In the design of industrial flue gas pipeline system, the reasonable arrangement of flue expansion joint directly determines whether the pipeline can operate safely and whether the expansion joint can play the normal compensation function. Improper arrangement can lead to premature failure of expansion joints, pipe deformation and even equipment damage. The expansion joint is not randomly installed in any position of the pipe. It needs to work together with the fixing bracket, guide bracket and limit bracket to form a complete compensation system. This article will systematically explain the technical key points of flue expansion joint arrangement from the arrangement principle, bracket configuration to typical examples.
1. Basic principles of flue expansion joint arrangement
The core objective of flue expansion joint arrangement is to absorb the axial and radial displacement of all connected equipment and flue with the most reasonable number and position of expansion joints, and at the same time control the thrust and torque of pipe to equipment within the safe range。
1. Principle of segmented compensation
The long straight pipeline is divided into several independent compensation pipe sections, and fixed brackets are arranged at both ends of each pipe section, and an expansion joint is arranged in the middle. Each expansion joint is only responsible for absorbing the amount of thermal elongation of this pipe section. The principle of dividing the length of the pipe section is: the upper limit of the compensation amount of each expansion joint determines the maximum length of the pipe section. Taking carbon steel pipe as an example, when the design temperature is 400℃, the thermal elongation of the pipe per 10 meters is about 45-50mm, and the length of the pipe section can be determined accordingly.
2. Principle of proximity to fixed bracket
The expansion joint should be arranged as close to the fixed bracket as possible. According to the engineering experience, when arranging the flue expansion joint, ensure that the distance from the expansion joint to the fixed bracket should be controlled within 4 times the pipe diameter (4D)。 The purpose of this requirement is to prevent transverse instability of the pipeline and to ensure that the expansion joint only bears axial displacement and does not bear additional bending moments.
3. Guide bracket matching principles
Both ends of the expansion joint must be provided with guide brackets, and the distance between the guide brackets and the expansion joint is strictly regulated. The first guide bracket shall be installed at a distance of 4D from the expansion joint, and the distance between the second guide bracket and the first guide bracket shall be 14D。 The function of the guide bracket is to ensure that the pipe expands and contracts along the axis direction and prevents lateral swing.
2. Arrangement requirements of fixed brackets
The fixing bracket is the most important component in the flue expansion joint arrangement. Its function is to limit pipe displacement and divide pipe sections into independent compensation units. The fixed bracket must be able to withstand the vector sum of the following loads:
| Load Type | Description |
|---|---|
| Elastic reaction force of expansion joint | Restoring force caused by bellows deformation |
| Blind plate force | F = P × A ( Pressure × effective area) |
| Pipe frictional resistance | Friction at the sliding bracket |
| self-weight | Pipe and insulation weight |
The research published in Pressure Vessel, Issue 2, 2024 clearly points out that the selection and rational arrangement of pipeline supports are the key to ensuring the safe and effective service of expansion joints。 In high temperature flue gas pipeline, if the strength of the fixed bracket is insufficient, the blind plate force will lead to the failure of the bracket and the instability of the expansion joint.
Third, the spacing of the guide bracket and the golden combination
The arrangement spacing of the guide brackets directly affects the working state of the expansion joint. The correct flue expansion joint arrangement should follow the following "golden combination":
Guide bracket spacing formula
According to the industrial pipeline construction specifications, the bracket configuration of flue expansion joint arrangement shall follow the combination mode of "fixed bracket-expansion joint-first guide-second guide-sliding bracket":
- Fixed bracket: end point of pipe section, bearing full load
- Expansion joint: mounted against fixed bracket
- First guide bracket: ≤4D from expansion joint
- Second guide bracket: distance from first guide bracket =14D
- Sliding brackets: The spacing is determined according to specification, allowing the pipe to slide freely
This "4D-14D" rule is derived from the engineering practice of diesel generator smoke exhaust pipe, and also applies to general industrial flue。
4. Difference between horizontal flue and vertical flue
1. Horizontal flue expansion joint arrangement
Horizontal flues are the most common scenario for flue expansion joint arrangements. In addition to following the above stent rules, note that:
- Drainage slope: When installing horizontal pipelines, the drainage slope should be ≥3‰ to ensure that the condensate can flow to the drainage point
- Drainage hole setting: Expansion joint contacting wet flue gas and located in horizontal flue section, drainage hole is at least DN150, located on the center line of horizontal flue section
- Material of drainage fittings: Drainage pipes should be made of corrosion-resistant materials such as FRP or nickel-based alloy steel
2. Vertical flue expansion joint arrangement
The expansion joint arrangement of a vertical flue is more complicated than that of a horizontal pipe because gravity factors can significantly affect the pipe stress:
- Guide brackets are required to prevent pipeline instability
- Load-bearing device: Consider the self-weight of the pipeline bearing by the expansion joint, and set a spring hanger if necessary
- Direction of inner sleeve: One end of the bellows expansion joint with welding seam, which should be placed on the upper part of the vertical pipe
3. Special arrangement of high-temperature flue
For high temperature flue such as SCR flue gas system, there are special requirements for flue expansion joint arrangement. According to DL/T 5121-2020 "Technical Regulations for Design of Flue, Wind and Pulverized Coal Pipeline in Thermal Power Plant", due to the high flue gas temperature, the lateral displacement and horizontal displacement of the flue are large, and the flue is not long enough, the metal expansion joint generally cannot meet the requirements of safe use, so the non-metal expansion joint is preferred。
Pre-offset setting: In order to ensure the uniformity of flue gas flow field, the expansion joint should be 100% pre-offset during installation, that is, the flue on both sides of the expansion joint is misaligned during installation, and the axis of the flue on both sides is on a straight line during operation。
V. Typical Arrangement Examples and Bracket Spacing Reference
Example 1: Smoke Exhaust Pipe for Diesel Generator
A 30-meter-long metal smoke pipe has a temperature rise of 520℃ and an expansion amount of 176mm. The correct arrangement scheme of flue expansion joint is as follows: a fixed bracket is arranged near the equipment side, the expansion joint is installed close to the fixed bracket, the first guide bracket is located at 4D, the second guide bracket is located at 14D, and the other positions are arranged with sliding brackets according to specifications。
Example 2: SCR Denitrification System Flue
The flue at the inlet and outlet of SCR reactor is subjected to three-way displacement, and non-metallic expansion joint is adopted. Special attention should be paid to the arrangement: fixing brackets with sufficient strength are set on both sides of the expansion joint; The spacing of guide brackets is according to the 4D principle; 100% pre-bias at installation。
Example 3: FRP Flue Gas Pipe
According to SH/T 3522-2017, the expansion joint arrangement of FRP flue gas pipe also follows the principle of distance from fixed bracket ≤4D. Set drainage slope ≥3‰ when installing horizontal pipeline。
6. Key control points in construction and installation
1. Pre-stretching/Pre-compression
Before installation, the expansion joint shall be pre-stretched or compressed according to the design documents, and the force shall be uniform。 The purpose of pre-stretching is to give the bellows an initial deformation in the cold state, so that the bellows are in optimal working condition in the hot state.
2. Temporary restraint device
When installing the bellows expansion joint, a temporary restraining device shall be installed, which shall be removed after the pipeline is installed and fixed。 After installation, the transport protection tie rod must be removed immediately (the positioning tie rod must not be removed) so that the expansion joint can be expanded and contracted freely.
3. Coaxiality Control
During installation, the bellows expansion joint shall be concentric with the pipe and shall not be skewed, and circumferential torsion of the expansion joint caused by installation shall be avoided。 The installation error of the pipeline shall not be adjusted by means of making the pipeline deform or compensation for the expansion joint.
4. Welding protection
During installation, welding slag is not allowed to splash on the surface of the bellows, temporary supports are not allowed to be welded on the wave nodes, and hoisting fixtures such as wire ropes are not allowed to be directly tied on the wave nodes, so as to avoid mechanical damage to the wave nodes。
VII. Common mistakes and correction of layout design
| Wrong practice | consequence | Correct practice |
|---|---|---|
| Expansion joint away from fixed bracket | Instability of pipeline and excessive lateral swing | Distance ≤4D |
| Missing or excessively spaced guide brackets | Expansion joint columnar instability, bellows buckling | First guide ≤4D, second guide distance from first 14D |
| Insufficient strength of fixed bracket | Bracket failure, blind plate force out of control | Strictly calculate the blind plate force to design the bracket |
| Lateral displacement not calculated | Torsional deformation of expansion joint | Complete calculation of three-way displacement |
| The expansion joint of the refractory section is not insulated | Burning of internal metal parts | Internal insulation is performed synchronously with the refractory section |
sum up
The arrangement of flue expansion joint is a systematic project. The correct arrangement should be closely integrated with the bracket system, and follow the cooperative arrangement principle of "fixed bracket-expansion joint-guide bracket":
- Section determination of fixed bracket: the pipeline is divided into several independent compensation pipe sections, and the fixed bracket shall bear blind plate force
- The expansion joint is close to the fixing frame: the distance is ≤4D, and the high-temperature flue should be 100% pre-biased
- The guide brackets are sequentially arranged: a first guide distance expansion joint 4D, a second guide distance first 14D
- Drainage design should not be neglected: DN150 drainage hole for horizontal wet flue expansion joint, material FRP or nickel-based alloy steel
- Welding specifications must be followed: Splash of welding slag is strictly prohibited, and the transport tie rod is removed after installation
The selection and reasonable arrangement of the support is the key to ensure the safety and effective service of the expansion joint。 A reasonably designed expansion joint arrangement scheme can avoid the failure problem from the source and ensure the long-term safe operation of the flue system.