In the engineering construction and maintenance of boiler flue gas system, the acceptance of boiler flue expansion joint is the last barrier to ensure the safe operation of equipment and eliminate hidden dangers in operation. After the expansion joint is installed, if the acceptance is not strict and the standard is unclear, it will lead to air and smoke leakage and energy consumption after operation, and in the worst case, it will lead to expansion joint failure, unplanned furnace shutdown or even safety accidents. So, what exactly should be checked for boiler flue expansion joint acceptance? What is the standard basis? This paper will systematically explain the professional and technical points of acceptance of boiler flue expansion joint from acceptance basis, inspection items to test methods.
1. Standard basis for acceptance of boiler flue expansion joint
The acceptance of boiler flue expansion joint shall strictly follow the following national standards and industry norms:
| Standard No. | Standard Name | Scope of application |
|---|---|---|
| GB/T 12777-2019 | General specifications for expansion joints of metal bellows | Design, manufacture and inspection of metal expansion joints |
| GB 50753-2012 | Code for design of dust collection in non-ferrous metal smelters | Installation and acceptance of flue compensator |
| GB 50517-2010 (2023 Edition) | Code for acceptance of construction quality of metal pipeline engineering in petrochemical industry | Pipeline System Pressure Test and Acceptance |
| DL/T 5121-2020 | Technical specification for design of smoke-wind and pulverized coal pipeline in thermal power plants | Expansion joint of flue duct in thermal power plant |
According to the provisions of Article 10.2.1 of GB 50517-2010, the expansion joint shall participate in the system test: the expansion joint of the self-restraining device shall participate in the system test; Externally constrained expansion joints shall participate in system tests; All expansion joints shall be subjected to system test during leakage test。
In the waste heat power generation project of cement factory, GB51005-2014 Code for Construction and Quality Acceptance of Waste Heat Power Generation Project of Cement Factory clearly lists "Installation of Expansion Joints of Smoke and Air Ducts" as an independent acceptance chapter, covering the installation quality inspection of expansion joints of smoke and air ducts。
2. Installation quality acceptance: full inspection from appearance to size
1. Appearance inspection
Appearance inspection for acceptance of boiler flue expansion joint shall include the following:
| Check Items | Acceptance criteria | Inspection method |
|---|---|---|
| Surface condition | No mechanical damage, scratches, corrosion | Visual inspection |
| Skin/Bellows | No breakage, crack, bulge | Visual + Touch |
| Weld appearance | No cracks, pores, unfused | Visual + NDT |
| Anticorrosive coating | Complete, no shedding, no leakage | Visual inspection |
| Nameplate identification | Clear, consistent with model specifications and design | Check drawings |
2. Dimensional inspection
The accuracy of the mounting dimensions directly affects the compensation ability of the expansion joint:
- Installation length: Check whether the distance from the flange surface to the flange surface of the expansion joint meets the design value, and the deviation shall not exceed ±5mm
- Coaxiality: The expansion joint shall be concentric with the pipe with a deviation ≤3mm
- Levelness: horizontally mounted expansion joint, level deviation ≤3‰
- Perpendicularity: Expansion joint installed vertically, perpendicularity deviation ≤3‰
3. Direction confirmation
This is the "one-vote veto item" in the acceptance of boiler flue expansion joint:
- Flow direction identification: The flow direction arrow on the expansion joint must coincide with the flue gas flow direction
- Direction of guide tube: The small end of guide tube must be facing the direction of flue gas flow
- Inner sleeve position: the end of the inner sleeve with weld, which should be located at the inflow end of the medium on the horizontal pipeline; It should be placed on the upper part of the vertical pipe
Special warning: A 410t/h boiler ignition air duct expansion joint fault case shows that the installation misalignment amount of 80mm is not reserved according to the design requirements during installation, and the expansion joint bellows is seriously deformed during operation, and the internal pouring material is damaged and fallen off, which finally leads to the expansion joint damage。
3. Inspection of transportation and limiting device
In the acceptance of boiler flue expansion joint, the treatment of transportation protection device is the most easily overlooked link.
1. Removal of transport tie rod
According to the provisions of Article 7.0.3 of GB 50753-2012, after the installation of the compensator, all the protective screws of the original belt shall be loosened, and the loosening distance shall be greater than the designed maximum compensation amount of the compensator.
Acceptance inspection points:
- Whether the transport protection tie rod of the metal expansion joint has been completely removed
- Whether the positioning limit pull rod is retained (not removed)
- Whether the release nut has been adjusted to the pre-bias position required by the design
2. Limit device adjustment
- Whether the limit tie rod nut has been adjusted in place according to the design drawing
- Whether the limit clearance meets the design requirements (generally about 1.2 times the axial compensation amount)
- Is the spring hanger reset (if any)
3. Temporary support removal
Transportation temporary supports added to large-diameter expansion joints when they leave the factory shall be completely removed after installation.
4. Sealability inspection and pressure test
1. Sealability Check
The tightness inspection of boiler flue expansion joint acceptance is the key link to verify the installation quality:
- Inspection method: Soap water application method or leak detector can be used
- Inspection site: Flange connection surface, weld, skin joint, pressure plate bolt
- Acceptance criteria: No bubble generation or no alarm from the detector is qualified
According to GB 50517-2010, all expansion joints shall participate in system test during leak test。
2. Pressure test precautions
When the expansion joint participates in the system pressure test, attention should be paid to:
- Confirm that the limit device of the expansion joint is set correctly before pressure test
- The expansion joint of the self-restraining device can directly participate in the system test
- The water used for hydraulic test shall be clean and non-corrosive, and the chloride ion content shall not exceed 25ppm
3. Airtightness test
For flue gas systems, air tightness tests should also be performed:
- The test pressure is 1.1 times the design pressure
- Hold time ≥30 minutes
- Pressure not reduced to qualified
V. Special acceptance items of non-metallic expansion joints
Acceptance of boiler flue expansion joints for non-metallic expansion joints requires additional attention to the following items:
1. Bolt tightening inspection
The skin of the non-metallic expansion joint is fixed by a platen bolt. Due to the large length of the platen (up to 4-6 meters), the distal bolt will loosen due to the compression deformation of the skin after one-time tightening.
Acceptance points:
- Check that all bolts have been tightened as per "diagonal tightening, fractional tightening"
- Use a hand hammer to gently tap the pressure plate, the sound should be crisp and consistent, without loosening
- Whether the bolt torque meets the design requirements (generally 50-80N·m)
2. Drain hole inspection
For horizontally installed boiler flue expansion joints (especially wet flue sections), drainage holes must be provided:
- Check Location: Whether the drain hole is located at the lowest point of the frame
- Pore size: ≥ DN50 (recommended DN50-DN100)
- Smoothness: Whether the drainage pipe is unobstructed and the dust net is intact
- Material: Whether drainage fittings are made of corrosion-resistant materials (FRP or stainless steel)
3. Skin lamination examination
The skin of the non-metallic expansion joint is a multi-layer composite structure, and the following shall be confirmed during acceptance:
- Whether the number of skin layers meets the design requirements
- Whether the layers are tightly attached and without delamination
- No wrinkles or scratches on the skin surface
VI. Acceptance of quality certification documents
The acceptance of boiler flue expansion joint should not only check the physical objects, but also check the documents. Complete quality certification documentation.
Common Acceptance Problems and Treatment
| Acceptance Issues | Probable cause | Treatment measures |
|---|---|---|
| Transport tie rod not removed | Installer negligence | Remove immediately and check for distortion of the expansion joint |
| Insufficient bolt tightening moment | Failed to tighten multiple times as specified | Re-tighten to required torque in diagonal order |
| Install the guide tube in reverse direction | Flow direction identification ignored | Remove and reassemble, the direction must be correct |
| Skin folded | Unflattened when installed | Re-paving the skin to ensure smoothness |
| Blocked drainage hole | Excessive density or dust accumulation of dust nets | Clean and dredge, adjust the aperture of dust-proof net |
| Damage to anticorrosive coating | Construction damage | Reapplying anticorrosive coating |
8. Work after acceptance
After the boiler flue expansion joint passes the acceptance, the following work shall be completed:
- Establish equipment ledger: record the model, specification, installation date and acceptance person of the expansion joint
- Photo archive: Take photos of the expansion joint after installation (including nameplate and flow direction identification)
- Formulate maintenance plan: specify the first tightening time (1 month, 3 months after commissioning) and inspection cycle
- Handover to Operation Department: Provide acceptance report and maintenance instructions
sum up
The acceptance of boiler flue expansion joint is the key link to ensure the safe operation of flue gas system. The standard acceptance shall follow the five-step method of "checking the appearance, nuclear size, checking the direction, removing the tie rod and testing the seal":
| procedure | Core Content | Acceptance criteria |
|---|---|---|
| Check the appearance | Surfaces, welds, coatings, skins | No damage, no cracks, intact coating |
| Nuclear size | Installation length, coaxiality, levelness | Deviation ≤ ±5mm, coaxiality ≤3mm |
| Inspection direction | Direction of flow direction arrow and guide tube | Small end towards incoming flow |
| pull-off rod | Transportation pull rod, limit device | Transportation rod removed, limit rod retained |
| Trial seal | Pressure test, airtightness test | No leakage |
Same Ensure that the quality certification documents are complete, establish the equipment ledger, and formulate the maintenance plan. A standardized acceptance can prevent the early failure of the expansion joint caused by installation quality problems from the source, and ensure the long-term safe and stable operation of the boiler flue gas system.