In the boiler flue gas recirculation system, the boiler flue gas return fan expansion joint is the key flexible component that connects the inlet and outlet of the fan with the flue, absorbs the heat displacement and isolates the vibration. The flue gas conveyed by the return fan has high temperature (up to 200-400℃), large dust content and strong corrosiveness, and the expansion joint is under the double erosion of high temperature alternating and corrosive medium for a long time. Once the expansion joint of the boiler flue gas return fan fails, it will lead to flue gas leakage and energy consumption increase, and in the worst case, the fan vibration exceeds the standard or even unplanned furnace shutdown. This paper will systematically explain the professional technical knowledge of expansion joint of boiler flue gas return fan from the characteristics of working conditions, key points of type selection to fault treatment.

1. Special requirements of return fan working conditions for expansion joints

1. High temperature flue gas environment

The boiler flue gas return system directs a portion of the flue gas from the tail flue back to the furnace or bellows to regulate the combustion temperature and NOx emissions. The flue gas temperature of the flue where the expansion joint of the boiler flue gas return fan is located is usually between 150-400℃, which is much higher than that of normal temperature air. High temperature environment puts forward higher requirements on the temperature resistance and thermal fatigue life of expansion joints.

2. Dusty and corrosive media

The flue gas contains dust particles and corrosive gases such as SO₂ and SO₃. Dust causes erosion wear on the inner wall of the expansion joint, and acidic gas may form dew point corrosion in the low temperature section. Therefore, the selection of expansion joint of boiler flue gas return fan must take into account both wear resistance and corrosion resistance.

3. Fan vibration transmission

The return fan generates constant mechanical vibration when it is operated. If the vibration isolation effect of the expansion joint is not good, the vibration will be transmitted to the boiler body along the flue, which will affect the safety of the equipment. Non-metallic expansion joints have good damping characteristics because of their multi-layer composite structure, which has become the first choice scheme for fan inlet and outlet.

2. Expansion joint type selection: non-metal is the first choice

1. Why are non-metallic expansion joints recommended?

According to the actual engineering experience, non-metallic fabric expansion joints are preferred for expansion joints of boiler flue gas return fan, mainly for the following reasons:

Non-metallic expansion joints can provide larger axial, angular and lateral displacements in smaller mounting sizes, and can compensate for multi-directional thermal displacements, which is greatly superior to metal compensators that can only compensate in a single way。 At the same time, the fiber fabric and heat insulation cotton themselves have the function of sound absorption and shock absorption, which can effectively reduce the noise and vibration of boiler, fan and other systems。

2. Typical Structural Composition

The non-metallic expansion joint consists of the following levels:

• Skin: Glass fiber cloth coated with fluororubber or silicone rubber for sealing and corrosion resistance
• Insulation: Aluminum silicate fiber cotton to reduce exterior surface temperature
• Stainless steel wire mesh: prevent the insulation material from scattering
• Deflector: Protects the skin from direct flushing by smoke
• Metal frame: fixed skin, providing structural strength

The practice of a large-scale coal-fired power station shows that the application of non-metallic expansion joint in flue duct can effectively absorb heat displacement, isolate vibration and have excellent corrosion resistance。

3. Key parameters and calculation of model selection

1. Calculation of thermal displacement

Correct selection of boiler flue gas return fan expansion joint requires accurate calculation of pipeline thermal displacement. The amount of thermal elongation is calculated as follows:

Δ L = α × L × Δ T

Among them:

• Δ L: thermal elongation (mm)
• α: linear expansion coefficient of pipeline (12×10⁻⁶/℃ for carbon steel)
• L: length of pipe section between two fixed brackets (mm)
• Δ T: Difference between operating temperature and installation temperature (℃)

Example: a section of 5 meters long return flue gas pipeline, installation temperature 20 DEG C, working temperature 350 DEG C, then:
Δ L =12×10⁻⁶ ×5000×330=19.8 mm

2. Key points of model selection

According to the practical engineering experience, the following factors should be paid attention to when selecting the expansion joint of the boiler flue gas return fan：

IV. Technical points of installation and replacement

1. Preparation before installation

Before installing or replacing the expansion joint of the boiler flue gas return fan, the following preparations should be made:

• Confirm that the new expansion joint model, specification and design are consistent
• Check the skin for damage or scratches
• Clean flue flange surface to remove old gasket residue
• Prepare tools such as electric wrenches and torque wrenches

2. Installation Steps

According to the engineering practice of non-metallic expansion joint replacementThe installation should follow the following process:

Step 1: Old expansion joint removal

• Remove the bolts holding the skin with an electric wrench
• Loosen the platen strip and remove the old skin
• For reusable platens, mark the installation sequence

Step 2: Frame Inspection and Repair

• Check that the internal deflector is intact
• Check the metal frame for corrosion deformation
• Clean up internal dust and replace damaged insulation cotton

Step 3: New Skin Installation

• Place the new skin flat on the flange surface of the metal frame to prevent wrinkles
• After the skin is tightened, it is tightened with pressure plates and bolts
• Installation starts at the bottom and unfolds progressively from the bottom point outward
• Tighten the bolts by "diagonal tightening and fractional tightening" with a torque of 50-80N·m

Step 4: Seal Inspection

• Perform airtightness test after installation
• Check for leaks with soapy water or a leak detector

3. Special precautions

• Transportation rod treatment: Transportation protection rod must be removed after installation so that the expansion joint can be expanded and contracted freely
• Welding protection: If welding is required, protective measures must be taken to prevent welding slag splashing from damaging the skin
• Bolt re-tightening: Tighten the pressure plate bolts once in 1 month and once in 3 months after operation

V. Common Faults and Treatment Measures

1. Skin breakage and leakage

Phenomenon: Smoke leakage, visible smoke and dust on site, increased energy consumption of fan

Cause Analysis:

• Long-term erosion and erosion of high-temperature smoke, skin aging
• The number of skin layers is insufficient or the material is not suitable for working conditions
• Insulation collapse causes skin overheating

Treatment measures:

• Temporary emergency: Seal with leak plugging glue stick, and tighten with stainless steel clamps
• Permanent Repair: Replace skin during scheduled downtime and increase skin layers if necessary

Typical case: The outlet expansion joint of induced draft fan of#2 boiler in a power plant has been eroded by high-temperature flue gas for a long time, resulting in smoke leakage and serious corrosion. After replacing the new corrosion-resistant expansion joint, the operating efficiency of fan is significantly improved。

2. Air leakage due to loose bolts

Phenomenon: Smoke leakage traces can be seen on the edge of the pressure plate

Reason: Failure to tighten multiple times according to specifications during installation, or loosening of bolts due to thermal expansion and contraction during operation

Treatment measures: Regularly re-tighten the pressure plate bolts, and enhance the frequency of inspection at the initial stage of operation

3. Frame corrosion

Phenomenon: The metal frame is corroded and perforated, and the skin is not firmly fixed

Cause: Poor quality of anti-corrosion coating, or long-term acidic wet environment

Treatment measures: After rust removal, recoat glass flake anticorrosive coating with a thickness of ≥2mm

VI. Preventive maintenance recommendations

1. Daily inspection: check the appearance of the expansion joint every week for smoke leakage, bulging and loose bolts
2. Temperature monitoring: Use infrared thermometer to monitor the surface temperature of the skin. Abnormal increase indicates damage to the insulation layer
3. Periodic tightening: Full tightening of platen bolts once a quarter
4. Replacement cycle: Non-metallic skin is usually replaced once every 3-5 years, and the cycle should be shortened in corrosive environment
5. Ledger management: record the installation date, inspection and maintenance records of each expansion joint

sum up

The expansion joint of boiler flue gas return fan is the key component to ensure the safe operation of flue gas recirculation system. Proper type selection, standard installation and regular maintenance are the three core elements to ensure its long-term reliable operation:

• Selection priority: It is recommended to choose non-metallic fabric expansion joint, which has the advantages of three-way compensation, excellent vibration isolation and no blind plate force
• Material suitable temperature: the skin is made of fluororubber composite material, the guide tube is made of wear-resistant steel plate, and the thickness of heat insulation layer is ≥100mm
• Installation specification: Install strictly according to the flow direction mark, paving the skin from bottom to top, tighten the bolts in stages, and tighten them again multiple times after commissioning
• Regular maintenance: Establish inspection ledger, monitor skin temperature changes, and replace old skin in time Chemical skin

An expansion joint of return fan with reasonable design and standard installation can run stably in high temperature and dusty flue gas environment for a long time. It is suggested that the requirements of flue gas temperature, corrosiveness and compensation amount should be fully considered in the selection stage, so as to avoid the failure of expansion joint from the source. When the skin is found to be aged or damaged, it should be replaced in time to prevent the leakage from expanding and affecting the system safety.