In direct and indirect air cooling systems of thermal power plants, the air-cooled expansion joint is the key flexible compensation component connecting the exhaust pipeline of steam turbine, the main steam pipeline of air-cooled island and the cooling water pipeline in air-cooled tower. The air cooling system pipeline has the characteristics of large diameter (up to 2000-7600mm), thin wall (only 8-18mm), high stiffness and low flexibility, so its thermal expansion cannot be satisfied only by the natural compensation of pipeline direction。 At the same time, the air cooling system faces multiple challenges such as vacuum conditions, temperature alternating, and high wind loads. This article will systematically explain the professional technical knowledge of air-cooled expansion joint from system type, expansion joint selection to installation and maintenance.
Types of air cooling systems and application scenarios of expansion joints
Air-cooled expansion joints are mainly used in two kinds of air cooling systems: direct air cooling system and indirect air cooling system.
1. Direct air cooling system
In the direct air cooling system, the exhaust steam of the steam turbine is directly sent to the air-cooled condenser through a large-diameter pipe for cooling. Because of the large diameter (2000-7600mm) and the thin wall of the exhaust pipe, the thermal expansion of the pipe cannot be satisfied only by the natural compensation of the pipe direction, and it must be solved by the scientific configuration of the expansion joint。
Typical Expansion Joint Configuration Scheme: According to the patented technology, the expansion joint configuration of the air-cooled exhaust pipe includes:
- Curved tube pressure balanced expansion joint: Set on horizontal main tube, can absorb both axial and transverse expansion
- Single hinge expansion joint: set on vertical branch pipe to absorb angular displacement
- Double hinge expansion joint: Set on vertical branch pipe, usually 2-3 combinations
2. Indirect air cooling system
In the indirect air cooling system, there are a large number of cooling water circulation pipelines in the air cooling tower. Taking the 2×660MW project of Helin Power Plant as an example, the purchase volume of expansion joints in the air cooling tower reached 1592 sets (two units), the specifications include DN200 (1568 sets), DN250 (2 sets) and DN1200 (22 sets), the design internal pressure is 1/0.01MPa, the nominal pressure is PN6 (0.6MPa), the operating temperature is-34.5~80℃, and the medium is desalinated water。
2. Main types and structural characteristics of air-cooled expansion joints
1. Curved tube pressure balance expansion joint
Structural characteristics: The expansion joint is set on the horizontal main pipe, which can absorb both axial displacement and transverse displacement。
Core advantages: It can compensate axial displacement and lateral displacement without generating internal pressure thrust on pipeline system or other equipment. It is commonly used in pumps, compressors, steam turbines and other load-sensitive pipeline systems, and can absorb vibration and reduce noise to ensure the safe operation of equipment.
Applicable scenario: Main horizontal pipe of direct air cooling system.
2. Double hinge expansion joint
Structural composition: It is composed of core components such as universal ring, two groups of hinge plates connected with end tubes, pins and bellows. The unique design contains two pairs of mutually perpendicular hinge structures connected by pins to form a flexible compensation unit。
Technical parameters:
Core Strengths:
- Can withstand the thrust generated by internal and external pressure of pipeline
- Gimbal rings, hinges and pins are capable of withstanding internal pressure thrust
- Suitable for complex pipe systems with limited space
3. Universal hinge expansion joint
The universal hinge expansion joint has a universal ring and two pairs of mutually perpendicular hinge structures in its circumferential direction. The universal ring, hinge and pin are designed to withstand the thrust of the internal pressure on the expansion joint, so it can absorb the angular displacement of any plane。
3. Failure Analysis and Improvement Measures of Air-cooled Expansion Joint
1. Typical failure modes
The waveform compensator installed in an air cooling inlet pipeline is damaged in actual operation. The analysis shows that when the device is started, the material is constantly heated and heated, the expansion joint on the pipeline is the free end, and the temperature difference stress causes the pipe and expansion joint to displace to the free end; The plastic deformation of the expansion joint cannot be completely restored to the initial state when the expansion joint is switched from the start to the stop stage。
The expansion joint is completely stretched due to frequent start-up and shutdown, and the expansion joint may have local fatigue damage. After the alternating load of subsequent start-up and shutdown, the expansion joint is subjected to tensile and compressive stress, which exceeds the material strength limit, resulting in plasticity and fatigue fracture。
2. Improvement measures
For the failure of the expansion joint of the air cooling system, the following improvement measures can be taken：
| Direction of improvement | Specific measures | Effect |
|---|---|---|
| Add attachments | Add guide joint, main fixed support and large pull rod, and add guide tube | Enhanced stability to avoid scouring |
| Single layer to multi-layer | Adopt multi-layer bellows structure | Improve fatigue life and reduce stiffness |
| Change the waveform | Replace small waveforms with large waveforms with small curvature and small stress | Reduce bellows stress levels |
| Hydroforming | Manufacture of Bellows by Hydroforming Process | Reduce residual stress and improve corrosion resistance |
| Pre-contingency measures | Install with proper pre-compression | Double life for every 10% reduction in expansion and contraction |
| Material upgrade | Select appropriate materials according to the flow medium, external environment and operating temperature | Enhanced corrosion and high temperature resistance |
3. Double-layer strip monitoring structure
In recent years, double-layer bellows structure has been adopted in important parts abroad: each layer can independently bear the pressure of working medium, and an alarm device is added between layers. When the inner layer leaks, an alarm device will be displayed to remind the operator to order spare parts, and the outer layer bellows can continue to be used until the scheduled maintenance is replaced。
Installation and maintenance of air-cooled expansion joint
1. Installation precautions
The installation of expansion joint is the key link to ensure the safe operation of air cooling system. In a power plant emergency repair case, the expansion joint at the outlet of the air preheater burst, and the hot air temperature at the leakage was as high as 360℃. The emergency repair team used steel pipe as the outer shield and welded it on the outside of the expansion joint. The inside of the shield was filled with ceramic fiber cloth and thermal insulation cotton, which successfully blocked the air leakage。
Installation Points:
- After installation, the transport protection tie rod must be removed so that the expansion joint can be expanded and contracted freely
- Guide bracket setting: the first guide bracket is less than 4 times the pipe diameter from the expansion joint
- The installation of vertical pipeline should be matched with load-bearing design
2. Quality Assurance and Service
Quality Air Cooled Expansion Joint suppliers should provide:
- Factory inspection report, material test certificate, product installation instruction
- 24-hour technical service support
- The air-cooled expansion joint is the core flexible component in the air-cooled system to compensate the thermal displacement and ensure the safe operation of pipelines. The following points should be followed in the correct selection:
| System Type | Recommended expansion joint types | Core Parameters | Key Requirements |
|---|---|---|---|
| Direct air cooling main pipe | Curved tube pressure balanced type | Large diameter (2000-7600mm) | Simultaneous absorption of axis and lateral displacement |
| Direct air cooling branch pipe | Single/double hinged | DN200-7000, vacuum ~ -0.1MPa | Use in combination to accommodate angular displacement |
| Indirect air cooling | Universal metal expansion joint | -34.5-80°C, PN6 | Mass installation to adapt to low temperature working conditions |
Core points:
- Special working conditions: the air-cooled pipeline has large diameter and thin wall, and the selection of expansion joint needs to consider the vacuum working conditions and high wind load comprehensively
- Various types: The selection of curved pipe pressure balance type (main pipe) and hinged type (branch pipe) should be accurately calculated according to the pipe layout
- Quality assurance: Required to provide factory inspection report and material test certificate
- Failure prevention: Focus on fatigue failure and instability issues, multi-layer structure + guide tube design can significantly improve reliability
The air-cooled expansion joint with reasonable design and standard installation can ensure the long-term safe operation of the air-cooled system under harsh working conditions such as vacuum and alternating temperature. It is recommended to carry out detailed pipeline stress analysis at the design stage, entrust professional manufacturers to carry out special selection of expansion joints, and ensure that the construction specifications are strictly followed during installation.