1. Discussion on the Necessity of High Temperature Flue Expansion Joint
In the design of high-temperature flue systems such as industrial furnaces, boilers and roasters, a frequently asked question is: Does the high-temperature flue need to be equipped with expansion joints? The answer: must be set in the vast majority of cases. During the operation of high-temperature flue, as the temperature rises from normal temperature to hundreds or even thousands of degrees Celsius, the flue material will undergo significant thermal expansion. If the flue is a rigid continuous structure and no expansion joint is set to absorb thermal elongation, the huge thermal stress will lead to flue weld cracking, bracket failure, flange leakage and even overall flue instability deformation. However, whether each high-temperature flue must be provided with expansion joints, how many to set them, and what type to adopt need to be comprehensively judged according to the pipe length, working temperature, direction layout and bracket form. This paper will systematically answer this question from the calculation of thermal expansion, the feasibility evaluation of no expansion joint to the selection of setting scheme.
Calculation of thermal expansion: the basis for judging whether an expansion joint is needed
2.1 Basic Formula of Thermal Expansion
Answer high-temperature flue need to set expansion joint, first of all, calculate the thermal expansion of the flue. The calculation formula is as follows:
Δ L = α × L × (T_WORK-T_INST)
Among them:
- Δ L: thermal elongation (mm)
- α: Coefficient of linear expansion of material (/℃)
- L: Calculate the length of the pipe section (m)
- T_work: Operating temperature (℃)
- T_inst: Installation or initial temperature (℃)
2.2 Thermal expansion coefficient of common materials in high temperature flue
| Materials | Linear expansion coefficient α (×10⁻⁶/℃) | Applicable temperature range |
|---|---|---|
| Carbon steel (Q235B/20#) | 11-13 | ≤450℃ |
| Low alloy steel (15CrMo) | 12-14 | ≤550℃ |
| Stainless steel (304/316L) | 16-18 | ≤750℃ |
| Heat resistant steel (310S) | 14-16 | ≤1000℃ |
2.3 Calculation Example
A section of carbon steel flue with a length of 20m is set, the working temperature is 450 DEG C and the installation temperature is 20 DEG C, then:
Δ L =12×10⁻⁶ ×20000× (450-20) =103.2mm
This means that a 20m-long flue, after warming up to 450°C, elongates by about 103mm – equivalent to a displacement of 10cm. If both ends of the flue are rigid and fixed, this displacement has nowhere to be released, which will inevitably produce huge internal stress. Therefore, in this case, the answer to whether the expansion joint needs to be set in the high-temperature flue is clear: it must be set.
2.4 The critical length of the expansion joint needs to be set
| Operating temperature (℃) | Critical length (m) | Description |
|---|---|---|
| ≤150 | 50-60 | Thermal displacement is small and can be compensated naturally by pipe flexibility |
| 150-300 | 30-40 | Recommended calculation validation, usually required to set |
| 300-500 | 20-25 | Expansion joints must be provided |
| 500-800 | 10-15 | Expansion joints must be provided |
| ＞800 | 5-10 | High strength expansion joint + thermal insulation |
3. Feasibility conditions of not setting expansion joints
Although the vast majority of high-temperature flues need to be provided with expansion joints, they may not be provided under certain conditions.
3.1 Natural Compensation (Flexible Pipe Design)
The flexibility of the pipe itself can absorb part of the thermal displacement when there are sufficiently long straight sections and elbows in the flue stroke. This is the most common alternative to high-temperature flues. Conditions for natural compensation include:
- The flue is arranged in an L-, Z-or U-shape, with elbows providing flexibility
- The length of the pipe between the two fixed points does not exceed the critical value (see table above)
- The pipe wall thickness is thin (≤6mm) and the stiffness is low
3.2 Short-distance straight pipe sections
For straight pipe sections of very short length (e.g. length ≤3-5m from the outlet of the equipment to the first turn), the amount of thermal elongation is very small (usually ≤10mm), which can be absorbed by the clearance of the connecting flange, the flexibility of the equipment interface or the elastic deformation of the pipe. At this time, no expansion joint can be provided.
Typical example: The connecting section from the roaster outlet to the settling chamber is usually only 2-4m long, and the working temperature is 800-900℃. It can adopt thick-walled tube + large flange structure, and the elasticity of flange bolts is used to absorb a small amount of heat displacement, without separate expansion joints.
3.3 Sliding bracket and elastic connection
For longer flues, if expansion joints are not provided, sliding brackets can be arranged throughout the length, allowing the flue to extend freely, with elastic connections at the ends (e.g. packing box seals). This scheme is commonly used in horizontal directly buried thermal pipes, but it is less used in high temperature flues because its sealing reliability is not as good as that of expansion joints.
4. Typical working conditions where expansion joints need to be set
The answer to whether the expansion joint needs to be set in the high-temperature flue under the following working conditions is yes:
4.1 Long-distance straight pipe sections
When the long straight section of the flue exceeds the critical length (see Section 2.4) and goes straight without turning, axial expansion joints must be provided. Common in:
- Connecting flue from boiler outlet to dust collector
- Straight section of annular flue of roaster
- Original flue at inlet of desulfurization tower
4.2 Connections between High Temperature Equipment
When both devices are fixed independently (such as gas turbine exhaust port and waste heat boiler inlet), there is no common basis between the two, and the relative thermal displacement difference is significant, expansion joints must be set. Typical operating conditions:
- Gas turbine boiler inlet flue: the gas turbine exhaust temperature is 500-650℃, the boiler inlet is about 120℃, and the displacement difference is 30-60mm
- Connecting flue between roaster and cooler
4.3 Where the pipeline changes direction
When the flue changes direction at the elbow, thermal expansion causes lateral displacement of the elbow, creating lateral thrust on adjacent equipment. At this time, hinge type or universal expansion joints should be set on both sides of the elbow.
4.4 Where the flue passes through the wall or floor
When a flue crosses a building structure, wall or floor constraints will limit the axial displacement of the flue, and expansion joints must be provided on both sides of the crossing.
5. Risks and consequences of not setting expansion joints
If the operating conditions that should be set are not set, the following problems will arise:
| Risk | Specific performance | consequence |
|---|---|---|
| flue deformation | Local bulging, warping, flange surface warping | Compromised aesthetics, failed connection |
| Weld cracking | Thermal stress exceeds weld strength and cracks appear | Flue gas leakage, environmental protection exceeds standard |
| Stent failure | The fixing bracket is bent or pulled off | Loss of support, sinking or displacement of pipeline |
| Flange leakage | Gasket failure caused by warping of flange surface | Seal failure, need to stop the furnace |
| Device interface is damaged | The expansion force is transmitted to the device interface | Equipment shell cracked, high maintenance cost |
6. Suggestions on the selection of high-temperature flue expansion joint
6.1 Select Material by Temperature
After confirming whether the expansion joint needs to be set in the high-temperature flue and deciding to set it, the material of the bellows should be selected according to the flue temperature:
| Flue temperature (℃) | Recommended Bellows Material | Description |
|---|---|---|
| ≤450 | 304 stainless steel | Economical type, suitable for general boiler flue |
| 450-600 | 321 or 316L stainless steel | Titanium-containing stabilization, anti-sensitization |
| 600-800 | 309S or 310S stainless steel | Excellent high temperature oxidation resistance |
| 800-1000 | Inconel 625 | Nickel-based alloy, suitable for roasting furnace outlet |
| ＞1000 | Ceramic fiber + air-cooled structure | Metal expansion joints cannot be directly touched |
6.2 Selecting Structure by Displacement Direction
| Displacement characteristics | Recommended expansion joint types |
|---|---|
| Mainly axial displacement | Axial type or double axial type |
| Axial + transverse combination | Large tie rod transverse type |
| Angular displacement | Hinge type or universal hinge type |
| Multi-directional small displacement | Non-metallic fabric compensator |
6.3 Special configuration of high temperature expansion joint
For high temperature flues (≥600℃), the expansion joint requires the following special configuration:
- Guide tube: Prevent high-temperature smoke from directly washing the inner wall of the bellows
- Insulation layer: filled with ceramic fibers to reduce the temperature of the outer wall
- Multi-layer bellows: Reduce single-layer stress and disperse heat load
- Air-cooled or water-cooled structure: External cooling is required under extreme high temperature conditions
VII. Example in which the expansion joint is not required
To give a more comprehensive answer to whether an expansion joint needs to be set in a high-temperature flue, the following example does not need to be set:
- Short-distance connection section: the distance from the outlet of the equipment to the first fixed point is ≤3m, the working temperature is 500℃, the thermal elongation is ≤12mm, and can be absorbed by the elasticity of the pipeline
- Fully suspended flexible flue: The flue is suspended by a hanger, and the full length can swing and telescope freely
- Masonry flue with expansion joints: lined with refractory bricks, with expansion gaps reserved in the brick joints, thermal expansion absorbed by the brick joints, and the metal shell separated in sections
- Small-diameter pipes with bellows compensator as connectors: such as instrument pipes, sampling pipes and other pipes with diameter ≤100mm
VIII. Summary
The core judgment basis of whether the expansion joint needs to be set in the high-temperature flue is whether the thermal expansion amount of the flue exceeds the bearing capacity of itself and the support. When the amount of thermal elongation exceeds 10 mm or the length of the pipe section exceeds a critical value (20-30 m, depending on the temperature), setting the expansion joint is a necessary and economical solution; For the flue with short distance (≤5m), natural compensation elbow or flexible suspension structure, it may not be set.
After deciding to set the expansion joint, the bellows material should be selected according to the working temperature (321/316L is recommended above 450℃, 310S or Inconel is recommended above 600℃), and the expansion joint structure (axial type, large tie rod type or hinge type) should be selected according to the displacement direction. For the high-temperature flue above 600℃, it is necessary to configure a guide tube, heat insulation layer and multi-layer bellows.
It needs to be emphasized that the pipeline stress damage caused by blindly omitting the expansion joint often appears after a period of operation-it may take months or even years for the crack to expand from microscopic to macroscopic leakage, but once it happens, the repair cost far exceeds the investment of the original configuration of the expansion joint. Therefore, in the design, the calculation results of thermal expansion should be used as the basis, and the scientific decision should be made whether the expansion joint should be set in the high-temperature flue, so as to avoid the long-term potential safety hazard due to saving initial investment.