In the flue gas system of coal-fired power stations and industrial boilers, rectangular boiler flue expansion joints are the key flexible compensation components connecting boilers, dust collectors, desulfurization towers and chimneys. Different from circular expansion joints, the rectangular flue section is non-circular, and the stress state and compensation characteristics are more complicated, which puts forward unique requirements for the selection, material and structure design of expansion joints. This paper will systematically explain the professional technical knowledge of rectangular boiler flue expansion joint from structure type, waveform selection to material matching.

1. Types and structural characteristics of rectangular boiler flue expansion joints

1. Rectangular non-metallic expansion joint

Rectangular non-metallic expansion joint is a new flexible pipe for smoke duct of power plants developed according to the US CE standard and international general products, and combined with the actual situation of power plants in China。 This product adopts a large circular arc transition corner structure, which is significantly different from the traditional right-angle welding design.

Core Strengths:

• Reduce stress concentration: the corners are transitioned by large circular arcs to form rectangular cross sections, and the stress level in the corner section is reduced, which eliminates the risk of deformation dead angle and local high stress damage, and prolongs the service life of the product
• Improve thermal compensation capability: Reduce the thrust of the pipe system under the condition that the whole is not instable and the basic working stiffness is maintained, and improve the thermal compensation capability of the product
• Improve corrosion resistance: It can work reliably in benign force system under the action of external tensile stress, compressive stress and alternating stress, and effectively improve the deterioration trend of stress corrosion fracture and corrosion fatigue fracture
• Weld optimization: When the length of a rectangular side is less than 2 meters, one weld can be reduced, and the amount of misalignment at the weld is greatly reduced

Applicable scenario: Operating temperature covering-20℃ to 1000℃, corrosion resistance and excellent sealing performance, mainly used in flue gas medium conveying systems in power plants, metallurgy, cement and petrochemical industries。

2. Rectangular metal bellows expansion joint

The expansion joint of rectangular metal bellows adopts rectangular cross section and rounded corner wave shape, which is commonly used in low pressure pipelines with design pressure ≤0.05MPa。 The main features of the product are as follows:

Structural form:

• V-shaped angle structure: suitable for low pressure environmental conditions
• U-shaped structure: suitable for high pressure environmental conditions

Applicable scenario: After internal insulation, the product can withstand the temperature up to 1200℃. The medium is mainly hot air (sometimes containing dust, SO₂, etc.), which can absorb axial, transverse and angular displacement. It is widely used in flue and air duct in electric power, metallurgy, petrochemical, cement and other industries。

Second, Waveform Selection of Rectangular Boiler Flue Expansion Joint

1. Waveform Classification and Expansion Calculation

The core of the selection of rectangular boiler flue expansion joint lies in the selection of waveform. According to the Typical Design Manual of Flue-wind and Pulverized Coal Pipeline Parts (D-LD2000), the waveforms are divided into full-height and half-height types。

Key points of expansion calculation:

• Full height type: maximum allowable expansion per wave ± 24mm
• Half-height type: maximum allowable expansion per wave ±12mm (no cold drawing)
• The wavenumber of a single bellows does not exceed 6 waves

2. 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 expansion joint during installation, so that the bellows is in the best stress state during hot working, which can effectively reduce the wave number demand and reduce the product cost.

3. Ash baffle setting

For dusty flues, the rectangular boiler flue expansion joint should adopt ash baffles (also known as deflectors). One end of the ash baffle is welded to the connection pipe and the other end is a free end, which is used to protect the bellows from direct washing of dust. Air ducts or ducts with less dust may not be used。

3. Common faults and improvement scheme of rectangular boiler flue expansion joint

1. Failure causes of traditional rectangular metal expansion joints

The rectangular metal expansion joint used on the smoke duct of a boiler in a power plant is made of ordinary Q235 A carbon steel. The metal corrugation is stamped and formed by 2mm steel plate, enclosed in a rectangle, and welded at four corners. This structure has significant drawbacks:

• Weak fillet welds: There are several welds on the four corners, which are the weak links of expansion joints
• Dust accumulation corrosion: There is a cavity between the inside of the corrugation and the baffle, and the dust in the flue gas is easy to accumulate in the cavity. When the flue gas containing SO₂ comes into contact with the metal wall surface whose temperature is lower than the acid dew point, a large amount of sulfuric acid vapor will condense down to form dilute sulfuric acid solution, resulting in low-temperature corrosion of the metal wall surface
• Three-dimensional expansion limited: the lateral displacement generated during the operation of the unit pulls open the welds on the four corners of the rectangular metal expansion joint, with cracks up to 10-15mm

2. Improvement of fillet corner wave expansion joint

In view of the above problems, the improvement direction of rectangular boiler flue expansion joint is to adopt rounded corner wave design. This design addresses the pain points of traditional expansion joints by:

• Large circular arc transition at edges and corners: eradicating dead angles of deformation and local high stress failure risk
• Integral forming corrugation: Reduce the number of welds and reduce the risk of leakage
• Structural reinforcement: The corners are reinforced by welding arc steel plates, and the interface is made of channel steel, which enhances the structural rigidity of the product

3. Novel structure of sintering flue gas purification device

In the sintering flue gas deep purification device, a novel rectangular metal expansion joint structure has appeared: a retractable ring plate bent into a ω-shape (or ω-shape) with steel plate, which can adapt to the micro-expansion of flue guard plate and effectively resist the erosion of corrosive gas。 Key features of the design include:

• Omega-shaped retractable ring plate: the opening faces the flue, providing micro-expansion compensation capability
• Vertical ear plates and horizontal cantilever plates: enhancing connection strength
• Groove closure plate: Prevent corrosive gas from contacting the bellows directly

4. Selection parameters of rectangular boiler flue expansion joint

1. Necessary selection parameters

Proper selection of rectangular boiler flue expansion joint requires the following parameters：

2. Judgment on the scope of application

According to D-LD2000 standard, the application scope of rectangular boiler flue expansion joint is as follows ：

• Half-height type: suitable for occasions where the cross-sectional area is less than 4.6 square meters, and one side of the outer dimension of the smoke duct is less than 1.5m but greater than 0.6m
• Full height type: suitable for all smoke and air ducts, not limited by cross-sectional area

3. Material selection

V. Key points of installation and maintenance

1. Installation direction confirmation: Expansion joint with guide tube, the small end of guide tube must be facing the direction of flue gas flow
2. Cold drawing treatment: 50% cold drawing according to design requirements to reduce wave number requirements
3. Bolt tightening: The pressure plate of non-metallic expansion joint should be tightened repeatedly by "diagonal tightening and fractional tightening"
4. Transportation rod removal: Transportation protection rod must be removed after installation

sum up

Rectangular boiler flue expansion joint is an indispensable flexible connection component in the flue duct system of coal-fired power station. Compared with circular expansion joints, rectangular expansion joints need to pay attention to the following points when selecting because of their special cross-sectional shape:

1. Structure selection: Rounded corner wave design (CE standard) is preferred. Large arc transition corner structure can significantly reduce stress concentration
2. Waveform determination: Full-height type (h =216mm, expansion amount ±24mm/wave) is suitable for large-section flue; Half-height type (h =108mm, expansion ± 12mm/wave) suitable for small section (
3. Cold Draw Design: Considering 50% Cold Draw Reduces Wave Number and Costs
4. Ash baffle configuration: Dusty flue must be equipped with ash baffles to protect bellows from washing
5. Material matching: Carbon steel, stainless steel or non-metallic material is selected according to flue gas temperature. Rectangular non-metallic expansion joint can achieve wide temperature range coverage from-20℃ to 1000℃

A rectangular boiler flue expansion joint with reasonable design and standard selection can operate stably for a long time under the harsh working conditions of high temperature, corrosion and dust, and effectively reduce the thrust of pipe system and construction cost。 It is recommended that the type selection calculation should be carried out according to D-LD2000 standard at the design stage, and the waveform, wavenumber and cold drawing requirements should be clearly marked in the drawing.