In the design and procurement of flue gas pipe system, the determination of flue expansion joint length is the primary problem faced by engineers and purchasers. The length is long, which wastes materials and takes up space; If the length is selected short, the compensation amount is insufficient, which may cause the bellows to be cracked or compressed during operation. So, how exactly should the length of flue expansion joint be determined? Is it a random choice of a standard size, or is there a scientific way to calculate it? This paper will systematically analyze the determination method and selection key points of flue expansion joint length for you.
1. The basic composition of the length of the flue expansion joint
Flue expansion joint length generally refers to the total length of the expansion joint, that is, the distance from flange face to flange face (flange connection) or the distance between the end faces of the connection pipe (welding connection). Different types of expansion joints have different length configurations:
| Expansion joint type | Length composition | Influencing factors |
|---|---|---|
| Metal bellows expansion joint | Bellows Length + Both End Connection Length | Wave number, wave pitch, length of nozzle |
| Non-metallic expansion joint | Metal frame width + skin mounting length | Flue section size and compensation requirements |
The bellows length of a metal expansion joint is determined by the wave number and wave pitch. Taking the rectangular metal corrugated expansion joint as an example, the standard full-height wave pitch is 86.4mm, and the wave height is 216mm； The half-height type has a wave distance of 59mm and a wave height of 108mm。 Based on the wave number, the basic length of the bellows can be calculated.
For non-metallic expansion joints, the overall length (flange face to flange face) is a key design parameter. The length of flue expansion joint of a denitrification reactor is 250mmThis is a typical non-metallic expansion joint length example.
2. The length is determined reversely by the compensation amount
The core design logic of the length of flue expansion joint is that the length is determined by the amount of thermal displacement compensation required by the pipe, rather than freely selected.
1. Calculation formula of thermal displacement
The calculation of the thermal elongation of the pipe is the basis for determining the length of the flue expansion joint:
Δ L = α × L × Δ T
Calculation example: A section of carbon steel flue with a length of 10 meters, the installation temperature is 20℃, the working temperature is 400℃, and the linear expansion coefficient is 0.012mm/m·℃:
Δ L =0.012×10× (400-20) =45.6 mm
2. Determination of each wave compensation amount and wave number
After the required compensation amount is determined, the required wave number can be calculated according to the single wave compensation amount:
Wave number ≥ required compensation ÷ single wave compensation
Allowable compensation per wave of metal expansion joint:
Note: The wavenumber of a single bellows generally does not exceed 6 waves。
3. Example: Correspondence between length of DN80 metal expansion joint and compensation amount
According to the technical parameters of the product, the compensation capacity of different lengths of the axial internal pressure corrugated compensator is as follows：
| Nominal diameter | model | wavenumber | Axial compensation amount (mm) | Total Length L (mm) |
|---|---|---|---|---|
| DN80 | 1.0YSTA80×8J/F | 8 waves | 30 | 244 |
| DN80 | 1.0 YSTA80×10 J/F | 10 waves | 37 | 274 |
| DN100 | 1.0YSTA100×6J/F | 6 waves | 33 | 223 |
| DN100 | 1.0 YSTA100×10 J/F | 10 waves | 56 | 287 |
| DN200 | 1.0YSTA200×4J/F | 4 waves | 56 | 244 |
| DN200 | 1.0YSTA200×6J/F | 6 waves | 84 | 298 |
It can be seen from the table that the more wave number, the larger the compensation amount and the larger the total length of the expansion joint. This is the core rule of determining the length of flue expansion joint.
3. Influence of cold drawing and pre-deformation on length
In order to reduce the number of wave nodes and the length of expansion joints of bellows, cold drawing (pre-deformation) technology is often used in engineering.
1. Principles of cold-drawn design
According to the technical specification of rectangular corrugated expansion joints, in order to reduce the number of nodes of corrugated pipes, cold drawing 50% should be considered。 The so-called cold drawing means that the expansion joint is pre-compressed or pre-stretched during installation, so that the bellows is in the best stress state when hot working.
2. Calculation of pre-deformation amount
For the axial type compensator, the calculation formula of the axial pre-deformation is：
Δ X = X × [1/2- (T₀-T_min) / (t_max-t_min)]
Among them:
- X: Axial compensation amount
- T_max: Maximum operating temperature
- T₀: Installation temperature
- T_min: minimum operating temperature
The length of the expansion joint can be reduced under the premise of satisfying the compensation requirements by setting the cold drawing amount reasonably.
4. Length reference of different types of expansion joints
1. Metal expansion joint length
The length of flue expansion joint of metal expansion joint is mainly determined by wave number and wave pitch:
| Waveform type | Wave length (mm) | Single wave length | Common total length range |
|---|---|---|---|
| Full height | 86.4 | ~86mm | 200-1000mm |
| Semi-high type | 59 | ~59mm | 180-600mm |
| Axial internal pressure type | Depending on specification | — | 220-650mm |
2. Nonmetallic expansion joint length
The length of flue expansion joint of non-metallic expansion joint is relatively fixed, which is usually designed by manufacturers according to flue section and compensation requirements. A typical example is as follows：
| Projects | parameter |
|---|---|
| Flue section | 1000mm ×3840mm |
| Thermal displacement (axis/transverse) | ±20mm/±40mm |
| Total length of expansion joint | 250mm (flange surface to flange surface) |
3. Comparison table of length of rectangular corrugated expansion joint
Comparison of wave number and length of rectangular corrugated expansion joint：
| wavenumber | Half-height length (mm) | Full height length (mm) |
|---|---|---|
| 1 wave | 184 | 290 |
| 2 waves | 243 | 376 |
| 3 waves | 323 | 572 |
| 4 waves | 382658 | |
| 5 waves | 464 | 854 |
| 6 waves | 521 | 940 |
Note: The above length is the basic bellows length, and the actual total length needs to be added with the size of the connectors or flanges at both ends.
V. Allowable value of deviation of installation length
The length of flue expansion joint is allowed to deviate at the time of installation. According to the installation specification of non-metallic expansion joints：
| Deviation Item | Allowable range |
|---|---|
| Mounting length deviation | ±5mm |
| Coaxiality deviation | ±5mm |
| Torsion angle | ≤10° |
| Offset angle | ±0.1° |
Excessive installation error may lead to distortion of the expansion joint and local stress concentration, which affects its normal compensation function.
VI. Decision-making process of length selection
The following steps should be followed to determine the length of the flue expansion joint:
Step 1: Calculate the pipe thermal displacement
Calculate the required compensation amount from Δ L = α × L × Δ T
Step 2: Determine Expansion Joint Type
- Mainly axial displacement → axial expansion joint
- Multidirectional displacement → non-metallic expansion joint or universal type
Step 3: Calculate the required wavenumber
Wave number = required compensation ÷ single wave compensation (considering safety factor 1.2)
Step 4: Determine the total length of the expansion joint
Total length = wave number × wave pitch + length of connection/flange at both ends
Step 5: Consider Cold Draw Optimization
If space is constrained, 50% cold drawing can be designed to reduce wave number
7. Common misunderstandings and precautions
Myth 1: The larger the length, the stronger the compensation ability
Positive solution: The compensation ability depends on the wavenumber rather than the mere length. With the same length of expansion joint, the wave number with smaller wave pitch is more and the compensation ability is stronger.
Myth 2: Just choose the standard length at will
Positive solution: The expansion joint length must be determined based on the thermal displacement calculation. Purchase only based on model or code, without providing working condition data, no service life can be guaranteed。
Myth 3: Installation error can be "hard adjusted" by expansion joint
Correct solution: It is strictly forbidden to adjust the deviation of pipeline installation by deforming the bellows。 The deviation of installation length shall be within the allowable range (±5mm).
Myth 4: The length of non-metal expansion joint can be customized arbitrarily
Correct answer: The length of non-metallic expansion joint needs to be comprehensively determined according to the flue section, compensation amount and installation space, and the longer the better.
sum up
The determination of the length of flue expansion joint is a technical work that requires accurate calculation. The core points can be summarized as follows:
The core of determining the length of flue expansion joint is "calculation" rather than "estimation". It is recommended that the thermal displacement calculation be completed according to the specification at the design stage, and the compensation requirement be clarified. Then, the manufacturer will determine the product length according to the wave number-length comparison table, and mark the allowable installation deviation in the drawing. A scientific length selection can avoid the failure of expansion joint caused by insufficient compensation.