In the flue gas treatment system, the fan is the key power equipment, but the high temperature, vibration and thermal displacement of the pipeline generated during its operation often become the main cause of system failure. The expansion joint at the inlet and outlet of the flue gas fan is just to solve the problems of stress concentration, leakage and even equipment damage caused by thermal expansion and contraction, mechanical vibration and installation deviation in the inlet and outlet pipelines of the fan. This paper will systematically explain why expansion joints must be installed at the inlet and outlet of flue gas fan and how to implement them correctly from the aspects of working principle, core benefit, selection key points and installation specification.
1. Why must expansion joints be installed at the inlet and outlet of flue gas fan
Flue gas fans are usually connected to dust collectors, desulfurization towers, or chimneys. When the fan is running, the inlet is negative pressure, the outlet is positive pressure, and the temperature difference can reach 150℃-400℃. If the inlet and outlet of the fan are connected by rigid pipes, there will be three major risks:
- Thermal stress damage: The high-temperature flue gas expands the pipeline, generating hundreds of kilograms of thrust, which directly acts on the fan housing, causing casing deformation, impeller friction and even bending of the main shaft.
- Vibration transmission: Imbalance of fan impeller or airflow pulsation will generate broadband vibration. Without flexible isolation, the vibration will be transmitted along the pipeline to buildings, supports and connected equipment, causing fatigue fracture.
- Installation deviation: In the field construction, the fan flange and the pipe flange are difficult to be absolutely centered, and the forced connection will generate additional bending moment and reduce the bearing life.
After the expansion joint is added to the inlet and outlet of the flue gas fan, the expansion joint absorbs the axial, transverse and angular displacement through the elastic deformation of its bellows or fabric layer, and cuts off the vibration propagation path, thus protecting the fan body and the pipeline system.
2. The core function of adding expansion joint to the inlet and outlet of flue gas fan
1. Isolate the vibration of equipment and prolong the life of fan
The fan rotates at high speeds (usually 1450-2980rpm) creating constant mechanical vibrations. After installing expansion joints at the inlet and outlet, the vibration transmission rate can be reduced by more than 80%. The measured data show that the bearing replacement cycle of the fan without expansion joint is about 6-12 months; Extended to 3-5 years after installation.
2. Compensate for thermal displacement and prevent pipeline stress concentration
The flue gas temperature difference between the inlet and outlet of the fan is large. For example, the flue gas at the inlet of the fan of the sintering head is about 150°C, which drops to ambient temperature when the machine is stopped. A section of carbon steel pipe with a length of 10 meters, a temperature rise of 130℃ will produce an axial elongation of about 15mm. The expansion joint easily absorbs this displacement, avoiding the duct from pushing and pulling the fan housing.
3. Simplify pipeline alignment and reduce installation difficulty
Ideally, the coaxiality between the fan flange and the pipe flange should be less than 0.5mm. In practical construction, the deviation often reaches 2-3mm. The lateral compensation capacity of the expansion joint (typically ±5mm to ±20mm) tolerates these errors, eliminating the need to repeatedly cut the rewelded pipe.
4. Absorb airflow pulsation and reduce noise
The fan blade passes through frequency and its harmonics will generate obvious pulsating pressure. The flexible wall surface of the expansion joint can convert the pressure fluctuation into a small deformation of the wall surface, thus attenuating the noise. After installing an expansion joint at the fan outlet, the noise at 1 meter can usually be reduced by 5-10dB (A).
5. Provide emergency protection to prevent the spread of accidents
When the duct behind the fan is blocked or the valve is misclosed, the outlet pressure may suddenly rise. The expansion joint with limit rod or blasting ring can be used as the weak link to prioritize pressure relief or deformation alarm to avoid the fan shell bursting.
3. Selection guide of expansion joint at inlet and outlet of flue gas fan
According to the fan type, flue gas temperature and media corrosiveness, the following three schemes are common:
| Expansion joint type | Applicable fan scenarios | Temperature range | Displacement compensation | advantage | shortcoming |
|---|---|---|---|---|---|
| Metal bellows expansion joint | High temperature flue gas fan (inlet/outlet) | ≤600℃ | Mainly axial, less lateral | High temperature and pressure resistance | Unable to absorb large lateral displacements |
| Non-metallic fabric expansion joint | Desulfurization booster fan, corrosive flue gas | ≤200℃ (special up to 400℃) | Multi-directional large displacement | Corrosion resistance, good vibration isolation and low cost | Not resistant to high temperatures, low strength |
| rubber elastic expansion joint | Low temperature flue gas, ventilator | ≤120℃ | Axial and transverse | Excellent vibration isolation and simple installation | Inresistant to high temperatures and oils |
Suggestions for selection:
- For high-temperature dusty flue gas fans (such as sintering and cement kiln tail), metal bellows expansion joints are preferred, and the material is not less than 316L. If it contains chloride ions, it will be upgraded to 254SMO or Inconel625.
- For wet desulfurization booster fans, non-metallic fabric expansion joints, lined with PTFE or fluororubber, are recommended due to the high corrosion of flue gas and low temperature (~80℃).
- For general ventilators or low-temperature flue gas, rubber elastic expansion joints are the most cost-effective.
4. Key technical points of installation position and direction
1. Difference between entrance and exit
- Fan inlet: negative pressure state (-2kPa to-10kPa). The expansion joint should have sufficient stiffness against negative pressure to prevent deflation. It is recommended to install an internal guide tube or reinforcing ring.
- Fan outlet: Positive pressure state (2kPa-30kPa). The expansion joint needs to withstand pressure thrust, and must be equipped with a limiting tie rod to prevent excessive elongation.
2. Installation sequence
- First install the fan and align and fix it.
- The expansion joint is pre-stretched or pre-compressed to 50% of the design displacement (for metal expansion joints).
- Connect the pipe flange to ensure that the expansion joint is not twisted.
- When tightening the bolts, tighten them evenly at diagonal angles, and the torque shall be according to the standard value.
- Welding or flame cutting after installation of the expansion joint is strictly prohibited to avoid burning the corrugation or fabric layer.
3. Guide bracket setting
Guide brackets must be provided at 1-2 times the tube diameter upstream and downstream of the expansion joint. For long pipelines, a guide bracket should be added every 4-6 meters to ensure that the displacement is concentrated in the direction of the expansion joint and avoid pipeline buckling.
V. Common faults and maintenance points
Even if you select and install correctly, you still need to pay attention after long-term operation:
- Corrugated pipe corrosion leaks: Check the flue gas dew point temperature. If it is lower than the exhaust temperature, condensate acid may be generated. Solution: Increase the exhaust temperature or upgrade the corrosion resistant alloy.
- Fabric layer breakage: Usually due to fly ash wear or high temperature aging. Check the appearance quarterly and replace if the fibers are found to be exposed or hardened.
- Limit rod bending: indicates that the actual displacement of the pipe exceeds the design value. The pipeline thermal expansion calculation should be reviewed and a second expansion joint should be added if necessary.
- Rubber expansion joint bulge: internal cord break. Shutdown for replacement immediately to avoid bursting and injuring people.
It is recommended to establish an inspection ledger, record the appearance and displacement indication value of the expansion joint every month, and conduct leak detection once a year.
VI. Call to Action: To ensure the safe operation of fan, start with the correct installation of expansion joints
The expansion joint at the inlet and outlet of the flue gas fan is by no means an indispensable accessory, but a core component to ensure the long-term stable operation of the fan. Ignoring it, you may face serious accidents such as fan shell cracking, bearing burning, pipe tearing, etc., and the loss of a single shutdown can be as high as hundreds of thousands of yuan.