In industrial boiler system, boiler flue gas expansion joint is the key component to connect flue, absorb heat displacement and vibration, and ensure the safe operation of equipment. Due to the high flue gas temperature and strong corrosion during boiler operation, once the expansion joint fails, it will lead to air leakage and energy consumption increase, and in the worst case, it will lead to safety accidents. This paper will focus on the selection, common problems and daily maintenance of boiler flue gas expansion joint, so as to help engineers and technicians better improve the reliability of equipment.

First, why is the boiler flue gas system inseparable from the expansion joint?

Significant thermal expansion and contraction occurs in the flue and connecting equipment during boiler start-stop and load change. If rigid connection is adopted, thermal stress will directly act on flue, desulfurization tower, dust collector, induced draft fan and other equipment, resulting in weld cracking, foundation deformation and even equipment damage. The boiler flue gas expansion joint absorbs thermal displacement in three directions through its flexible structure, such as metal bellows or non-metallic fabric layer, while reducing the transfer of mechanical vibrations to sensitive equipment, such as SCR denitrification reactor.

In addition, modern boiler systems are often equipped with desulfurization and denitrification devices, and the flue gas composition is complex, which may contain SO₂, NO, water vapor and dust particles. The expansion joint needs not only high temperature resistance, but also corrosion resistance, good sealing performance and the ability to adapt to short-term overtemperature. Therefore, reasonable selection directly determines the continuous operation cycle of smoke and air system.

II. Main types and applicable scenarios of boiler flue gas expansion joints

According to material and structure differences, common types can be divided into two categories:

1. Metal expansion joint

Consists of stainless steel bellows, end pipe and guide tube. It has the advantages of strong pressure bearing capacity and high temperature resistance (up to above 600℃), and is suitable for flue in high temperature section, such as the area from the boiler body outlet to the economizer and air preheater. However, metal structure has limited ability to compensate lateral displacement, and its corrosion resistance is weaker than that of non-metal materials.

2. Non-metallic expansion joint

Use multi-layer fabric composite materials (such as glass fiber, PTFE film, silicone rubber cloth, etc.). It can absorb axial, lateral and angular displacements simultaneously, and has excellent vibration isolation and noise silencing performance. The boiler flue gas expansion joint is more suitable for non-metal types in low-temperature wet flue gas scenarios such as the inlet of desulfurization tower and the inlet and outlet of electrostatic precipitator. Its corrosion resistance is strong, and its price is relatively economical, but its temperature resistance generally does not exceed 400℃.

Type selection suggestions: Metal type should be preferred in the high-temperature drying section; Non-metallic type is recommended for moisture-containing, sulfur-containing and multidirectional displacement sections; For the special working conditions with both high temperature and corrosion, the composite expansion joint lined with heat insulation layer can be used.

3. Analysis of common failure modes and causes

Mastering the failure law of expansion joint will help to intervene in advance and avoid unplanned shutdown.

• Corrugated pipe corrosion perforation: mostly occurs on non-metallic expansion joints or low-grade stainless steel materials. When the flue gas temperature is lower than the acid dew point, condensed acid is generated on the inner wall, resulting in pitting. Typical manifestations are leakage of the outer fabric and dampness of the insulating cotton.
• Mechanical overload of bellows: due to improper pre-stretching/pre-compression during installation, or missing flue brackets, guide limiting devices, causing the expansion joint to withstand displacement beyond the design value. Local yield deformation and even tearing may occur in metal bellows.
• The guide tube falls off or wears: The guide tube is used to reduce the flushing of the bellows by dusty flue gas. If the wall thickness of the guide tube is insufficient, the welding strength is insufficient or the length design is too short, the high-speed dust will directly wear the bellows crest and eventually cause leakage.
• Failure of insulation layer: For high-temperature non-metallic expansion joints, the inner insulation cotton shrinks and powders under heat for a long time, resulting in the outer skin temperature exceeding the limit and accelerating the aging of fabric.

Main points of whole life cycle maintenance and inspection

To prolong the service life of the boiler flue gas expansion joint, the whole process management should be implemented from installation, operation to maintenance.

Installation link

• Check the displacement compensation amount in the design drawings to ensure that the expansion joint model matches the actual pipe section on site.
• During installation, the expansion joint shall be ensured to be in a natural state, and it is strictly prohibited to adjust the installation deviation by forced tension or compression.
• Check the direction of the guide tube: It must be consistent with the flow direction of the medium, and avoid smoke backward flowing and flushing the bellows.
• Reasonable pipe support and guide frame are set to prevent the expansion joint from bearing torsion or excessive transverse force.

Operation Monitoring

• Establish a ledger for detecting the surface temperature of the expansion joint. For the non-metallic type, an abnormal increase in the temperature of the outer layer usually means that the inner insulation layer has been damaged.
• Pay attention to the boiler start-stop rate. Too fast heating or cooling will subject the expansion joint to severe alternating stress and accelerate fatigue.
• Regularly check whether the flue drainage device is unobstructed to prevent condensate accumulation from corroding the bottom of the expansion joint.

Overhaul and replacement standards

• Metal bellows should be replaced as a whole when they have cracks, deep corrosion pits, or significant permanent deformation.
• Local damage of non-metallic skin can be repaired, but if more than 3 layers of fabric are damaged or brittle by aging, it is recommended to replace it in a whole circle.
• When replacing, clean the old sealant on the flange surface simultaneously, and use high-temperature resistant sealant with stainless steel bolts to tighten it to ensure no leakage.

Suggestions on professional selection and supplier collaboration

Due to the great differences in fuel (coal, biomass, gas), combustion mode and tail purification process of different boilers, the selection parameters of boiler flue gas expansion joint need to be clarified item by item: design pressure, maximum/minimum operating temperature, flue gas composition (especially sulfur content and moisture), displacement (X, Y, Z three directions), cycle life requirements and whether there are fire protection specification limitations. It is recommended that the purchaser provide a detailed working condition data sheet and require the supplier to issue a calculation sheet and a pre-factory pressure/displacement test report.

In addition, for the renovation project, the failure site of the original expansion joint should be photographed and the cause analysis should be carried out, and the conclusion should be fed back to the new selection scheme. For example, if the original product is damaged by condensate corrosion, it should be upgraded to an acid-resistant material (such as 316L or 904L stainless steel) and lined with a PTFE film layer.