In the wet flue gas desulfurization (WFGD) system, the inlet flue of the absorption tower is connected with the upstream dust removal equipment or booster fan and the absorption tower body. This area has high flue gas temperature, high concentration of corrosive medium, and is subject to severe thermal expansion and contraction and equipment vibration. As a key flexible compensation element, the flue expansion joint at the inlet of absorption tower directly affects the sealing and operation safety of desulfurization system. However, many power plants have unplanned shutdowns due to improper selection or lack of maintenance, resulting in leakage, corrosion or even bursting of expansion joints. This paper will systematically analyze the technical points, common faults and optimization schemes of the flue expansion joint at the inlet of absorption tower.

1. Challenges in the working conditions of the flue at the inlet of the absorption tower

The flue at the inlet of the absorption tower is usually treated with raw flue gas that has not been desulfurized, and the temperature is between 120℃ and 180℃, which may instantaneously reach above 200℃. The flue gas contains high concentration of SO₂, SO₃, HCl, HF and a large amount of dust, which is easy to form strong corrosive condensate acid when exposed to cold. In addition, the flue and absorption tower are made of different materials (carbon steel and lined with anti-corrosion layer), and the thermal expansion coefficient is significantly different. Without reliable compensation, huge thermal stress will be generated at the welded structure or flange connection, resulting in deformation and cracking. Therefore, the expansion joint must meet the three core requirements of high temperature resistance, corrosion resistance and large displacement compensation at the same time.

2. Common structural types of flue expansion joints at the inlet of absorption tower

According to the principle of manufacturing materials and compensation, the current mainstream products are divided into the following three categories:

1. Non-metallic fabric expansion joint
   Adopt multi-layer structure: fluororubber or silicone rubber coated glass fiber cloth as medium isolation layer, with glass fiber cloth and stainless steel wire mesh reinforcement layer in the middle, and weather-resistant glue layer on the outer layer. Its advantages are large compensation amount (can be absorbed in axial, transverse and angular directions), and a single piece can compensate the displacement of more than 50mm; Good elasticity, can effectively isolate vibration; And excellent in acid resistance. Absorption tower inlet flue is preferred to use non-metal type in more than 90% of the occasions, because its cost is lower than that of metal expansion joint, and it is not easy to accumulate dust.
2. Metal bellows expansion joint
   It is composed of stainless steel thin-walled bellows, end pipe and guide tube. It is suitable for high temperature (> 400℃) or high pressure (> 0.1MPa), but the compensation amount is relatively small, and it is easy to pitting or stress corrosion cracking under corrosive condensate erosion. In practical applications, it is rarely directly used in the raw flue gas section at the inlet unless the flue gas is pretreated and cooled.

3. Key selection parameters and pit avoidance guidelines

To ensure the long-term reliability of the expansion joint, the following parameters must be specified and provided to the manufacturer:

• Design temperature: Take the highest instantaneous flue gas temperature plus 20℃ balance. If there is a bypass flue, the accident condition should be considered.
• Media composition: emphasize the SO₃ concentration and dew point temperature. When the inner wall temperature is lower than the acid dew point (about 110℃ ~130℃), the isolation layer must be made of acid-resistant fluororubber instead of ordinary silicone rubber.
• Amount of displacement: including axial elongation/compression, lateral offset, and angular displacement. It is obtained by pipeline stress analysis and cannot be simply estimated.
• Pressure level: Usually ±5kPa ~ ±15kPa, positive pressure or negative pressure should be indicated (negative pressure may be after induced draft fan).
• Flue section shape and flange size: Rectangular flue should pay attention to the radius of fillet to avoid stress concentration.

Common error: Ignore flue bottom condensate accumulation. It should be required that the expansion joint should have its own condensate discharge port, or a drainage pipe should be set at a low position during installation. Otherwise, long-term soaking of the acid will accelerate the hydrolysis failure of the fabric layer.

Key Points of Installation, Operation and Maintenance

4.1 Inspection before installation

• Check whether the length of the expansion joint matches the reserved clearance of the flue (usually the reserved compression amount is 20~30mm, which is used to absorb the cold installation deviation).
• Check the direction of the lining guide tube: The interface end of the guide tube must face the incoming flue gas to prevent the air flow from directly washing the flexible layer.
• It is strictly prohibited to damage corrugated or fabric surfaces during hoisting.

4.2 Installation Process Critical Controls

• A temporary positioning rod is adopted to fix the expansion joint to the cold installation length; After the flue and absorption tower are all welded and the heat insulation installation is completed, the positioning rod is removed to freely expand and contract.
• Acid-resistant gaskets (e.g. expanded polytetrafluoroethylene) are used for flange connection, and the bolts are tightened diagonally evenly, and the torque meets the design value.

4.3 Daily inspection and failure warning

• Visually check the external surface for bulging, discoloration and fiber shedding every month. Silicone rubber is powdered in the initial age stage.
• Use an infrared temperature measuring gun to detect the surface temperature distribution of the expansion joint every quarter. If the local temperature is obviously low (lower than the acid dew point), it means that the insulation layer is damaged or the internal condensation is serious.
• During annual shutdown, enter the flue to check whether the inner fluororubber membrane is hard and cracked. Use a flashlight to illuminate the inside from the outside, and the light transmitting point is the precursor of leakage.

Typical failure modes and countermeasures:

V. Summary

Although the flue expansion joint at the inlet of absorption tower is a non-standard accessory, the rationality of its selection directly determines the availability and environmental compliance of wet desulfurization system. The core conclusions are as follows:

• The non-metallic fluororubber fabric expansion joint is preferred, which has both corrosion resistance and large displacement compensation ability, and is suitable for most power plant working conditions.
• The acid dew point conditions and condensate discharge design must be specified in the technical specifications to avoid the cost reduction trap of "silicone rubber instead of fluororubber".
• Keep the cold pre-compression amount during installation, check the state of the inner layer regularly during operation, especially pay attention to the corrosion of liquid accumulation at the bottom.
• Guided by full life cycle cost (LCC), moderately increasing initial procurement standards can significantly reduce the risk of unplanned outages.

By systematically mastering the above key points of selection, installation and maintenance, the operation and maintenance personnel can reduce the failure rate of the flue expansion joint at the inlet of the absorption tower to less than one time/5 years, thus ensuring the long-term safe and stable operation of the desulfurization system.