In the thermal pipeline system of power plant, expansion joint (also called compensator) is the key component to ensure safe operation. It is able to absorb the thermal displacement of pipes due to temperature changes, reduce equipment vibration, and protect core equipment such as steam turbines, boilers, pumps and valves from additional stress damage. Because of the complex operation environment of power plant-high temperature and high pressure steam, corrosive flue gas, frequent start-and-stop conditions-the correct choice of expansion joint type directly relates to the reliability and life of the system. So, how many kinds of expansion joints in power plants? This paper will carry out detailed analysis from three core dimensions of axial type, transverse type and non-metallic expansion joint, so as to help engineers and technicians quickly grasp the key points of type selection.
1. Axial expansion joint: the first choice for straight pipe section
Axial expansion joints are the most common type in power plants, which are mainly used to absorb the telescopic displacement of pipes along the centerline direction. Its typical structure is a single axial expansion joint, which consists of a set of bellows and connecting tubes at both ends. When the pipe temperature increases, the bellows is compressed; When the temperature decreases, the tensile reset.
In power plant scenarios, axial expansion joints are widely used in water supply pipelines, low-pressure steam pipelines, and some auxiliary thermal pipelines. It has the advantages of compact structure, relatively low cost and small installation space requirements. However, it should be noted that the axial expansion joint can only withstand axial displacement and cannot absorb transverse or angular displacement. Therefore, the guide bracket and the fixed bracket must be strictly set during installation to avoid twisting or lateral bending of the bellows.
Typical axial-type products include single-type axial expansion joints and duplex axial expansion joints. The latter is connected in series by two sets of bellows, which can obtain a greater amount of axial compensation, and is suitable for long-distance straight pipe sections.
2. Lateral expansion joint: coping with the lateral displacement of pipeline
When the pipe needs to absorb the transverse displacement perpendicular to the axis, the transverse expansion joint is the perfect choice. The structure of this kind of expansion joint is relatively complex, and the typical representatives include large tie rod transverse type, small tie rod transverse type and hinge transverse type.
Among them, the transverse expansion joint of large tie rod is widely used in desulfurization flue and hot air duct of power plant. It can effectively absorb large lateral displacement through two sets of bellows and large tie rod structure, and at the same time, the tie rod bears the thrust generated by internal pressure, thus reducing the load of the fixed bracket. Transverse-type expansion joints tend to be the best solution in "Z"-shaped pipes in power plants or pipe sections where space is constrained and obstacles need to be bypassed.
Unlike the axial type, the transverse type expansion joint allows for large lateral movement but relatively limited axial compensation capability. Therefore, in practical applications, it is usually necessary to use it in combination with an angular type or an axial type.
3. Non-metallic expansion joint: the core of flue duct and desulfurization system
If metal expansion joint is the pillar of high temperature and high pressure pipeline, then non-metal expansion joint is an indispensable choice in flue duct system and wet desulfurization device of power plant. The flexible part of the non-metallic expansion joint is composed of multi-layer composite materials, commonly including silicone cloth, fluorine cloth, glass fiber cloth and intermediate heat insulation layer.
The core advantages of non-metal expansion joints are reflected in three aspects:
- Strong multi-directional compensation ability: It can absorb axial, transverse and angular displacements at the same time, especially suitable for rectangular flues with complex shapes.
- The effect of vibration isolation and noise reduction is remarkable: the fabric material itself has damping characteristics, which can effectively isolate the vibration transmitted by fans, vibrating devices and other equipment.
- Outstanding corrosion resistance: In the original flue gas pipeline of wet desulfurization, the medium often contains high concentration of chloride ions and dilute sulfuric acid, and the metal expansion joint is prone to pitting corrosion, while the non-metal material can run stably for a long time.
However, non-metallic expansion joints also have obvious limitations: the temperature resistance generally does not exceed 400℃, and the pressure bearing capacity is usually less than 0.1MPa. Therefore, it is mainly used in low pressure, high temperature or corrosive environments such as boiler smoke duct, dust collector inlet and outlet, desulfurization tower inlet, etc.
4. How to select the appropriate type according to the working conditions
After understanding several types of expansion joints in power plants, the actual selection needs to combine the following key parameters:
- Temperature: When it exceeds 400℃ and the pressure is high, high temperature resistant alloy metal expansion joints (such as Inconel 625 or 310S stainless steel) are preferred; Below 400°C and corrosive flue gas, non-metallic expansion joints are preferred.
- Pressure: only metal expansion joints can be selected for medium and high pressure pipelines (≥1.0MPa); A non-metallic type may be used for the low pressure or slightly negative pressure flue.
- Displacement direction: axial type for pure axial displacement; When there is obvious lateral displacement, use transverse type or large tie rod type; For multi-directional composite displacement, non-metallic expansion joint or universal hinge type should be considered.
- Medium corrosivity: desulfurization flue gas and chemical wastewater pipelines preferably select non-metal or high nickel alloy; Rubber expansion joints can be selected for clean media at room temperature such as circulating water.
V. Key points of installation and maintenance
Regardless of the type of expansion joint you choose, installation and maintenance are equally critical. First, metal expansion joints usually need to be pre-stretched or pre-compressed when installed in the cold state to properly distribute the stress range in the hot state. Secondly, the position of the guide bracket and the fixed bracket must be constructed strictly according to the design drawings, and the distance is too large or too small, which will lead to abnormal deformation of the bellows. For non-metallic expansion joints, attention should be paid to whether the fabric layer is damaged, leaked or has excessive dust accumulation during inspection. Once found, it should be replaced in time to avoid environmental protection risks caused by smoke leakage.
conclusion
To sum up, several types of expansion joints in power plants can be clearly divided from two dimensions: structure and application scenario: axial expansion joints are suitable for straight pipe sections to absorb heat expansion and contraction, transverse expansion joints should cope with lateral displacement of pipelines, while non-metal expansion joints occupy a dominant position in flue ducts and desulfurization systems by virtue of their multi-directional compensation and corrosion resistance advantages. Correctly identifying the characteristics and applicable boundary of these three types of expansion joints, and scientifically selecting them according to the temperature, pressure, displacement and medium conditions of power plants can not only ensure the safe and stable operation of pipeline system, but also effectively prolong the life of equipment and reduce the frequency of maintenance. It is hoped that this paper can provide a practical and reliable reference for engineers engaged in the design, operation and maintenance of power plants.