In industrial flue gas treatment system, the selection of expansion joint is important, but the scientific and reasonable expansion joint arrangement of flue gas pipeline is the core to ensure the long-term stable operation of the system. Improper arrangement can lead to premature failure of expansion joints, pipe deformation and even equipment damage. Starting from engineering practice, this paper systematically explains the principles, common misunderstandings and optimization schemes of expansion joint arrangement of flue gas pipeline, so as to help technicians avoid risks from the source.
First, why is the layout of flue gas pipe expansion joint crucial?
Flue gas pipelines are usually connected to boilers, dust collectors, desulfurization towers, induced draft fans, chimneys and other equipment, and the operating temperature ranges from normal temperature to above 600℃. Pipes can produce significant thermal elongation in the hot state, and if the expansion joint is not set in the appropriate position or is arranged in a wrong way, thermal stress can be transmitted through the pipe to the equipment interface, resulting in flange leakage, foundation cracking or equipment shell deformation.
Correct flue gas pipe expansion joint arrangement can: effectively absorb the thermal displacement and vibration of the pipe; Reduce the thrust of the pipe on the fixed bracket and equipment; Preventing weld cracking due to accumulation of thermal stress; At the same time, it is easy to overhaul and replace in sections. Conversely, layout defects are often exposed after months of system operation, but are extremely costly to fix – involving furnace shutdowns, scaffolding erection, and extensive cutting and welding operations. Therefore, it is of significant economic value to master the standard layout method at the design stage.
2. Basic principles of expansion joint arrangement of flue gas pipeline
Regardless of whether metal or non-metal expansion joints are used, the following five principles are generally applicable:
1. Principle of targeted compensation
The expansion joint shall be arranged between the two fixed brackets of the pipe, specifically to absorb the amount of thermal elongation of the pipe section. An expansion joint should not compensate for displacement in multiple directions at the same time, unless a gimbal type construction is adopted.
2. Close to displacement source principle
For equipment inlet and outlet pipes, the expansion joint should be arranged as close to the equipment interface as possible (usually ≤4 times the pipe diameter) to directly absorb the thermal displacement of the equipment body. For example, it is most reasonable to arrange the expansion joint of the flue gas pipe at the inlet and outlet of the induced draft fan at a distance of 1.5-2 meters from the fan housing.
3. Principle of guiding and limiting cooperation
Guide brackets must be provided at both ends of the expansion joint, and the distance between the guide frame and the expansion joint should be controlled within 4 times the pipe diameter. At the same time, a limit bracket is arranged on one side of the expansion joint to prevent excessive lateral swing of the pipeline due to unexpected pressure pulsation.
4. Principle of avoiding blind plate force impact
When arranging the expansion joint at the elbow or blind end position of the pressurized flue gas pipe, the blind plate force generated by internal pressure must be considered. This force can reach several tons or even tens of tons and must be withstood by the main fixing bracket. Do not place the expansion joint directly at the end of a straight pipe section without a fixed bracket.
5. Principle of avoiding high-temperature accumulation zones
The expansion joint of flue gas pipeline of non-metallic expansion joint should be arranged to avoid the direct flushing surface of flue gas, especially not on the flushing side of sharp turn of flue. Install deflectors or insulation liners if necessary.
3. Layout scheme under typical working conditions
Scheme 1: Arrangement of long and straight horizontal flue
For horizontal flue gas pipes exceeding 30 meters in length, a "segmented compensation" strategy should be adopted: a set of fixed brackets every 15-20 meters with an axial-type expansion joint arranged between them. Each expansion joint absorbs the axial thermal elongation of the segment, and adjacent tube segments do not interfere with each other. Note: The expansion joint should be arranged close to the main fixing frame in the fixing bracket, while the guide frame is equally spaced over the pipe section.
Option 2: Vertical flue and equipment connection
On the vertical flue at the inlet of the desulfurization tower or the outlet of the dust collector, the flue gas pipe expansion joint arrangement should take into account the influence of gravity. It is recommended to use an axial type expansion joint with a load-bearing ring and set a spring hanger below it to avoid the expansion joint bearing the flue self-weight. At the same time, the spacing between the guide brackets at both ends of the expansion joint on the vertical pipe should be shortened to less than 3 times the pipe diameter to prevent instability.
Scheme 3: Universal compensation for space-constrained areas
When the flue direction is complex and limited by the building structure, a single expansion joint cannot meet the multi-directional displacement requirements. In this case, a combination arrangement of hinge-type or universal-type expansion joints may be used. For example, a hinge-type expansion joint is arranged on both sides of the horizontal elbow, and it is matched with an intermediate fixing bracket to absorb angular displacement in both directions. This scheme is commonly found in the flue of the inlet and outlet of the denitrification reactor.
Scheme 4: Protective arrangement of high-temperature dusty flue
The flue gas in the tail flue of coal-fired boiler (from the outlet of air preheater to the inlet of dust collector) has high dust content and the temperature is about 150-180℃. When arranging expansion joints of flue gas pipelines in such areas, wear-resistant guide tubes must be installed, and the length of the guide tubes should extend to at least 50mm after the trough of the expansion joints. At the same time, the expansion joint should be arranged at the lower position of the horizontal central axis of the flue section to avoid dust accumulation.
4. Common layout errors and correction methods
| Wrong practice | consequence | Correct practice |
|---|---|---|
| The expansion joint is arranged directly within 1 m behind the elbow | Flue gas bias flow scour bellows | Keep the length of straight pipe section ≥4 times the pipe diameter |
| Missing or excessively spaced guide brackets on both sides | Columnar instability occurs in expansion joint | The spacing between guide frames is ≤4D, and the spacing between first frame and expansion joint is ≤2D |
| Insufficient strength of fixed bracket | Expansion joint receives additional thrust | Design of the main fixing frame according to the calculated blind plate force |
| An Axial Expansion Joint for Multidirectional Displacement | Lateral deformation and tearing of bellows | Switch to double universal type or increase the number of expansion joints |
| Cold tightening amount not reserved | Displacement out of compensation range in hot state | Calculate pre-offset by design temperature |
V. Key Points of Parameter Calculation in Layout Design
The following key parameters must be obtained before arranging the expansion joint of the flue gas pipe:
- Thermal elongation: Δ L = α × L × Δ T, where α is the linear expansion coefficient of the pipeline (12×10⁻⁶/℃ for steel), L is the length of the pipe between the two fixed frames, and Δ T is the difference between the installation temperature and the working temperature.
- Allowable compensation amount of expansion joint: It should be greater than 1.2 times of the calculated thermal elongation, and the safety margin should be reserved.
- Blind plate force: F = P × A, P is the working pressure (kPa) and A is the effective area of the bellows (m²). For large diameter flues (diameter> 2m), the blind plate force may exceed 30 tons, and a heavy-duty fixing bracket must be designed.
- Guide frame spacing: L_max ≤0.25× (E × I/P_c) ^0.5, where P_c is the critical instability load of the pipeline.
In practical engineering, it is recommended to use professional pipeline stress analysis software (such as CAESAR II) for calibration, especially for pipeline systems involving high temperature, large diameter or complex strike.
VI. Precautions during construction and acceptance
After the layout design is completed, the on-site construction stage still needs to focus on:
- Check whether the actual installation length of the expansion joint is consistent with the design drawing and whether the cold tightness value is correctly marked.
- Check that the clearance of all guide brackets and limit brackets meets the design (usually guide clearance 2-5mm).
- After the expansion joint is installed, remove the transport protection rod (Note: the limit rod with the positioning function cannot be removed).
- During the system pressure test, temporary constraints should be set on the expansion joint area to prevent overpressure deformation.
Call to Action
The reasonable arrangement of flue gas pipe expansion joints is directly related to the continuous production safety and maintenance cost of your plant. If you're planning a new flue system or facing frequent problems with existing pipes, feel free to contact our duct design team today.