In the flue gas system of coal, gas and biomass fired boilers, the role of boiler flue gas expansion joint is often underestimated, and it is not paid attention to until leakage, deformation or even equipment damage occurs. As the key flexible component connecting the boiler body with the subsequent flue, dust collector, desulfurization tower and chimney, the reliability of the expansion joint directly relates to the safe operation of the whole flue gas system. This paper will systematically analyze the function of boiler flue gas expansion joint, from thermal displacement compensation to equipment protection, to help engineers and technicians comprehensively understand the value of this key component.
1. Absorption of thermal displacement of pipeline: core function
The most basic and important function of the boiler flue gas expansion joint is to absorb the heat displacement caused by the temperature change of the pipe. When the boiler is running, the flue gas temperature can rise from normal temperature to hundreds or even thousands of degrees Celsius, and the thermal expansion coefficient of flue steel is about 12×10⁻⁶/℃. Take a section of carbon steel flue with a length of 20 meters as an example. When the temperature rises from 20 °C to 400 °C, the thermal elongation can reach 91.2 mm.
If the boiler flue gas expansion joint does not absorb this thermal displacement, the thermal stress will make the flue bend and deform, the weld crack, and even transmit the force to the boiler body and fan equipment, resulting in serious consequences. The expansion joint undergoes elastic deformation through its flexible structure (metal bellows or non-metallic fabric layer), converting thermal displacement into its own deformation, thereby protecting the structural integrity of the pipeline system.
2. Isolate mechanical vibration: protect equipment safety
Equipment such as boiler induced draft fan and dust collector pulse cleaning system will produce continuous mechanical vibration when running. The function of the boiler flue gas expansion joint is embodied here as an effective vibration isolation function, which prevents vibration from propagating to the boiler body or other sensitive equipment along the flue.
Non-metallic fabric expansion joints are particularly good at this function. Its multilayer composite structure has good damping characteristics and can absorb high-frequency vibrations. The non-metallic expansion joint in the inlet flue of the desulfurization tower can significantly reduce the damage to the anticorrosion coating of the tower body caused by vibration.
3. Reduce equipment thrust: prevent interface damage
Boilers, desulfurization towers, dust collectors and other equipment have strict restrictions on the thrust and torque of inlet and outlet pipelines. One of the functions of the boiler flue gas expansion joint is to reduce the thrust generated by the thermal expansion of the pipe on the equipment interface.
Different types of expansion joints have different control capabilities on thrust:
- The axial expansion joint will produce large blind plate force, which should be borne by the fixed bracket
- The hinge type expansion joint is subjected to internal pressure thrust through the hinge plate and is not transmitted to the equipment
- The pressure balanced expansion joint can completely eliminate the blind plate force
In the inlet pipeline of flue gas turbine of catalytic cracking unit, the adoption of planar three-hinge expansion joint is based on this principle-no blind plate force, small deformation reaction force, and the thrust and torque on the interface of flue gas turbine are controlled within the allowable range.
4. Compensate for installation errors: reduce construction difficulty
Some dimensional deviation and coaxiality error inevitably exist in the field installation of flue system. The function of the boiler flue gas expansion joint is also reflected in the ability to compensate the installation error.
The flexible structure of the expansion joint can absorb a certain amount of axial, transverse and angular deviations, reducing the requirement of pipeline matching accuracy and speeding up the installation progress. This feature is particularly useful in the installation of large flue systems (more than 2 meters in diameter) -accurate centering is almost impossible entirely by rigid connections, and the presence of expansion joints provides the necessary tolerance space.
5. Isolate different settlements: adapt to foundation deformation
The foundation settlement of large equipment such as boilers, desulfurization towers, and chimneys varies. In long-term operation, the foundations between different devices can produce relative settlements of millimeters or even centimeters. The function of boiler flue gas expansion joint here is to absorb the displacement caused by foundation settlement and prevent flue from cracking due to uneven settlement.
This function is especially important in the wet desulfurization system-the desulfurization tower is large in volume and the foundation load is high, while the foundation conditions of the connected original flue support are different, and the relative settlement between the two must be absorbed by the expansion joint.
6. Convenient equipment maintenance: Provide disassembly interface
The equipment in the flue gas system (such as induced draft fan, dust collector, desulfurization tower spray layer) needs regular maintenance. The role of the boiler flue gas expansion joint also includes providing a convenient disassembly interface-by loosening the connecting bolts at both ends of the expansion joint, the equipment can be separated from the flue without cutting and welding.
This function greatly shortens the overhaul period and reduces the maintenance cost. In large thermal power units, the quick disassembly function of the expansion joints at the inlet and outlet of induced draft fans can shorten the maintenance time of a single fan by 1-2 days.
Reducing system noise: Improving working environment
Airflow noise and mechanical noise are produced when the boiler flue gas system is operated. The non-metallic fabric expansion joint has good sound insulation performance, which can reduce the propagation of noise along the flue. The function of the boiler flue gas expansion joint is to improve the working environment and reduce noise pollution. The multi-layer composite material structure can absorb part of the sound energy and play the role of a silencer.
8. Comparison of functional emphasis of various expansion joints
| Expansion joint type | Thermal displacement compensation | Vibration isolation and noise reduction | Thrust control | Installation tolerance | Cost |
|---|---|---|---|---|---|
| Metal axial type | Strong (predominantly axial) | normal | Have blind plate force | normal | Middle |
| Metal hinge type | Strong (angular) | normal | No blind plate force | normal | tall |
| Non-metallic fabric type | Strong (three-way) | Excellent | No blind plate force | Strong | Medium Low |
| Pressure balance type | Strong | normal | Complete elimination | normal | tall |
Analysis of Typical Application Scenarios
Scenario 1: Boiler outlet to economizer
Here, the flue gas temperature is high (300-600℃), which mainly needs to absorb axial thermal displacement. The role of boiler flue gas expansion joint is realized by high temperature metal expansion joint, focusing on high temperature resistance and fatigue life.
Scenario 2: Import and export of desulfurization tower
Here, the flue gas temperature is low (45-80℃), corrosiveness is strong, and multi-directional displacement coexists. Non-metallic fabric expansion joint is the first choice because of its three-way compensation and corrosion resistance, which is reflected in absorbing equipment thermal displacement and isolating vibration while resisting acid corrosion.
Scenario 3: Inlet and outlet of induced draft fan
Here, the vibration is large and it is sensitive to thrust. The function of the expansion joint is to isolate the vibration of the fan and reduce the thrust of the pipeline to the fan shell. It is recommended to use non-metal expansion joint or metal expansion joint with limit structure.
sum up
The function of boiler flue gas expansion joint can be summarized as six core functions:
- Absorbing heat displacement: compensates for thermal elongation of flue due to temperature change and prevents stress damage
- Isolate vibration: Block vibration propagation of equipment, protect boiler and sensitive equipment
- Reduce thrust: Control the thrust and torque of the pipeline to the equipment to prevent damage to the interface
- Compensation for installation errors: Reduce the requirements of pipeline matching accuracy and speed up the construction progress
- Adapting to foundation settlement: Absorbing relative displacement between different equipment foundations
- Easy maintenance and disassembly: Provide quick disassembly interface to shorten maintenance period
Correctly understanding and giving full play to the various functions of boiler flue gas expansion joint is an important guarantee to realize the safe, stable and long-term operation of flue gas system. When selecting the model, the priority of each function should be defined according to the specific working conditions-temperature resistance and fatigue life should be given priority in the high-temperature section, corrosion prevention and drainage should be given priority in the corrosion section, and vibration isolation and blind plate-free structure should be given priority in the vibration-sensitive section. Only the differentiated design of "one district, one policy" can give full play to the value of expansion joints.