In the design and selection of industrial flue gas pipeline system, the temperature resistance of flue expansion joint is one of the most core technical parameters. Selecting the wrong temperature resistance grade can lead to rapid aging and leakage failure of the expansion joint, and safety accidents such as skin burning and unplanned shutdown in the worst case. The temperature resistance range of expansion joints of different types and materials is very different-from low-temperature silicone rubber at-60℃ to ceramic fiber composite at 1200℃, with a span of more than 1200℃. This paper will systematically explain the technical knowledge of temperature resistance of flue expansion joint from temperature classification, material selection to engineering application.

1. From metal to non-metal: the temperature resistance basis of two types of expansion joints

The temperature resistance of flue expansion joints first depends on the type of expansion joint. There are essential differences between metal expansion joints and non-metal expansion joints in temperature resistance mechanism and scope:

1. Temperature resistance characteristics of metal expansion joints

The metal bellows expansion joint uses stainless steel bellows as the compensation element. Selection of corrugated pipe material according to operating temperature is key：

The limitation of metal expansion joints is that when the temperature exceeds 800 DEG C, even if special alloys are used, the cost will increase sharply, and the thermal displacement compensation efficiency will decrease by 30% to 50%。

2. Temperature resistance advantages of non-metallic expansion joints

The non-metallic expansion joint is made of multi-layer composite material, which has unique performance advantages in high temperature environment because its main body is fiber fabric：

• Higher upper temperature resistance limit: up to 1300℃ instantaneous high temperature
• Strong thermal shock resistance: The fiber material has very low thermal expansion coefficient and does not crack under quenching and sudden heating
• Excellent comprehensive performance: It can resist corrosive media attack at high temperature

For most high temperature flue gas conditions, non-metallic expansion joints have become the preferred scheme.

2. Temperature classification and material combination of non-metallic expansion joints

The core of temperature resistance of flue expansion joint lies in the correct selection of temperature grade of skin material. Depending on the operating temperature, non-metallic expansion joints can be classified into the following grades：

Critical Material Properties by Temperature Class

Ceramic fiber cloth: Temperature resistance ≥1260℃, low thermal conductivity (0.1W/m·K), strong thermal shock resistance。

High silicon oxide cloth: Temperature resistance 900℃, silica protective layer will be formed when exposed to high temperature, enhancing durability。

Metal foil layer: Inconel 601 is resistant to oxidation temperature up to 1150℃, which can effectively insulate corrosive media。

Fluorine rubber and silicone rubber: Silicone rubber has temperature resistance of-60℃ ~200℃, fluorine rubber has temperature resistance of-20℃ ~250℃, and has excellent acid and alkali resistance。

3. Selection points of high-temperature flue expansion joint

Correct temperature resistance selection of flue expansion joint needs to pay attention to the following key parameters:

1. Distinguish between medium temperature and design temperature

This is the most common model selection misconception. The medium temperature is the flue gas temperature at which the skin is in direct contact, while the design temperature contains a safety margin. For example, when the actual flue gas is 900℃, the design temperature should be 1100℃ grade skin。

2. Consider local overheating and temperature fluctuations

In boiler flue and other working conditions, the temperature of local superheating zone may far exceed the design value. According to engineering practice, the medium temperature in the flue duct can reach 1000℃。 The flue gas temperature distribution should be fully evaluated during model selection to avoid premature failure of the expansion joint due to local overtemperature.

3. Dual challenges of high temperature + corrosion

In the sulfur-containing flue gas environment such as desulfurization system, the temperature resistance of flue expansion joint needs to be considered as a whole with the corrosion resistance. The combination of fluorine-based hydrocarbon polymer and reinforcing material of Garlock 8400-HT nonmetallic expansion joint has extremely high resistance to SO₂, H₂SO₄ and other corrosive substances at high temperature, and the continuous working temperature is higher than 400°F, instantaneously up to 1200℃。

4. Installation length of large-size high-temperature expansion joint

For ultra-high temperature, large diameter expansion joints, the installation length needs to be determined according to the calculation results of thermal displacement. For example, a cement factory φ 4000 high temperature metal expansion joint, design temperature 450℃, design pressure ±7500Pa, installation length 2000mm and 1500mm respectively。

4. Heat insulation design of high temperature flue expansion joint

The realization of temperature resistance of flue expansion joint not only depends on skin material, but also needs perfect heat insulation structure.

1. Multi-layer insulation design

The working logic of a typical high-temperature non-metallic expansion joint is：

Hot flue gas → ceramic fiber layer reflection/barrier → insulation cotton further cooling → sealing layer temperature

This temperature gradient design ensures that the temperature of the outer skin and exposed metal frame is always within a safe range.

2. External surface temperature control

According to the industry standard, the long-term working temperature of the outer skin is ≤250℃, which can reach 350℃ in a short time. After installation (with heat insulation filler), the outer surface temperature of the skin is ≤50℃, which meets the safety requirements of anti-scald。

V. Special high temperature working conditions: pure titanium expansion joint scheme

For the desulfurization system with wet flue gas at 45-55℃ for a long time, although the temperature is not extreme, it is extremely corrosive, so it is also necessary to pay attention to the synergistic performance of temperature resistance and corrosion resistance of materials. It is found that the average life of 316L expansion joint is not more than two years in the actual use of desulfurization wet flue。

The pure titanium TA2 expansion joint becomes an innovative scheme for this working condition. Titanium has a high melting point (1668 °C) and sufficient toughness at temperatures of − 250 °C to 510 °C. Its most outstanding property is corrosion resistance, which exhibits good corrosion resistance in harsh corrosive media such as chloride, sulfide and wet chlorine。 The comprehensive price of pure titanium expansion joint is the same as that of non-metallic expansion joint, and it can achieve "zero leakage" operation.

VI. Guide to selection and pit avoidance

Myth 1: Only look at the upper limit of temperature resistance and ignore temperature fluctuation

For high temperature and frequent start-and-stop conditions (such as batch kilns), it is necessary to choose materials with good thermal shock stability, such as ceramic fibers containing ZrO₂, instead of pursuing temperature resistance values alone。

Mistake 2: Blind selection of non-metals in high temperature section

Although the upper temperature resistance limit of the non-metallic expansion joint is high, when the pressure exceeds 0.05MPaMetal expansion joints may be more suitable when the medium contains high-velocity hard particles.

Myth 3: Ignoring high temperature selection under negative pressure

When the negative pressure value of the system is> -5000Pa, even if the temperature is within the temperature resistance range of the non-metallic expansion joint, the steel wire mesh layer must be lined to resist collapse。

7. Quick look-up table of temperature resistance grade and material selection

sum up

The selection of temperature resistance of flue expansion joint is a systematic project, and the correct decision should follow the following points:

A reasonable temperature resistance selection of flue expansion joint is a key step to ensure the long-term stable operation of flue gas system. When facing multiple challenges such as high temperature, high pressure and strong corrosion, it is recommended to cooperate with professional manufacturers to formulate the optimal technical scheme through detailed working condition analysis.