In the central heating system of northern cities, the densely underground heating pipe network is like the "blood vessels" of the city, which transports the hot water or steam from the heat source plant to thousands of households. However, these pipes elongate significantly at high temperatures – a 100-meter steel pipe rising from normal temperature to 120°C will elongate about 140mm. If left untreated, huge thermal stress will lead to pipe twisting, weld cracking, bracket damage and even pipe burst accident. The key equipment to solve this problem is the expansion joint of heating network pipeline. This paper will systematically analyze the multiple functions of pipeline expansion joints in heating network from core functions, system protection to type selection and application.
1. Absorption of heat displacement: the pipe network is no longer "stretching"
The most basic and core function of pipeline expansion joint in heating network is to absorb the thermal elongation of pipeline due to temperature change。 The temperature of the heating pipeline can reach 120-150℃ or even higher during operation, but it is in a normal temperature state during shutdown. This huge temperature difference leads to significant thermal expansion and contraction of the pipeline.
Its working principle is: when the temperature rises, the expansion joint is stretched or compressed like a spring, absorbing the linear elongation of the pipe; When the temperature drops, it returns to its original state。 This is like the elastic waist of pants, whether thermal expansion or cold contraction, can be adaptively adjusted to ensure that the whole pipe will not be deformed or damaged due to excessive force。
Compared with traditional compensation methods (such as π-shaped elbow), bellows expansion joint has obvious advantages such as small footprint, good sealing performance, less leakage and large compensation ability, so it has been widely used in modern thermal pipe network。
2. Reduce equipment thrust: protect sensitive equipment from stress
Another important role of pipe expansion joint in heat network is to reduce the thrust generated by pipe thermal expansion on equipment。 In heat source plants, heat exchange stations and other places, pipelines are directly connected with steam turbines, heat exchangers, circulating water pumps and other equipment. If the thrust generated by the thermal expansion of the pipe acts directly on the equipment interface, it may cause the equipment to be deformed, damaged or even shut down.
For the pump inlet and outlet pipeline, the installation of universal expansion joint can compensate the thermal expansion of the pipeline, reduce the thrust of the pipeline thermal expansion to the pump, and absorb the vibration generated by the pump operation。 The bending force balance corrugated expansion joint is used for the connection between equipment and equipment, which can prevent equipment from being affected by the blind plate force generated by internal pressure, improve the stress condition of equipment and simplify the fixing mode of equipment。
3. Absorb vibration and shock: make the system run "quietly"
The pipeline expansion joint of heating network also has the function of absorbing vibration and pressure shock in heating system。 The circulation pump and water supply pump in the heating pipe network will generate continuous mechanical vibration when they are operated. If the connection is hard, vibration can be transmitted along the pipe to the entire system, creating noise and accelerating pipe fatigue.
The metal corrugated structure in the corrugated expansion joint can effectively eliminate the vibration caused by fluid impact and avoid metal fatigue。 In addition, when the pump is started, stopped or the valve is quickly closed, water hammer impact will occur in the pipeline, and the expansion joint can absorb part of the impact energy to protect the safety of the pipe network.
4. Compensation for installation errors and foundation settlement: leaving room for construction
In the construction and installation of thermal pipelines, it is difficult to achieve absolute precision in the alignment and leveling of pipelines. The pipeline expansion joint of heating network has the characteristics of flexible connection, which can compensate the installation error of pipeline, accommodate the assembly deviation and reduce rework。 Even if the pipeline is not fully aligned during construction, the flexibility of the expansion joint can accommodate this part of the error, reducing the demanding requirements for construction accuracy。
In addition, the geological conditions along the heating pipeline are complex, and the natural subsidence of the foundation may cause the pipeline to displace. The expansion joint can absorb the displacement change caused by uneven settlement and prevent the damage of connecting pipelines and equipment。 In seismic zones or high-rise buildings, expansion joints can also absorb the displacement caused by structural deformation。
5. Balance pipe system stress: make the whole pipe network work "harmoniously"
The reasonable arrangement of the expansion joint of the heat network pipeline can balance the stress distribution of the whole pipe system. According to the pipeline layout, designers can use fixed brackets and secondary fixed brackets to reasonably divide the pipe network into typical pipe sections such as straight pipe section, L-shaped pipe section and Z-shaped pipe section, and select suitable expansion joint types in each pipe section。
Follow the proportion rule of "one fixation and one compensation", that is, an expansion joint is set between a fixed bracket, which can ensure the balance of the force of the pipe system。 The fixed bracket must bear the composite loads such as internal pressure thrust, friction force and elastic reaction force of the expansion joint, and the design of the expansion joint can effectively disperse these loads and avoid stress concentration.
6. Thinning the pipe wall to reduce the project cost: the invisible value of economy
Because the expansion joint effectively absorbs the thermal stress and vibration stress of the pipeline, the thinner wall thickness of the pipeline itself can be adopted in the design, thus reducing the overall cost of the project. Although the traditional method of natural compensation (π-shaped elbow) can also absorb thermal displacement, it needs to occupy a large space, increase the length of pipe and the number of elbows, and the material and construction cost are higher. The compact size of the bellows expansion joint provides sufficient compensation capacity in a limited space。
7. The role of different types of expansion joints
VIII. Standard basis and selection principle
The manufacture and inspection of the expansion joint of the heating network pipeline shall strictly comply with the standard of CJ/T 402-2012 Bellows Compensator for Urban Heating Pipeline. This standard is applicable to bellows compensators for urban heating pipelines with design pressure ≤2.5MPa and design temperature ≤350℃。
You should pay attention to when selecting the model:
- Calculate the expansion and contraction amount according to the pipe temperature rise and pipe length, select the expansion joint that can meet the compensation and leave the margin amount
- The complex pipe network is divided into straight pipe section, L-shaped pipe section, Z-shaped pipe section, and calculated separately
- The expansion joint should be set in a reasonable position, and the hinge type should be selected at the bend. Vertical pipe is better than horizontal pipe
- It is strictly prohibited to be installed in curves or places that are easily twisted. If it must be installed on horizontal pipes, guide slides must be installed at both ends
sum up
The function of heating network pipeline expansion joint can be summarized as six core functions:
| function | Core Value |
|---|---|
| Absorbed heat displacement | Compensate the thermal expansion and contraction of pipeline to prevent stress damage |
| Reduce equipment thrust | Protect the interface of sensitive equipment such as pumps and heat exchangers |
| Absorb vibration shock | Reduce vibration and noise to prevent water hammer damage |
| Compensate for installation errors | Accommodates assembly deviations and accommodates foundation settlement |
| Balancing pipe system stress | Reasonable distribution and balanced pipe network load |
| Reduce project cost | Replaces π-shaped elbows for space and material savings |
To give full play to the functions of pipeline expansion joints in heating network, it is necessary to select and calculate the type strictly according to CJ/T 402-2012 standard in the design stage, follow the specifications of "consistent flow direction marking, removal of transportation rod, and strictly prohibiting the use of expansion joints to adjust pipeline deviations" in the installation stage, and establish a regular inspection and maintenance system. A heating network pipe expansion joint with reasonable design and standardized installation is a "safety guard" to ensure the safe, stable and efficient operation of urban heating pipe network.