In boilers, desulfurization towers and industrial flue systems, the expansion joint skin (also known as the non-metallic compensator skin) plays a key role in absorbing heat displacement and isolating corrosive media. However, in actual operation, the failure accidents such as skin burning, carbonization and delamination frequently occur, and the root cause often lies in the insufficient understanding of the temperature resistance characteristics of flue expansion joint skin. Different skins of different materials have great differences in continuous use temperature, instantaneous temperature resistance and acid dew point corrosion resistance. This paper will systematically expound temperature resistance classification, misunderstanding of type selection and verification methods to help engineers match the most reliable flexible compensation scheme.

1. Why is temperature resistance the primary indicator of skin

The flue gas temperature in the flue is usually between 120℃ and 400℃, and it is accompanied by large fluctuations. If the upper temperature resistance limit of the skin of the flue expansion joint of the skin is lower than the actual working condition, the following cascading failure will occur:

• Carbonization of inner barrier film: When the temperature exceeds the thermal decomposition temperature of fluororubber or silicone rubber, the polymer chain breaks, the material becomes hard, cracks and loses airtightness.
• Glass fiber cloth brittle fracture: Although the glass fiber itself can withstand 550℃, after the coupling agent and coating on the surface are burned, the lubrication between the fibers is lost, and brittle fracture occurs.
• Interlaminar exfoliation: Multilayer composites generate shear stress at high temperature due to inconsistent thermal expansion coefficients, resulting in delamination bulging.

Therefore, defining the temperature resistance limit of each functional layer of skin is the basis of type selection and operation and maintenance.

2. Temperature resistance classification of common skin materials

According to the structural order from the inside to the outside, a standard flue expansion joint skin usually includes the following four layers, and the temperature resistance of the respective flue expansion joint skin is as follows:

Key conclusions:

• If the flue gas temperature is ≤180℃ and there is a risk of condensation acid, the fluororubber isolation layer (acid resistant and long-term stable at 200℃) is preferred.
• If the flue gas temperature is 180℃ ~250℃ and it is dry flue gas (the acid dew point has been eliminated), silicone rubber isolation layer can be selected, which is less costly.
• If the flue gas temperature is> 250℃ or there are open flame sparks, ceramic fiber insulation layer + stainless steel wire mesh protection must be added, and the skin should be arranged away from the high-temperature area.

3. Selection misunderstanding: temperature and acid resistance cannot be considered separately

A common mistake is to only pay attention to the "temperature resistance 300℃" marked on the skin, but ignore the damaging effect of acidic condensate on the temperature resistance of materials. For example:

• Case: The flue exhaust temperature of a sintering machine is 140℃, but the SO₃ concentration is as high as 800ppm, and the acid dew point is 135℃. The selected silicone rubber skin can withstand 230℃ under dry hot air. However, after 3 months of production, the bottom of the skin leaks frequently. The sampling test found that after absorbing condensed acid, the thermal decomposition temperature of silicone rubber dropped sharply from 230℃ to about 120℃, resulting in acid-catalyzed hydrolysis at 140℃, and the surface was sticky and cracked.
• Correct practice: When the smoke temperature is lower than the acid dew point or there is condensate of the start-stop machine, even if the actual operating temperature is only 150℃, fluororubber skin must be selected. The fluorine content of fluorine rubber is more than 65%, the C-F bond energy is large, and the comprehensive performance of acid and temperature resistance is far better than that of silicone rubber. "Thermal stability under medium conditions" should also be indicated in the technical specification for temperature resistance of flue expansion joint skin, instead of single dry heat aging data.

4. Strengthening design in high temperature working conditions

For flues with large fluctuation of flue gas temperature and occasional overtemperature (such as boiler oil combustion and denitrification failure), the following measures can be taken to improve the temperature resistance margin of the skin:

4.1 Adding insulation

A ceramic fiber mat with a thickness of 3~5mm (temperature resistance 800℃) is added between the isolation layer and the reinforcement layer, which can block the instantaneous high temperature impact. The experiments show that the temperature of the inner surface of the skin without insulation layer can reach 260℃ (exceeding the limit of fluororubber) when the temperature of the flue gas side suddenly rises to 350℃ for 10 minutes; However, the temperature of the same position with ceramic fiber felt is only 180 ℃.

4.2 Deflector + Cold Air Dilution

A heat-resistant steel plate guide tube is arranged in the flue duct upstream of the expansion joint to prevent the high-temperature flue gas from directly washing the inner wall of the skin. At the same time, cold air (about 50 DEG C) can be introduced near the expansion joint, and the flue gas temperature near the skin can be reduced by 30~50 DEG C. This is a mature protection scheme for high dust flue gas section of power plant.

4.3 Metal + non-metal composite structure

When the design temperature of the flue exceeds 300℃ for a long time and cannot be cooled, the metal bellows expansion joint can be used instead, or the composite type of "metal frame + non-metal skin" can be adopted: the metal bellows bears high temperature and pressure, and the peripheral skin only plays the role of sealing and dust prevention, so the temperature resistance requirements are reduced.

V. On-site temperature resistance verification method

No matter how rigorous the selection, the temperature resistance of the flue expansion joint skin should be verified by actual measurement after installation:

1. Thermocouple embedding: Apply K-type thermocouple to the inner surface of the skin (near the smoke side), and lead to the external digital display instrument. Continuously record the temperature curve during the load lifting process of the unit, and confirm that the maximum temperature does not exceed 90% of the continuous working temperature of the skin material (safety factor).
2. Infrared thermal imaging scanning: Scan the skin surface from the outside. If it is found that the local temperature difference is greater than 30℃ and the outer surface temperature of the corresponding area is> 120℃, it means that the inner insulation layer may collapse or the inner layer has been burned through.
3. Shutdown sampling testing: 50mm ×50mm skin samples (sampled from discarded parts) are intercepted during overhaul every year, and thermogravimetric analysis (TGA) is performed to test the initial decomposition temperature of materials. If the drop is more than 50℃ compared to the new sample, it should be included in the replacement plan even if the appearance is intact.

6. How to deal with overtemperature events during operation

When a boiler or industrial furnace has an operational error that causes the smoke temperature to suddenly rise beyond the allowable value of the skin, the following emergency measures are taken:

• Immediate Load Downgrade: Reduce smoke temperature below rated value in 15 minutes.
• Check for leaks: Scan all seams of the skin with ultrasonic leak detector every 4 hours within 24 hours after overtemperature. Over-temperature will soften the sealant, attenuate the bolt pre-tightening force, and easily produce tiny gaps.
• Record the overtemperature curve: fill the peak temperature and duration into the equipment file. If the single overtemperature exceeds the instantaneous tolerance limit of the material (for example, fluororubber> 250℃ for 1 hour), it should be replaced in the near future, and the operation should not be continued until the next overhaul.

Actual case: Due to the deflagration of the combustion chamber in a waste incineration power plant, the flue temperature instantaneously reached 400℃ for 5 minutes. After the inspection, it was found that the inner layer of silicone rubber skin had hardened and had many cracks. If it continues to run, it is expected that it will leak within 30 days. After emergency repair and replacement with fluororubber + ceramic fiber composite skin, when the same overtemperature occurs again, the skin is intact, which proves that the heat insulation layer is effective.

VII. SUMMARY

The temperature resistance of flue expansion joint skin is not an isolated number, but a systematic index determined by the intrinsic heat resistance of materials, chemical environment (acid/alkali), thermal shock frequency and thermal insulation structure. The core conclusions are as follows:

• Dry heat, acid-free, ≤230℃ working condition: silicone rubber skin is selected, which is cost-effective.
• Condition containing condensed acid, ≤200℃: fluororubber isolation layer must be selected and must not be degraded.
• Instantaneous overtemperature> 250℃ or long-term> 200℃: Add ceramic fiber insulation layer + guide tube, and composite metal structure if necessary.
• Verification cannot be omitted: the actual temperature measurement and annual sample analysis at the initial stage of production are the last line of defense to prevent the accident of "false standard temperature resistance".

Through scientific selection of temperature resistance grade matching working conditions and supporting heat insulation measures, the actual life of flue expansion joint skin can be prolonged from 1~2 years to 5~8 years on average, and the risk of unplanned shutdown and maintenance cost can be greatly reduced. When encountering uncertain working conditions, it is recommended to consult us.