In the flue duct system of coal-fired power plants, gas-fired power plants and biomass power plants, the expansion joint is an important part to absorb heat displacement and isolate vibration. Its maintenance quality is directly related to the operation safety and environmental protection standards of the unit. However, many power plants lack uniform quality standards when overhauling expansion joints, resulting in re-leakage or damage within a short period of time after overhaul. So, what are the quality requirements for maintenance of flue expansion joints in power plants? This paper will systematically sort out the specification basis, inspection items, quality indicators and acceptance methods of expansion joint maintenance, and provide operable technical guidelines for power plant maintenance personnel.
1. Specification basis for maintenance of flue expansion joint in power plant
In order to clarify the quality requirements for maintenance of flue expansion joints in power plants, we should first know the technical specifications on which they are based. At present, the commonly used standards in China include:
- GB/T 12777-2019 General Technical Specifications for Metal Bellows Expansion Joints
- JB/T 12235-2015 Non-Metallic Compensator
- DL/T 5437-2019 Code for Design of Flue Wind Pulverized Coal Pipeline in Thermal Power Plant
- DL/T 782-2017 Technical specifications for metal expansion joints in thermal power plants
In addition, the maintenance procedures of each power plant and the technical documents provided by the expansion joint manufacturers are also important bases. Maintenance work shall follow the principle of "safety first, quality first", and strictly implement the relevant requirements of inspection, disassembly, replacement and acceptance in the above specifications.
Quality requirements for preparatory work before maintenance
Before the official commencement of maintenance, the following prerequisites must be met, otherwise the construction shall not be started:
| Preparing projects | Quality requirements and standards |
|---|---|
| Shutdown isolation | The system has been completely shut down, the fan is off and tagged, and the baffle door is closed and locked |
| Cooling ventilation | The internal temperature of the flue drops below 50 DEG C, and the flue is continuously ventilated for more than 30 minutes |
| Gas detection | The oxygen content is 19.5% ~23.5%, and the concentration of toxic gases such as CO and H₂S is below the limit |
| Scaffolding/Platform | Firmly erected, load-bearing capacity ≥270kg/m², guardrail height ≥1.2m |
| Technical Information | Expansion joint drawings, instructions and maintenance record cards are fully prepared |
| Spare Parts Confirmation | The model, specification and material of the new expansion joint are consistent with the original or drawing, and the factory certificate is complete |
3. Quality requirements for maintenance of expansion joint body
The core of the quality requirements for maintenance of flue expansion joints in power plants is the inspection and treatment of expansion joints. The sub-requirements for different materials and structures are as follows:
1. Requirements for maintenance of non-metallic expansion joints
| Check Items | Quality qualification criteria | Treatment method (when unqualified) |
|---|---|---|
| Flexible cloth (fabric layer) | No crack, no perforation, no delamination, no bulge, no water leakage | A small area can be temporarily repaired with repair tape of the same material; Extensive damage must be replaced |
| Heat insulation (ceramic fiber/glass fiber) | Uniform thickness, no collapse, no moisture clumping | Supplement or replace insulation at a thickness not less than 90% of the original design |
| Plates and bolts | The pressure plate has no deformation, the bolts have no rust death, and the torque meets the design value (generally 50~80N·m) | Replacement of deformed platen and corroded bolts |
| Sealing gasket | Complete and no fracture, the material matches the working conditions (acid and temperature resistance) | Replace new gasket, do not reuse old gasket |
| Overall appearance | No abnormal bumps or depressions, correct installation orientation (flow direction marks aligned) | Calibrate or reinstall |
2. Requirements for maintenance of metal expansion joints
| Check Items | Quality qualification criteria | Detection Methods |
|---|---|---|
| Bellows surface | No cracks, no pitting perforations, no obvious scratches | Visual + colored flaw detection or magnetic particle detection (important parts) |
| Bellows geometry | Uniform wave pitch, wave height deviation ≤ ±5% design value | Caliper or special sample measurement |
| weld | No pore, slag inclusion, unfused, good appearance shaping | Visual + Penetration (PT) |
| guide tube | No shedding, no severe wear (thickness reduction | Endoscopic or manual access examination |
| limit tie rod | The rod body is not bent, the nut is not loose, and the positioning pin is complete | Visual + Wrench Inspection |
| flange surface | Flat without warpage, flatness of flange surface ≤2mm/m | Ruler or gauge inspection |
3. Key Testing Items
Quantitative testing must be carried out for the key indicators in the quality requirements for maintenance of flue expansion joints in power plants:
- Wall thickness test: The measured wall thickness of metal bellows shall not be less than 85% of the designed wall thickness
- Hardness test: Hardness HB ≤190 of austenitic stainless steel. If there is obvious hardening, it means that cold work hardening or deformation exceeds the limit
- Air tightness test: The expansion joint after maintenance shall be subjected to 0.1MPa compressed air pressure holding test, and the pressure drop within 5 minutes is ≤0.005MPa. It is qualified
Quality requirements for installation and replacement
When the expansion joint needs to be replaced, the quality requirement standard for maintenance of flue expansion joint in power plants puts forward the following requirements for the installation process:
1. Dimensional review before installation
- Deviation of center distance of flue interface flange: ≤ ±5mm in axial direction and ≤ ±3mm in radial direction
- Parallelism of flange surface: ≤1/1000 of outer diameter of flange and ≤5mm
- Cleanliness: No foreign matter or burr on flange surface and bolt hole
2. Installation Process Control
- Forced centering is strictly prohibited: the expansion joint shall not be stretched or compressed to compensate for the error of flue installation
- Pre-stretch/pre-compression: Proceed as designed. If not specified in the design, the pre-stretch can be performed at 50% of the thermal displacement
- Bolt tightening: Tighten 3 times in diagonal order (30% → 70% → 100% torque) with final torque deviation ≤ ±10%
- Direction of guide tube: The arrow must be consistent with the flow direction of smoke gas, and it is strictly prohibited to install it backwards
3. Post-installation inspection
| Inspection Items | Specification |
|---|---|
| Coaxiality | The angle between the axis of expansion joint and the axis of flue is ≤1° |
| Bellows state | No distortion, no local bulge |
| drainage hole | Located at the lowest point, unblocked |
| Insulation restoration | The thickness is not lower than the original design, and the outer guard plate is well sealed |
Maintenance records and acceptance standards
The quality requirement standard for maintenance of flue expansion joints in power plants emphasizes the traceability of the whole process, and the maintenance unit must fill in the following records:
Required Records
- Pre-service inspection records: including original defect description, photographs, measuring point data
- Maintenance process records: disassembly status, list of replacement parts, installation data (pre-tension amount, bolt torque)
- Quality inspection records: test results and test reports
- Completion Report: Overhaul Summary, Description of Remaining Problems, Suggestions for Next Overhaul
Acceptance signature process
- Level 3 acceptance: self-inspection by maintenance team → re-inspection by professional engineer → final inspection by equipment management department/owner
- Acceptance qualification conditions: All inspection items meet the standard, the records are complete, and the test passes
- Treatment of unqualified acceptance: rework until qualified and shall not be released
VI. Common maintenance quality problems and prevention
In actual maintenance, the quality requirements for maintenance of flue expansion joints in power plants are often neglected:
| Frequently asked questions | Standard requirements violated | Precautionary measures |
|---|---|---|
| Old flange gasket not cleaned | The flange surface should be flat and clean before installing the new gasket | Use a blade + wire brush to clean thoroughly and apply sealant |
| Wrong sequence of bolt tightening | Must be diagonally tightened in fractional stages | Post tightening sequence diagram, use torque wrench |
| Forget to install or install the guide tube backwards | Must be consistent with the flow direction | Check the drawings before installation, and the flow direction mark is eye-catching |
| Drain hole not opened or blocked | Drainage device must be installed at the lowest point | Clearly marked on drawings and verified by water supply after installation |
| Insulation layer not fully restored | The insulation thickness shall not be reduced | Take pictures and compare before completion, and measure the thickness |
7. Differences in maintenance quality of different types of expansion joints
| Expansion joint type | Maintenance Key Points | Unique quality indicators |
|---|---|---|
| Non-metallic fabric expansion joint | Sealability of fabric layer and integrity of heat insulation layer | No leakage, no bulging, no loosening of pressure plate |
| Stainless steel bellows expansion joint | Fatigue crack and pitting corrosion of bellows | Colored flaw detection without crack, wall thickness ≥85% design value |
| rubber expansion joint | Aging cracking, swelling deformation | No visible cracks, Shore hardness decrease ≤15% |
| Lined PTFE expansion joint | Liner detached, bulging | No alarm for EDM leak detection, no liner peeling |
VIII. Summary
The quality requirements for maintenance of flue expansion joints in power plants are the technical basis to ensure the safe, environmental protection and economic operation of flue duct system. This paper systematically elaborates from six dimensions: specification basis, preparation requirements, ontology inspection, installation control, record acceptance and common problems. The core points can be summarized as follows:
- Standards first: Maintenance work must be in accordance with GB/T 12777, DL/T 782 and other national standards and industry standards, and no random construction based on experience
- Check full coverage: non-metallic expansion joints focus on flexible cloth cracks, insulation collapse, platen fastening; Metal expansion joints focus on bellows cracks, deflector shedding, wall thickness reduction
- Strict control of quantitative indicators: wall thickness ≥85% design value, wave height deviation ≤5%, bolt torque deviation ≤ ±10%, pressure drop ≤0.005MPa in 5 minutes of airtightness test-these figures are the key standards to measure maintenance quality
- Installation process specification: forced centering is strictly prohibited, pre-stretching/pre-compression must be required, diagonal fractional tightening, correct flow direction of guide tube-each step of installation stage directly determines the service life after maintenance
- Records are traceable: Level 3 acceptance signature, photo retention, and maintenance report filing to ensure that every maintenance is well documented
- Prevention is better than treatment: "Small problems" such as blockage of drainage holes, reuse of gaskets and thinning of insulation layer are the main causes of early leakage after maintenance, and must be included in the quality inspection list
Power plant maintenance personnel should take this standard as an action guide for on-site operations, and strictly implement the whole process from preparation to acceptance to recording. Only by truly "overhauling one unit, qualifying one unit and recording one unit" can we effectively prolong the service life of the expansion joint, reduce the risk of unplanned shutdown, and ensure the safe and economical operation of the unit.