In the SCR (selective catalytic reduction) denitrification system, the flue is connected to the boiler outlet, the ammonia injection grid, the catalyst layer and the air preheater. Because the flue gas temperature is as high as 300℃ ~420℃, and it contains high concentration of dust and corrosive components such as NH₃ and SO₃, the problems of thermal expansion displacement and vibration are particularly prominent. Whether the structure of denitrification flue expansion joint is reasonable or not directly affects the sealing efficiency, catalyst life and equipment safety of denitrification system. In this paper, the special requirements of denitrification conditions for expansion joints will be deeply analyzed, the scheme of disassembling non-metal and metal structures will be detailed, and the core parameters of structure selection will be provided.

I. Special challenges of denitrification environment

Compared with ordinary flue, denitrification flue expansion joint faces three unique working conditions:

1. Wide temperature range alternating: When the boiler starts and stops and the load changes, the smoke temperature can fluctuate drastically between 30℃ ~420℃, and the thermal expansion displacement can reach 50~100mm.
2. High dust scour: The dust concentration of the flue gas is usually 10~30g/m³, and the hard particles cause abrasion on the inner wall of the expansion joint and the guide tube.
3. Ammonium salt deposition risk: When the flue gas temperature is below 230℃, the escaped ammonia reacts with SO₃ to form ammonium bisulfate (ABS), which adheres to the cold surface and may cause the moving parts of the expansion joint to get stuck.

Therefore, the structure of denitrification flue expansion joint must simultaneously meet the following requirements: large displacement compensation ability, anti-dust accumulation design, high temperature and corrosion resistant materials, and easy maintenance.

2. Main structural types of denitrification flue expansion joints

According to the cross-sectional shape, displacement direction and pressure level of flue, the following three types of structures are currently commonly used:

2.1 Non-metallic fabric expansion joint

This is the most common choice for SCR denitrification inlet and outlet flue, and the structure of its denitrification flue expansion joint from outside to inside is as follows:

• Outer layer: silicone rubber coated fiberglass cloth, weather resistant, moisture resistant, fire resistant.
• Intermediate reinforcement layer: 2~4 layers of alkali-free glass fiber cloth or polytetrafluoroethylene (PTFE) cloth to provide tensile strength.
• Inner isolation layer: fluororubber or PTFE film, direct contact with flue gas, temperature resistance below 250℃ can be selected with fluororubber; PTFE-coated fiberglass cloth must be used above 280℃.
• Thermal insulation filler layer (optional): ceramic fiber felt for reducing the temperature of the external surface below 60℃ to prevent scalding.
• Metal frame and platen: Usually Q235B or 316L angle/channel steel, bolted skin.

Advantages: Large compensation (can absorb three-dimensional displacement), good sealing, no lubrication, vibration and sound insulation.
Disadvantages: Low pressure resistance (usually ≤30kPa), cannot withstand excessive negative pressure or positive pressure impact.

2.2 Metal bellows expansion joint

It is suitable for high temperature (> 400℃), high pressure (> 30kPa) or in situations with strict requirements for fire rating. Its core structure is:

• Bellows: Multi-layer (2~4 layers) austenitic stainless steel (304, 316L, INCONEL625) hydroforming with wall thickness of 0.8~2.0mm.
• End pipe: stub welded to the flue, made of the same material as bellows or carbon steel lined with heat-resistant layer.
• Guide tube: lined on the inner wall of the bellows, with a thickness of 3~6mm, one end is fixed, and the other end expands freely to prevent the air flow from directly impacting the corrugation.
• Tie Rod/Hinge: Used to restrain bellows from pressure thrust and limit excessive lateral displacement.

Applicable scenarios: vertical flue, high temperature section, above and below the catalyst layer. However, it should be noted that the metal bellows is prone to stress corrosion cracking in denitrification flue gas (especially in chlorine-containing and fluorine-containing conditions), and the angular displacement cannot be compensated.

3. Design and calculation of key structural parameters

Regardless of the construction, the following parameters directly determine the life of the expansion joint:

3.1 Displacement distribution

For non-metallic expansion joints, the sum of axial compression and transverse displacement should not exceed 80% of the total compensation capacity, and a safety margin should be kept. For example, if the total lateral displacement is designed to be 40mm, the axial compression amount should be controlled at ≤30mm.

3.2 Guide tube structure

• Length: The one-side overhanging length of the guide tube should be 1/3~1/2 of the total length of the expansion joint, so as to ensure that the high-temperature flue gas does not directly scour the flexible section.
• Gap: Leave 5~10mm annular gap between the guide tube and the inner wall of the expansion joint, which is used to absorb thermal expansion but does not cause dust blockage.
• Thickness: When the dust concentration is> 15g/m³, the thickness of the guide tube should not be less than 4mm, and the material should be upgraded to 316L or wear-resistant surfacing.

3.3 Bolts and pressure plates

The pressure plate connecting bolts of non-metallic expansion joints shall be hot-dip galvanized or high-strength bolts (grade 8.8 or above) treated with Dacromet. The value of torque applied during installation shall be calculated according to the following formula:

T = K × D × F

Where K is the torque coefficient (0.2), D is the nominal diameter of the bolt (mm), and F is the preload force (N). Typical value: M16 bolt torque is 120~150N·m. Excessive torque tightening is strictly prohibited to avoid deformation of the platen and tearing the skin.

4. Typical faults and structural improvement

4.1 Expansion failure due to ash accumulation

Phenomenon: After half a year of operation, the expansion joint appears local bulging, and the skin is stuck and cannot be freely expanded and contracted.
Reason: The gap between the guide tube and the skin is too small, and the dust accumulates into hard lumps, blocking the movement gap.
Improved structure: the gap is increased from 5 mm to 12 mm; Or a purge hole is added on the outside of the guide tube, and the compressed air is connected for regular backwashing.

4.2 ABS corrosion in low temperature section

Phenomenon: The flue expansion joint at the inlet of the air preheater at the denitrification outlet, the inner fluororubber hardens and cracks, accompanied by white crystals (ammonium bisulfate).
Cause: When the smoke temperature is below 230℃, ABS condenses and penetrates into the skin layers.
Improved structure: replaced with all PTFE (polytetrafluoroethylene) composite skin, PTFE is completely non-adhesive to ABS and temperature resistance can reach 260℃; At the same time, an electric heat tracing system is added on the outside to maintain the temperature of the inner surface of the skin higher than the dew point of ABS.

4.3 Weld cracking of metal bellows

Phenomenon: Penetrating cracks appear at the peak of the bellows, and smoke leaks.
Cause: Residual chloride ions (from coal or denitrifier) in denitrification flue gas cause stress corrosion cracking of austenitic stainless steel.
Improve the structure: Upgrade the bellows material. At the same time, the number of bellows layers is increased from single layer to double layer to improve the fatigue resistance life.

V. Key points of installation and inspection

In order to ensure that the performance of the designed structure can be realized, it is necessary to control during construction:

1. Cold state pre-compression: When installing non-metallic expansion joints, compress the length by 5% ~8% (relative to the free length) to reserve thermal expansion space. Fix it with a temporary positioning rod, and release it after the flue is welded and the support is removed.
2. Coaxiality check: When installing the expansion joint of metal corrugated pipe, the deviation of the center of the pipe at both ends shall not exceed ±3mm, otherwise the corrugated pipe will be subjected to bending stress and accelerated fatigue.
3. Welding slag protection: When welding the end pipe, the skin or bellows surface must be covered with asbestos cloth to prevent the welding slag from scalding the flexible components.
4. Airtightness test: After installation, conduct soap bubble leak detection at 30% working pressure, or use ultrasonic leak detector to scan all joints, and the leak rate should be less than 0.5%.

VI. Structure Selection Decision Table

VII. SUMMARY

The structural design of denitrification flue expansion joint is a systematic project, which must be considered in combination with the special denitrification working conditions (high temperature, high dust, ammonium salt, alternating thermal stress), material mechanics and fluid dynamics. The core conclusions are as follows:

• The non-metallic fabric structure is preferentially used for the rectangular flue at the inlet and outlet of SCR, which has good economy and strong compensation ability; For the high-temperature and high-pressure section, metal bellows are selected.
• The guide tube is an indispensable protective component, and its thickness, gap and material should be calculated separately according to the dust content and corrosiveness.
• Anti-dust accumulation and anti-ABS design are the key to distinguish the special expansion joint for denitrification from the ordinary flue expansion joint, and the purge hole or heater interface should be reserved.
• The installation of pre-compression and coaxiality control directly affects the actual life, and the accuracy is one grade higher than that of conventional flue.

By deeply understanding and optimizing the above structural details, engineers and technicians can select and match the expansion joint scheme suitable for the whole life cycle of denitrification, reduce the leakage failure rate to below 0.2 times/year per unit, and provide reliable guarantee for the stable discharge standard of SCR denitrification system.