What exactly is ripple height? Let's break down this concept first
The vertical distance from the crest to the trough is the ripple height. Together with wave pitch and wave thickness, it constitutes the three basic geometric parameters of bellows. Don't underestimate this size – it directly determines how much axial displacement the bellows can absorb and is the starting point for calculating stiffness and fatigue life. Our general-purpose corrugated expansion joint and high-temperature axial expansion joint, the first thing when designing is to set the corrugation height. But the doorway behind this number is far more complicated than you think.
The effect of corrugation height on performance is not as simple as you think
The greater the height, the greater the amount of single-wave compensation — that's intuitive. But what about the cost? The stiffness decreases, and the pressure bearing capacity also weakens. On the other hand, the corrugation height is too small, the compensation ability is insufficient, the stiffness is large, and the pipeline stress is easy to concentrate. Two days ago, I met a customer who asked, "Can you make the corrugation height bigger for more compensation?" I told him directly: Be careful, the bigger the corrugation height, the better. If it is too large, the meridional stress will exceed the standard, and the fatigue life will fall by a cliff. Supported by data: the same 304 stainless steel material, the corrugation height is increased from 50mm to 80mm, and the single wave compensation can be increased by 30%, but the fatigue life may be reduced from 100,000 times to less than 20,000 times. It's not worth it.
How to choose the corrugation height under different working conditions? Discuss by situation
Let's talk about the high temperature working condition first. For example, the corrugated expansion joint used in power station industry, the yield strength of the material at 600℃ is only 30%-40% of that at normal temperature, and the corrugated height should be appropriately reduced to prevent plastic instability. In the design specification of our high temperature axial expansion joint, the corrugation height is usually 15%-20% lower than that of the normal temperature type. Let's talk about high pressure conditions. The medium pressure faced by large-diameter thick-walled expansion joints is more than 2.5MPa, and the corrugation height is limited by wall thickness and pressure. Deadpulling will only lead to trough cracking. At this time, it is more secure to design with wavelet height and multiple wavenumbers-for example, the wave height is 30mm, but it is made into six waves, and each wave shares a small displacement and a small strain. Finally, talk about the super-large displacement requirement. The design displacement of double hinge expansion joint of air-cooled island vacuum pipeline may reach 300mm. What should I do? Instead of simply pulling up the corrugation, multi-layer bellows are used. Two layers, three layers or even four layers, by increasing the number of layers to improve the compensation amount, and at the same time, the wave height of each layer is controlled in a reasonable range to avoid stress concentration.
The three easiest pits to step on in design
The first pit: only look at the compensation amount and ignore stiffness matching.The corrugation height was chosen to be large, and the compensation amount went up, but the stiffness dropped sharply, and the pipe bracket couldn't withstand the thrust-the bracket was deformed, the pipe was offset, and finally the side of the corrugated pipe was flattened. We have seen at least five such accidents at the scene, all of which are only displacement but not stiffness when selecting.
The second pit: Ignore the influence of wave height on the guide tube.If the wave height is too large, the gap between the guide tube and the bellows becomes smaller. When the medium flows at high speed, the flow velocity at the small gap increases suddenly, and the erosion effect is aggravated. The metal corrugated expansion joint in the cement industry often deals with dusty hot air, and the dust particles are ground like sandpaper in high-speed airflow, which can wear the guide tube through in half a year. In our scheme, the corrugation height will be determined together with the wear-resistant guide tube to ensure that the gap is greater than 3 times the maximum diameter of the medium particles.
The third pit: copying the standard atlas without verification.For the same corrugation height, the fatigue life differs several times under different materials. 304 may meet 100,000 times at room temperature, 150,000 times when changed to 316L, but it may only be 30,000 times when changed to superalloy Inconel 625 at 600℃. External pressure single type axial expansion joint especially emphasizes the matching relationship between wave height and pressure, because the bellows is more prone to instability under external pressure. The corrugation height on the standard atlas is based on the standard working conditions. If you use it directly on high-pressure steam pipelines, it will be weird if nothing happens.
Looking back at our product line, the corrugation height actually hides a lot of design details
For example, the wave height of metal corrugated expansion joint in cement industry is usually 40-60mm, with thickened guide tube and wear-resistant bushing. The high temperature axial type expansion joint adopts wavelet height multi-wave structure with wave height of 30-45mm, and the number of corrugated layers is calculated according to pressure. The corrugation height of each model is not determined by patting the head, and behind it is a lot of fatigue tests and stress analysis. A while ago, a customer chose the double hinge expansion joint of air-cooled island vacuum pipe, and when he came up, he said, "Give me the model with the maximum compensation amount". After careful calculation, the pipeline pressure is 0.1MPa vacuum, the temperature is 80℃, and the displacement is 250mm. Finally, we recommend the scheme of double-layer corrugated pipe with wave height of 50mm. If he uses a single layer of big wave height according to his requirements, it will leak in half a year.