In the flue gas pipeline system, the plugging and repair of flue gas expansion joint is the most common urgent task faced by operation and maintenance personnel. Whether it is non-metal skin seepage, metal bellows crack, or flange bolt leakage, if it can't be treated effectively in time, it will lead to aggravated corrosion, exceeding environmental protection standards or even unplanned shutdown. This paper systematically sorts out three mainstream methods of plugging and repairing flue gas expansion joints-temporary plugging, local repair and overall replacement, and compares them from three dimensions: applicable scenarios, operation steps and effect cycle, so as to help technicians make quick decisions and make accurate construction.

1. Leak diagnosis: the first step before plugging the leak

The location, type and severity of the leak must be accurately determined before any flue gas expansion joint plug and repair operation is performed.

1. Leak Location Identification

• Water seepage from skin surface: Non-metallic expansion joints are the most common, manifesting as moist patches or dripping
• Leakage at pressure plate bolts: caused by loose or corrosion fracture of bolts
• Cracks in metal bellows: usually located at crests or troughs, accompanied by a whistling sound
• Flange end face leakage: aged gasket or insufficient bolt preload
• Weld cracking: metal expansion joint frame weld or pipe connection weld

2. Leak Severity Classification

3. Downtime and Online Decision Making

• Shutdown: Local repair or whole replacement is preferred for the most reliable effect
• Unable to shutdown: On-line plugging technology is used, but safety risks are assessed

2. Method 1: Temporary blocking (emergency treatment without stopping)

Suitable for temporary flue gas expansion joint plug repair during primary leakage or waiting for spare parts.

Applicable scenarios

• Small area water seepage of non-metallic skin
• Fine crack in metal bellows
• Slight leakage of flange surface

Material Preparation

• High temperature plugging cement (temperature resistance ≥200℃)
• Stainless steel cable ties or clamps
• Acid-resistant rubber sheet or PTFE film
• Special leak plugging fixture (prefabricated)

Operation Procedures

1. Surface cleanup: Use a wire brush or sandpaper to remove dust, rust, and old sealant from around the leakage area
2. Pre-treatment: For water seepage, dry the surface with compressed air first
3. Construction of leak-plugging cement: After kneading the leak-plugging cement evenly, press it firmly at the leakage point to form a covering layer with a thickness of 5-10mm
4. Reinforcement: For the parts with high pressure or high vibration, wrap stainless steel cable ties or install leak plugging fixtures around the outer layer of the cement
5. Curing: Stand to cure according to the product instructions (usually 30-60 minutes)
6. Inspection: After curing, check whether there is still any leakage, and apply a layer if necessary

Effects and Cycles

• Temporary occlusion can be maintained for 1-6 months
• Suitable for transitional treatment, not as a permanent option

3. Method 2: Local repair (shutdown treatment)

Suitable for secondary leakage, it is the most cost-effective plugging and repair scheme of flue gas expansion joint.

Applicable scenarios

• Local damage of non-metallic skin (area ≤0.5m²)
• Single wave crack of metal bellows
• Platen Bolt Corrosion Replacement

Material Preparation

• Skin patch of the same material (non-metal) or bellows patch of the same specification (metal)
• High temperature sealant (silicone sealant or fluororubber glue)
• Stainless steel bolts and nuts
• sanding tool

Operation steps (take non-metallic skin repair as an example)

1. Shutdown Cooling: Confirm that the system has been shut down and the temperature has dropped to ambient temperature
2. Area isolation: Blind plates or baffles are set before and after the expansion joint to prevent smoke from returning
3. Clean up the damaged area: Cut the damaged skin with a knife to form a regular rectangular opening (be careful not to damage the internal insulation layer)
4. Base surface grinding: Use grinding wheel to grind the metal frame around the repair area to expose the metallic luster
5. Prefabricated patch: cut the skin patch of the same material according to the opening size enlarged by 50mm
6. Glue application: Evenly apply high temperature sealant to the back of the patch and the frame attachment surface
7. Fit and compress: Cover the repair piece on the opening, and fix it with pressing strips and bolts. The spacing between bolts is ≤100mm
8. Curing: Stand curing for 24 hours
9. Airtightness test: Airtightness check after system restoration

Effects and Cycles

• 50%-70% of the original design life can be restored after local repair
• Suitable for local skin damage, not suitable for large area aging

4. Method 3: Overall replacement (complete repair)

It is suitable for tertiary leakage, large-area aging or repeated failure, and is the most thorough plugging and repair scheme of flue gas expansion joint.

Applicable scenarios

• Large-area aging and brittle cracking of non-metallic skin (aging area ≥50%)
• Multiple cracks or severe corrosion of metal bellows
• More than 2 repairs in one year at the same location

Operation Procedures (Take Non-Metal Expansion Joint Replacement as an Example)

1. System isolation: after shutdown, cut off flue and install blind plate
2. Old expansion joint removal: remove the pressure plate bolts and remove the old skin; Check frame for corrosion
3. Frame repair: polishing and removing rust from the corroded frame, and welding and reinforcing if necessary; Re-apply anti-corrosion coating (glass flake cement, thickness ≥2mm)
4. Infill insulation: new aluminum silicate fiber cotton is laid in the frame in the original design thickness
5. Installation of new skin: Lay the new skin on the frame to ensure even allowance all around
6. Fixation of pressure plate: Install pressure plate and bolts, operate according to the principle of "diagonal tightening and fractional tightening", with a torque of 50-80N·m
7. Drain hole setting: Drill drain hole at lowest point for horizontal installation (DN50-DN100)
8. Re-tightening: Re-tighten the bolts once in 1 month and once in 3 months after operation

Effects and Cycles

• All-new performance can be restored after overall replacement, with a service life of 3-5 years (non-metal) or 5-8 years (metal)
• Is the most fundamental measure to solve repeated leakage

5. Comparison and selection suggestions of three methods

Suggestions for selection:

• Select temporary plugging for emergency production guarantee
• Local repair for planned overhaul
• Reach the replacement cycle or repeat failure, select whole replacement

VI. Preventive measures after leak plugging and repair

After completing the plugging and repair of the flue gas expansion joint, the following measures can effectively prolong the next failure interval:

1. Establish a patrol inspection system: check the repair area every week for re-leakage
2. Control start-stop rate: avoid sharp temperature change and reduce thermal stress shock of expansion joint
3. Tighten bolts periodically: Tighten the non-metallic expansion joint pressure plate bolts once a quarter
4. Clean drainage holes: Check drainage holes for blockage every month to ensure timely drainage of condensate
5. Ledger record: Record the time, location, method and effect of each repair to provide the basis for subsequent replacement