In the industrial flue gas pipeline system, how to keep the flue expansion joint warm is the key technical problem to ensure the safe operation of equipment, reduce the heat loss and prevent personnel from burning. The expansion joint should not only absorb thermal displacement and isolate vibration, but also keep the external surface temperature within a safe range under high temperature conditions (usually requires ≤60℃). Improper insulation design can lead to increased heat loss and energy waste, or overtemperature aging of expansion joint, burning of outer skin and even safety accidents. This paper will systematically explain the professional technical knowledge of how to insulate flue expansion joint from insulation structure, material selection to construction steps.
1. Why does the flue expansion joint need to be insulated?
How to insulate the flue expansion joint First of all, we should understand the necessity of insulation. The thermal insulation of expansion joints is mainly due to the following four considerations:
1. Prevent over-temperature aging
The outer skin of the expansion joint is usually made of non-metallic composite materials (fluororubber, silicone rubber, ceramic fiber cloth, etc.), which will accelerate aging and brittleness when exposed to high temperature for a long time. According to industry experience, the problem of overtemperature of expansion joint not only affects its life, but also affects the operation safety of boiler。 Effective heat preservation can reduce the working temperature of the skin and prolong the service life.
2. Reduce heat loss
High temperature flue heat loss will reduce the system thermal efficiency and increase energy consumption. Standardized insulation can significantly reduce heat loss.
3. Anti-scald protection
According to safety specifications, the surface temperature of equipment accessible to personnel shall not exceed 60°C. Insulation of expansion joint is an important measure to ensure the safety of maintenance personnel.
4. Prevent embrittlement in winter
Under the low temperature environment in winter, non-metallic skin materials are easy to harden and brittle. Low temperature resistant modified materials such as low temperature fluororubber can be adapted to environment ≤ -30℃Cooperate with heat preservation measures to effectively prevent embrittlement.
Second, heat insulation structure design of flue expansion joint
The core of how to insulate the flue expansion joint lies in the reasonable design of the insulation structure. According to the latest patented technology, the insulation structure of the expansion joint mainly includes two parts: the inner insulation layer and the outer skin。
1. Inner insulation layer structure
The inner insulation layer is the core of expansion joint insulation, and the structural design of multi-layer insulation cotton + steel mesh barrier is usually adopted:
| Structural hierarchy | Materials | function | Thickness reference |
|---|---|---|---|
| Innermost layer (smoke contact) | Ceramic fiber cloth + fine mesh steel wire mesh | Smoke gas isolation to prevent fiber from falling off | 2-3mm |
| Main insulation layer | Multi-layer insulation cotton (aluminum silicate fiber cotton) | Primary insulation function | 150-200mm |
| Insulation separation | First wire mesh | Fixing insulating cotton, dispersing stress | Mesh ≤10mm |
| Outer wrap | Ceramic fiber cloth | The insulation layer is wrapped as a whole and prevented from flying | 2-3mm |
| Outer cortex | Steel wire mesh + ceramic fiber cloth + PTFE cloth + PTFE membrane | Integral sealing, waterproof and anti-corrosion | 5-10mm |
Key technical points:
- The adjacent insulation cotton layers are separated by steel wire mesh to prevent the insulation layer from settling
- The heat insulation cotton shall be wrapped and fixed with stainless steel wire mesh, and the heat insulation nails shall be distributed to be fixed smoothly
- The innermost layer is laid with ceramic fiber cloth and fine mesh steel wire mesh to enhance the overall strength
2. Thermal insulation extension of flange part
The metal frames (flanges) at both ends of the expansion joints also need to be insulated. The patent technology proposes that that inner side of each folding plate-shaped flange is fixed with an inner lining folding plate, the inner lining folding plate is filled with heat preservation cotton, and an expansion gap is left between the two inner lining folding plates。
The advantages of this design are:
- The insulation layer is continuous and uninterrupted to prevent heat from escaping from the flange
- The inclined opening of the inner lining folding plate is consistent with the flow direction of smoke gas, so as to reduce the flow of smoke gas into the gap
- The expansion gap meets the thermal displacement requirements
3. Guide for selection of insulation materials
The key link of how to preserve the flue expansion joint is the material selection. Select the appropriate insulation material according to the working condition temperature:
1. Main insulation material
| Temperature range | Recommended insulation materials | Density (kg/m³) | Thermal conductivity (W/m·K) |
|---|---|---|---|
| ≤400℃ | Aluminum silicate fiber cotton | 96-128 | ≤0.07(500℃) |
| 400-600℃ | High aluminum type aluminum silicate wool | 128-160 | ≤0.09(600℃) |
| 600-800℃ | Zirconium-containing aluminum silicate wool | 160-192 | ≤0.12(800℃) |
| 800-1000℃ | Polycrystalline mullite fiber | 80-100 | ≤0.05(1000℃) |
2. Multilayer composite structure of outer skin
A typical outer skin structure (from the inside out) comprises：
| Sequence | Materials | Thickness (mm) | function |
|---|---|---|---|
| Level 1 | steel wire mesh | 1-2 | Supporting, anti-scour |
| Levels 2-3 | Ceramic fiber cloth | 2-3 | Heat preservation and fire protection |
| Level 4 | PTFE cloth | 0.5-1 | Corrosion-resistant, sealed |
| Level 5 | Ceramic fiber cloth | 1-1.5 | Thermal insulation |
| Level 6 | Tetrafluorine membrane | 0.3-0.5 | Waterproof, sealed |
| Level 7 | PTFE cloth | 1.5-2 | External protection |
3. Supplementary heat insulation in low temperature working conditions
For expansion joints running in winter, anti-embrittlement measures should be considered:
- Avoid using non-metallic skin materials with low molecular weight and easy freezing
- Use low temperature resistant modified materials such as low temperature fluorine rubber to adapt to the environment ≤ -30℃
- Add anti-brittle coating, such as silicon-based protective agent or thermal insulation film, to reduce temperature stress and moisture erosion
Four, flue expansion joint heat insulation construction steps
The construction of how to insulate the flue expansion joint should follow the standard process:
Step 1: Construction Preparation
- Check that the outer diameter size, skin width and technical parameters of the expansion joint meet the requirements
- Check the quantity and quality of insulation materials (insulation cotton, wire mesh, skin, bolt assembly)
- Clean up the surrounding flammable objects and prepare fire extinguishing tools
Step 2: Installation of the inner insulation layer
How to insulate the flue expansion joint, the inner insulation layer is the most critical link:
- Laying the second wire mesh: fixing the second wire mesh between the left flange and the right flange as the base frame of the insulation layer
- Laying bottom layer protection: the steel wire mesh is concave to form the installation area. First, a layer of ceramic fiber cloth is laid, and then a layer of fine mesh steel wire mesh is laid
- Laying insulation cotton layer by layer: Laying insulation cotton layer and first steel wire mesh layer layer by layer, adjacent insulation cotton layers are separated by steel wire mesh
- Insulation cotton fixation: Fix flat with insulation nails, and insulation cotton should be wrapped with stainless steel wire mesh
- Outer wrap: Ceramic fiber cloth is laid on the outer part of the outermost insulation cotton, and ceramic fiber cloth is sewn on the edge
Thickness requirements: The total thickness of insulation cotton is determined according to the design temperature. Generally, low-temperature flue (400 °C) at 200-250 mm.
Step 3: Outer skin laying
- Laying sequence: one layer of steel wire mesh, two layers of ceramic fiber cloth, one layer of PTFE cloth, one layer of ceramic fiber cloth and one layer of PTFE membrane are laid sequentially from the inside to the outside
- PTFE membrane welding: Weld the PTFE membrane to the previous layer of ceramic fiber cloth to ensure sealing
- Hemming and stitching: Finally, the outer side is hemmed and stitched with ceramic fiber cloth
Step 4: Skin fixation
- Flat skin: Flat skin on the flange surface of metal frame to prevent wrinkles
- Install from bottom to top: Install from the bottom, make sure that the inner insulation cotton has been installed sufficiently, and spread out from the bottom point to the outside in sequence
- Tightening and compressing: After the skin is tightened, it is compressed with pressure plates and bolts
- Bolt tightening: All compression bolts are tightened with an electric wrench, and no missing screwing is strictly prohibited
V. Installation precautions
How to insulate the flue expansion joint? The following matters should be paid attention to in the construction:
1. Prevent dust accumulation and blockage
For the non-metallic expansion joint of horizontal flue, the design of anti-plugging structure should be considered to prevent ash from entering the expansion joint and affecting the heat insulation effect。
2. Interface waterproof
For the expansion joint skin of horizontal flue, the upper interface should be pressed down at the interface of the outermost skin to prevent water seepage at the interface。
3. Welding protection
If welding is required during thermal insulation construction, protective measures must be taken to prevent welding slag splashing from damaging the skin.
4. Drainage design
According to the technical requirements, the expansion joint in contact with wet flue gas and located in the horizontal flue section shall be provided with a drainage hole, the drainage hole shall be at least DN150, and the drainage fitting shall be made of FRP or nickel-based alloy steel material。 Drainage design should be carried out in coordination with heat insulation construction, and drainage channels should be reserved.
5. Insulation thickness verification
After the installation of the insulation layer, the surface temperature shall be measured to confirm that the external surface temperature is ≤60℃ (or the design requirement value).
VI. Maintenance and Inspection
After the heat insulation construction is completed, how to insulate the flue expansion joint needs to be included in daily maintenance:
- Regular inspection: check the outer skin for damage, aging and cracking
- Inspection of insulation layer: Local overheating indicates collapse or missing insulation cotton
- Winter inspection: Check the skin for embrittlement cracks during winter operation, and repair it in time
- Drainage inspection: Check whether the drainage hole is unobstructed to prevent the accumulation of acidic condensate [ci^tation:2]
sum up
How to keep the flue expansion joint warm is the key technology to ensure the long-term safe operation of the expansion joint. The core points can be summarized as follows:
| Link | Core Essentials |
|---|---|
| Thermal insulation structure | Inner insulation layer (multi-layer insulation cotton + steel mesh spacer) + outer skin (multi-layer composite), the insulation of flange part should be continuously extended |
| Material selection | Aluminum silicate fiber cotton (thickness 150-200mm) is selected as the main insulation layer, and the thickness is increased under high temperature conditions; The outer skin is multi-layer composite of ceramic fiber cloth + PTFE cloth + PTFE membrane, and low-temperature resistant fluorine rubber is selected for low-temperature working conditions |
| Key points of construction | The heat insulation cotton is laid layer by layer, the steel wire mesh is fixed in separate layers, the skin is installed smoothly from bottom to top without wrinkles, and the bolts are repeatedly tightened in stages |
| Affiliate design | The horizontal section of the wet flue is provided with DN150 drainage hole (made of FRP/nickel-based alloy), and the interface is pressed up and down to prevent seepage water to prevent dust accumulation from blocking the insulation layer |
A flue expansion joint with reasonable heat insulation design and in-place construction specification can effectively reduce external surface temperature, prevent over-temperature aging, reduce heat loss, and realize long-term safe and stable operation under harsh working conditions such as high temperature and corrosion.