1. Importance of assembly and connection of flue expansion joint
In the installation engineering of flue system, expansion joint is the key component to absorb heat displacement and compensate installation deviation. The assembly and connection quality of flue expansion joint directly determines the reliability and service life of the system after it is put into operation. According to statistics, more than 30% of the accidents of premature failure of expansion joints are caused by improper assembly or wrong connection process. Unlike other equipment in the pipeline system, expansion joints are metal bellows structures that are extremely sensitive to installation stress, centering accuracy, and welding heat input. Forced matching during assembly, high temperature burning through bellows during welding, and uneven tightening of flange bolts-these seemingly subtle problems may evolve into leakage or even fracture accidents during operation. Therefore, it is a compulsory course for every installation engineer and construction worker to master the standardized assembly and connection process of flue expansion joint. This article will systematically elaborate the complete site construction guide from pre-assembly preparation, flange joining, welding joining to quality control.
Preparation before assembly
2.1 Unpacking inspection and acceptance
Before assembling and connecting the flue expansion joint, the following inspections must be completed:
- Check the model specification: confirm that the nominal diameter, pressure level and compensation amount of the expansion joint are consistent with the design drawings
- Appearance inspection: No mechanical damage, arc burn, indentation or crack on the bellows surface
- Dimension review: measure installation length, flange bolt hole distance, guide tube position
- Check transport tie rod: Verify that tie rod is in good condition and record pre-tension/pre-compression value
- Check the material certificate: the materials of bellows, flange and guide tube shall be consistent with the design requirements
2.2 Preparation of Flue Interface
Before installation of the expansion joint, the flue interface connected to it should be treated:
- Smooth cutting: The end face of the flue shall be neatly cut with allowable deviation ≤3mm
- Clean bevels: Remove scale, oil, rust and weld slag
- Preparation of grooves: Process grooves according to welding process requirements, angle 60-70°, blunt edge 1-2mm
- Measure actual spacing: Measure the actual distance between the two flue interfaces and compare it with the length of the expansion joint installation
2.3 Confirmation of installation conditions
The assembly and connection of the flue expansion joint can only be started after the following conditions are met:
- Flue support installed in place, bracket adjusted
- The flue has been cleaned and there is no debris
- The working space meets the requirements and there are enough operating positions
- Safety measures in place, fire approval completed
3. Flange connection process
3.1 Flange Centering and Clearance Control
For flanged expansion joints, the assembly and connection of flue expansion joints first requires precise centering:
- Parallelism of two flange surfaces ≤1.5mm (check with a level or gauge)
- Even flange clearance with deviation ≤0.5mm
- Bolt hole alignment, deviation ≤1mm, bolt should be free to penetrate
- It is strictly prohibited to use a crowbar to force holes
3.2 Selection and installation of sealing gaskets
Proper installation of gaskets is key to flange connection sealing:
| Gasket Type | Applicable working conditions | Installation Points |
|---|---|---|
| Non-asbestos fiber mat | General flue, ≤250℃ | Apply sealant on both sides and align bolt holes |
| Stainless steel clad graphite pad | High temperature flue, ≤600℃ | Check that the cladding is intact and not bent |
| Ceramic fiber mat | High temperature and corrosive working conditions | Uniform thickness without moisture |
| Metal winding pad | High pressure, high temperature | Pay attention to that direction of the inner and outer ring |
For installation, the spacers should be centered and not offset or overlapping. The flange bolts shall be tightened cross-symmetrically and gradually screwed to the specified torque in 2-3 times.
3.3 Bolt tightening specifications
The tightening quality of flange bolts directly affects the sealing effect of assembly and connection of flue expansion joints:
- High-temperature anti-bite agent should be applied to bolts and nuts
- Fastening sequence: first radial, then circumferential, cross-symmetric
- Fastening in three times: 30% → 70% → 100% specified torque
- Finally check the flange clearance, any point clearance ≤0.5mm
Four, welding connection process
4.1 Selection of Welding Method
For welded expansion joints, the following processes are recommended for the assembly and connection of flue expansion joints:
| Welding position | Recommended Method | Description |
|---|---|---|
| Bottom Layer | Argon Tungsten Arc Welding (TIG) | Ensure root penetration and no oxidation |
| fill layer | Manual arc welding (SMAW) | High efficiency, suitable for medium and thick walls |
| Cover layer | TIG or SMAW | Ensure beautiful shaping and no biting edge |
4.2 Welding Process Parameter Control
In the assembly and connection of flue expansion joints, the control of welding heat input is crucial:
| Parameter Item | Recommended value | Description |
|---|---|---|
| Welding current | 80-120A (TIG) /90-140A (SMAW) | Small current to prevent overheating |
| Arc voltage | 10-14V (TIG) /22-26V (SMAW) | Short arc operation |
| Interlayer temperature | ≤150℃ | Strict control to prevent intergranular corrosion |
| welding material | Matching with base metal | 308/309 welding wire for stainless steel |
4.3 Welding sequence and deformation control
- Symmetrical and segmented welding along the circumference to reduce shrinkage deformation
- Single-section welding length ≤100mm, alternate jump welding
- Remove slag after each weld and check for defects
- If cracks or pores are found, they should be polished and removed in time and then welded
4.4 Precautions for welding dissimilar steels
When welding stainless steel expansion joints to carbon steel flues, special attention should be paid to the assembly and connection of flue expansion joints:
- Select transition layer welding material (e.g. E309L)
- Before welding, the transition layer is welded on the carbon steel side, and then butt jointing is carried out
- Strict control of heat input and interlayer temperature
- Small current, fast welding and narrow bead process can be adopted
Adjustment and acceptance after assembly
5.1 Adjustment of pull rod and limit device
After the welding or flange connection is completed, the final step in the assembly and connection of the flue expansion joint is to adjust the tie rod:
| Expansion joint type | Tie rod handling method |
|---|---|
| Axial type (no tie rod) | No processing required |
| With transport tie rod | Completely loosen or remove tie rod |
| With limit tie rod | Adjust limit nut position according to design |
| Large tie rod type | Tighten the tie rod nut to bear the internal pressure thrust |
5.2 Welding Quality Inspection
For expansion joints connected by welding, the following inspections shall be performed:
- Appearance inspection: welds are well formed, no cracks, unfused, bitten edges
- Penetration testing: 100% penetration testing of all welds, grade I qualified
- Radiographic inspection: Proportional radiographic inspection of butt welds, not less than Grade II
5.3 Tightness Test
After the assembly and connection of the flue expansion joint is completed, a tightness test shall be performed:
- Install temporary blind plates on both sides of the expansion joint and fill with 0.05-0.1MPa compressed air
- Apply leak detection fluid at welds and flange joints
- Hold pressure for 30 minutes without bubbles and pressure drop is qualified
VI. Common Assembly Errors and Corrective Measures
| Error | consequence | Corrective action |
|---|---|---|
| Forced counterpart | Increase installation stress and reduce fatigue life | Adjust flue bracket to center naturally |
| Corrugated pipe damaged by conductive arc during welding | Corrugated pipe surface pitting | It is strictly prohibited to put ground wires on bellows |
| Welding slag splashes on bellows | Stress concentration point, corrosion source | Cover protection during welding, clean up after welding |
| Flange bolt unilateral tightening | Gasket bias, seal failure | Cross-symmetric fractional fastening |
| Forgot to loosen the transport tie rod | The expansion joint cannot compensate, and the tie rod is bent | Check and release before commissioning |
Safety Precautions
When assembling and connecting flue expansion joints, the following safety regulations must be observed:
- Use special spreaders when hoisting expansion joints, and it is strictly prohibited to collide with bellows
- During the welding operation, the ground wire should be placed on the connectors, and it is strictly prohibited to be placed on the bellows
- Confirm that the high temperature flue has been cooled to normal temperature before construction
- Welding in limited space requires restricted space operation ticket and maintains ventilation
- Equipped with fire extinguishing equipment and supervised by special personnel
VIII. Summary
The assembly and connection of flue expansion joint is the key process in the installation of flue system, and its quality directly determines whether the expansion joint can operate safely and reliably within the design life. Successful assembly and connection can be summarized into four key points: "accurate centering, standardized welding, even tightening, and tie rods in place".
Before assembly, the expansion joint and flue interface must be fully inspected to confirm that the specifications, models, appearance quality and actual dimensions match. In the connection process, the centering accuracy, gasket quality and bolt tightening sequence should be strictly controlled for flange connection; For welding connection, the process of TIG primer + SMAW filling should be selected, the interlayer temperature should be strictly controlled to ≤150℃, and the deformation should be controlled by symmetric segmented welding. After the connection is completed, adjust the tie rod as specified (the transport tie rod must be loosened or removed), and carry out the weld non-destructive testing and system tightness test.
In particular, it is strictly forbidden to overlap the welding ground wire on the bellows, and it is strictly forbidden to use the deformation of the expansion joint to compensate for the deviation of the flue. These two are "high-voltage lines" in the installation of the expansion joint. Through the standardized assembly and connection construction, the expansion joint can ensure the normal compensation function in the flue system, avoid the early failure caused by improper installation, and lay a solid foundation for the long-term safe operation of the flue system.