At the end position of the flue duct system, the flue end expansion joint plays a key role in connecting the chimney, absorbing the heat displacement of the flue end, and ensuring the tightness of the system. Unlike ordinary flue expansion joints, end expansion joints usually withstand larger flue settling displacements, more complex stress states, and more severe environmental conditions. The expansion joint installed at the entrance of the chimney is particularly special-it not only absorbs the thermal expansion of the last section of the flue, but also adapts to the relative settlement and vibration between the chimney and the flue. This paper will systematically explain the professional technical knowledge of flue end expansion joint from functional characteristics to fault treatment.

1. Special working conditions and functional positioning of flue end expansion joint

The flue end expansion joint is usually installed at the end of the flue duct system, most typically at the chimney entrance. This location has the following special operating conditions:

1. Complexity of end operating conditions

• Various types of displacement: not only to absorb the axial thermal expansion of the flue, but also to adapt to the relative settlement between the chimney and the flue, the swing caused by wind load, etc
• The stress state is complex: the constraint condition of the end position is different from that of the intermediate pipe section, and the expansion joint may bear multi-directional loads such as tension, compression and torsion
• Severe corrosive environment: Especially at the end of desulfurization wet flue, the flue gas temperature is as low as 45-55℃, which is in a completely saturated state, and the condensate is highly corrosive

2. Unique role of expansion joint at chimney entrance

The engineering practice of Huadian Luohe Power Generation Co., Ltd. shows that the expansion joint at the wet flue of the chimney entrance is the most corroded part. The installed capacity of the plant is 2×330MW, and the design exhaust temperature of the desulfurization system is 45℃. The original design is non-metallic expansion joints. With the operation of the unit, the flue expansion joints are corroded and leaked many times, and the leakage problem occurs in a short time after the operation。

In this position, the flue end expansion joint bears:

• Thermal displacement difference between absorption chimney and desulfurization tower
• Isolate chimney vibration and reduce transmission to desulfurization equipment
• Ensure that the drain system is sealed to prevent condensate from leaking

2. Key points of selection of expansion joint at the end of flue

1. Select the structure form according to the end working condition

The selection of the expansion joint at the end of the flue is different from the middle position, and the following structure is more suitable for the end working condition:

2. Material selection of end expansion joint

Taking the desulfurization wet flue end as an example, the material selection of flue end expansion joint faces severe corrosion challenges. Traditional non-metallic expansion joints have the following problems at the end of wet desulfurization:

• The glass flake on the end face of the flange is damaged by the bolt extrusion, forming corrosion channel
• The 316L metal expansion joint has weak corrosion resistance to Cl⁻¹ ions, and its average life is not more than two years

Upgrade scheme: Pure titanium TA2 expansion joints are the preferred scheme for desulfurizing wet flue ends. Titanium has excellent corrosion resistance-it performs well in harsh corrosive media such as chloride, sulfide and wet chlorine, and its passivation film is stable, which can be quickly repaired after scratches on the surface。

3. Dimensional design of end expansion joint

The design size of the flue end expansion joint needs to be fully considered:

• Calculation of thermal displacement: Calculate the amount of thermal expansion based on flue length and operating temperature
• Settlement margin: the relative settlement between the chimney and the flue
• Installation space: The end position is usually limited in space and requires a compact design

Design parameters of pure titanium expansion joint applied by Luohe Company: titanium TA2 material, thickness 2mm, expansion joint width 300mm, expansion margin ±50mm, designed as a split single corrugated structure。

3. Installation specification of flue end expansion joint

The installation of flue end expansion joints requires strict compliance with specifications, especially at critical points connected to the chimney.

1. Check points before installation

Before installation, check whether the model and specification of the corrugated compensator and the pipe support configuration meet the design requirements。 For the end expansion joint connected to the chimney, it is also necessary to confirm that:

• Flatness of flange surface of chimney and integrity of anti-corrosion coating
• Flow direction identification of expansion joint (with guide tube, the direction of liner tube must coincide with the direction of media flow)

2. Installation direction confirmation

The flow direction identification of the flue end expansion joint is crucial. For compensators with liners, care should be taken to make the direction of the liner cylinder coincide with the direction of media flow (install according to the flow direction sign of the compensator)。 Installation in reverse direction will cause flue gas to directly flush the bellows or skin, accelerating failure.

3. Special installation requirements for end expansion joints

Because that expansion joint at the end of the flue is located at the terminal of the system, the connecting end of the flue and the flue can be connected by welding, and the expansion joint connected with the absorption tower should be connected by flange on the equipment side, so that the arrangement can ensure that the expansion joint can be replaced。 This design takes into account the convenience of overhaul and maintenance of the end part.

4. Transportation and limiting device handling

After installation, the flue end expansion joint needs to perform the following operations:

• Disassembly of transportation protection device: The auxiliary positioning mechanism and firmware used for installing transportation protection on the compensator shall be disassembled immediately after the pipe system is installed
• Adjustment of limiting device: Adjust the limiting device to the specified position according to the design requirements, so that the pipe system has sufficient compensation ability under environmental conditions
• Forced adjustment of deviation is strictly prohibited: Except for pre-tension or "cold tightening" pre-deformation required by design, it is strictly prohibited to adjust the installation deviation of the pipeline by deforming the bellows

5. Welding and Protection

Welding slag is not allowed to splash onto the bellows surface and other mechanical damage to the bellows during installation。 For the installation of titanium expansion joints, the welding principles of titanium to titanium and steel to steel should be strictly implemented. The side of titanium-steel composite plate shall be welded to the inside of the flue, and the inside and outside shall be fully welded and the weld seam shall be inspected。

4. Drainage and sealing of flue end expansion joint

The flue end expansion joint is located at the end of the flue gas discharge, and drainage and sealing are particularly important.

1. Drain hole setting

Expansion joints exposed to wet flue gas and located in the horizontal flue section shall be drained through the expansion joint frame with drain holes of at least DN150 and located on the centerline of the horizontal flue section. The drainage material is made of FRP or nickel-based alloy steel material, and the drainage is returned to the drainage pit in the FGD area。

2. Sealing Requirements

The end expansion joint shall be designed to be leak-free and capable of withstanding the maximum design positive/negative pressure of the system plus a margin of 1000Pa。 The expansion joint on the cryogenic flue shall be considered to be anti-corrosion, and the steel structure part connected to the flue shall be treated with anti-corrosion。

3. Insulation requirements

The expansion joint at the end of the flue shall be insulated, and the manufacturer shall provide the insulation design drawing。 The insulation design should ensure that the external surface temperature meets the safety requirements and prevent personnel from burning.

Five, Failure and Treatment of Expansion Joint at Flue End

1. Typical failure: non-metallic expansion joint leakage

The application of non-metallic expansion joints at the end of desulfurization wet flue faces serious corrosion problems. The practice of Luohe Company shows that the non-metallic expansion joint is easily formed in the wet flue due to the extrusion and damage of the glass flake on the flange end face by the bolt, which is easy to form a vicious circle of "the more leakage gets tighter, the more tight the more leakage"。

Treatment plan: Groove filling technology can be used for the leakage part, flattening and sealing with acid-resistant glue, and maintenance for 6 hours can be put into use。 Material upgrades should be considered in case of structural damage.

2. Upgrade scheme: pure titanium expansion joint

In 2016, Luohe Company innovatively upgraded the flue end expansion joint to a pure titanium TA2 expansion joint, and achieved "zero leakage" throughout the year after continuous operation for one year. The inner and outer surfaces are as clean as new, and the titanium welds are full and intact 。

3. Analysis of corrosion causes

Corrosion at the end of desulfurization wet flue is mainly caused by the following reasons:

1. Chemical corrosion induced by SO₂ and SO₃
2. Electrochemical corrosion induced by Cl⁻and F⁻¹
3. Mechanical corrosion caused by fluid flushing

Precautionary measures: select corrosion-resistant materials (TA2 titanium), and do a good job in the quality control of anti-corrosion construction during installation.

VI. SUMMARY

As the terminal compensation device of smoke and air duct, the expansion joint at the end of flue is the last line of defense to ensure the safety at the end of the system. The core points can be summarized as follows:

Although the terminal expansion joint is only a "joint", it is the guardian of the safe operation of the terminal of the flue gas system. Correct selection, standard installation and regular maintenance can effectively prevent end leakage, prolong the life, and ensure the safety and reliability of end discharge of the whole flue gas system.