The thicker the thicker the better
When I get the expansion joint selection table, the first sentence is "Thicken the wall thickness for me". This intuition is correct when selecting steel plates, but if it is rested on the bellows, it may directly eliminate the expansion joint.
Why can bellows compensate for displacement? It depends on the flexible deformation of the wave structure. As the wall thickness increases, the stiffness immediately catches up, and the same amount of displacement needs to be pulled with more force. You think about it, if a tin spring is made thicker and harder to press, how can it absorb thermal expansion? To put it bluntly, the bellows is not a pressure-bearing vessel, it is a flexible component, and the harder it is, the less it works.
Before designing wall thickness, find out these three numbers: pressure, temperature, and displacement
The underlying logic of wall thickness design is actually to find a balance between three parameters.
Pressure is the force that pushes the wall thickness up. The higher the internal pressure, the greater the circumferential stress bearing by the wave shell, and if the wall thickness is not enough, it may bulge or even crack. Temperature determines the reduction coefficient of the allowable stress of the material-it is also 316L stainless steel, and the stress that can be carried at 600℃ is a bit lower than that at normal temperature, so the wall thickness in high temperature conditions has to be increased.
Displacement is the force that pulls the wall thickness downward. The greater the compensation amount, the softer the wave shell will be, and the thicker the wall thickness, the displacement absorbed by a single wave will shrink. The design formula in GB/T 12777 "General Technical Conditions for Expansion Joints of Metal Bellows", to put it bluntly, is: first set the lower limit of wall thickness according to pressure and temperature, and then check the displacement and fatigue life. If it doesn't work, adjust the wave number and layer number instead of the wall thickness of a bar pile.
Two days ago, I met a customer who was doing a cement plant project, and the opening required an 8mm thick bellows. I asked him three numbers: How much pressure? What's the temperature? What is the amount of compensation? As a result, the pressure was only 0.15MPa, the temperature was 200℃, and the compensation amount was required to be ±50mm-in this working condition, the 8mm thick wall was made, and the bellows couldn't be pulled at all, so it was only for myself to dig a pit.
How to calculate the account of wall thickness, life and stiffness
Take a bellows with the same nominal diameter as a comparison. If the wall thickness is increased from 1.2mm to 2.0mm, the stiffness may not increase by 40%, but it may double. When the stiffness is large, the displacement absorbed by each wave energy becomes smaller, and the wave number can only be increased to achieve the same compensation amount. As a result, the overall length of the expansion joint is stretched longer and longer, and the pipeline layout suffers.
Under the cyclic displacement, the stress concentration at the trough is more serious, and the fatigue life of thick-walled corrugated pipe is shorter than that of thin-walled multilayer structure. If you think about it, the thick wall looks strong, but every time it stretches and contracts, the stress is stuck at the point of the trough, bending repeatedly, and cracking sooner or later. This is why the large-diameter thick-walled expansion joint of this station is only used in the scenario where high-pressure large-diameter and single-layer wall thickness really can't bear the internal pressure-that is forced by working conditions, not to show off.
On the other hand, most of the general corrugated expansion joints adopt multi-layer thin-walled structures. With the same total thickness, two layers of 1.0mm are superimposed, and the stiffness is much smaller than that of a single layer of 2.0mm, and the fatigue life is longer. This account must be calculated clearly when selecting the model.
Different industries, different media, how big is the difference in wall thickness selection
The difference in actual working conditions is much more complicated than simply looking at pressure gauges and thermometers.
The typical characteristics of flue gas pipeline in power station are low pressure, large diameter and fast temperature fluctuation. This kind of working condition does not need a thick wall thickness, but the key is the stability of corrugation-if there is a real problem, it can be solved by reinforcing the ring or increasing the number of layers. This idea is basically the idea of corrugated expansion joints used in power station industry.
The high-temperature air duct in cement industry has high temperature and large dust content, so abrasion is the main contradiction. At this time, the wall thickness should take into account both temperature resistance and wear margin, but the material selection is often more critical than the wall thickness-it is better to change the material from 304 to heat-resistant alloy as well as doubling the wall thickness.
Desulfurization systems are the other extreme. The medium has acidic corrosive components. In this case, no matter how thick the wall thickness is, it can't bear the corrosion. What really works is the material grade and anti-corrosion measures. For example, the expansion joint in the desulfurization flue gas baffle door system focuses on the selection of corrosion-resistant alloy and lining protection, not on the wall thickness figure.
Incidentally, the deflector also has an indirect effect on the wall thickness. If the flow rate of the medium in the pipeline is high and the particles are washed, the high-speed medium can be separated from the bellows by adding a guide tube. At this time, the wall thickness can be appropriately thinned. Many customers ignore the value of the guide tube, and it will thicken when it is washed, resulting in stiffness and fatigue being dragged down.
These are the four things to really keep an eye on when choosing a model
Stop worrying about "thick or not" and focus on these four numbers-pressure grade, compensation amount, fatigue life and corrosion margin.
- Pressure Class:Determine the lower limit of wall thickness, which is not bad, directly related to safety.
- Compensation amount:Decide how soft the bellows should be, and the wall thickness is too thick equals locking yourself.
- Fatigue life:How many times does the pipeline cycle start and stop? According to the actual working conditions, don't copy the general parameters of the design institute.
- Corrosion margin:The corrosiveness of the medium determines how the wall thickness and the material are matched. Sometimes changing the material is a hundred times more useful than adding the wall thickness pipe.
When proposing requirements, write these four items clearly, for example: "DN600, design pressure 0.25MPa, temperature 350℃, axial compensation amount 80mm, fatigue life 1000 times, medium containing SO₂." Given this set of parameters, any serious manufacturer can calculate a reasonable wall thickness and number of layers.
The wall thickness and the number of layers are designed with each other. The same total thickness, 2.0mm single layer and 1.0mm +1.0mm two layers, the performance is quite different. Multi-layer structure has less stiffness and better flexibility under the same pressure, which is also the reason why multi-layer structure is commonly used in general-purpose corrugated expansion joint. Unless the medium is extremely corrosive or particularly worn, preference is given to multi-layer schemes. Don't let the word "thick wall" bias your judgment.
The selection of expansion joint is a compromised arithmetic problem, and wall thickness is only one of the variables. It is much more reliable to arrange the numbers of pressure, temperature, displacement, life and medium, and let the formula and working conditions speak for themselves than to listen to people blowing "our wall thickness is solid".