by 2~3 times.

4. Protect adjacent equipment: prolong the life of expensive components

Flue systems do not exist in isolation and are rigidly connected to expensive equipment such as induced draft fans, heat exchangers (GGH), absorption towers, dust collectors and chimneys. When the flue is displaced due to thermal expansion and contraction and no expansion joint is provided, all displacement loads will be transferred to the equipment interface, resulting in:

• The casing of the induced draft fan is deformed, the impeller rubs against the casing, and in severe cases, it is scrapped by "sweeping the chamber" (the value of a single induced draft fan is tens to millions of yuan)
• The heat exchanger tube sheet is cracked, and the tube bundle leaks, resulting in a short circuit of flue gas
• The rubber lining of the flue at the inlet of the absorption tower falls off due to repeated stretching, and the steel plate of the tower body is corroded and perforated

The effect and function of flue expansion joint also lies in "liberating" the above-mentioned equipment. Through the rational arrangement of the expansion joints, the flue heat displacement at both ends of each equipment is absorbed by the expansion joints individually, and the additional stress bearing by the equipment body is close to zero. Case of a power plant: After installing a large compensation non-metallic expansion joint at the entrance of the absorption tower, the rubber lining life at the entrance of the tower body was extended from 3 years to 9 years, and this alone saved more than 600,000 yuan in maintenance costs.

5. The intrinsic correlation of the three core functions

These three do not exist independently: good sealing reduces erosion of the insulation layer and metal frame by corrosive gas spillage; Effective vibration damping prevents leakage of the seal interface due to fretting wear; While the anti-corrosion design ensures that the expansion joint maintains its elasticity and sealing ability during long-term operation. It can be said that the efficacy and function of flue expansion joint is a trinity system engineering of "vibration reduction-sealing-anticorrosion".

6. How to ensure full play of the efficacy

To make the expansion joint maintain the above three functions for a long time, it is necessary to do:

1. Correct selection: Select the corresponding material (fluororubber vs silicone rubber; non-metal vs metal) according to flue gas temperature, acid dew point and displacement.
2. Specification installation: cold state pre-compression 5% ~8%, the direction of the guide tube must face the flue gas, and the bolts are evenly tightened diagonally.
3. Regular maintenance: check the skin for bulging and powdering every quarter; Measure annually whether the displacement is within the design range; The inner layer is detected for carbonization or acid etching.
4. Timely replacement: When the non-metallic expansion joint runs for 5~6 years or hardens or cracks, it should be replaced as a whole, and should not run with disease.

Ignoring any link will lead to attenuation of vibration damping, sealing or anti-corrosion functions, which in turn will lead to cascading failure.

VII. SUMMARY

The efficacy and function of flue expansion joint can be highly summarized as three core values: vibration reduction, sealing and anti-corrosion. Vibration reduction protects the fan and flue structure to avoid fatigue damage; Seal and block smoke leakage to ensure environmental protection standards; Anti-corrosion Resist acid dew point corrosion and extend system life. These three functions support each other and are indispensable.

In practical engineering, the type should be selected scientifically according to the specific working conditions (temperature, medium, displacement), and the installation and maintenance regulations should be strictly implemented. Only by fully understanding and correctly giving full play to the vibration reduction, sealing and anti-corrosion functions of flue expansion joints can we realize the transformation from "passive emergency repair" to "active prevention", minimize the whole life cycle cost of flue system, and ensure the long-term safe and stable operation of desulfurization and denitrification equipment.