In the flue gas treatment system of coal-fired power plant, chemical industry and metallurgy industry, flue expansion joint corrosion is a common problem that affects the safe operation of equipment and the environmental protection standard. Whether it is the cracking of the bellows of the metal expansion joint or the leakage of the skin of the non-metal expansion joint, the corrosion problem not only leads to flue gas leakage and pollutes the environment, but also increases the energy consumption of the fan and shortens the equipment life. So, what about flue expansion joint corrosion? This paper will start with the analysis of corrosion mechanism, systematically explain the corrosion causes, coping strategies and engineering practice cases of different types of expansion joints, and provide professional solutions for technicians.
1. Main types and causes of flue expansion joint corrosion
To answer "What to do about flue expansion joint corrosion", you first need to clarify the type and root cause of corrosion. According to the engineering failure analysis, the corrosion of flue expansion joint is mainly divided into the following categories:
1.1 Dew Point Corrosion
When the flue gas temperature is below the acid dew point, SO₂ and SO₃ in the flue gas will combine with water vapor to form dilute sulfuric acid, which condenses and deposits on the surface of the expansion joint. This condensate is strongly corrosive to carbon steel and ordinary stainless steel。
Typical characteristics: The corrosion is evenly distributed, mostly occurring at the fluid accumulation at the bottom of the expansion joint, showing honeycomb or pit erosion.
1.2 Stress Corrosion Cracking (SCC)
Stress corrosion is the most common cause of metal expansion joint bellows failure. While subjected to pressure and displacement stresses, corrugated pipes are exposed to corrosive media (sulfur-containing flue gas), which can crack under conditions well below the yield strength of the material。
Typical features: The crack is dendritically branched, originating from the inner surface of the bellows and propagating along the crystal. Failure analysis of Inconel 625 bellows in a catalytic cracking unit shows that the NiS eutectic (melting point only 645 ℃) formed at grain boundary is the key factor inducing stress corrosion。
1.3 Intergranular corrosion
Because the grain boundary of the welding heat affected zone or the material itself is chromium depleted, the grain boundary is preferentially eroded under the action of the corrosive medium, resulting in cracking of the bellows along the grain boundary。
1.4 Chemical Corrosion and Liquid Corrosion of Non-Metallic Expansion Joints
In the desulfurization flue, although the main material (fluororubber, PTFE) of the non-metallic expansion joint skin is corrosion-resistant, there are inherent defects in its fixation mode. Grooves will be formed at the connection between the skin and the flange, and condensed acid water will accumulate during operation. The acid water will gradually penetrate through the skin fabric layer and corrode the fixing bolts, resulting in bolt breakage and skin loosening and leakage。
2. Countermeasures of flue expansion joint corrosion
For different types of corrosion, what to do about flue expansion joint corrosion needs to take differentiated technical measures:
2.1 Anti-corrosion measures of metal expansion joints
(1) Material upgrade
| Operating condition | Recommended Material | Applicable temperature | Corrosion resistance characteristics |
|---|---|---|---|
| Conventional high temperature flue | 304 stainless steel | ≤600℃ | Generally antioxidant, not resistant to chloride ions |
| Sulfur-containing corrosion flue | 316L stainless steel | ≤650℃ | Added Mo, resistant to sulfide and chloride ions |
| Severe corrosion of flue | Inconel625 | ≤700℃ | Nickel-based alloy, resistant to oxidation and vulcanization at high temperatures |
Of particular interest is the application of pure titanium expansion joints. The practice of Huadian Luohe Power Plant proves that the 2mm thick TA2 pure titanium expansion joint is adopted in the desulfurization wet flue, and the surface is as clean as new after one year of continuous operation, thus achieving "zero corrosion and zero leakage". Because that density of titanium is low (4.5g/cm³), the weight of titanium is more than 40% lighter than that of stainless steel under the same size, and the total cost is basically the same as that of non-metallic expansion joint。
(2) Structural Optimization
In view of stress corrosion cracking, it is suggested that the expansion joint bellows be upgraded from a single-layer structure to a double-layer structure, and the concept of "single-layer pressure bearing and double-layer design" is adopted. The double-layer structure can cooperate with the interlayer leakage online monitoring system. Once the inner layer leaks, the outer layer plays a buffer and protection role, and at the same time sends out early warning signals to avoid unplanned shutdown caused by sudden failure。
(3) Operation and maintenance measures
- Furnace shutdown protection: During maintenance, hot air is used to purge the inside of the expansion joint to prevent the condensation and deposition of sulfur-containing gas
- Temperature control: Increase the exhaust temperature above the acid dew point to avoid the formation of condensate
- Regular replacement: Replace the bellows regularly according to the design life (usually 3-5 years) to avoid overdue service
2.2 Anti-corrosion measures of non-metallic expansion joints
(1) Skin material optimization
The skin of non-metallic expansion joint should adopt multi-layer composite structure, and the materials of each layer should be accurately selected and matched according to the working conditions:
| horizon | RECOMMENDED MATERIAL | function |
|---|---|---|
| Outer layer (weather resistant layer) | Fluorine rubber (FKM) | High temperature, corrosion and aging resistance |
| Intermediate layer | PTFE membrane + alkali-free glass fiber cloth | Chemical isolation, enhancement |
| inner layer | Fluorine rubber + stainless steel wire mesh | Corrosion and scour resistance |
For the desulfurization clean flue, the skin material should be fluororubber instead of silicone rubber. Fluororubber has extremely high resistance to acid and alkali and high temperature saturated wet smoke。
(2) Structural improvement-radical cure scheme for effusion problem
The root cause of leakage of non-metallic expansion joints is the accumulation of acid water in the grooves at the skin platens。 Solutions include:
- Drainage design: Add a drainage pipe above DN100 at the bottom of the expansion joint to discharge the condensate in time
- Anti-corrosion of flange end face: Laying glass flake anti-corrosion layer and glass fiber reinforced plastic anti-corrosion layer on steel substrate in sequence, and then installing rubber elastic expansion joint, and fill the layers with silica gel seal
- Bolt protection: The fixing bolts are made of 316L stainless steel, coated with anti-loosening glue during installation, and periodically tightened
(3) Installation process control
- When the skin is installed, it should be unfolded from bottom to top, and the pressure plate bolts should be tightened repeatedly for 2-3 times
- Horizontal flue expansion joint, the outermost skin interface shall adopt the "up-pressing-down" overlap mode to prevent rainwater infiltration
3. Overall replacement scheme: thoroughly solve frequent corrosion
For expansion joints with repeated leakage and low repair value, the whole replacement with expansion joints made of high performance materials can be considered.
Project case: The outlet expansion joint of induced draft fan of 2#boiler of a company has been eroded and washed by high-temperature flue gas for a long time, resulting in serious leakage. Through the accurate formulation of special maintenance plans, the removal of old expansion joints, installation of new corrosion-resistant expansion joints, seal inspection and other procedures were completed, and finally one-time debugging was successful. After the replacement, the corrosion loss of the equipment is significantly reduced, and the operating efficiency of the induced draft fan is greatly improved。
4. Suggestions on anti-corrosion selection of different types of flues
| Flue type | Characteristics of working conditions | Recommended expansion joint types | Key anti-corrosion measures |
|---|---|---|---|
| Boiler tail flue | 120-180℃, dusty | 316L Metal Expansion Joint | Inner lining guide tube anti-scour |
| Desulfurization raw flue | 120-150°C with SO₂ | Fluororubber non-metallic expansion joint | Skin multi-layer composite + bottom drainage |
| Clean flue for desulfurization | 45-80℃, saturated wet flue gas | Pure titanium expansion joint/fluororubber + TA2 composite | Titanium monolithic structure or TA2 skin |
| Catalytic cracking flue | 650-700℃, containing sulfur | Inconel625 Double Bellows | Material Upgrade + Interlayer Monitoring |
| bypass flue | Instantaneous high temperature, intermittent operation | High temperature resistant non-metallic expansion joint | Strengthening insulation |
V. Summary
Faced with the engineering problem of how to deal with the corrosion of flue expansion joint, the core lies in "prescribing the right medicine"-choosing the appropriate technical scheme according to the corrosion type and working conditions. Summary points are as follows:
- Identify corrosion types: dew point corrosion requires temperature control and condensation prevention; Stress corrosion requires material upgrading and stress relief; Intergranular corrosion needs to control the welding process and material selection.
- Material is the key: conventional desulfurization net flue recommends fluororubber non-metallic expansion joint.
- Anti-corrosion of metal bellows: Nickel-based alloys such as Inconel625 are selected for high-temperature sulfur-containing working conditions; Consider double-layer structure + interlayer leakage monitoring to avoid sudden failure; Prevent dew point corrosion measures during furnace shutdown.
- Non-metallic expansion joint anti-corrosion: the skin material is preferably fluororubber; A bottom drainage pipe is added on the structure; The pressure plate bolts are periodically tightened.
- Active maintenance: Establish a regular inspection system for expansion joints, and deal with leakage in time. For the parts with low repair value, decisively replace them with high-performance expansion joints to fundamentally eliminate hidden dangers.
It is recommended that users carry out corrosion risk assessment according to specific working conditions, invite professional and technical personnel to conduct failure analysis when necessary, and formulate targeted anti-corrosion transformation schemes. Only in this way, the corrosion problem of flue expansion joint can be solved from the root cause, and the long-term safe and stable operation of smoke and air system can be ensured.