In the flue gas treatment systems of coal-fired power plants, iron and steel plants and chemical enterprises, the non-metallic expansion joints of low temperature gas-saving flues play a crucial role. This equipment is located after saving gas (flue gas heat exchanger), and faces the clean flue gas environment of low temperature, high humidity and strong corrosion for a long time. This paper will deeply analyze the structural characteristics, material selection, technical advantages and maintenance points of non-metallic expansion joint of low-temperature gas-saving flue, so as to provide professional reference for engineering design and equipment operation and maintenance personnel.

First, what is low-temperature gas-saving flue non-metallic expansion joint?

Low-temperature gas-saving flue refers to the channel section after flue gas is desulfurized or cooled by heat exchange. The flue gas temperature here is usually below 80°C, even as low as about 50°C, and contains a large amount of water vapor and residual acidic medium (such as dilute sulfuric acid)。 The non-metallic expansion joint of low temperature gas flue is the flexible connection part installed in this section, which is used to compensate the displacement caused by temperature change and mechanical vibration of the pipeline.

Compared with traditional metal expansion joints, non-metal expansion joints have unparalleled advantages under such low temperature corrosive working conditions, mainly reflected in no thrust conduction, strong multi-directional compensation ability and excellent corrosion resistance。

Core Structure Composition

A standard cryogenic gas-saving flue non-metallic expansion joint usually consists of the following parts：

1. Non-metallic band (skin): This is the core flexible component of the expansion joint, usually in multi-layer composite structure.
2. Metal Frame: Includes flanges and deflectors for connecting flue and protecting loop straps.
3. Heat insulation layer: Although it mainly prevents condensation under low temperature conditions, it still needs to isolate heat when the temperature difference is large.
4. Compression device: The skin is pressed against the metal frame by bolts to ensure the sealing performance.

Design Logic of Multilayer Skin

In view of the corrosion characteristics of low temperature and gas saving, the skin structure is usually designed as：

• Outer layer (weather-resistant layer): silicone rubber or fluororubber coated glass fiber cloth, with good aging resistance, UV resistance and rain and snow resistance。
• Middle Layer (Reinforcement Layer): Use polyester fiber cloth or alkali-free fiberglass cloth to provide tensile strength and withstand system pressure.
• Sealing layer: PTFE film is the key because it has excellent chemical stability and can resist acid attack formed by residual sulfur oxides in flue gas。
• Inner layer (anti-stick/anti-corrosion layer): direct contact with flue gas, using fluororubber or butyl rubber to prevent water vapor penetration and resist chemical corrosion of condensate.

3. Key technical parameters of model selection

When selecting suitable non-metallic expansion joint for low temperature gas-saving flue, the following parameters should be focused on：

1. Temperature resistance range

Although it is named "low temperature", it is still necessary to consider temperature fluctuations under start-stop or accident conditions. The applicable temperature of conventional low temperature type is ≤200℃, which is sufficient to cover normal working conditions below 80℃ and short-term overtemperature。

2. Corrosion resistance grade

This is the core indicator of model selection. After the flue gas is desulfurized, the humidity is saturated, and the residual SO₂ and SO₃ will form strong corrosive condensate. At this time, fluororubber or polytetrafluoroethylene must be selected as the material of contact medium, and standard silicone rubber cannot meet the long-term corrosion resistance requirements。

3. Compensation amount calculation

The expansion joint should absorb the axial, transverse and angular displacements of the pipeline. The formula reference is:$\mathrm{Delta}L=\mathrm{Alpha}\times L\times \mathrm{Delta}T$DeltaL=Alpha×L×DeltaTBecause of their large flexibility, non-metallic expansion joints can usually compensate for large multidirectional displacements, and there is no need to accurately calculate stiffness forces like metal expansion joints。

4. Pressure Class

The flue gas desulfurization system mostly belongs to low pressure and micro negative pressure (usually ≤0.05Mpa), and the strength of glass fiber fabric of non-metallic expansion joint is enough to meet the requirements。

4. Solution to the problem of winter embrittlement

In the northern winter, non-metallic expansion joints of gas flues in low temperature province face a special challenge-embrittlement. When the ambient temperature is extremely low, the common rubber material will become hard and brittle, and the vibration during starting and stopping the machine will easily lead to skin cracking。

Here is the solution：

• Material upgrade: Avoid the use of low molecular weight materials that are easy to freeze, and use low temperature resistant modified fluororubber instead to ensure that its glass transition temperature adapts to the environment of ≤ -30℃.
• Increase thermal insulation layer: Install thermal insulation clothing or silicon-based protective coating outside the expansion joint to reduce temperature difference stress.
• Avoid man-made injury: It is strictly prohibited to bend or knock the skin during installation in winter. Before installation, the material should be placed at room temperature to soften.

Application Scenarios and Summary

The non-metallic expansion joint of low temperature saving gas flue is widely applicable to：

• Desulfurization system of thermal power plant (absorption tower inlet and outlet, clean flue);
• Desulfurization flue of sintering machine in iron and steel plant;
• Acid gas treatment systems for chemical enterprises.

sum up

The non-metallic expansion joint of low-temperature gas flue is the key component to ensure the long-term stable operation of desulfurization system. The core of its selection lies in "anti-corrosion" and "sealing". Users must abandon the old concept of "non-metal is cheap" when purchasing, and focus on whether the skin material is fluororubber/PTFE combination, and whether the metal frame has glass flake anti-corrosion coating。 When applied in cold areas, it is necessary to give priority to the low temperature embrittlement resistance of the material. By adopting low temperature resistant modified fluororubber and installing heat insulation layer to optimize the maintenance strategy, the expansion joint can run reliably in winter。

Only when the characteristics of low-temperature condensation corrosion are fully considered in the design stage, combined with accurate displacement calculation and reasonable material selection, can the stubborn leakage at the flue interface be completely cured, and the environmental protection facilities can operate worry-free