In the operation of thermal power plants and industrial boilers, flue skin expansion joint, as a key flexible connection compensation device, is subjected to high temperature flue gas, corrosive medium and alternating thermal stress for a long time. Once a leakage occurs, it will not only lead to an increase in the output of the induced draft fan and an abnormal exhaust temperature, but also lead to environmental protection exceeding the standard and unplanned furnace shutdown. The emergency repair of boiler flue skin expansion joint leakage is an emergency work with high requirements of timeliness and technology. Combined with the actual combat experience, this paper systematically expounds the leakage cause diagnosis, emergency repair scheme selection and operation key points, and helps the operation and maintenance personnel to quickly restore the airtightness of equipment.

1. Common causes of leakage and emergency assessment

After receiving the leak alarm or finding smoke during inspection, the emergency repair team needs to complete the preliminary assessment within 15 minutes. Boiler flue skin expansion joint leakage is usually caused by the following four conditions:

1. High-temperature ablation: During the boiler start-stop process, hot and cold alternate, and the fluororubber or silicone rubber coating in the inner layer of the skin becomes carbonized and brittle, resulting in cracks or holes.
2. Chemical corrosion: The sulfur content in the fuel is high, the acid dew point of the flue gas rises, and the condensed acid soaks the bottom of the skin for a long time, resulting in delamination or acid lysis of the fabric layer.
3. Mechanical damage: the flue support sinks, the limit tie rod breaks or the guide tube falls off, and the skin is repeatedly stretched exceeding the designed compensation amount, resulting in tearing.
4. Aging and cracking: the skin that has been run for more than 5 years, the polymer material naturally ages, the surface is powdered and loses elasticity.

Emergency Assessment Points:

• Locate the leakage point: Use an infrared thermal imager or the back of your hand close to the suspected leakage area (pay attention to anti-scalding) to find out the main leakage location.
• Judge the leakage level: slight leakage (wet marks visible to the naked eye but no open flame) can be plugged with pressure; Perforated fire breathing or large area tearing must apply for furnace shutdown for emergency repair.
• Measure smoke temperature and pressure: If the flue gas temperature is higher than 200℃ or the static pressure exceeds ±10kPa, it is necessary to reduce the load before processing.

2. Safety and preparation before emergency repair

2.1 Safety Isolation

• Strictly implement the work ticket system, close the inlet baffle of the induced draft fan, and open the flue vent valve to ensure that the negative pressure of the furnace returns to zero.
• Carry out gas detection (CO, SO₂, O₂) on the emergency repair area, and you can only enter after passing the qualification.
• Wear high temperature resistant gloves, acid resistant face mask and heat insulation clothing.

2.2 List of Tools and Spare Parts

Key reminder: Emergency fire extinguishers (dry powder or CO₂ type) must be prepared at the site, as sparks may be accompanied by skin leaks.

3. Temporary plugging operation under pressure (non-stopping furnace scheme)

When the furnace cannot be shut down immediately and the leakage is small, pressure plugging can be used to maintain the operation until the planned furnace shut down. The procedure is as follows:

1. Surface cleaning: Use a wire brush to remove dust, rust and oil around the leakage point, exposing the metal frame and skin body. It is prohibited to use water for cleaning to avoid crack propagation caused by quenching.
2. Bottom sealing: Inject silicone sealant with temperature resistance of 300℃ into the crack and scrape it flat. For holes less than 5mm in diameter, they can be directly cross-covered with high temperature resistant aluminum foil tape.
3. Cover reinforcement layer: Cut a piece of stainless steel wire mesh that is not less than 3 times the area of the leakage area, cover it above the tape, and then fix it to the adjacent flange edge with stainless steel cable ties or throat hoops.
4. Pressure test: Slowly raise the frequency of the induced draft fan to 70% of the normal load, and check the edge of the patch for bubbles with soapy water or a leak detector. If there is no new leak, it can resume operation, but it needs to be inspected every 2 hours.

Limitations: Pressure plugging is only a temporary measure, the overall strength of the skin has decreased, and the longest maintenance time does not exceed 72 hours. Always schedule a shutdown replacement during this time.

4. Standard process for overall replacement and emergency repair after furnace shutdown

For the emergency repair of leakage of boiler flue skin expansion joint with large area tearing, multiple perforations or temporary plugging failure, shutdown replacement must be performed. The complete process includes:

4.1 Old skin removal

• Loosen the pressure plate bolt (note that the rusty bolt is infiltrated with loosening agent for 30 minutes and then gas cut).
• Remove the platen and old skin, and retain the original metal frame. Check the frame deformation: If the diagonal deviation exceeds 10mm, it should be corrected or replaced first.
• Check the internal guide tube: the overlapping direction of the guide tube must face the flue gas flow direction. If there is any peeling or corrosion perforation, replace it together (the material is not less than 316L).

4.2 Prefabrication and installation of new skin

• Length review: Set the cold installation length to compress 5% ~8% of the design length (reserve thermal expansion space). For example, the theoretical length is 2000mm, but the actual production should be 1900~1940mm.
• Multilayer structure laying: from the inside to the outside: acid-resistant fluororubber isolation layer, thickness ≥2mm; Glass fiber cloth reinforcement layer, 2 layers; Stainless steel wire mesh anti-scour layer, 1 layer; The outer layer of silicone rubber is coated with fiberglass cloth for weather protection.
• Compression and fixation: Grade 12.9 high strength bolts are used to pre-tighten diagonally to a torque of 80N·m, and then the final tightening torque of 120N·m is reached in three times in sequence. It is strictly forbidden to screw to death once, so as to avoid stress concentration caused by deformation of the pressure plate.

4.3 Seal Verification

• Before the flue manhole door is closed, illuminate the skin edge with a fluorescent leak detection lamp from the inside to verify that no light is transmitted.
• Start the induced draft fan to 30% load, maintain the negative pressure of 500Pa, scan all joints with ultrasonic leak detector, and the signal strength is less than 20dB.

5. Debugging and preventive measures after emergency repair

5.1 Thermal Tracking

Within 24 hours after resumption of operation, focus on monitoring the following parameters:

• Skin surface temperature: It should be 20~40℃ lower than the flue gas temperature. If the temperature difference is too small, it means that the heat insulation is missing.
• Expansion joint displacement: Mark a scale on the outside of the skin to record the cold and hot displacement values. The measured displacement should not exceed ±90% of the design value.
• Exhaust temperature and oxygen content: If the oxygen content rises abnormally and the exhaust temperature is locally high, it indicates that there are still undiscovered leakage points.

5.2 Long-term prevention strategies

• Skin surfaces are visually inspected quarterly and photographed for archiving to compare aging trends.
• Cut 50mm ×50mm skin samples at shutdown every year and send them to the laboratory for testing tensile strength and elongation at break. When the value drops to 60% of the initial value, it will be included in the replacement plan for the next year.
• Install a short condensate discharge pipe (with a drain valve) at the lowest point of the skin at the bottom of the flue to avoid long-term soaking of acid.

VI. SUMMARY

The emergency repair of boiler flue skin expansion joint leakage is a systematic emergency project. The key to success lies in: quickly and accurately judging the leakage grade (deciding to plug the leakage under pressure or shut down the furnace for replacement), preparing special materials for high temperature and acid resistance (fluororubber layer + stainless steel pressure plate), strictly implementing the installation process (cold pre-compression amount, diagonal uniform fastening, guide tube alignment) and hot state verification after emergency repair (double confirmation of displacement and sealing). By establishing the mechanism of "daily preventive inspection + standardized emergency repair plan + quota reserve of key spare parts", the number of unplanned outages caused by skin leakage can be reduced to less than 0.5 times per year, and the reliability of boiler operation can be significantly improved.

For the old units, it is recommended to upgrade to perfluorororubber + double guide tube structure in the next overhaul, and optimize the stiffness of flue support from the design level, so as to reduce the leakage frequency of boiler flue skin expansion joint from the root source.