In the design and construction of industrial flue gas pipeline system, the placement of flue expansion joint is the key factor to determine whether the expansion joint can play the compensation function normally and ensure the safe operation of the system. The wrong position is selected, which can lead to early failure of the expansion joint, air and smoke leakage, or pipeline deformation and equipment damage. So, how should the flue expansion joint position be determined? Which parts must be set? This paper will systematically explain the technical knowledge of flue expansion joint position from the layout principle, typical position to the specification requirements.
1. Core principle of flue expansion joint position arrangement
The arrangement of flue expansion joint is not arbitrary, but needs to be comprehensively determined on the basis of pipeline stress analysis, combined with thermal displacement direction, equipment interface requirements and space conditions. According to GB 50235-2010 "Construction Code for Industrial Metal Pipe Engineering", the following core principles should be followed in determining the position of flue expansion joint：
Principle 1: Close to the fixed bracket
The expansion joint should be arranged between the two fixed brackets and not too far away from the fixed brackets to ensure that the compensator can effectively absorb the thermal elongation of the pipe section.
Principle 2: Keep an appropriate distance from the device interface
The distance between the expansion joint and the equipment interface should be controlled within a reasonable range, which can not only absorb the thermal displacement of the equipment body, but also not produce excessive reverse thrust on the equipment interface.
Principle 3: Avoid Stress Concentration Areas
The position of the expansion joint should avoid the stress concentration areas such as the elbow and diameter reduction of the flue to prolong the service life.
Principle 4: Easy maintenance and replacement
The position of the flue expansion joint shall be convenient for inspection, maintenance and replacement after installation. The expansion joint connected with the absorption tower shall be connected by flange on the equipment side for easy replacement。
2. Typical arrangement of flue expansion joint position
1. Equipment entry and exit location
In the import and export of fans, hoods, boilers and other equipment, expansion joints are essential. It should be positioned close to the equipment interface to absorb the thermal displacement and mechanical vibration generated during the operation of the equipment.
Silent diesel generator smoke pipe arrangement: Must be connected to the silent diesel generator smoke exhaust outlet through a bellows to absorb thermal expansion, displacement and vibration。
Arrangement of the inlet pipe of the cigarette hood: Two balance ring expansion joints are arranged at the horizontal pipe near the inlet of the cigarette hood, so that the connection of the inlet pipe of the cigarette hood is changed into a soft connection, so that the influence of the misalignment, deflection, deformation and settlement of the inlet pipe in the transverse and vertical directions on the cigarette hood is greatly reduced, and the long-term safe operation of the cigarette hood is ensured。
In the catalytic cracking unit, the rational arrangement of flue expansion joint position is crucial. The horizontal smoke pipe adopts spring hanger and center rolling support to reduce friction resistance when the pipe is moved by thermal expansion; Three hinge expansion joints or three balance ring expansion joints are used for pipe thermal compensation to absorb the influence of pipe thermal expansion on the hood。
2. Pipeline Turning and Direction Change Position
It is recommended to install expansion joints to counteract the thermal expansion of the pipe during operation where the direction of the pipe is changed。 The elbow is the concentrated area of thermal displacement, and the inner bending radius of the 90-degree elbow should be 3 times the diameter of the pipe。
3. Section Location of Long Straight Pipe Section
For larger straight pipe sections, they should be divided into several independent compensation pipe sections by fixed brackets, and expansion joints should be set at appropriate positions in each pipe section. This arrangement can effectively prevent stress concentration caused by the accumulation of thermal expansion of long pipes.
4. Location requirements of high-temperature flue
For high temperature flue such as SCR flue gas system, the arrangement of flue expansion joint position has its own particularity. For the expansion joint of SCR flue gas system, it is necessary to withstand axial, radial and angular displacement and absorb vibration effectively, so the non-metallic expansion joint with inner baffle and high temperature resistance should be preferred。
In SCR engineering, 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, so the metal expansion joint can not meet the requirements of safe use. Therefore, the non-metal expansion joint is used to compensate the thermal displacement。
In order to ensure the uniformity of flue gas flow field, if the flue is wrong edge during operation, it will inevitably affect the flow field distribution and the use effect of ammonia spray grid. Therefore, the installation of such flue expansion joint position has a special requirement: the installation should be 100% pre-offset, 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。
5. Location requirements of low-temperature wet flue
Expansion joints that are in contact with wet flue gas and are located in the horizontal flue section, need to be drained through the expansion joint frame. Therefore, when selecting the flue expansion joint position, it must be ensured that the position meets the drainage conditions: the drainage hole is at least DN150 and is located on the centerline of the horizontal flue section。
In addition, the expansion joint connected to the absorption tower shall be flanged on the equipment side, and the arrangement shall ensure that the expansion joint can be replaced。
3. Installation specification of flue expansion joint position
After determining the position of flue expansion joint, correct installation is the key to ensure its compensation function.
1. Directional Requirements
The installation direction of the expansion joint directly determines its compensation effect. According to the provisions of GB 50235-2010, the end of the bellows expansion joint with the welding seam shall be located at the inflow end of the medium on the horizontal pipe, and at the upper part on the vertical pipe。 For compensators with liner, care should be taken to make the direction of the liner coincide with the direction of media flow。
2. Concentricity requirements
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 guiding and fixing supports shall be reasonably set at both ends of the expansion joint of the bellows, and the installation error of the pipeline shall not be adjusted by the method of deforming the pipeline or compensating the expansion joint。
3. Pre-stretch/pre-compression requirements
For compensators requiring "cold tightening", the auxiliary components used for pre-deformation shall be removed after the pipeline is installed。 Before installation, the bellows expansion joint shall be pre-stretched or pre-compressed according to the design documents, and the force shall be uniform。
4. Temporary restraint device
When installing the bellows expansion joint, a temporary restraining device shall be installed, and the temporary restraining device shall be removed after the pipeline is installed and fixed。
5. Welding protection
During installation, welding slag is not allowed to splash on the wave node, temporary supports are not allowed to be welded on the wave node, and hoisting fixtures such as wire ropes are not allowed to be directly tied on the wave node, so as to avoid mechanical damage to the wave node。
6. Removal of transport tie rod
After the pipe system is installed, the auxiliary positioning members and fasteners used for installation and transportation on the expansion joint should be removed as soon as possible, and the limiting device should be adjusted to the specified position according to the design requirements, so that the pipe system has sufficient compensation ability under environmental conditions。
4. Key Points of Flue Expansion Joint Position Selection under Different Working Conditions
| Type of operating condition | Location Essentials | Special requirements |
|---|---|---|
| Equipment import and export | Immediate device interface | A balance ring expansion joint is arranged at the inlet of the cigarette hood, and a spring crane is adopted to reduce friction |
| SCR denitrification flue | High temperature zone compensation position | 100% pre-offset installation to ensure axis alignment on both sides during operation |
| Wet desulfurization flue | Horizontal segment arrangement | The DN150 drain hole is set at the center line of the lowest point |
| bend of pipe | Appropriate distance behind elbow | Inner bend radius of 90° elbow ≥3 times pipe diameter |
| Connect with absorption tower | Equipment side flange connection | Ensure that the expansion joint is replaceable |
V. Common position arrangement errors and correction
| Wrong practice | consequence | Correct practice |
|---|---|---|
| Install the expansion joint close to the elbow | Flue gas bias flow scour bellows | Keep the length of straight pipe section ≥4 times the pipe diameter |
| Horizontal installation without drainage holes | Condensate accumulation corrosion | Horizontal wet flue DN150 drainage hole |
| Installation not identified by flow direction | Corrugated pipe damaged by washing | The weld end of that inner sleeve is located at the medium inflow end |
| High temperature section without pre-bias | Flue misalignment affects flow field during operation | 100% pre-offset installation |
sum up
The reasonable arrangement of flue expansion joint position is the basis of ensuring the safe operation of flue gas system. The following core points should be followed to correctly determine the position of the expansion joint:
| gist | Specific content |
|---|---|
| Device-side arrangement | Close to that inlet and outlet of the fan and the cigarette hood, the inlet of the cigarette hood is arranged with a balance ring expansion joint |
| Arrangement at elbow | An expansion joint shall be set at the point where the direction of the pipeline is changed, and the inner bending radius of the 90° elbow shall be ≥3 times the pipe diameter |
| High temperature section arrangement | SCR flue with non-metallic expansion joints, 100% pre-biased installation |
| Wet flue arrangement | The horizontal section must be provided with DN150 drainage hole, located at the center line of the lowest point |
| Installation Specifications | The weld end of the inner sleeve is located at the medium inflow end, and the transport tie rod is removed after installation |
| Criterion basis | Design according to DL/T 5121-2020, installation according to GB 50235-2010 |
A reasonable position arrangement and standard installation of the expansion joint can fundamentally ensure the long-term stable operation of the expansion joint under harsh working conditions such as high temperature and corrosion. It is suggested that the position of the expansion joint should be determined according to the specification requirements in the design stage, and the installation quality should be strictly controlled in the construction stage.