In flue system arrangements, it has become routine practice to install expansion joints in horizontal flues. However, when flues need to run vertically, many engineers have questions: Can vertical flues be fitted with expansion joints? The answer is yes-vertical flues can be fitted with expansion joints perfectly and are necessary in many operating conditions. However, due to the gravity, the change of displacement direction and condensate accumulation, the design and installation requirements of the flue are significantly different from those of the horizontal flue. This paper will systematically expound the feasibility, special considerations and engineering practice points of installing expansion joints in vertical flue.
1. Necessity of installing expansion joint in vertical flue
1.1 Thermal expansion also exists in the vertical segment
Vertical flues, like horizontal flues, produce axial thermal elongation at high temperatures. For example, a section of carbon steel vertical flue with a height of 15 meters, the temperature of the flue rises from 20°C to 350°C, and its thermal expansion is about:
Δ L = α × L × Δ T =0.012×15×330 ≈ 59.4 mm
If the expansion joint is not provided to absorb this displacement, thermal stress can cause flue bending, bracket failure, or bottom interface cracking. Therefore, the answer to whether an expansion joint can be installed in a vertical flue is clear-it must be installed, usually in the middle of the vertical section or near the equipment interface.
1.2 Special requirements for vertical segment
- Isolation equipment vibration: When the inlet and outlet of the induced draft fan are vertical flues, the expansion energy saving prevents the vibration from being uploaded
- Compensation for foundation settlement: The settlement of the bottom and top of the towering flue is different, and the expansion joint can absorb the difference
- Easy maintenance: The expansion joint can be set to remove the vertical flue in sections
2. Structure selection of vertical flue expansion joint
The deeper question of whether an expansion joint can be installed in a vertical flue is: Which expansion joint is suitable for vertical installation? The applicability of various types of expansion joints is as follows:
2.1 Non-metallic fabric expansion joints-applicable, subject to conditions
The non-metallic fabric expansion joint is lightweight and compensating, making it ideal for vertical flues. But note that:
- Must be provided with a guide tube: When installed vertically, the guide tube should not only guide the air flow, but also prevent the insulation cotton from falling
- Installation of supporting structure: Add stainless steel wire mesh or supporting plate at the bottom of the expansion joint to prevent the filled ceramic fiber cotton from settling down to the bottom due to gravity, resulting in the formation of hollow in the upper part
- Installation direction: The pressure plate bolts of the skin should be arranged horizontally to avoid uneven force in the vertical direction
Applicable scenario: Vertical flue with flue gas temperature ≤250℃ and pressure ≤ ±30kPa.
2.2 Metal bellows expansion joint-applicable, self-weight should be checked
Metal bellows expansion joints have proven applications in vertical flues. The key questions are:
- Bellows are subjected to dead weight loads: The flue weight of the vertical segment may be transferred through the bellows, causing the bellows to be compressed or stretched beyond the design value. Solution: Load-bearing brackets are set on both sides of the expansion joint, and the brackets bear the weight of the flue. The bellows only absorbs heat displacement and does not bear gravity
- Select type with tie rod: the tie rod can withstand pressure thrust while limiting excessive axial displacement and preventing bellows from sagging or jumping
Applicable scenarios: high temperature (> 250℃), high pressure, circular vertical flue.
2.3 Sleeve Expansion Joints-Use with caution
Sleeve expansion joint relies on packing seal. When vertically installed, packing is easy to sink due to gravity, resulting in uneven seal, large gap in the upper part, squeezing in the lower part, and high leakage risk. Therefore, it is not recommended for use on vertical flues.
3. Special design requirements of vertical flue expansion joint
3.1 Condensate discharge issues
In the vertical flue, condensed acid or condensed water will flow down the inner wall of the flue and collect at the expansion joint. If the expansion joint does not have the ability to drain liquid, the liquid will soak the bottom skin or bellows for a long time, causing rapid corrosion.
Design requirements:
- Provide a drain hole at the lowest point of the expansion joint (usually at the lower flange) to connect the drain pipe and the valve
- An acid-resistant water baffle shall be installed on the inside of the bottom skin of the non-metallic expansion joint to guide the condensate to the water-phobic port and avoid direct contact with the skin
- The trough of the expansion joint of metal bellows shall be provided with a φ 6~8mm drainage hole at the lowest position (under the guidance of the manufacturer)
3.2 Anti-insulation cotton drop design
For non-metallic expansion joints, the key to whether the expansion joints can be installed in vertical flue lies in the treatment of thermal insulation cotton. Due to gravity, thermal insulation cotton has a tendency to sink when installed vertically.
Standard practice:
- Select high-density ceramic fiber blanket with bulk density ≥128kg/m³ to reduce settlement
- The heat insulation cotton is divided into short segments (length of each segment ≤300mm), sewn and fixed on the inner reinforcing cloth
- A stainless steel wire barrier net is arranged every 200mm inside the expansion joint to support the heat insulation cotton in layers
- It is strictly prohibited to use loose cotton in vertical expansion joints, and blanket materials must be used
3.3 Limit and Guide Bracket Arrangement
After installing the expansion joint on the vertical flue, the fixing bracket and guide bracket must be reasonably arranged:
| Type of stent | position | effect |
|---|---|---|
| Fixed bracket | Above the expansion joint (≤2m from it) | Bear the flue weight and prevent sliding down |
| guide bracket | Below the expansion joint (≤2m from it) | Ensures vertical alignment and prevents bending |
| Limit rod (expansion joint comes with itself) | Expansion joint structure | Limit excessive stretching and protect bellows |
Key principle: The expansion joint itself must not bear the weight of the flue and insulation. The full gravity load of the vertical flue shall be borne by the fixed bracket, and the expansion joint shall only be responsible for thermal displacement compensation.
IV. Installation precautions
4.1 Pre-compression/pre-tension direction
Horizontal flues usually use cold pre-compression (due to hot elongation). However, the vertical flue needs to be determined according to the specific layout:
- The flue below the expansion joint is longer: in the hot state, the lower flue extends downward, and the expansion joint is stretched → it needs to be pre-compressed in the cold state (reserve stretching space)
- The flue above the expansion joint is longer: the upper flue extends upward in the hot state, and the expansion joint is compressed → it needs to be pre-stretched in the cold state (reserve compression space)
Calculation example: If the total height of the vertical flue is 20m, the expansion joint is installed 5m from the bottom (upper part 15m, lower part 5m). Upper thermal elongation =0.012×15×330 ≈ 59 mm (upward), lower thermal elongation =0.012×5×330 ≈ 20 mm (downward). The actual net displacement withstood by the expansion joint is 59-20=39mm (tension). Therefore, it should be pre-compressed about 20mm during installation.
4.2 Hoisting and temporary fixing
- When hoisting the vertical flue expansion joint, temporary supports must be set. It is strictly prohibited to apply the hoisting load to the bellows or skin
- Before welding the end pipe or flange, use temporary bolts or positioning rods to fix the expansion joint at the designed length to prevent sag due to gravity
4.3 Installation direction of guide tube
In the vertical flue, the fixed end of the guide tube should be located above (the flue gas inlet side), and the free end should be downward. In this way, the condensate can flow down smoothly along the outer wall of the guide tube without accumulating inside the expansion joint.
Special Concerns for Operation and Maintenance
5.1 Check condensate discharge periodically
The hydrophobic hole of the vertical flue expansion joint is the most prone to blockage. Recommendations:
- Open the trap monthly to check for accumulated fluid discharge
- If there is no drainage for a long time, dredge the drainage hole with thin steel wire (be careful not to pierce the inner isolation membrane)
5.2 Monitor expansion joint length changes
Permanently mark both sides of the vertical flue expansion joint and measure cold and hot lengths with a steel tape measure. If the deviation between the thermal length and the design value is found to exceed ±10mm, explain:
- The fixing bracket may fail and the flue has slipped down or topped up
- Plastic deformation may have occurred inside the expansion joint (e.g. bellows varus or skin tear)
5.3 Check the drop of insulation cotton
After shutting down, tap the external surface of the expansion joint at different height positions by hand. If the upper sound is hollow and the lower sound is dull, it indicates that the insulation cotton has sunk and accumulated to the bottom. The skin needs to be cut, refilled and barrier mesh added.
6. Common mistakes and avoidance
| Wrong practice | consequence | Correct practice |
|---|---|---|
| The vertical expansion joint is not provided with drainage opening | Condensed acid accumulation, bottom corrosion perforation | Open a drainage hole at a low position to drain water regularly |
| The flue weight is bear by that expansion joint | Crushing/tearing of bellows or skin | Load-bearing bracket beside expansion joint |
| Insulation cotton is not layered and fixed | Drop accumulation, loss of insulation in upper part | Stainless steel wire mesh compartments every 200mm |
| Install pre-compression in wrong direction | Exceeding limit tension or compression in hot state | Calculate the net displacement direction based on the expansion joint position |
| Use loose cotton fill | Settle to bottom, upper cavity | Blanket material must be selected and sewn in sections |
VII. SUMMARY
Whether vertical flues can be fitted with expansion joints-Not only can, but it is a must in most vertical flues. The core points of proper installation of a vertical flue expansion joint can be summarized as follows:
- Selection priority: Non-metallic fabric expansion joints are used in ≤250℃ working conditions, and metal bellows are used in high temperature and high pressure. Sleeve type not recommended
- Key design: It is necessary to solve two vertical unique problems: condensate discharge (with hydrophobic holes) and insulation cotton drop (segmented support + barrier net)
- Load-bearing separation: the flue self-weight shall be borne by the fixed bracket, and the expansion joint shall only be responsible for thermal displacement compensation, and shall not bear gravity load
- Pre-compression direction: Calculate the net displacement direction according to the position (upper or lower) of the expansion joint in the vertical section to determine the cold pre-compression or pre-tension
- Key points of operation and maintenance: regularly check the patency of hydrophobic holes, monitor the change of length, and judge the position of insulation cotton by tapping
As long as the above principles are followed, the reliability and service life of a vertical flue expansion joint can be comparable to that of a horizontal flue. Engineers should incorporate expansion joints into the system as standard when designing vertical flues, not as an afterthought remedy.