In the industrial flue gas emission system, the pipeline has been subjected to multiple tests such as high temperature, corrosive medium and mechanical vibration for a long time.Function of Expansion Joint in Flue Gas PipeIt is the key to solve the stress damage caused by thermal expansion and contraction, compensate the displacement of pipeline, absorb vibration and prolong the life of equipment. This paper will systematically analyze the irreplaceability of expansion joint in flue gas pipeline from the aspects of working principle, core function, selection key points and maintenance strategy.
1. Working principle of expansion joint in flue gas pipeline
The flue gas pipeline is usually connected to boilers, dust collectors, desulfurization towers and other equipment, and the temperature can be as high as 200℃-600℃ or even higher during operation. Metallic materials can develop significant thermal expansion or cold contraction under temperature changes. If the pipeline is a rigid connection, the thermal stress will lead to weld cracking, flange leakage, bracket deformation, and even safety accidents.
Expansion joints (also known as compensators) absorb axial, lateral and angular displacements of the pipe through their flexible structural elements, such as bellows, rubber bodies or layers of fabric. When the flue gas temperature rises, the pipe is elongated and the expansion joint is compressed and deformed; When the temperature decreases, the pipe shrinks and the expansion joint springs back. This dynamic compensation process makes the pipeline system always in a low stress state.
Second, the core function of expansion joint in flue gas pipeline
1. Absorb heat displacement and prevent structural damage
The temperature difference of flue gas pipe from cold state to hot state is extremely large. For example, a 30-meter-long section of carbon steel pipe can thermally elongate up to about 100 mm at a temperature rise of 300 °C. If the expansion joint is not installed, the thrust can easily exceed the support bearing capacity. Expansion joints protect pipes and connecting equipment by converting linear displacement into elastic deformation through corrugated or bent structures.
2. Isolate mechanical vibration and reduce noise
Fans, compressors and other equipment will generate low-frequency vibration when they are running. If vibration is transmitted along the pipeline to buildings or sensitive instruments, it will cause fatigue damage. The flexible element in the expansion joint can effectively cut off the vibration propagation path, and at the same time absorb the noise generated by airflow pulsation, so as to improve the factory environment.
3. Compensate for installation errors and simplify alignment requirements
In the field construction, the pipeline axis inevitably has millimeter-level deviation. The expansion joint can compensate for the lateral or angular deviation of ±5mm to ±30mm, avoid the additional stress caused by forced matching, and speed up the installation progress.
4. Resist corrosive media and extend system life
Flue gas often contains sulfur oxides, nitrogen oxides and water vapor, and acidic condensate is easy to form at low temperature. Expansion joints lined with corrosion-resistant alloys (such as 316L, Inconel) or fluororubber can effectively resist pitting and intergranular corrosion, and avoid leakage caused by thinning of pipeline wall thickness.
5. Adapt to emergency conditions and improve safety
The flue gas temperature or pressure may suddenly change instantaneously under the condition of starting and stopping the furnace, load shedding or accident. The expansion joint can be used as a "fuse" to deform preferentially, absorb impact energy and prevent the main pipe from rupturing. Some metal expansion joints also have a limit rod design to prevent catastrophic failure caused by excessive stretching.
3. Comparison of common types of expansion joints in flue gas pipelines
| Type | Applicable temperature | Displacement compensation capability | Corrosion resistance | Cost |
|---|---|---|---|---|
| Metal bellows expansion joint | ≤800℃ | Axial/transverse/angular | Medium (optional alloy) | Medium and high |
| Non-metallic fabric expansion joint | ≤200℃ (special up to 400℃) | Multi-directional large displacement | Excellent (PTFE/silica gel) | Middle |
| rubber elastic expansion joint | ≤120℃ | Axial/transverse | Good (EPDM, etc.) | Low |
| Sleeve expansion joint | ≤450℃ | Axial only | poor | Low |
For high-temperature sulfur-containing flue gas,Metal bellows expansion jointStill the mainstream choice; The desulfurization inlet pipeline with large dust content and frequent cleaning is recommendedNon-metallic fabric expansion jointBecause of its ease of replacement and corrosion resistance.
4. Key points of selection and installation
- Accurate calculation of displacement: Calculate axial, transverse and angular displacements according to pipeline length, temperature difference and material linear expansion coefficient respectively. It is recommended to reserve 20% margin to cope with the fluctuation of working conditions.
- Matching Media Characteristics: The contents of Cl⁻,SO₃ and H₂O in flue gas and the dew point temperature were measured. When the concentration of chloride ion is higher than 100ppm, stainless steel containing molybdenum or nickel-based alloy must be selected.
- Consider fatigue life: When metal expansion joints are designed according to ASME standards, the number of cycles (usually not less than 1000 times) should be clearly required. Low-stress design should be selected for flue gas pipelines with frequent start-and-stop.
- Install guide brackets: The expansion joint alone cannot control the instability of the pipeline. Guide brackets and fixed brackets must be provided at specified positions to ensure that the displacement is concentrated in the direction of the expansion joint.
- Retaining Cold Draw Value: For long pipelines, the expansion joint is pre-stretched (cold drawn) during installation, which can effectively reduce the actual stress amplitude at high temperature
V. Routine maintenance and fault warning
Even with proper design, expansion joints can still age. It is recommended to check the following items quarterly:
- appearance: Whether the metal bellows has scratches, pits or corrosion pits; Whether the fabric layer is delaminated and hardened.
- leak: Use soapy water or ultrasonic leak detector to detect welds and flange surfaces.
- displacement indication: Check that the limit lever position exceeds 80% of the design stroke.
- guide tube: If wear and fall off, the high-speed flue gas will directly impact the bellows, resulting in early fatigue fracture.
Once an abnormality is found, it should be replaced in time. Never weld or apply additional loads to the expansion joint body.
PROFESSIONAL ADVICE AND CALL TO ACTION
Correctly understand and applyFunction of Expansion Joint in Flue Gas PipeIt can not only ensure the stable operation of environmental protection facilities such as desulfurization and dust removal, but also reduce the annual shutdown loss caused by pipeline maintenance.