What is stiffness? Don't equate it with "hard."
"Is the softer the expansion joint, the better?” Alas, I have encountered this problem dozens of times. To put it bluntly, it's easy to confuse "stiffness" with "hardness". Stiffness does not refer to whether the material is hard or not, but the ability of the bellows to resist deformation. Think about it, there is high-temperature and high-pressure steam running in the pipe. If the bellows is as soft as noodles, it will be squashed as soon as the pressure comes up, how can it compensate for the displacement?
From the data I have on hand, like the one in our stationExternal pressure single axial expansion jointAndThe high-temperature axial expansion joint has a large number of corrugated layers and sufficient wall thickness, so its stiffness naturally goes up. These models are specially designed to carry high pressure and large displacement conditions. The stiffness is actually an advantage-it allows the expansion joint to maintain a stable shape when subjected to internal pressure, without instability or bulging. So don't just hear "high stiffness" and think it is a disadvantage. It is protecting your plumbing system.
Behind the high stiffness, the corrugated structure and materials are supporting the waist
For the same bellows, the stiffness difference between single-layer wave and multi-layer wave can be several times. You think you can do it with a thin-walled piece? Not realistic. The key is to look at the material: the elastic modulus of stainless steel 304 and 316L is different, and 316L is slightly higher, which has better stability at high temperatures. And guess what? Some working conditions have to be usedLarge-diameter thick-walled expansion joint, the wall thickness is directly piled up to more than 6mm, and the stiffness is full, but the compensation amount is still sufficient-the key to this matter depends on the matching of wave height and wave pitch.
One of our clients has chosenStraight pipe pressure balance type expansion joint, pipe diameter DN800, pressure 2.5MPa, temperature 450℃. The number of corrugated layers is 6, the wall thickness is 4.5mm, and the stiffness value is more than twice that of the ordinary model. Run for three years after installation, no problem at all. Why? Because the wave height/wave pitch ratio was calculated in the design, the compensation amount was not discounted, but the reaction force was a little larger. Therefore, stiffness and compensation amount are not opposite, but can be both achieved through structural design.
Which scenes must be "rigid"?
Steam pipes in power plants, hot air pipes in the cement industry, desulfurization flues-these are places with high pressures and large temperature fluctuations. Expansion joints that are not stiff enough are squashed or cracked by fatigue as soon as they are installed. Two days ago, a customer who was doing a power station project choseStraight tube pressure balance type expansion joint, it is because its overall stiffness can balance the internal pressure thrust, without adding additional fixing bracket. Think about it, how many man hours will you save by welding a few fewer brackets?
Another exampleIn the pipeline near the desulfurization flue gas baffle door, the flue gas contains corrosive medium, and the temperature changes back and forth from normal temperature to 150℃. At this time, if the rigidity of the bellows is too low, it will deflate when the vacuum is pumped. We made it for a steel millCompound hinge transverse expansion joint, after the stiffness has been checked, can not only absorb transverse displacement, but also resist negative pressure, and has not been changed for five years.
Choosing a model with high stiffness will make the pipeline "hard carry" displacement?
It won't be. Large stiffness does not mean no compensation, but the reaction force during compensation is larger. The thrust of the bracket should be calculated when designing, such asCompound hinge transverse expansion joint andThe pressure balance expansion joint of curved pipe shares the force generated by stiffness through the tie rod or hinge, which not only keeps the compensation amount, but also makes the pipeline bear uniform force.
How to understand it specifically? Assuming that the stiffness of an expansion joint is 1000N/mm and the displacement is 10mm, the reaction force it exerts on the pipe is 10000N. You have to let the brace bear this force. But structures with tie rods, such asCurved tube pressure balance type expansion joint, the tie rod directly bears the internal pressure thrust, and the bellows is only responsible for compensating the displacement, and the reaction force is much smaller. Therefore, the model with high stiffness, with the correct structural design, will not make the pipeline hard to carry at all.
How to judge whether the stiffness of an expansion joint is enough?
Look at the stiffness value (N/mm) in the product specification table, and then compare the allowable thrust and displacement of the pipeline system. In our stationFAQThere is a special article inStiffness and Calculation Formula of BellowsThen you will have the confidence in your heart. Don't choose according to your feeling, otherwise it will be fatigued and cracked after two months of installation-that has something to do with too little stiffness.
You get an expansion joint and ask the supplier for the stiffness curve first. Such asUniversal corrugated expansion joints, stiffness values are usually marked on nameplates. If it is not found, it can be estimated using the formula: stiffness = (modulus of elasticity × wall thickness³) / (wave height² × wave pitch × correction factor). To put it bluntly, stiffness is directly proportional to the cube of the wall thickness and inversely proportional to the square of the wave height. Therefore, if you want to improve the stiffness, you should give priority to increasing the wall thickness instead of decreasing the wave height.
Recently, a customer working on a cement plant project choseCement Industry Metal Corrugated Expansion Joint, I suggest that he look at it at the same timeCase data for large diameter thick wall expansion joints. In the final selection, the stiffness matched the pipeline thrust, and now it has been running for half a year, and the thermal displacement is completely normal. That's the benefit of data talking.
To summarize in one sentence
The high stiffness of metal expansion joint is not a bad thing, but it is just needed for high pressure, high temperature and large displacement conditions. When selecting a model, focus on the stiffness value, calculate the reaction force, and match the appropriate structure, so you can have safety and compensation ability at the same time. Don't be led off by the concept of "soft"-soft ones are not necessarily good, but rigid ones are stable.