In high temperature flue system, the expansion joint should not only absorb the heat displacement and seal the flue gas, but also have good heat preservation performance. Especially for non-metallic fabric expansion joints, if not filled with thermal insulation cotton, its external surface temperature may be as high as above 200℃, which not only causes huge heat loss, but also poses potential safety hazards of burning personnel and burning surrounding cables. Insulation cotton filling of flue expansion joints is a key process that affects the energy efficiency and service life of expansion joints. This paper will systematically explain the material selection, filling structure, construction steps and quality acceptance criteria of thermal insulation cotton.

1. Why do expansion joints need to be filled with thermal insulation cotton

Flue expansion joints are usually installed in high temperature flue gas environments. Taking the boiler outlet flue as an example, the flue gas temperature can reach 350℃ ~420℃. The skin of non-metallic expansion joints is composed of multi-layered composite materials, but their thermal insulation capacity is limited. If the flue expansion joint insulation cotton is not filled, the following problems will occur:

• The temperature of the outer surface of the skin is too high (up to 150℃ ~200℃), which can easily burn the inspectors
• High temperature accelerates the aging and pulverization of the outer layer of silicone rubber, and the service life is shortened by more than 50%
• Heat loss causes flue gas temperature drop to increase, which affects the efficiency of downstream denitrification reaction
• Adjacent cable trays and instrument lines are damaged due to heat radiation

Therefore, for flue expansion joints with operating temperatures exceeding 180°C, it is necessary to fill the interior or exterior with insulating cotton. For ultra-high temperature (> 400℃) working conditions, thermal insulation cotton is an indispensable component of expansion joint.

2. Material selection of thermal insulation cotton

2.1 Types of commonly used insulation cotton

Recommended choice: For most desulfurization and denitrification flue expansion joints, ceramic fiber cotton is the best choice, with high temperature resistance, good chemical stability and long service life.

2.2 Morphology of thermal insulation cotton

Flue expansion joint insulation cotton filling can adopt the following forms:

• Felt/blanket: Thickness 5~25mm, easy to cut, suitable for layered filling of rectangular expansion joints.
• Loose cotton: It is used to fill complex shaped cavities, but it needs to be wrapped with steel wire mesh to prevent airflow from blowing away.
• Module prefabricated parts: molded according to the expansion joint size, quick installation, but high cost.

Ceramic fiber blanket is the most commonly used in site construction, with a recommended thickness of 20~30mm and bulk density of 128kg/m³.

3. Filling structure of heat insulation cotton in expansion joint

3.1 Inner fill structure (most common)

For non-metallic fabric expansion joints, thermal insulation cotton is filled between the inner barrier film and the reinforcing layer. Typical structures from the inside to the outside are:

1. Inner isolation membrane (fluororubber/silicone rubber/PTFE) -contact smoke
2. Insulation cotton layer (ceramic fiber blanket) -core insulation layer
3. Intermediate reinforcement layer (glass fiber cloth + stainless steel wire mesh) -mechanical load bearing
4. Outer protective layer (silicone rubber coated fiberglass cloth) -weather-resistant seal

Such a configuration can reduce the outer surface temperature from 200°C to below 60°C.

3.2 External additional insulation

For expansion joints that have been put into operation and the external surface temperature is too high, a layer of thermal insulation cotton + aluminum leather shell can be wrapped on the exterior. Method:

• The outside of the expansion joint was covered with a 50 mm thick ceramic fiber blanket
• The outer layer is wrapped with 0.5mm galvanized iron sheet or aluminum sheet and fixed with cable ties
• Care should be taken not to hinder the free expansion and contraction deformation of the expansion joint

3.3 Thermal Insulation Filling of Metal Expansion Joints

The expansion joint of metal bellows is usually not filled with internal insulation cotton, but an insulation jacket or an insulation layer is installed on the outside. Because bellows need to expand and contract freely, internal filling can limit its movement. For metal expansion joints in high temperature section, thermal insulation nails can be welded on the outside, and ceramic fiber cotton + razor wire + galvanized iron shell can be laid.

4. Construction steps: practical operation of filling the heat insulation cotton of the flue expansion joint

Hereinafter, a standard procedure for filling the flue expansion joint with insulating cotton will be described in detail taking the inner filling of the non-metallic fabric expansion joint as an example.

4.1 Preparation before construction

• Verify that the expansion joint is installed in place but the outer skin has not been sealed.
• Clean the dust and oil on the surface of the inner isolation film and keep it dry.
• Check the appearance of insulation cotton: no moisture, no clumping and uniform thickness.

4.2 Cutting and laying of thermal insulation cotton

1. Cutting: Cut the ceramic fiber blanket to the corresponding width and length according to the inner cavity size of the expansion joint. The corner of the rectangular expansion joint should be cut into a 45° bevel butt to avoid gaps generated by folding.
2. Number of layers laid: Calculated according to the required insulation temperature difference. Empirical formula: Reduce the external surface temperature by about 40~50℃ per 10mm ceramic fiber blanket (128kg/m³). For example, the flue gas is 350℃ → the target surface is 60℃, and the temperature difference is 290℃, so it is necessary to fill about 60~70mm (that is, 2~3 layers of 20mm thick blankets).
3. Laying Method:
   • The first layer: lay close to the inner layer of isolation film, and the joint is overlapped by 50mm.
   • Second layer: The seams are staggered from the first layer by at least 100mm and arranged in a "pin" shape.
   • With stainless steel wire (φ 1.0mm) every 200mm, the insulation cotton "Z"-shaped seam is fixed to the inner reinforcing cloth to prevent slipping or displacement.

4.3 Compaction and inspection

• The insulation cotton should be full and even after filling, and the rebound rate should be ≥80% when pressed by hand.
• Check the corner fill density with a gauge, there must be no voids or gaps.
• The four corners of the rectangular expansion joint need to be extra filled with loose cotton because the blanket material is difficult to fit here.

4.4 Cover outer layer

• After confirming that the thermal insulation cotton filling is qualified, cover the outer reinforcing layer and the protective layer.
• When the pressure plate bolts are tightened, be careful not to over-compress the insulation cotton to avoid reducing the heat insulation effect. It is recommended that the compression rate be controlled at 10% ~15%.

V. Key Precautions in Construction

5.1 Moisture is strictly prohibited

Once ceramic fiber cotton and aluminum silicate cotton are damp, the thermal conductivity will increase by 2~3 times, and the evaporation of water at high temperature will lead to bulging and cracking of the insulation layer. Sunny weather should be chosen during construction, and insulation cotton that has been damp should not be used.

5.2 Prevention of fiber inhalation

Ceramic fibers are carcinogens (IARC Class 2B) and construction workers must wear N95 masks, goggles and long-sleeved gloves. Cutting should be done in a well-ventilated area.

5.3 Treatment of expansion joints

The expansion joint will undergo expansion and contraction deformation during operation, and the insulation cotton must be able to compress and rebound with it. Therefore:

• Do not stick the insulation cotton to the inner wall or skin of the flue.
• The compression space should be reserved at the edge of the platen to avoid the compaction failure of the thermal insulation quilt.

5.4 Isolation of acidic condensate

For flues with acid dew points (smoke temperature

VI. Quality Acceptance Criteria

After completing the filling of flue expansion joint insulation cotton, the following inspections shall be performed:

Common Problems and Treatment

Maintenance and replacement cycle

• Daily inspection: Touch the external surface of the expansion joint with your hands every month (pay attention to anti-scalding), and mark the abnormal high temperature points.
• Annual shutdown inspection: Open the inspection hole of the expansion joint to observe whether the insulation cotton sinks, powders and is damp. Use an endoscope to view the status of the cotton layer from the flange gap.
• Replacement cycle: ceramic fiber cotton can be used for 5~8 years in dry and acid-free environment; If there is condensate acid infiltration, the life may be reduced to 2~3 years, and it needs to be replaced in advance.

IX. SUMMARY

Insulation cotton filling of flue expansion joint is the key link for energy saving and safe operation of high temperature flue system. The core points are as follows:

• The preferred material is ceramic fiber blanket, which has temperature resistance above 800℃, bulk density of 128kg/m³, and the thickness is calculated according to the temperature difference (about 10mm/40~50℃ temperature difference).
• Filling structure: inner insulation film + insulation cotton layer (multi-layer staggered laying) + reinforcement layer + outer protective layer. The insulation cotton shall be fixed by sewing with stainless steel wire to prevent it from falling.
• Key to construction: It is strictly prohibited to be damp, and the corners need to be filled additionally. The compression rate of the pressure plate is controlled at 10% ~15%, and the expansion space is reserved.
• Acceptance criteria: External surface temperature ≤60℃, no cavity, rebound rate ≥80%.

Correct thermal insulation cotton filling can reduce the external surface temperature of the expansion joint from above 200℃ to below 60℃, and at the same time prolong the skin life by 2~3 times. For the working conditions below the acid dew point, it is necessary to add PTFE moisture-proof layer to prevent the condensate from eroding the insulation cotton. Through standardized construction and regular inspection, it can ensure the long-term stable operation of flue expansion joint under high temperature, and achieve the comprehensive goal of energy saving, safety and durability.