Bellows can absorb thermal expansion and contraction, but there is a premise. It does not rely on the material deformation to carry hard, but relies on the corrugated structure to transform the axial displacement, transverse displacement and angular displacement of the pipeline into its own elastic deformation. Note that the bellows mentioned here is not the kind of gas hose, but the corrugated expansion joint commonly used in industrial pipelines, also called corrugated compensator. You asked, "Can the bellows play the role of thermal expansion and contraction?" When it comes time to select the type, the expression of this sentence has to be changed. The accurate statement is that "the bellows can compensate for the displacement caused by thermal expansion and contraction". One word difference, the logic is completely different.
Why can bellows "telescope"? The key is the corrugated design
A corrugation is equivalent to a spring, and multiple corrugations are connected in series, and the overall stiffness is reduced. When the pipe expands under heat, the corrugations are compressed; When the pipe cools and shrinks, the corrugations are stretched. The essence of this process is elastic deformation, so repeated displacement does not destroy the corrugation itself-provided the amount of displacement is controlled within the design range. There is a key parameter here called bellows stiffness. The lower the stiffness, the smaller the reaction force generated when compensating for displacement, and the smaller the thrust on the pipe support. But this does not mean that the lower the stiffness, the better. Too low the stiffness will cause the bellows to become unstable, so it is done when designing.
It is not enough to have ripples alone, and the structure should be selected according to the working conditions
Similarly, the bellows used in different pipes vary greatly. For high-temperature steam pipes and flue gas pipes, the selection is completely different. For example, when corrugated expansion joints are used in power station industry, the basic problem of "whether they can absorb displacement" is not considered, but the comprehensive performance under high temperature, high pressure and large displacement; The metal corrugated expansion joint in cement industry should be wear-resistant and corrosion-resistant, and other materials can't hold it in the dust environment. If the medium is strongly corrosive, it must be lined with PTFE hose or PTFE compensator. Ordinary stainless steel bellows may not last a maintenance cycle under this working condition.
When selecting the model, we can't only look at "can absorb displacement", but also look at fatigue life
The bellows absorbs thermal expansion and contraction repeatedly, and each expansion and contraction produces stress concentration at the trough. The design must be calculated according to the actual number of cycles, and it is not just a general-purpose corrugated expansion joint that can be used. For example, directly buried (fully buried) expansion joint and external pressure single axial expansion joint are optimized structures for specific working conditions, and their fatigue life is not the same order of magnitude as that of ordinary type. The bellows of directly buried pipelines are buried in the soil and cannot be repaired, so the structure must be reliable; External pressure type corrugated pipe bears external pressure, has better stability, and is suitable for high-pressure pipeline. If you choose the wrong structure, the displacement will be absorbed, and the life may only be one tenth of the design value.
Another category is easily overlooked: non-metallic expansion joints
Many people think that only metal bellows can compensate for thermal displacement. In fact, fabric fiber expansion joints and rubber compensators are more flexible in low-pressure and large displacement scenarios. In particular, the rectangular non-metallic expansion joint, which is used on the flue gas pipeline, can not only absorb thermal expansion and contraction, but also isolate vibration and noise, and be corrosion resistant. Why? Because the elastic modulus of non-metallic materials is low, the reaction force generated under the same displacement is small, and it can be made into a rectangular cross section to match the shape of the flue. So back to the question at the beginning, "Can bellows do the role of thermal expansion and contraction?" — the phrase itself implies a presupposition, as if only bellows can do the job. When it comes to the selection stage, you still have to know that in addition to bellows, there are non-corrugated structures such as sleeve pipe expansion joints and rotary compensators that can also compensate for displacement, but the applicable scenarios are different.
Final reminder: Installation and constraints determine whether bellows can function properly
Tie rods, hinges, duplex structures, these are not decorations. For example, the transverse expansion joint of the compound hinge can only absorb the transverse displacement. If you take it hard to carry the axial displacement, the bellows will be scrapped soon. For example, the expansion joint of the large tie rod, whether the screw should be disassembled during installation, and how to adjust the nut, all of which directly affect the compensation effect. Choose the right type, install it in the wrong position, and still have problems. Alas, two days ago, I met a customer who used the double hinge transverse expansion joint as a general-purpose type. As a result, the bellows bulged directly when the pipeline was tested for pressure. Therefore, if the working condition analysis is not in place, it will be useless to change any compensator.
Can bellows play the role of thermal expansion and contraction? The answer is simple: yes, but it also depends on how you choose and install it. Understand this layer, and you won't be led by the parameter table when selecting a model.