In the wet desulfurization system, the vertical section of the original flue is the key transition area connecting the horizontal flue with the inlet of the desulfurization tower. The expansion joint installed on the vertical section faces different technical requirements from the horizontal section because of its special installation orientation and stress state. It is of great significance to deeply understand the design points, selection principles and common faults of expansion joints in the vertical section of the original flue of desulfurization for ensuring the long-term stable operation of desulfurization system. This paper will systematically analyze the special working conditions, structure selection and maintenance strategies of vertical section expansion joints, and provide practical guidance for engineers and technicians.
1. Special working conditions of expansion joint at vertical section of desulfurization original flue
The expansion joint at the vertical section of the original desulfurization flue is installed in the steering area of the flue from horizontal to vertical, and its working environment has the following remarkable characteristics:
- The flue gas temperature is relatively high: usually between 120~180 ℃, instantaneously reaching above 200 ℃
- Large dust content: the dust concentration of the original flue gas can reach 30~50g/Nm³, and the dust may settle and accumulate in the vertical section
- High concentration of corrosive gases: contains acidic gases such as SO₂, SO₃, HCl
- Downward accumulation of condensate: The condensed acid in the vertical section will flow downward along the inner wall of the flue and concentrate at the low point of the expansion joint
- Bearing self-weight load: the flue and the expansion joint body in the vertical section bear large axial tension
- The displacement direction is complex: both axial displacement caused by flue thermal expansion and lateral displacement caused by flue oscillation
Different from horizontal flue expansion joints, the expansion joints at the vertical section of the original flue of desulfurization should not only compensate for thermal displacement, but also bear the tensile force caused by the self-weight of the flue, and at the same time, it should deal with the corrosion problem caused by the downward accumulation of condensate. These special working conditions put forward higher requirements for its structural design and material selection.
2. The core function of the expansion joint at the vertical section of the original flue of desulfurization
1. Absorb vertical thermal expansion
The vertical section flue produces axial (vertical) thermal elongation when the temperature increases. Taking a section of 15-meter-high vertical flue as an example, the working temperature is 150℃, the ambient temperature is 20℃, and the temperature difference is 130℃. The expansion amount of carbon steel material is about: Δ L =0.012×15×130=23.4mm. The expansion joint at the vertical section of the desulfurization original flue needs to absorb this axial displacement.
2. Bear flue dead weight load
Unlike the horizontal section, the entire weight of the flue in the vertical section needs to be transferred to the building structure through the expansion joints and brackets. If the expansion joint is improperly selected, it may cause fatigue failure or seal failure due to excessive tension for a long time.
3. Compensate for flue swing displacement
The vertical flue will oscillate to a certain extent under wind load, earthquake or airflow pulsation, which requires the expansion joint to have sufficient lateral compensation ability.
4. Prevent condensate accumulation and corrosion
The acidic condensate produced by the inner wall of the vertical section will flow down the flue. The structural design of the expansion joint should avoid the formation of liquid accumulation areas and prevent local corrosion and perforation.
3. Main points of selection of expansion joint at vertical section of desulfurization original flue
Correct type selection is the prerequisite to exert the performance of the expansion joint at the vertical section of the original flue of desulfurization. The following are the key selection parameters:
1. Structural form selection
The structural form of vertical section expansion joint is directly related to its stress state and service life:
| Structure Type | peculiarity | Applicable vertical segment height | Recommendation |
|---|---|---|---|
| single bellows type | Simple construction, but unable to withstand large pulling forces | ≤5 meters | Not recommended |
| Tie rod type with limit | The limit pull rod can bear the flue weight and protect the bellows | 5~15 meters | recommend |
| Load-bearing ring type | Set up dedicated load-bearing structures to transfer loads to the building | ≥15 meters | Highly recommended |
| hinged type | Can withstand large lateral displacement, suitable for high chimney | Chimneys | Special operating conditions |
For most vertical sections of desulfurization original flue (height 10~20 meters), it is recommended to choose non-metallic or metallic expansion joints with limiting tie rods, which bear the self-weight of the flue, and bellows are only responsible for absorbing heat displacement.
2. Material selection
The material of the expansion joint at the vertical section of the original desulfurization flue shall meet the requirements of high temperature resistance, corrosion resistance and fatigue resistance at the same time:
- Non-metallic expansion joint: preferential choice of fluororubber or PTFE composite fabric, can withstand 180℃ high temperature and acid condensate corrosion, light weight, easy vertical installation
- Metal expansion joint: choose 316L or 254SMO stainless steel, but pay attention to the risk of chloride ion corrosion. If the chlorine content in the flue gas is high, it is recommended to choose the metal expansion joint lined with PTFE
3. Condensate discharge design
This is the most overlooked selection point of vertical section expansion joint. The expansion joint at the vertical section of the desulfurization original flue must be equipped with a reliable condensate discharge device:
- Opening DN25~ DN50 drainage holes at the lowest point of the expansion joint
- The drainage hole shall be connected to the sump or ditch to avoid acid liquid corrosion of the surrounding equipment
- For non-metallic expansion joints, drainage holes should be passed through flanges and fabric layers, and leakproof sleeves should be provided
4. Guide tube setting
The direction of flue gas flow in the vertical section may be upward (into the desulfurization tower) or downward (leading out of the boiler). Regardless of the flow direction, a guide tube shall be set inside the expansion joint at the vertical section of the original desulfurization flue:
- The guide tube can prevent smoke from directly washing the inner wall of the bellows or fabric
- The guide tube should extend to 1/2~2/3 of the length of the expansion joint to avoid vortex
- The material is 304 or 316L stainless steel with a thickness of ≥3mm
4. Installation points of expansion joint at vertical section of desulfurization original flue
Correct installation is the key to ensure the long-term stable operation of the expansion joint at the vertical section of the original flue of desulfurization. There are significant differences between the vertical segment installation and the horizontal segment:
1. Load-bearing measures
Before installing the vertical section expansion joint, the upper flue must be temporarily supported to prevent the flue from falling during installation. Once the installation is complete, gradually transfer the load to the limiting tie rod or load-bearing structure.
2. Pre-stretching/Pre-compression
The vertical section expansion joint shall be pre-stretched during installation to compensate for the initial deformation caused by the flue dead weight. The pre-stretching amount is usually 30% ~50% of the designed axial compensation amount, and the specific value is determined by calculation.
3. Requirements for centering accuracy
The vertical section expansion joint must be installed to ensure that its concentricity deviation from the flue is ≤5mm, otherwise it will cause additional bending stress to the bellows or fabric layer.
4. Drain line connection
Water seal or valve shall be provided on the drain hole connection to prevent smoke from leaking from the drain pipe. The water seal height should be calculated according to the flue pressure, usually not less than 200mm water column.
V. Common Fault Modes and Diagnosis
According to the field statistics, the faults of the expansion joint at the vertical section of the original flue of desulfurization are mainly concentrated in the following aspects:
| Fault phenomenon | Main reasons | Diagnostic Methods | Treatment measures |
|---|---|---|---|
| Falling deformation of expansion joint | Limit rod failed or not set | Comparison between the measured length of expansion joint and the design value | Add limit pull rod to adjust flue support |
| Fluid corrosion at bottom | Blocked or uninstalled drainage hole | Open the drain hole to check the water flow; Endoscopic examination of the inner wall | Dredge or add drainage holes and replace corroded parts |
| Water stain leakage of fabric layer | Condensate seeps out of the flange connection | Check flange bolt torque; pH strip test exudate | Tighten bolts, replace gaskets |
| Fatigue crack of bellows | Displacement exceeds limit or frequent start-stop | Coloring flaw detection or airtightness test | Replace expansion joints and optimize operation mode |
| Guide tube falling off | Welding fatigue or vibration | Shutdown internal inspection | Re-weld and add riveted fixing |
Operation, Maintenance and Life Management
Key points of daily inspection
- Every week: Observe the appearance of the expansion joint, and record whether there is leakage, deformation and abnormal noise
- Monthly: Check whether the drainage hole is unobstructed and whether the pH value of the discharged condensate is abnormal (normal should be> 4)
- Quarterly: Check flange bolt torque, re-tighten loose bolt
- Every half year: measure the actual length of the expansion joint and compare it with the original record to determine whether there is permanent deformation
Factors affecting life span
The service life of the expansion joint at the vertical section of the original flue of desulfurization is mainly affected by the following factors:
- Fluctuation amplitude of smoke temperature: Frequent alternation of hot and cold accelerates material fatigue
- Condensate corrosion degree: the higher the SO₃ concentration, the higher the acid dew point, the more serious the condensation corrosion
- Number of starts and stops: More than 50 starts and stops per year can significantly shorten life
- Installation quality: Whether pre-stretching is accurate and drainage holes are unblocked is the key
Expected useful life
On the premise of reasonable selection, standardized installation and regular maintenance:
- Non-metallic expansion joints: 5~8 years
- Stainless steel metal expansion joint: 6~10 years (the chloride ion concentration should be controlled
- Lined with PTFE metal expansion joint: 8~12 years
7. Main differences with horizontal section expansion joints
| Comparative Item | Vertical section expansion joint | Horizontal expansion joint |
|---|---|---|
| Primary force | Axial tension (flue dead weight) | Axial pressure/tension (thermal expansion) |
| displacement direction | Mainly in vertical direction | Predominantly horizontal |
| Condensate accumulation | Concentrated at the bottom, forced drainage is required | Evenly distributed, discharged naturally |
| Key points of selection | With limiting tie rod or load-bearing structure | Ordinary axial type is enough |
| Installation difficulty | Higher (load bearing and pre-stretching required) | normal |
| Failure mode | Falling, liquid accumulation and corrosion are mainly | Mainly fatigue and wear |
VIII. Summary
The expansion joint at the vertical section of the original desulfurization flue is different from the conventional horizontal flue expansion joint in terms of type selection, installation and maintenance because of its special installation orientation and stress state. The core conclusions can be summarized as follows:
- Special design is required for special working conditions: the expansion joint of vertical section must consider three major problems: thermal displacement compensation, flue self-weight bearing and condensate discharge, all of which are indispensable
- The core of type selection is load-bearing and drainage: the structure with limit tie rod is the preferred scheme of vertical section; Drainage holes must be provided at the lowest point, which is the key to prevent corrosion failure
- Installation quality determines initial life: Vertical segment installation must be temporarily supported, accurately pre-stretched and strictly aligned, any negligence may lead to early damage
- The guide tube is essential: Whether it is a non-metal or metal expansion joint, the internal guide tube can effectively prevent the direct flushing of smoke and prolong the service life
- Regular maintenance focuses clearly: monthly inspection of drainage holes, quarterly tightening of bolts, and annual internal inspection of deflector tubes. These three actions can cover more than 80% of maintenance needs
For desulfurization system designers and operation and maintenance engineers, it is an important link to ensure the long-term safe operation of the whole original flue system to fully understand the particularity of the expansion joint at the vertical section of the original flue of desulfurization and give specific consideration in the design and selection. A vertical section expansion joint with reasonable design, standard installation and proper maintenance can completely achieve a stable service period of 5~8 years or even longer.