First, the core position of electrostatic precipitator flue expansion joint in power plant
In the flue gas treatment system of coal-fired power plant, electrostatic precipitator is the key equipment to remove dust from flue gas, and the flue expansion joint connected with the inlet and outlet of electrostatic precipitator undertakes multiple tasks of absorbing heat displacement, isolating vibration and sealing flue gas. The selection and running state of the flue expansion joint of electrostatic precipitator in power plant are directly related to the efficiency of dust removal, system energy consumption and the continuous and stable operation of the unit. Different from ordinary flue expansion joints, the inlet flue of the electrostatic precipitator bears high-temperature dusty flue gas from the boiler outlet (usually 130-180℃, and some working conditions can reach above 300℃), while the outlet flue faces the risk of induced draft fan vibration and corrosion of subsequent desulfurization system. Once the expansion joint fails, the energy consumption of the induced draft fan will rise from air leakage to short circuit of the electrostatic precipitator, exceeding the standard of smoke and dust emission or even unplanned shutdown. This paper will systematically expound the complete technical scheme of flue expansion joint of electrostatic precipitator in power plant from working condition analysis, key points of type selection to operation and maintenance measures.
2. Analysis of working conditions of electrostatic precipitator flue
2.1 Inlet flue operating conditions
To correctly select the expansion joint of the flue of the electrostatic precipitator of the power plant, we must first understand the characteristics of the inlet flue:
- Flue gas temperature: usually 130-180℃, may drop below 90℃ during low boiler load or air preheater bypass operation
- Dust concentration: 10-30g/m³ (depending on boiler load and coal type), dust is abrasive
- Pressure state: Mainly slight negative pressure (-2~ -5kPa), maintained by the suction force of induced draft fan
- Corrosion risk: When the smoke temperature is below the acid dew point, the condensate can corrode the expansion joint
2.2 Outlet flue operating conditions
Most of the dust has been removed from the flue gas at the electrostatic precipitator outlet, and the content is usually reduced to ≤50mg/m³, but there are other challenges:
| parameter | trait | Effects on expansion joints |
|---|---|---|
| Flue gas temperature | 130-160℃ | Thermal displacement remains |
| Dust content | Very low (≤50mg/m³) | Scour wear is greatly reduced |
| Pressure fluctuation | Negative pressure at the inlet of induced draft fan (-3~ -8kPa) | Expansion joint may be compressed and deflated |
| Vibration | Low frequency vibration transmitted by induced draft fan | Accelerated bellows fatigue |
2.3 Particularity of Start-Stop Conditions
The start-stop process of power plant units often challenges the expansion joint of electrostatic precipitator flue of power plant more than the steady-state operation:
- Fume temperature rises rapidly and heat displacement is released concentrately in the initial period of load rise
- The temperature rise rate of electrostatic precipitator and flue is not synchronized, resulting in relative displacement difference
- Fatigue life consumption of expansion joints in power plants with frequent start-and-stop (peak shaving units) is faster
3. Key points of selection of flue expansion joint of electrostatic precipitator
3.1 Selection of Inlet Flue Expansion Joint
According to the characteristics of high temperature and high dust in the inlet flue, the selection of the inlet side of the expansion joint of the electrostatic precipitator flue in power plants should focus on:
Structural form: Axial type single or double metal bellows expansion joint is recommended
- The flue at the inlet of electrostatic precipitator is generally a horizontal straight section, mainly axial displacement
- Single structure is suitable for short pipe sections (≤15m), and duplex structure is suitable for long pipe sections
Material selection: 316L stainless steel is preferred for bellows
- 316L contains molybdenum, and its resistance to pitting and acid condensate corrosion is better than that of 304
- Using temperature 150-180℃ is completely within the tolerance range of 316L
Critical configuration:
- A guide tube must be set to prevent dust-containing smoke from directly washing the inner wall of the bellows
- The thickness of the guide tube shall be ≥4mm, and the length shall cover the entire bellows section
- The fixed end of that guide tube is located upstream of the flue gas flow, and the free end is pointed downstream
3.2 Selection of Expansion Joint of Outlet Flue
The selection of the expansion joint of the electrostatic precipitator flue of the power plant for the outlet flue is relatively flexible:
Structural form: metal bellows expansion joint or non-metal fabric compensator can be used
- Low dust content and low risk of skin wear for non-metallic compensators
- The vibration isolation effect of non-metallic compensator is better than that of metal structure
Material selection: 304 stainless steel is sufficient (the outlet flue gas is weak in corrosion)
Special attention to:
- The negative pressure of the outlet flue is high, and the vacuum stability of the bellows should be considered
- If necessary, reinforcing rings are arranged in the trough to prevent deformation by deflation
3.3 Selection Comparison Table
| Projects | Inlet expansion joint | Outlet expansion joint |
|---|---|---|
| Type of recommendation | Axial metal expansion joint | Metallic or non-metallic |
| Corrugated pipe material | 316L | 304 |
| guide tube | Must be set | Suggested settings |
| Wear Resistance Measures | Required (guide tube + wear resistant layer) | Generally not required |
| Vibration isolation requirements | normal | Higher (induced draft fan side) |
IV. Key points of installation and construction technology
4.1 Inspection before installation
Before installing the electrostatic precipitator flue expansion joint of the power plant, the following inspections should be completed:
- Check whether the model, compensation amount and pressure level are consistent with the design drawings
- Check the bellows surface for scratches, indentations, and arc burns
- Verify that the guide tube is firmly fixed and free end is not stuck
- Record the protection position of the transport pull rod, and release it according to the requirements after installation
4.2 Docking Accuracy and Welding
The installation accuracy of flue expansion joint of electrostatic precipitator in power plant directly affects the service life:
| Controlled items | Allowable deviation | Detection Methods |
|---|---|---|
| Axial deviation | ≤3mm | Ruler measurement |
| Radial misalignment | ≤1.5mm | gauge inspection |
| Flange parallelism | ≤1.5mm | Level |
Care should be taken when welding:
- Adopt argon arc welding bottoming + manual arc welding filling
- Strictly control the interlayer temperature ≤150℃
- Use E309L transition welding material for different steel welding (stainless steel + carbon steel)
4.3 Pre-displacement and cold drawing
Due to the cold installation and hot operation of the flue expansion joint of the electrostatic precipitator in power plants, pre-displacement treatment is usually required:
- Pre-tension or pre-compression at 50% of the design displacement
- Retain transport tie rod until welding is complete
- Loosen or remove the tie rod as specified before the system heats up
Operation, maintenance and fault handling
5.1 Key points of daily inspection
The flue expansion joint of electrostatic precipitator of power plant after commissioning shall be included in regular inspection:
| Check Items | Frequency | Contents |
|---|---|---|
| Appearance inspection | Weekly | The presence or absence of cracks, soot accumulation and the integrity of the insulation layer in the bellows |
| Seal Inspection | Monthly | Check for leakage with smoke generator in negative pressure flue |
| Wall thickness detection | Annual (downtime) | Ultrasonic thickness measurement to evaluate corrosion thinning |
| Infrared temperature measurement | Quarterly | Are there any abnormal hot spots in surface temperature distribution |
5.2 Common Troubles and Handling
Guide tube falling off
- Cause: Fatigue fracture of fixed end weld
- Treatment: Planned shutdown for maintenance, re-welding and fixing and adding reinforcing ribs
Bellows cracking
- Cause: exhaustion of fatigue life or wrong installation pre-displacement
- Treatment: Temporary pressure plugging, scheduled shutdown for replacement
Outlet expansion joint suction deflation
- Reason: Excessive negative pressure or no reinforcing ring
- Treatment: Reduce fan outlet resistance and increase internal support
Flange leakage
- Cause: Aging gaskets, loose bolts or deformed flange surfaces
- Treatment: Replace the gasket and tighten it according to the specified torque. Serious deformation needs on-site machining and repair
5.3 Lifespan Management
Establish the life management ledger of electrostatic precipitator flue expansion joint in power plant:
- Record the commissioning date and design fatigue life
- Counting the number of starting and stopping units (one fatigue life consumed per cycle)
- Record the problems found in previous maintenance and the treatment measures
- When the cumulative number of cycles reaches 80% of the design life, it will be included in the scheduled replacement list
6. Typical problems and countermeasures
| Question | Probable cause | Treatment measures |
|---|---|---|
| Expansion joint surface temperature is too high | Insulation collapse or moisture | Check lining, replenish insulation |
| Induced draft fan current abnormally increased | Expansion joint leakage, increased air leakage | Detect leaks and plug leakage points |
| Fluctuation of negative pressure at inlet of electrostatic precipitator | Local blockage or deformation of expansion joint | Shutdown to inspect deflectors and bellows |
| Abnormal vibration of expansion joint | Airflow excitation or resonance | Adjust the uniform distribution plate of air flow and change the stiffness of expansion joint |
VII. SUMMARY
The selection and maintenance of flue expansion joint of electrostatic precipitator in power plant is an important link to ensure the safe and economical operation of electrostatic precipitator system. The main points of successful application can be summarized as follows: "Heavy wear resistance at the inlet, heavy vibration isolation at the outlet, corrosion resistance of material selection, and precision of installation control".
In the selection stage, the inlet flue expansion joint must be equipped with a guide tube and 316L or above corrosion-resistant materials to prevent high-dust flue gas erosion and acid condensate corrosion; The outlet flue expansion joint needs to pay attention to the vacuum stability under negative pressure and the vibration transmitted by induced draft fan. During the installation stage, strictly control the matching accuracy, and carry out pre-displacement treatment according to the design requirements to ensure that the guide tube is firmly fixed and the free end is not stuck. During the operation and maintenance stage, a regular inspection system is established, focusing on bellows cracks, leakage points, wall thinning and the integrity of the guide tube, and actively replacing them when the cumulative start-stop cycle approaches the design fatigue life.
In particular, it should be emphasized that the state of the guide tube of the flue expansion joint of electrostatic precipitator is often neglected, but the fall of the guide tube is the main reason for the early scour failure of the bellows-the guide tube weld should be taken as a must-inspect item during the maintenance of the inlet expansion joint. Through standardized life cycle management, the expansion joint of electrostatic precipitator flue of power plant can effectively guarantee its service life of 8-10 years, avoid the risk of dust removal efficiency decrease and environmental protection exceeding standard caused by expansion joint failure, and provide reliable support for the safe and environmentally friendly operation of coal-fired power plants.