What exactly is the number of ripples?
Don't underestimate the "folds" on the expansion joint. Each wave is the basic unit that absorbs displacement. The number of corrugations directly determines the compensation ability, stiffness and fatigue life of the expansion joint. For example, the general-purpose corrugated expansion joint we often say, the number of corrugations can vary from several to dozens. But many people only focus on the quantity and ignore the mechanical logic behind it-it's like just looking at how big the bowl is, no matter how hot the rice is.
The more ripples, the greater the displacement that can "eat in", but at the cost of softening the stiffness and decreasing the ability to resist pressure. Under high-pressure conditions, more wavenumbers are prone to accidents.
What is the relationship between the number of corrugations and the compensation amount and stiffness?
Compensation amount = single wave compensation amount × wave number. But don't be too happy, the more ripples, the lower the axial stiffness and the weaker the ability to resist pressure thrust. It is like a spring. It is indeed soft with more turns, but it is easy to deform when pulled.
The power station industry uses corrugated expansion joints, and the pressure is always several MPa. At this time, it is necessary to reduce the wavenumber and increase the wall thickness, and it is preferable to have fewer compensation points than instability. On the other hand, in scenarios with low pressure and large displacement, such as smoke ducts, many wavenumbers are an advantage-it doesn't matter if the stiffness is low under low pressure, as long as the compensation amount is enough.
To put it bluntly, the wave number is not determined by slapping the forehead, but depends on three parameters: medium pressure, temperature and displacement. Missing one is blindness.
How to slap the number of plate corrugations under different working conditions?
Two days ago, I met a customer who made high-temperature steam pipelines. When I came up, I asked, "Can I give a 30-wave expansion joint?" I asked him about the medium pressure, temperature and displacement, but he couldn't answer it. This is not difficult for him, but the material strength of the high-temperature axial expansion joint decreases at high temperature, and it is easy to become unstable if the wave number is too large. We generally suggest 6~12 waves for high-temperature steam pipelines, with guide tube protection.
Looking at the metal corrugated expansion joint in the cement industry, the dust is large, the temperature difference is large, and the wave number design should take into account the fatigue life and the protection of the guide tube. Where the dust is seriously washed, the guide tube is easy to wear out if the wavenumber is small, and the stiffness is not enough if the wavenumber is too large, and it can't bear the pressure. At this time, the combination of multi-wave + thick wall is often adopted, and the material of the guide tube has to be considered.
High temperature axial expansion jointAndMetal Corrugated Expansion Joints in Cement IndustryThe design logic of wave number is completely different. The former looks at material creep, while the latter looks at fatigue life and wear resistance.
Common Pits: The More Number of Ripples, The Better?
Don't be led by experience. Much corrugation does compensate for a large amount, but the cost is: too low stiffness leads to pipeline instability, trough stress concentration accelerates fatigue, and the cost doubles. We have tested that a project used 4-wave expansion joints to replace the original design of 6-wave, but the fatigue life increased by 30%. Why? Because the single wave displacement is more reasonable, the deformation undertaken by each wave is within the elastic range, rather than approaching the limit.
In addition, the role of the guide tube of the expansion joint is more prominent when the wave number is small-it can effectively prevent the medium from washing the corrugated body. When the number of waves is more, the length of the guide tube has to be increased, and the cost goes up, but the effect is not necessarily good.
So stop being superstitious about the saying "more wave numbers are better". Choose more and choose less, depending on the specific working conditions.
Is there a quick way to estimate?
Wave number ≈ total compensation ÷ single wave allowable compensation. The allowable value of single wave can be checked in GB/T 12777 standard, but note that this is only a preliminary estimate, and the actual selection must consider the pressure, temperature and number of cycles.
The total compensation amount is 100mm, the allowable single wave is 25mm, and the theoretical wave number is 4. However, if the medium pressure is 2.5MPa, the temperature is 400℃, and the number of cycles requires 10,000 times, it has to be discounted-the allowable compensation amount of single wave should be reduced when the pressure is high, the material strength should be reduced when the temperature is high, and the fatigue life should be checked when the number of cycles is high. Finally, the actual wave number may become 5-6, and it has to be adjusted with the wall thickness.
The most reliable thing is to throw the working condition parameters to the manufacturer. For example, when we make straight pipe pressure balance expansion joint, the wave number, layer number and wall thickness should be calculated together, not determined by patting the head.
Differences in Wavenumber Design of Different Product Types
Different structural forms of expansion joints, wave number design logic is very different. You flip through the products of this site:
- External pressure single axial expansion jointThe wave number can be done a lot, because its external pressure structure can resist instability, and the corrugation is subjected to external pressure, so it is not easy to bulge.
- Compound hinge transverse expansion jointThe wave number is very small, and the angular displacement is absorbed by the hinge. The corrugation mainly plays the role of flexible connection.
- For the same absorption displacement, the wave number design logic of general-purpose corrugated expansion joint and vacuum special hose is completely different-the former looks at stiffness, while the latter looks at stability under vacuum. In the vacuum environment, the corrugation is easy to deflate, so the wave number can't be too much, and a support ring has to be added.
Therefore, don't just look at the parameter table when selecting a model, you must know the design principles behind it. One parameter corresponds to one scene, and there is no universal product.