What is it about thickness tubes: triangulation between stiffness, fatigue life, and compensation
Many people think that the thickness of the bellows is a pressure-bearing parameter, and knocking on the steel plate feels thick and solid. That's really not what happened. The thickness determines the stiffness and fatigue life of the expansion joint at the same time. These three are twisted together, like a love triangle.
As the thickness increases, the pressure-bearing capacity does go up, but the stiffness also increases, and the compensation amount decreases instead. Think about it, for the same wave, if the plate is too thick to break, the ability to absorb displacement will naturally become worse. Even more troublesome is that the effect of thickness on fatigue life is not linear — under certain pressure cycles, thin-walled bellows have a longer life instead, because it can spread the stress through greater elastic deformation. I met a customer two days ago, and he asked for a 6mm thick bellows, saying that he was afraid of explosion. I asked him about the working conditions, the pressure was 0.4MPa, the temperature was 200℃, and the displacement was quite large. In this working condition, 3mm double layer is completely enough. If you don't go to 6mm single layer, the stiffness will more than double. The bracket can't bear the stress, and the compensation amount will shrink. What's the picture?
So thickness is not a simple arithmetic problem, but an optimization problem. Don't choose expansion joints with the thinking of "thick is good". That's to buy bricks, not elastic elements.
Pressure, temperature, medium: How three variables force thickness to give in
Look at the pressure first. The higher the internal pressure, the greater the circumferential stress bearing by the wave shell, and it is easy to bulge or even crack if the thickness is not enough. This is the most intuitive. But just looking at the pressure is not enough. As soon as the temperature rises, the allowable stress of the material falls. At 600℃, the strength of 304 is less than half that of normal temperature. At this time, it should be thickened or the materials should be changed. If you are cheap and insist on using 304 without thickening it, that's equivalent to joking about the life of the pipe.
The medium cannot be ignored either. Corrosive media will directly reduce the wall thickness, so a corrosion margin should be allowed in the design. In some working conditions, the medium is still worn, such as the air duct in the cement industry. Dust erosion is more lethal than pressure. At this time, it is better to install a guide tube or wear-resistant lining simply thickening. This is how the metal corrugated expansion joint in the cement industry of this station is treated. Once the guide tube is installed, the particle dust is directly grinded to the lining, and the bellows body is clean. You see, thickness is not a panacea, and sometimes it's better to think differently than stacking materials.
Single layer, multi layer, and wave number: thickness is not the only card
Many customers come up and ask, "What is the wall thickness of your expansion joint?" In fact, what they should ask is "several layers and waves". Multilayer bellows have better flexibility and higher fatigue life at the same total thickness because each layer of sheet shares the stress more evenly. For example, high-temperature axial expansion joints often adopt multi-layer structures to take into account both pressure resistance and flexibility. You have to carry it hard with a thick plate, and all the stress is concentrated on the outer surface, and cracks will appear from there sooner or later.
An increase in the wave number also reduces the single-wave stress, but takes up the axial length. Some pipes are tight in space and can't hold so many waves, so you have to rely on the number of layers to make up. Therefore, whether to choose single layer thick wall or multi-layer thin wall in design must be comprehensively judged according to displacement, pressure and space limitation. The large-diameter thick-walled expansion joint of this station is suitable for high-pressure and large-diameter pipes, but thick-walled pipes should not be used in all occasions. If you are a low-pressure and large-displacement pipeline, using thick walls is just asking for trouble, and the stiffness is so great that even the bracket trembles.
How to set the thickness of the standard: not patting the head, it is calculated
The commonly used design basis in the industry is GB/T 12777 "General Technical Specifications for Expansion Joints of Metal Bellows" and EJMA (American Association of Expansion Joint Manufacturers) standards. The thickness calculation should consider the design pressure, wave height, wave pitch, material yield strength, and check the column instability and plane instability. These are not patted on the head by experience, each term has formulas and coefficients.
In actual selection, the manufacturer will give a parameter table including thickness, number of layers and wave number. For example, the general corrugated expansion joint has a clear pressure-displacement correspondence. You don't know for yourself? The easiest way is to provide the working condition data and let the manufacturer calculate it with professional software. Don't estimate a thickness by yourself and let the quotation. That is not to save trouble, but to lay a mine. We often meet customers who say "I want 4mm" and ask him about the pressure and temperature displacement, but they can't answer it. We don't dare to accept this kind of order. If something happens, who will be on it?
Common misconceptions about choosing thick walls: thickness does not equal safety and may be counterproductive
Some people think that thick-walled expansion joints must be more durable, which is only half true. The increase of wall thickness will lead to greater stiffness, and the pipeline fixing bracket will have to bear greater reaction force, which may break the bracket. You save money on expansion joints, but you end up breaking several brackets, which is more expensive to fix.
In addition, the fatigue performance of thick-walled bellows is not necessarily good, especially under high cycle times, thin-walled multilayer design is often more reliable. It's the same as springs-have you ever seen springs wrapped in thick steel bars? Can that still bounce? There is another point that is easily overlooked: thick walls require higher welding, and inadequate post-welding heat treatment is easy to produce residual stress, which becomes the source of cracks instead. Some thick-walled bellows are cracked in welds, not because the material is not good, but because the welding is not done well.
Therefore, when selecting the model, don't just focus on the thickness. Report the pressure, temperature, displacement, cycle times and media to the manufacturer, so that professional people can do professional things. You have to specify a thickness, and the manufacturer will also do it, but if something goes wrong, the responsibility will be unclear. These days, it is easy to shake the pot, but difficult to rework.