In industrial flue gas treatment systems, flue expansion joints (also customarily called compensators or expansion joints) are the key flexible compensating components connecting boilers, dust collectors, desulfurization towers and chimneys。 Because the flue gas pipeline is under the bad working conditions of high temperature, corrosion and dust for a long time, if the stress caused by thermal expansion and contraction is not absorbed, it will lead to pipeline deformation and weld cracking, and in the worst case, it will lead to equipment damage or even safety accidents. This paper will systematically explain the core technical knowledge of flue expansion joint from structural principle, key points of type selection to installation and maintenance.

1. What is the flue expansion joint?

Flue expansion joint is a flexible connection device that takes advantage of the effective deformation of elastic elements to absorb the displacement (axial, transverse, angular) of the pipe due to temperature change and mechanical vibration。 Its core functions include: compensating the dimensional changes caused by thermal expansion and contraction, isolating equipment vibration, reducing the thrust of pipes to equipment, and at the same time facilitating the installation and disassembly of pipes.

According to the difference of structure and material, flue expansion joints are mainly divided into two categories: metal expansion joints and non-metal expansion joints.

1. Metal expansion joint

The metal expansion joint consists of stainless steel bellows, end pipe and guide tube. Its advantage are strong pressure bearing capacity, high temperature resistance (up to 600℃ or above), and it is suitable for flue in high temperature section, such as the area from the boiler outlet to the economizer and air preheater。 Common types of metal expansion joints include:

2. Non-metallic expansion joint

Non-metallic expansion joints are made of multi-layer composite materials (fluororubber/silicone rubber coated fiberglass cloth, PTFE membrane, fiberglass cloth, etc.)。 Its advantages are that it can simultaneously absorb three-way displacement, has good vibration isolation and noise reduction effect, excellent corrosion resistance, no reverse thrust transmission, and can simplify the design of pipeline support。 It is suitable for low temperature sulfur-containing flue gas environment such as wet desulfurization system, gas turbine exhaust passage, dust collector inlet and outlet。

For the smoke duct with rectangular section, the rectangular fillet expansion joint is a better choice. It adopts a rectangular cross-section structure with large circular arc transition at the edge corners, which can effectively reduce the stress level at the edge corners, eradicate the risk of deformation dead corners and local high stress damage, and prolong the service life of products。

Second, the main points of the selection of flue expansion joint

1. Select according to working conditions

2. Selection core parameters

The latest national standard for flue expansion joints is GB/T 12777-2019, which was officially implemented in December 2019, replacing the old GB/T 12777-2008。 The following parameters should be focused on when selecting the model:

• Nominal diameter (DN): Consistent with pipe inner diameter
• Design temperature: Not lower than maximum operating temperature +50℃
• Design pressure: Not less than maximum working pressure
• Compensation amount: determined according to thermal displacement calculation (rated compensation amount ≥1.2× calculated thermal displacement)
• Material: The material of the guide tube is not lower than the material of the bellows

Calculation formula of thermal displacement: Δ L = α × L × Δ T

Where α is the line expansion coefficient of the pipeline (carbon steel 12×10⁻⁶/℃, stainless steel 16×10⁻⁶/℃)。

3. Material selection specification

Special reminder: The fault analysis of the expansion joint of the ignition air duct of a 410t/h boiler shows that the metallic properties of the part in direct contact with the flue gas should not be lower than 0Cr25Ni20Si2 (310S). However, the factory actually uses ordinary stainless steel, which leads to the rapid failure of the expansion joint under high temperature。 High-temperature sulfur-containing environmental materials shall not be downgraded.

3. Installation specification of flue expansion joint

The installation quality of flue expansion joint directly affects its service life. The following specifications should be followed during installation：

1. Preparation before installation

• Check whether the model, specification and pressure level are consistent with the design
• Check bellows/skin surface for mechanical damage
• Verify that the direction of the liner cylinder (guide cylinder) coincides with the flow direction of the medium (the small end faces the incoming flow)

2. Critical controls during installation

3. Special requirements for non-metallic expansion joints

• Installation starts from the lower part, and the skin is placed flat on the flange surface of the metal frame to prevent wrinkles
• The marking direction of the wear-resistant bushing must be consistent with the flow direction of the pipe medium
• Tighten the bolts by "diagonal tightening and fractional tightening"

4. Common faults and treatment

1. Leak Cause Analysis

The most common cause of leakage in non-metallic flue expansion joints is water accumulation in grooves。 When the skin is installed, an annular groove is naturally formed between the pressure plate and the skin, and a large amount of acid water is accumulated during the operation of the unit. Acid water penetrates the fabric layer to reach the position of the fixing screw, causing the screw to loosen and corrode and break. Finally, the acid liquid flows out from the broken screw hole and the damaged part of the skin, and seriously appears the "small waterfall" phenomenon。

Other causes of leakage include: the skin material is not corrosion-resistant (silicone rubber is not acid-resistant, so fluororubber should be used instead)The bolts are not tightened repeatedly and loosened due to vibrationFlue overall displacement pulling and cracking expansion joint。

2. Treatment Methods

V. Operation and maintenance suggestions

1. Daily inspection: check the appearance of the expansion joint every week for smoke leakage, bulging and loose bolts
2. Regular tightening: Full tightening of non-metallic expansion joint platen bolts on a quarterly basis
3. Temperature monitoring: Infrared temperature measurement, if the surface of non-metallic expansion joint abnormally heats up, it indicates that the inner heat insulation layer is damaged
4. Drainage inspection: Horizontally installed expansion joints should be provided with drainage holes (above DN50) at the lowest point of the frame, and should be cleaned every month to prevent blockage
5. Replacement cycle: Non-metallic skin is usually replaced once every 3-5 years; If the metal frame is severely corroded, it should be replaced as a whole

sum up

Flue expansion joint is the key component to ensure the safe operation of flue gas system. The correct type selection shall be based on the flue gas temperature, corrosiveness, displacement direction and other factors to determine the type and material. The material shall not be degraded under high temperature conditions (321, 310S or nickel-based alloy shall be selected)Wet flue gas environment shall be provided with drainage holes and use fluororubber skin。 During installation, the flow direction sign shall be strictly followed. The non-metallic expansion joint shall be repeatedly tightened with bolts and the transportation rod shall be removed in time。 Regular maintenance, including inspections, tightening and replacement, can effectively extend the life of the equipment. It is suggested to select and calculate according to the latest national standards in the design stage to ensure the safety of the system from the source.