In the installation or maintenance of flue system, the welding of expansion joint is the key process to determine the sealing performance and service life. Unqualified welding quality will lead to leakage, deformation and even early failure of the expansion joint. However, many on-site construction workers have misunderstandings about how to weld the expansion joint of the flue-the expansion joint is welded equally with the ordinary flue pipe, which causes serious consequences. This paper will systematically explain the standard welding method of flue expansion joint from four aspects: welding preparation, process parameters, operation steps and quality inspection.
I. Preparation before welding
Before discussing how to weld the expansion joint of the flue, an important principle must be clarified first: it is strictly prohibited to pass welding current through the flexible components of the expansion joint. Whether it's a metal bellows or a non-metal skin, the welding current can cause irreversible damage to it.
1.1 Ground Wire Location Selection
- The ground wire must be clamped on the flue body on the same side as the part to be welded and must not be connected to the other side across the expansion joint.
- For metal bellows expansion joints, the grounding wire cannot be clamped on the bellows crest, otherwise the arc will ablate the thin wall of the bellows.
1.2 Protection of flexible elements
- Non-metallic expansion joint: When welding end pipe or flange, the skin surface must be completely covered with asbestos cloth or fire blanket to prevent welding slag from splashing and burning the fabric layer.
- Metal expansion joint: Cover the bellows trough with thick cardboard or rubber sheet to prevent welding slag from embedding in the corrugated gap.
1.3 Group-to-Size Review
- Before welding, measure whether the actual length of the expansion joint is consistent with the designed cold length (non-metallic expansion joint needs to be pre-compressed by 5% ~8%).
- Check the concentricity of the pipes (or equipment interfaces) at both ends; the deviation shall be ≤3mm.
| Check Items | Allowable deviation | Measurement tool |
|---|---|---|
| Flange parallelism | ≤3mm | Size gauge, straight ruler |
| Pipe coaxiality | ≤3mm | Laser centering instrument or pull wire |
| counter clearance | 2~4mm | Welding inspection ruler |
2. Welding process of metal expansion joint
There are usually two ways to connect the expansion joint of metal bellows with the flue: flange connection and direct welding. How to weld the expansion joint of the flue is mainly aimed at direct welding.
2.1 End pipe welding
Both ends of the metal expansion joint come with end pipes (short joints), and only butt welding of the end pipe and the flue pipe is necessary during construction.
Welding process parameters (taking carbon steel flue, thickness 8mm as an example):
- Welding method: Manual arc welding (SMAW) or CO₂ gas shielded welding (GMAW)
- Electrode/Wire Model: E5016 (J506) or ER50-6
- Electrode diameter: φ 3.2mm (base) → φ 4.0mm (filling cover)
- Welding current: 110~130A (φ 3.2), 160~190A (φ 4.0)
- Interlayer temperature: ≤150℃
2.2 Groove Form
- When the wall thickness of the flue is ≤6mm, the groove may not be opened, but the gap of 2~3mm should be ensured.
- When the wall thickness is> 6mm, a V-shaped groove should be opened with an angle of 60° ~70° and a blunt edge of 1~2mm.
Key point: The root of the weld must be welded through, but the internal guide tube must not be burned through. There is usually only 10~15mm gap between the guide tube and the inner wall of the bellows, and excessive welding current will break down the end tube and damage the guide tube.
2.3 Welding sequence and deformation control
- Symmetric segment welding is adopted: for circular flue, segment according to clock point (12 o'clock → 6 o'clock → 3 o'clock → 9 o'clock), each segment is 50~100mm long, and welding is applied alternately.
- For rectangular flue, welding should be applied from the midpoint of the long side to both ends, then the short side, and finally the corner.
- After each weld, tap the weld and the heat affected zone with a wooden hammer to release the welding stress.
2.4 Prohibited Matters
- No welding (including spot welding, repair welding) shall be performed on the bellows. The wall thickness of the bellows is only 0.8~2.0mm, and the welding will immediately burn through or cause stress cracking.
- When welding the end pipe to the flue, do not clamp the ground wire to the bellows or the opposite flue.
3. "Welding" related procedures in the installation of non-metallic expansion joints
The non-metallic expansion joint itself has no welding parts, and its connection to the flue relies on flanges and bolts. However, during installation, it is necessary to weld flanges or angle steel platen frames at the end of the flue. This part of the welding also requires caution.
3.1 Flange welding
How to weld the expansion joint of the flue For non-metallic expansion joints, it refers to welding the flange ring or frame used to compress the skin.
Step:
- First, spot weld the flange ring and fix it at the end of the flue to ensure that the flange surface is perpendicular to the axis of the flue, and the deviation is ≤2mm/m.
- Adopt segmented jump welding method: 100mm per weld, skip 100mm, and repair welding after cooling. Prevent flange deformation and warpage caused by continuous welding.
- The weld should be continuous, pore-free, and the weld slag should be ground to smooth, without sharp bumps-otherwise it will puncture the skin.
3.2 Plate Bolt Seat Welding
Some non-metallic expansion joints need to be welded with nut plates or bolt seats. Care should be taken when welding:
- Remove splashes after welding and re-pass the thread with a tap to prevent weld slag from clogging the thread.
- The position of the bolt seat must correspond to the hole position of the skin pressure plate, and the deviation shall be ≤1.5mm.
3.3 Deflector welding
The guide tube is welded on the inside of the flue, which belongs to a concealed process, and is especially important:
- The fixed end of the guide tube shall be continuously welded with the inner wall of the flue, and no spot welding shall be allowed.
- The free end is strictly prohibited from welding and must be kept in a free sliding state.
- Before welding, confirm the installation direction of the guide tube: the bell mouth or overlap end faces the incoming flue gas.
Welding quality inspection
After completing the construction of how to weld the expansion joint of the flue, the following inspections must be carried out:
4.1 Appearance inspection
- There are no cracks, pores, slag inclusions and biting edges on the weld surface (depth ≤0.5mm).
- The weld residual height is ≤3mm, and the transition is smooth.
4.2 Dimensional inspection
- The parallelism of the flanges or end tubes at both ends of the expansion joint after welding is ≤3mm.
- The overall length variation of flue shall be within the design allowable range (the non-metallic expansion joint shall be kept in the pre-compressed state).
4.3 Sealability Test
After completion of welding and before heat insulation, conduct air tightness test:
- Methods: Apply soapy water to the weld, and fill the flue with compressed air to 1.1 times the working pressure (but not exceed the pressure resistance limit of the expansion joint).
- Criteria: No bubbles are continuously generated as qualified.
- For non-metallic expansion joints, it is also necessary to check the flange pressure plate bolts for leakage.
4.4 Non-destructive testing (according to design requirements)
- The butt welds of important flues (such as denitrification inlet and absorption tower inlet) shall be subjected to 20% ~100% radiographic inspection (RT) or ultrasonic inspection (UT) according to NB/T 47013 standard, and are qualified for Class II.
V. Common welding problems and prevention
| Problematic phenomenon | Main reasons | Treatment method |
|---|---|---|
| Welding slag burns non-metallic skin | Uncovered with asbestos cloth or lax coverage | Immediately replace the damaged skin, and strictly prohibit perfunctory tape repair |
| Bellows breakdown | Ground wire crosses expansion joint or arc accidentally touches bellows | Replace the entire metal expansion joint without repair welding |
| Flange welding deformation | Continuous welding unsegmented | Cut the weld and re-segment symmetric welding |
| Guide tube falling off | The free end is fixed by miswelding | Cut solder joints, smooth them and restore them to free state |
| Air tightness of weld unqualified | The root is not welded through or has pores | Carbon arc gouging to remove defects and re-weld |
6. Welding matching of different flue materials
| Flue material | Expansion joint end tube material | Welding material selection | Special requirements |
|---|---|---|---|
| Q235B carbon steel | Q235B or 20# | E5016 (J506) | Preheat 100℃ before welding (winter) |
| 16Mn low alloy steel | 16Mn | E5016 or E5515 | Heat treatment after welding 600~650℃ |
| 304 stainless steel | 304 | A102 or ER308 | Argon-filled protection on the back, control the interlayer temperature ≤150℃ |
| Carbon Steel and Stainless Steel Dissimilar Steel | Carbon steel end pipe + stainless steel bellows | A302 (E309) | Use of transition layer beads |
Special Note: When welding dissimilar steel, the ground wire must be clamped on the carbon steel side to avoid the tendency of intergranular corrosion in stainless steel.
Welding Safety and Protection
- Fire prevention: combustible gas (gas, VOCs) may remain in the flue. Gas detection and hot fire ticket must be carried out before welding.
- Anti-scalding: Bellows and skin will heat up under welding heat radiation. The surface of non-metallic skin should not be exposed to high temperature (> 80℃) for a long time, and wet asbestos cloth can be used to cool down.
- Ventilation: When welding in the flue, forced ventilation and dust mask must be worn.
VIII. Summary
The core principle of how to weld the expansion joint of the flue can be summarized as three sentences: "protecting flexible parts, controlling deformation by sections, and strictly checking and sealing":
- Protect flexible parts: Cover with asbestos cloth (non-metal) or protective plate (metal) before welding, the grounding wire is strictly prohibited from crossing the expansion joint, and the arc shall not touch the bellows or skin.
- Sectional deformation control: Symmetric sectional jump welding method is adopted to control the interlayer temperature and prevent flange warping or bellows instability.
- Strict sealing inspection: Air tightness test must be carried out after welding, and non-destructive testing must be carried out for important welds.
For non-metallic expansion joints, the so-called "welding" is actually the welding of the flange frame and the guide barrel, and the skin itself is not involved in the welding. Workers must distinguish the process differences of different types of expansion joints, and avoid directly applying the welding method of metal expansion joints to non-metallic products. Correct welding process is the first guarantee for the long-term reliable operation of expansion joints, and any negligence may lead to premature failure of equipment or even safety accidents.