In industrial pipeline systems, non-metallic expansion joints are becoming an important alternative to metal expansion joints under high temperature, corrosion and dust conditions by virtue of their unique technical advantages. Different from traditional metal bellows, non-metallic expansion joints are made of flexible materials such as fiber fabric, silicone rubber and polytetrafluoroethylene, which can absorb multi-dimensional displacement and have comprehensive properties such as vibration reduction, sound insulation and corrosion resistance. This paper will systematically introduce the professional knowledge of non-metallic expansion joints from five aspects: structure composition, core advantages, applicable working conditions, key points of type selection and installation and maintenance, so as to help engineers and operation and maintenance personnel make scientific decisions.

I. What is a non-metallic expansion joint?

Non-metallic expansion joint, also known as fiber compensator or fabric expansion joint, is a pipeline connection compensation device with flexible non-metallic materials as the main body. Typical structures include a multi-layer composite fabric belt (usually composed of glass fiber cloth, polytetrafluoroethylene membrane, silicone rubber coated cloth, etc.), a metal flange, a thermal insulation layer (ceramic fiber blanket), and a flow guide tube. Unlike metallic expansion joints, which rely on corrugation elastic deformation, non-metallic expansion joints rely on bending and folding of fabric materials to absorb displacements, so they can simultaneously compensate for large displacements in axial, transverse and angular directions.

2. Core advantages of non-metallic expansion joints

To understand the characteristics of non-metallic expansion joints, we need to start with the comparison between it and metal expansion joints. Here are its five core strengths:

1. Strong multi-dimensional compensation ability

Metal expansion joints mainly absorb axial displacements and have limited compensation ability for lateral and angular displacements. The fabric belt of non-metallic expansion joint has extremely high flexibility, which can absorb the displacement in three directions simultaneously in a limited space. It is especially suitable for the occasions of installation deviation, uneven thermal expansion and contraction or foundation settlement in pipeline system.

2. No rebound force, friendly to support

When the metal bellows is deformed, it will produce a large elastic reaction force (usually tens of kilonews), which requires the pipe support to have sufficient strength. The non-metallic expansion joint hardly produces reaction force during the deformation process, and has no additional load on the original bearing and connecting equipment, so it is very suitable for the reconstruction of old equipment or the occasion where the bearing capacity of the bearing is limited.

3. Excellent corrosion resistance

For flue gas pipes containing acidic gases (such as SO₂, SO₃, HCl) or high humidity environments, metal expansion joints are prone to pitting, intergranular corrosion and even stress corrosion cracking. The composite ring belt of non-metallic expansion joint is made of polytetrafluoroethylene (PTFE), fluororubber and other materials, which has excellent acid and alkali resistance and moisture resistance, which is an ideal choice for flue gas desulfurization (FGD) system and waste incineration tail gas pipeline.

4. The effect of sound insulation and vibration reduction is remarkable

The fabric structure of the non-metallic expansion joint has natural damping properties, which can effectively absorb airflow pulsations and mechanical vibrations while reducing wind noise by 10 – 20 dB. This characteristic is especially important in the inlet and outlet of induced draft fan, ventilation and air conditioning system of power plant.

5. Cost and maintenance advantages

Non-metallic expansion joints are typically 30% – 50% less expensive to manufacture than metallic expansion joints of the same specification in large-diameter pipe applications (over 2 meters in diameter). In addition, when the non-metal ring belt is damaged, only the ring belt part needs to be replaced without removing the entire expansion joint flange, which is convenient and low-cost to repair.

Applicable working conditions and typical application fields

Non-metallic expansion joints are not panacea, but they have obvious advantages in the following operating conditions:

It should be noted that the operating temperature of the non-metallic expansion joint usually does not exceed 600 °C (the ceramic fiber composite type can reach more than 1000 °C, but the cost increases significantly). For high temperature pipes exceeding 800℃ (such as cement tertiary air ducts), the scheme of metal expansion joint + lining castable is still recommended.

IV. Key points of selection: How to choose the appropriate non-metallic expansion joint?

When selecting a non-metallic expansion joint, the following key parameters are required to be provided to the supplier:

1. Medium temperature and composition: maximum continuous temperature, instantaneous overtemperature temperature, whether it contains acid gas or oil mist. For example, fluororubber or PTFE should be used as the airtight layer for flue gas containing SO₃.
2. Displacement Amount and Direction: Specify the specific values of axial, transverse and angular displacement. The compensation capacity of non-metallic expansion joints is proportional to the width of the band, and the wider the band, the greater the transverse displacement that can be absorbed.
3. Working pressure and negative pressure: Ordinary non-metallic expansion joints are suitable for within ± 30kPa. If the negative pressure is high, it is necessary to add an internal support ring or choose a reinforced structure to prevent the ring belt from being deflated.
4. Pipe diameter and connection method: DN100 to DN8000 can be customized. There are usually two kinds of connection methods: flange connection and welding connection, and flange connection is convenient for the replacement of ring belts in the future.
5. Whether it contains a guide tube: For dusty air flow or pipelines with flow velocity greater than 15m/s, wear-resistant guide tubes (materials can be selected as Q235B, 304 or 316L) must be installed to prevent the air flow from directly washing the fabric band.

V. Installation and maintenance precautions

Proper installation and maintenance are key to ensuring long-term stable operation of non-metallic expansion joints:

• Inspection before installation: Check model, specification and displacement mark. The belt must not be scratched by sharp objects during transportation.
• Pre-stretching is strictly prohibited: Unlike metal expansion joints, non-metal expansion joints should not be artificially stretched or compressed during installation, and should be kept in a natural state.
• Pay attention to the direction of airflow: The direction of installation of the guide tube must be consistent with the direction of airflow, otherwise it will cause the ring belt to be torn by the reverse airflow.
• Regular Inspection: Check the surfaces of the bands every 3 – 6 months for breakage, aging, cracking or leakage. If a small area is damaged, special repair tape can be used for temporary treatment. If a large area is damaged, the loop tape should be replaced.
• Prevent dust accumulation: The bottom of horizontally installed expansion joint is easy to accumulate dust, so it is necessary to set drainage/dust discharge port, or clean it regularly to avoid dust accumulation and corrosion of fabric.

VI. Typical case: Application of non-metallic expansion joint in desulfurization system of power plant

Stainless steel metal expansion joints were used in the original flue of the desulfurization system of a 2×300MW coal-fired power plant. After one year of operation, corrosion, perforation and leakage were found in many expansion joints. Replace it with a non-metallic expansion joint (structure: the outer ring is silicone rubber coated glass fiber cloth, the inner layer is PTFE film, and the middle is sandwiched with ceramic fiber insulation layer). After three years of operation, the surface of the ring belt is only slightly aged, without leakage, and the flue vibration is obviously reduced. Subsequently, the factory replaced a total of 12 expansion joints, such as the inlet and outlet of the induced draft fan and the clean flue, with non-metallic types, and the maintenance cost was reduced by about 60%.

VII. Call to Action: Obtain the exclusive selection plan of non-metallic expansion joints

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