1. Flue expansion joint of desulfurization tower: indispensable flexible connection in wet desulfurization system
In the wet flue gas desulfurization system, the original flue gas is washed by the absorption tower and turned into clean flue gas, and then discharged into the chimney through the flue. However, there are significant thermal displacements, vibrations, and corrosive media between the desulfurization tower and the flue, and between the flue and the chimney. The purpose of flue expansion joint of desulfurization tower is in this complicated working condition, which plays multiple functions such as absorbing heat displacement, absorbing shock and noise, sealing flue gas and resisting corrosion. Different from ordinary flue expansion joints, the flue gas at the inlet of desulfurization system is high-temperature raw flue gas (up to 150-180℃), and the outlet is low-temperature saturated net flue gas (45-60℃). The temperature difference between the two is large and corrosive, which puts forward higher requirements for the performance of expansion joints. Without a reliable expansion joint, the rigid connection between the desulfurization tower and the flue will crack under the action of thermal stress, resulting in acid flue gas leakage, environmental protection emissions exceeding the standard and even system shutdown. This paper will systematically expound the core use of flue expansion joint of desulfurization tower from function decomposition, structure selection to engineering application.
2. Absorb heat displacement: prevent damage of rigid connection between flue and tower body
2.1 Relative displacement due to temperature difference
To understand the use of flue expansion joint of desulfurization tower, we must first recognize the relative thermal displacement between desulfurization tower and flue. The desulfurization tower is usually carbon steel lined with glass flakes or rubber, and the operating temperature is stable at 50-60℃; However, the original flue at the inlet may bear high-temperature flue gas of 120-180℃. The temperature difference between the two can reach over 100℃, and the relative thermal elongation produced is very considerable.
Taking a 20m long inlet flue as an example, when rising from normal temperature (20°C) to 150°C, the thermal elongation of the carbon steel flue is about:
Δ L = α × L × Δ T =12×10⁻⁶ ×20m ×130℃ ≈ 31mm
If there is no flue expansion joint of desulfurization tower to absorb this elongation, the connecting weld or flange between flue and tower body will bear huge thermal stress, which will inevitably crack under long-term action.
2.2 Compensation of foundation settlement and installation deviation
In addition to the thermal displacement, the desulfurization tower and the flue have independent supporting foundations, respectively. Over running time, uneven settlement may occur between the two, resulting in misalignment at the connection. One of the purposes of flue expansion joints of desulfurization towers is to absorb these lateral and angular installation deviations and settlement displacements, so as to avoid stress concentration at rigid joints.
Sealing flue gas: the core function of ensuring environmental protection emission standards
3.1 Prevention of sulfur-containing flue gas leakage
In the desulfurization system, the flue gas contains corrosive gases such as SO₂ and SO₃. Once leaked, it not only causes heat loss, but also causes the risk of environmental protection exceeding the standard. In the use of flue expansion joint of desulfurization tower, sealing function is the most direct and important aspect.
The sealing performance of the expansion joint is reflected in the following aspects:
- Bellows Integrity: The metal bellows itself is non-porous and completely blocks smoke
- Flange connection seal: Match with corrosion-resistant gasket (e.g. EPDM, fluororubber or PTFE) to ensure that the interface is not leaky
- Guide tube protection: built-in guide tube to prevent smoke from directly washing the inner wall of the bellows and prolong the sealing life
3.2 Adapt to low pressure and large flow conditions
The flue at the inlet of the desulfurization tower is usually slightly negative pressure (-1~ -3kPa) or slightly positive pressure (1~5kPa), which belongs to the low pressure condition. The purpose of flue expansion joint of desulfurization tower is in this pressure range, not only to ensure sufficient rigidity and sealing performance, but also to not excessive rigidity to affect the compensation ability. Therefore, the expansion joint for desulfurization usually uses thinner wall thickness bellows (1.0-1.5mm) to reduce the stiffness while ensuring the seal.
4. Resistance to corrosion: coping with the acidic environment of wet desulfurization
4.1 Corrosion Risk of Inlet Original Flue
The raw flue gas in the inlet flue of the desulfurization tower contains SO₂, SO₃, HCl, HF and dust particles. When the flue wall temperature is below the acid dew point, a strong acidic condensate will form on the inner wall (pH can be as low as 1-2). In the application of flue expansion joint of desulfurization tower, corrosion resistance is the core consideration of material selection.
For the original flue at the inlet, the corrugated pipe should be made of at least 316L stainless steel, which has better pitting resistance than 304 in the environment containing chloride ions. For more corrosive conditions, 2205 duplex stainless steel or higher grade alloys should be considered.
4.2 Condensate corrosion of outlet clean flue
The net flue gas at the outlet of the desulfurization tower is low-temperature saturated flue gas (45-60℃, relative humidity 100%), which is easy to form condensate in the flue and the inner wall of the expansion joint. These condensates absorb residual SO₂ and are likewise strongly acidic (pH 2 – 4). The use of the desulfurization tower flue expansion joint at the outlet side likewise includes resistance to such condensate corrosion.
For the expansion joint of the clean flue, in addition to corrosion resistance of the corrugated pipe material, a condensation discharge port should be set to discharge the condensate accumulated at the bottom in time to avoid local corrosion caused by liquid accumulation.
4.3 Comparison of commonly used corrosion-resistant materials
| Materials | Applicable working conditions | Resistance to Cl⁻Capacity | Relative cost |
|---|---|---|---|
| 304 | Dry flue gas, no condensation | poor | 1.0 |
| 316L | General desulfurization conditions | medium | 1.3-1.5 |
| 317L | High Cl⁻¹ desulfurization condition | Better | 1.8-2.0 |
| 2205 duplex steel | Strong corrosion desulfurization condition | Excellent | 2.5-3.0 |
5. Vibration absorption: Protect the desulfurization tower and flue structure
5.1 Isolation of fan vibration
The desulfurization system is equipped with a booster fan or induced draft fan, and the operation of the fan will produce continuous low-frequency vibration. If the flue is rigidly connected to the desulfurization tower, the vibration will be transmitted directly to the tower body, which may cause fatigue cracking of the tower liner (glass flakes, rubber). One of the uses of the flue expansion joint of desulfurization tower is to act as a flexible connector to absorb and isolate the vibration transmitted by the fan.
5.2 Flue Gas Pulsation and Airflow Excitation
Airflow pulsations and pressure fluctuations occur during flue gas flow, especially through baffle doors, elbows, and reducers. The flexible structure of the expansion joint can cushion these pulsating shocks and protect downstream equipment.
6. Structural selection: Different uses in different locations
6.1 Desulfurization tower inlet expansion joint
The expansion joint located between the desulfurization tower and the original flue is mainly used for:
- Absorb the high temperature thermal displacement of the original flue (large temperature difference)
- Resistance to SO₂/SO₃ corrosion of raw flue gas
- Exposure to transient overpressure that may exist
Recommended construction: metal bellows expansion joint with guide tube, 316L or 2205 material.
6.2 Desulfurization tower outlet expansion joint
The expansion joint located between the desulfurization tower and the clean flue is mainly used for:
- Compensate for the relative displacement between the net flue and the tower body
- Resistance to cryogenic condensate corrosion
- Set a bottom condensation discharge port to prevent condensate from accumulating
Recommended construction: metal bellows expansion joint with guide tube and condensation discharge port.
6.3 Bypass flue expansion joint (if applicable)
For systems with bypass flues, the expansion joint also needs to withstand the instantaneous high temperature impact when the bypass is opened. The purpose of flue expansion joint of desulfurization tower increases the requirement of high temperature resistance under bypass condition, and the corrugated pipe material should be heat-resistant stainless steel.
| Installation location | Primary Use | Type of recommendation | Material requirements |
|---|---|---|---|
| Tower entrance | Absorb high temperature thermal displacement and resist corrosion | Axial metal expansion joint | 316L/2205 |
| Tower outlet | Compensate for displacement and resist condensation corrosion | Axial type belt condensation | 316L |
| bypass | High temperature shock resistance | Metal expansion joint with thermal insulation | 321/309S |
VII. Key points of installation and maintenance
7.1 Cold drawing treatment during installation
In order to ensure the full use of flue expansion joint of desulfurization tower, cold drawing (pre-displacement) treatment should be carried out according to the design temperature during installation. Generally, pre-stretching or pre-compression is carried out according to 50% of the design displacement, so that the expansion joint is in the most balanced position of stress at the working temperature.
7.2 Setting and maintenance of condensation outlet
For net flue expansion joints, the bottom condensation vent is an essential configuration. During operation, the condensation discharge tube should be kept unblocked, and the condensation discharge tube should be checked regularly for In winter, heat tracing and heat preservation measures should be taken to prevent the condensate from freezing and damaging the expansion joint.
7.3 Periodic Inspection and Life Management
| Check Items | Frequency | Contents |
|---|---|---|
| Appearance inspection | Monthly | Corrugated pipe crack, soot trace, condensation exhaust state |
| Wall thickness detection | Annual (downtime) | Ultrasonic thickness measurement to evaluate corrosion thinning |
| Leak detection | Quarterly | Smoke for negative pressure, leak detector for positive pressure |
VIII. Summary
The uses of flue expansion joints in desulfurization towers can be summarized as four core aspects: "absorbing heat displacement, sealing flue gas, resisting corrosion, and isolating vibration". These four uses together ensure the safe, environmental protection and stable operation of the desulfurization system under complex working conditions.
In terms of absorbing heat displacement, the expansion joint compensates for the relative thermal elongation between the desulfurization tower and the flue due to temperature difference (which can reach more than 30mm) and the misalignment caused by foundation settlement. In terms of sealing flue gas, the bellows and flange sealing structure of the expansion joint effectively prevent the leakage of sulfur-containing flue gas and avoid exceeding the environmental protection standard. In terms of corrosion resistance, according to the different corrosive environments of raw flue gas at the inlet and clean flue gas at the outlet, corrosion resistant materials such as 316L and 2205 are reasonably selected, and condensation discharge ports are arranged to prevent condensate accumulation. In terms of vibration isolation, the flexible structure of the expansion joint absorbs the vibration generated by the fan and airflow pulsation, protecting the tower liner and flue structure.
In engineering practice, expansion joints of corresponding types and materials should be selected according to different installation positions (tower entrance, tower exit and bypass), and maintenance measures such as cold drawing, condensation drainage setting and regular inspection should be strictly implemented. By correctly understanding and applying the purpose of flue expansion joint of desulfurization tower, the leakage risk and shutdown probability of desulfurization system can be significantly reduced, the service life of equipment can be prolonged, and reliable technical guarantee can be provided for the emission standard of flue gas desulfurization in coal-fired power plants and industrial kilns.