In the on-line flue gas monitoring system (CEMS), the expansion joint of the straight pipe section of the flue gas sampling port is the key component to connect the sampling probe with the flue, absorb the heat displacement and ensure the representativeness of sampling. The sampling pipeline is usually led out from the high-temperature flue and sent to the analyzer cabinet through the heat tracing pipeline. During this process, the temperature of the pipeline changes significantly, and the thermal displacement can not be ignored. Once the expansion joint of the straight pipe section of the flue gas sampling port is improperly selected or installed, it will lead to the deformation of the sampling pipeline and the cracking of the weld, and in the worst case, it will affect the representativeness of sampling and cause the distortion of monitoring data. This paper will systematically explain the professional technical knowledge of the expansion joint of the straight pipe section of the flue gas sampling port from the characteristics of working conditions, key points of type selection to the installation specification, and introduce the technical advantages and service ability of our company in this field.
1. Special requirements of sampling port working conditions for expansion joints
In the flue gas denitrification and desulfurization system, the working environment where the expansion joint of the straight pipe section of the flue gas sampling port is located has the following characteristics:
1. Significant temperature change
The flue gas sampling port is usually set at the horizontal flue, and the flue gas temperature can reach 120-400℃. After the sampling line is led out of the high-temperature flue, it is sent to the analyzer through the heat tracing line, and there is a significant temperature gradient in this process. Without expansion joints, thermal stress in the pipe may cause cracking of the weld or deformation of the sampling probe.
2. Constrained space and complicated pipelines
The flue gas sampling platform is usually located outside the flue, and the sampling line needs to be led out of the horizontal flue and then connected to the analyzer cabinet through a vertical downward arrangement. In this process, there are multiple elbows and direction changes in the pipeline, which put forward high requirements for the flexible compensation ability of the expansion joint.
3. High sealing requirements
The accuracy of flue gas analysis system depends on the tightness of gas path. If the expansion joint of the straight pipe section of the flue gas sampling port is not tightly sealed, external air will be sucked into the sampling pipeline, resulting in high oxygen detection value and distortion of monitoring data. Therefore, the sealing performance of expansion joint is directly related to the measurement accuracy of CEMS system.
2. Main points of selection of expansion joint of straight pipe section of flue gas sampling port
1. Expansion joint type selection
For the expansion joint of the straight pipe section of the flue gas sampling port, it is recommended to choose a single axial metal corrugated expansion joint. This kind of expansion joint is compensated by a single bellows, which is mainly used to absorb axial displacement of straight pipe section, but also can absorb a small amount of transverse displacement. It has the advantages of simple structure, convenient installation and reliable sealing, and is especially suitable for the working conditions of sampling pipelines.
Selection basis:
- Nominal diameter: Usually DN32-DN100, matches the sampling diameter
- Operating pressure: 0.25-1.0MPa (selected according to sampling system pressure)
- Displacement direction: mainly axial
Nantong Chuangxin Machinery Co., Ltd. can provide a full series of single axial metal corrugated expansion joints, which are designed and manufactured in strict accordance with GB/T 12777 national standard to meet the working conditions of various flue gas sampling systems.
2. Material selection
Select the bellows material according to the flue gas medium composition:
| Flue gas medium | Recommended Bellows Materials | Description |
|---|---|---|
| Hot air | SUS316, 316L | High temperature oxidation resistance |
| SO₂-containing flue gas | SUS316, 316L | Sulfur dioxide corrosion resistance |
| Steam-containing flue gas | SUS316, 316L | Moisture corrosion resistance |
| Catalytic cracking high temperature flue gas | SUS321, INCONEL | Resistance to high temperature and intergranular corrosion |
For the desulfurization flue gas sampling port, the medium contains SO₂ and steam, and SUS316 or 316L stainless steel bellows should be selected, which has good corrosion resistance.
Nantong Chuangxin Machinery Co., Ltd. has rich experience in material selection, which can accurately recommend the most suitable bellows material according to the flue gas working condition parameters (temperature, composition, pressure, etc.) provided by customers, and provide material certification documents.
3. Structural requirements
The expansion joint of the straight pipe section of the flue gas sampling port shall meet the following structural requirements:
- With liner tube (guide tube): Protect the bellows from direct flushing of smoke gas. Note that the direction of liner tube is consistent with the flow direction of media during installation
- Flange connection: easy to disassemble and maintain the sampling probe
- Compact design: accommodates the limited space of the sampling platform
The expansion joint of flue gas sampling port produced by Nantong Chuangxin Machinery Co., Ltd. adopts precision welding technology, and the matching precision between the guide tube and the bellows is high, which effectively prevents the flue gas from being biased and washed, and the service life of the product is longer than that of ordinary products by more than 30%.
Installation Specifications and Key Control Points
1. Confirmation of installation position
According to engineering experience, the expansion joint of the straight section of the flue gas sampling port is usually installed at the appropriate position before the sampling pipe finally enters the flue. This position can not only absorb the thermal displacement of the sampling pipeline, but also facilitate subsequent maintenance.
2. Direction confirmation
The flow direction identification on the expansion joint must be confirmed before installation:
- Expansion joint with liner, the direction of liner must coincide with the direction of media flow
- Installation in reverse direction will lead to flue gas directly washing the bellows, accelerating failure
3. Connection to Sampling Probe
The sampling probe and the flue are usually inserted obliquely at an angle of 30°, which is consistent with the flow direction of flue gas, and are connected with flanges. The expansion joint of the straight pipe section of the flue gas sampling port shall be installed between the probe and the subsequent heat tracing pipeline, and the flange size shall be consistent with the connecting flange of the probe.
4. Installation precautions
According to the general installation requirements of bellows expansion joints, the following specifications shall be observed when installing expansion joints of straight pipe sections of flue gas sampling ports:
- Unable to bear load: The expansion joint should be hoisted separately, and it is not allowed to be hoisted together with the pipe after welding
- Deformation adjustment is strictly prohibited: It is strictly prohibited to adjust the installation deviation of the pipeline by deforming the bellows
- Welding protection: No welding slag is allowed to splash onto the bellows surface during installation
- Removal of transport tie rod: The auxiliary positioning mechanism for installing transport protection on the expansion joint shall be removed immediately after the pipe system is installed
5. Sealability Check
After the installation is completed, the sealing check shall be performed on the expansion joint of the straight pipe section of the flue gas sampling port:
- Inspect all joints, seals and gaskets to ensure that they are not broken and that the seals are intact
- Can be verified by airtightness test or leak detector
Nantong Chuangxin Machinery Co., Ltd. provides installation technical guidance services, and can send professional engineers to the site to carry out installation disclosure and process supervision to ensure that the installation quality of expansion joints meets the standard.
Typical Application Scenarios
Scenario 1: Denitrification flue gas sampling system
In denitrification devices, the flue gas sampling port is usually set up at the horizontal flue leading to the air preheater after the lower layer. After the sampling pipeline is led out from the horizontal flue, it is connected to the heat tracing pipeline through the expansion joint of the straight pipe section of the flue gas sampling port, and then arranged vertically downward to the analyzer cabinet. The role of the expansion joint here is:
- Thermal displacement of absorption sampling line
- Isolate flue vibration and protect analyzer
- Disassembly and assembly interface is provided to facilitate probe maintenance
Scenario 2: Desulfurization flue gas sampling system
In the desulfurization unit, the sampling pipeline is in a corrosive environment with low temperature and high humidity. The expansion joint of the straight pipe section of the flue gas sampling port should be made of SUS316L material, which has good corrosion resistance. At the same time, the sealing of the expansion joint is directly related to the accuracy of the analysis data-if the seal is not tight, the outside air will be inhaled, which will cause the oxygen detection value to be high.
V. Common problems and treatment measures
| Question | Probable cause | Treatment measures |
|---|---|---|
| Expansion joint weld cracking | Incomplete absorption of thermal displacement, deviation in installation | Check line support and reinstall |
| High sample oxygen content | Loose sealing of expansion joint | Check gasket, tighten bolts |
| Corrugated pipe corrosion | Material N/A Flue Gas Medium | Upgrade to 316L or 321 |
| Expansion joint deformation | Transport tie rod not removed | Inspect and remove transport tie rods |
VI. Maintenance Suggestions
- Periodic inspection: Check the appearance of the expansion joint for leakage or deformation every time the CEMS system is calibrated
- Sealability verification: Conduct airtightness inspection every six months to ensure that there are no leakage points in the system
- Replacement cycle: Determine the replacement cycle according to the working conditions. It is recommended to replace it in 2-3 years for corrosive environments
- Ledger management: record expansion joint model, installation date, inspection
sum up
The expansion joint of straight pipe section of flue gas sampling port is an indispensable compensation element in CEMS sampling system. Its selection and installation should follow the following core principles:
- Correct selection: Single type axial metal corrugated expansion joint is selected, and the material is selected according to the flue gas medium SUS304/316/316L/321
- Installation specifications: Consistent flow direction marking, separate hoisting, strictly prohibited deformation adjustment, removal of transportation rod
- Reliable sealing: The sealing performance of the expansion joint directly affects the accuracy of the analysis data, and the airtightness check should be carried out after installation
- Regular maintenance: Establish inspection ledger to discover and deal with potential leakage hazards in time
One-time standard selection and installation can ensure the long-term stable operation of the flue gas sampling system, and provide reliable guarantee for the accuracy of CEMS monitoring data.
Nantong Chuangxin Machinery Co., Ltd. -specializing in the research and development, design and manufacture of metal corrugated expansion joints and non-metal expansion joints, the products are widely used in electric power, metallurgy, chemical industry, environmental protection and other industries. The company has advanced production equipment and testing means, one-stop service from scheme design, product manufacturing to installation guidance and after-sales maintenance. For technical consultation or product selection support, please contact the technical team of Nantong Chuangxin Machinery Co., Ltd.