In industrial flue gas pipeline system, flue corrugated expansion joint is one of the most core flexible compensation devices. It utilizes elastic deformation of stainless steel bellows to absorb axial, transverse and angular displacements of flue caused by thermal expansion and cold contraction, and at the same time isolates vibration of equipment to protect safe operation of pipeline system。 Correct selection and installation of flue corrugated expansion joint is the key to ensure the long-term stable operation of flue gas system.
1. Structure and working principle of flue corrugated expansion joint
The flue corrugated expansion joint is composed of a corrugated pipe (an elastic element) which constitutes its working main body and accessories such as end pipe, bracket, flange and conduit。 Bellows are made of elastic stainless steel (SUS304, SUS316L, SUS321, 310S, etc.) with an annular or spiral corrugated structure。
When the flue is elongated by heat, the bellows is compressed; When the temperature drops, the bellows are stretched – this elastic deformation ability allows it to efficiently absorb thermal displacement, converting thermal stress into elastic potential energy of the bellows, thus protecting pipes and equipment from damage。 The flue corrugated expansion joint can simultaneously compensate the axial, transverse and angular cold and hot deformation of the absorption pipeline, facilitate the installation and disassembly of the valve pipeline, absorb the vibration of the equipment, and reduce the influence of the vibration of the equipment on the pipeline。
2. Classification of flue corrugated expansion joint
According to the cross-sectional shape, flue corrugated expansion joints can be divided into two categories: circular and rectangular.
1. Circular corrugated expansion joint
Circular corrugated expansion joints are suitable for circular section flues and consist of bellows, end tubes and flanges. The common diameter range is DN32-DN3000mm, operating temperature covering-10℃ to +300℃, special materials can reach higher temperatures。 In engineering procurement, typical specifications such as TN1800*1-J-546, material 321, thickness 3mm, wave height 157mm, connector material 304, connector size φ 1820×10。
2. Rectangular corrugated expansion joint
The rectangular corrugated expansion joint adopts rectangular cross-section and rounded corner wave shape, which is suitable for rectangular flue duct。 According to different wave height, it can be divided into two standard forms: half-height type and full-height type：
| Type | Wave height h | Wave pitch q | Wall thickness t | Compensation per wave | Applicable section |
|---|---|---|---|---|---|
| Semi-high type | 108mm | 59mm | 1.5mm | ±11mm (±12mm) | |
| Full height | 216mm | 86.4mm | 2.6mm | ±24mm | Arbitrary section |
The half-height expansion joint is suitable for the occasion where the cross-sectional area of the rectangular flue duct is less than 4.6 square meters。 The full-height expansion joint is suitable for rectangular smoke ducts except for the above conditions。
3. Key parameters of model selection
The correct selection of flue corrugated expansion joint requires clarification of the following parameters:
1. Basic operating condition parameters
According to the design specification, at least a minimum of：
- Circulating medium in the pipe: air, flue gas, steam, gas, etc
- Smoke duct design pressure: Maximum working pressure
- Maximum temperature during operation: Determines bellows material
- Flue cross-sectional dimensions: length × width (rectangular) or diameter (circular)
2. Corrugation selection
The waveform and wavenumber are selected according to the size of the pipe section and the amount of thermal displacement. Maximum allowable expansion of single wave：
- Full height: Δα = ±24mm/wave
- Half-high type: Δα = ±12mm/wave
The wave number of a single bellows generally does not exceed 6 waves。
3. Cold-drawn design
To reduce the number of nodes of the bellows, cold drawing 50% should be considered。 Cold drawing refers to the pre-compression or pre-stretching of the bellows during installation, so that it is in the best stress state when working in hot state.
4. Ash baffle setting
Ash baffle (guide tube) shall be used for dusty flue。 Ash baffles are used to protect the bellows from direct flushing by dust.
4. Material Selection Guide
The material of the corrugated pipe of the flue corrugated expansion joint should be selected according to the working temperature and media corrosiveness:
In engineering practice, typical procurement specifications such as expansion joints of material 321, thickness 3mm, and connection material 304。 For pulverized coal pipes, the bellows are made of 1Cr18Ni9Ti and the connectors are made of low carbon alloy wear-resistant steel。
V. Standards, specifications and installation requirements
1. Implementation standards
The design, manufacture and inspection of flue corrugated expansion joints shall comply with GB/T 12777-2019 General Technical Specifications for Expansion Joints of Metal Bellows 。 This standard was implemented on December 1, 2019, replacing the old GB/T 12777-2008。
2. Install Core Points
According to GB/T 12777 and other specifications, the installation of flue corrugated expansion joints shall follow：
- It is strictly prohibited to use expansion joint deformation to adjust pipeline deviation: Except for pre-tension and compression as required in the design, it is strictly prohibited to use the method of deforming bellows to adjust pipeline installation deviation
- Consistent Flow Direction Identification: For expansion joints with liner, the direction of liner must be consistent with the direction of media flow
- Removal of transport tie rod: The auxiliary positioning mechanism used for installing transport protection on the expansion joint shall be removed immediately after the pipe system is installed
- Welding protection: Weld slag is not allowed to splash onto bellows surface during installation
- Bracket setting: Fixing and guiding supports shall be reasonably set at both ends of the compensator
The bellows cannot bear weight and should be hoisted separately。
Typical Application Scenarios
Flue corrugated expansion joints are widely used in：
- Smoke system, steam turbine and dust removal device of boiler in thermal power plant
- Iron and steel plant, smelting furnace, incinerator flue
- Flue gas pipeline of petrochemical enterprise
- High temperature air duct in cement plant
- Gas-solid two-phase flow conveying system for pulverized coal pipeline
In pulverized coal pipeline, flue corrugated expansion joint (three-dimensional corrugated compensator) can absorb displacement in any direction, high temperature resistance, no powder leakage, no powder accumulation, and low displacement stiffness。 In the electric power industry, the wind and smoke system of power station boiler is the typical application place。
VII. Guide to selection and pit avoidance
Common misunderstandings and correct practices in the selection of flue corrugated expansion joint:
Cold drawing 50% can effectively reduce the wave number requirement, which should be considered when selecting the type。
sum up
Flue corrugated expansion joint is the core flexible connection component to ensure safe operation in flue gas pipeline system. The correct type selection shall be carried out according to the following steps based on the working condition parameters:
The flue corrugated expansion joint with reasonable design and standard installation can run stably for a long time under harsh working conditions such as high temperature and corrosion. It is recommended that a professional organization be entrusted to conduct pipeline stress analysis and special selection of expansion joints during the design stage, and the acceptance shall be strictly according to GB/T 12777-2019 standard.