What does thickness affect? Pressure, life, safety, three things in one parameter tube
Anyone who does pipeline compensation knows that the parameter of metal expansion joint thickness is stuck at the front. If you choose it as an ordinary steel plate thickness, something will happen sooner or later. A bellows, every 0.2mm difference in thickness, the pressure bearing capacity may be more than 30% difference. But if you only focus on the pressure and ignore the fatigue life-for example, under high-temperature cycling conditions, thick plates are prone to stress concentration, and cracks run out from the trough, which is called a fast one.
Not to mention safety. Two years ago, there was a thermal station project, which used 0.8mm 304 material with a design pressure of 1.0MPa, and it leaked after less than 3,000 cycles. When I turned back and checked, I felt that 0.6mm was too thin when I selected the model, so I thickened it directly to 0.8mm-I thought it was safer, but as a result, the stiffness became larger, and the pipeline thrust crooked the fixed bracket. Thickness is not a simple addition or subtraction.
High temperature, corrosion, large diameter-how to determine the thickness under different working conditions
Let's start with the heat. When the temperature exceeds 400℃, the material yield strength drops very quickly. For example, the strength of 316L at 600℃ is only one third of that of room temperature. At this time, if you still choose the thickness according to normal temperature experience, the bellows may bulge in a heat cycle. Therefore, manufacturers usually suggest thickening 0.3~0.5mm for high-temperature axial expansion joints, and increasing the number of layers to reduce the stress of single layer.
What about the corrosive environment? Especially the wet flue gas behind the desulfurization flue gas baffle door, which contains chloride ions and has a pH value as low as 2. 304 can't hold it at all, so you have to use 316L or even 254SMO. But you thought it was done with high nickel? The corrosion rate is inversely proportional to the wall thickness, but the increase of the wall thickness will reduce the corrugation flexibility. At this time, the smarter way is to keep the proper wall thickness, increase the number of corrugated layers, and make an outer sheath at the same time-for example, PTFE is added to the inner layer of PTFE-lined hose in this station to isolate corrosive media.
The large caliber is more headache. For pipes above DN2000, the bellows diameter is large, and the film stress is greater under the same pressure. You calculate: Under the internal pressure of 1.0MPa, the circumferential stress of DN2000 pipeline is about four times that of DN500. Simply by thickening? That cost soared, and the difficulty of forming corrugations increased dramatically. At this time, the large-diameter thick-walled expansion joints in the station usually adopt multi-layer thin-walled structures, such as 3 layers ×0.6mm instead of a single layer of 1.8mm-which not only maintains pressure, but also maintains flexibility.
Thickness is not isolated: How to match the number of corrugated layers, wave height and material strength
Many people think that thickness is the design input, just adjust it. In fact, the thickness is tied with the number of corrugated layers, wave height and material strength. For example, the same design pressure of 1.6MPa, DN800 pipeline: using 304 material, single layer wall thickness of 1.2mm, wave height of 40mm, can be done. However, if you reduce the wave height to 30mm, the stress directly exceeds the limit under the same wall thickness. What to do? Either thicken to 1.5mm or add a layer. The general-purpose corrugated expansion joint of this station clearly indicates the recommended wall thickness range corresponding to different wave heights in the selection table.
Material strength is also critical. With the same wall thickness, SUS321 can carry 5000 more cycles at high temperatures than 304. However, the 321 welding process is complicated and costly. Some project drawings are cheap, and 304 is used instead of 321. As a result, the thickness remains unchanged, the temperature goes up, and the life is cut in half. You say it was wrong or not?
Thicker thickness is better? The people who said this probably didn't calculate the cost
When a customer comes up and says "Give me the thickest", I usually ask first: Have you calculated the pipeline thrust? Double the thickness and the rigidity of the bellows may increase three or four times. As the stiffness becomes larger, the thrust of the pipeline to the fixed bracket also becomes larger, and the cost of the bracket and the foundation cost all rise accordingly. Not to mention the material cost-304 stainless steel is more than 20,000 yuan per ton. You can casually thicken it by 0.2mm, calculate hundreds of expansion joints in the whole project, and spend hundreds of thousands more easily. What is this not waste?
I have seen a power plant project, the original design used 1.0mm thick 316L, and it ran for 5 years without any problems. Later, the person in charge of purchasing was changed, and it had to be changed to 1.5mm "safety first". As a result, the bellows was installed. Because the stiffness was too high, the sliding brackets of adjacent pipes were all displaced, and the maintenance cost cost 300,000 yuan. Tsk, the thicker the better? That depends on who pays the bill.
A real case: Metal corrugated expansion joint in a cement plant was scrapped in advance due to wrong thickness selection
Last year, a cement factory approached us and said that the metal corrugated expansion joint of the cement industry on their kiln tail pipeline had leaked after less than a year of use. When removed, there is a penetrating crack at the trough position of the bellows. Our analysis: they selected 304 material, wall thickness 0.8mm, single layer. What's the problem? The temperature of flue gas at the end of the kiln is measured at 480℃, but intergranular corrosion begins to occur when 304 is above 450℃. In addition, the flue gas contains SO₂ and dust, which can be washed and corroded, and 0.8mm can't bear it at all. Moreover, once the single-layer bellows is partially thinned, the whole pressure bearing capacity collapses.
Replace with 321 material, with a wall thickness of 1.2mm, divided into two layers: 0.6mm +0.6mm, and install a guide tube to reduce dust erosion. It has been two and a half years since it was installed, and there has been no problem. The wrong thickness is chosen, and the cost is that the whole production line is shut down for three days, resulting in a loss of millions. Do you say it's worth it?
You have a large diameter, thick-walled expansion joint in your station, but don't push it as soon as you get up
We do have this product of large diameter thick wall expansion joint, which is specialized in high pressure and large diameter working conditions. However, every time sales communicate with customers, I will remind you: first calculate the working condition parameters. If it is a low-pressure, normal temperature and small-diameter pipeline, the ordinary general-purpose corrugated expansion joint is completely enough, and the upper thick wall is a knife to kill chickens. The customer's budget is limited, and you push expensive products to him, which seems to have a good performance, but when he looks back, he finds that he can't use them, and his trust is gone.
Real professionalism is to help customers find that "just enough" thickness. For example, the wall thickness of the transverse expansion joint of the compound hinge in this station is often thinner than that of the axial type when the transverse displacement needs to be compensated, because the displacement depends on the hinge to share the stress. However, the wall thickness of the double-hinge expansion joint used for air-cooled island vacuum pipeline is a little thicker, because the external pressure instability under vacuum is the main risk.
The parameter of thickness is, in the final analysis, a systematic project. You isolate it, it gives you color to see. Next time you select a model, you might as well ask yourself one more question: Does this thickness really match my temperature, medium and number of cycles?