In coal-fired power plants, iron and steel sintering and cement kiln tail waste gas treatment systems, the flue expansion joint at the outlet of induced draft fan is under the bad working conditions of high humidity, corrosion and strong vibration for a long time, and the skin aging and corrosion and cracking of metal bellows are common faults. When the expansion joint fails, the replacement of the flue expansion joint of induced draft fan becomes a key maintenance work that must be completed with high quality. Different from ordinary flue expansion joint replacement, the working space on the side of induced draft fan is narrow, the negative pressure system requires extremely high sealing performance, and the vibration conduction of fan needs to be balanced. Starting from engineering practice, this paper systematically explains the applicable conditions, detailed operation steps, quality control standards and safety precautions of replacing the flue expansion joint of induced draft fan, and provides a practical guide for maintenance engineers and construction personnel.
When do I need to replace an induced draft fan flue expansion joint?
Before starting the replacement of the flue expansion joint of the induced draft fan, the replacement timing should be accurately judged. The following six situations should be replaced immediately:
- Skin penetrating rupture: Visible penetrating cracks or holes appear in the skin of non-metallic expansion joints, and obvious smoke leakage.
- Corrosion leakage of metal bellows: Stress corrosion cracks appear in the trough of 316L bellows, and bubbling in soapy water test.
- Large-area skin aging: surface hardening, powdering, cracking, or delamination and bulging between layers.
- Deflector falling off damage: After the deflector falls off, it wears through the expansion joint body.
- Exceeds the designed service life: Non-metallic expansion joints run for more than 5-6 years and metal expansion joints for more than 8-10 years.
- The energy consumption of induced draft fan increased significantly: the current of induced draft fan increased by more than 5% for unknown reasons, which may be related to the internal leakage of expansion joint.
Three Preparations Before Replacement
The replacement of flue expansion joint of induced draft fan belongs to the restricted space operation involving safety and environmental protection, and the quality of preparation work directly determines the construction safety and replacement effect.
1. Parameter review and new parts customization
The following raw data must be recorded before removing the old expansion joint: flue section size (rectangular length × width or inner diameter of circular tube), flange bolt hole arrangement, original expansion joint type and number of structural layers, design compensation amount. For non-metallic expansion joints, the material and thickness of each layer of skin should be recorded. Submit the above parameters to professional manufacturers to customize new expansion joints, and avoid blind purchase when the parameters are unknown.
2. Safety isolation measures
- The induced draft fan is shut down and powered off, and the tag is locked.
- Close the inlet and outlet baffle doors of the induced draft fan, and install blind plates if necessary.
- The internal ventilation of the flue was replaced, and the O₂ content was detected to be ≥19.5%, and the SO₂ and CO concentrations were below the occupational exposure limit.
- Workers wear acid-proof clothing, gas masks and seat belts.
3. Preparation of tools and materials
Includes: angle grinder, pneumatic shovel, hand hoist (2-5 tons), torque wrench, stainless steel bolt (original specification), acid resistant sealant, acetone cleaning agent, measuring ruler, infrared thermometer.
Nine Step Operation Procedure for Expansion Joint Replacement
Standardized induced draft fan flue expansion joint replacement should strictly follow the following steps:
Step 1: Remove the old expansion joint
Loosen the flange bolts one by one, and remove the old expansion joint and pressure plate. Pay attention to protect the flange surface and avoid strong prying injury. If the expansion joint is welded and connected with the flue, it should be separated by gas cutting or plasma cutting, and the peripheral equipment should be protected during cutting.
Step 2: Clean the flange surface
Use an angle grinder with a wire brush to thoroughly remove the old sealant, rust and burrs on the flange surface until the metallic shine is exposed. When the flatness deviation of the flange exceeds 3mm/m, it should be trimmed or fitted with gaskets for leveling.
Step 3: Check the Flue Centering Status
Measure the actual displacement deviation (horizontal, vertical and angular deflection) of the flanges on both sides. The deviation value should not exceed 50% of the design compensation amount of the new expansion joint. When the limit is exceeded, the hand-pulled hoist should be used to pull the flue and adjust it to the allowable range.
Step 4: Check the Deflector
Verify that the inner deflector of the flue is complete and in the correct direction (along the flue flow direction). If the deflector is worn more than 40% of the original thickness or the weld is cracked, it shall be replaced or welded simultaneously.
Step 5: Apply sealant and install new expansion joints
Apply acid-resistant sealant uniformly on the flange surface (thickness 2-3mm, continuous without breakpoints). Hoist the new expansion joint into place and align the bolt holes to ensure that the expansion joint centerline coincides with the flue centerline. For non-metallic expansion joints, the skin must not be twisted.
Step 6: Install bolts and pre-tighten
Place the platen and install the new stainless steel bolts. Fasten in three times symmetrically from the middle to the sides: 30% target torque for the first time, 70% for the second time, and 100% for the third time. Target torque reference: M12=40-50 N m, M16=70-90 N m.
Step 7: Check the seal and flatness
After tightening, check that the surface of the skin (non-metal) or bellows (metal) should be flat without abnormal deformation, and the sealant around the flange should be evenly extruded to be qualified.
Step 8: Remove Transport Fixtures
This step is very easy to miss! Transportation fixtures on non-metallic expansion joints must be removed completely before trial operation, otherwise the expansion joints cannot expand and contract freely and will tear at the first heating.
Step 9: Air tightness test and heat tightening
After restoring the system, start the induced draft fan (low load) and do an air tightness check. The test pressure is 1.05 times of the working pressure (but not more than 10kPa), all interfaces are painted with soapy water, and the pressure is kept for 10 minutes without bubbles. After the system has operated stably to the operating temperature, heat tighten the bolts again to the specified torque.
5 Common Mistakes and Avoidance
| Common Errors | consequence | avoidance measures |
|---|---|---|
| Transport fixtures not removed | Skin tears when it first heats up | Set eye-catching marks to remind removal after installation |
| Use carbon steel bolts | Short-term corrosion fracture | Mandatory 304 stainless steel bolts |
| One-sided disposable fastening | Flange deflection, seal failure | Symmetrical three-time tightening |
| Deflector mounted in wrong direction | Smoke washes the root of the skin | Check orientation marks before installation |
| Missing heat tightening | Bolt slack leakage in hot state | Write Operation Ticket Enforcement |
Acceptance criteria after replacement
After completing the replacement of the flue expansion joint of the induced draft fan, the following acceptance items shall be confirmed item by item:
- Appearance: The expansion joint is installed correctly, without mechanical damage, and the skin is free of wrinkles.
- Bolts: All tightened in place, exposing 2-3 threads, made of stainless steel.
- Airtightness: Soapy water check for no bubble leakage.
- Displacement: The deviation between the measured displacement and the design value in the hot state is ≤15%.
- Vibration: There is no abnormal increase in the vibration amplitude of the induced draft fan bearing.
- Energy consumption: The induced draft fan current returns to the normal range.
epilogue
The replacement of flue expansion joint of induced draft fan is not a simple dismantling of old and replacing new, but a systematic project involving parameter review, safety protection, fine construction and verification test. Strictly following the standardized process, the service life of the new expansion joint can reach 3-5 years (non-metal) or 5-8 years (metal), while effectively avoiding environmental penalties and unplanned shutdown caused by leakage.