First, find out what the metal expansion joint is for-the premise of performance description is to understand what problems it wants to solve
Two days ago, I met a customer and asked, "How many layers are the bellows? How thick is it?" I asked him back: Does your pipe run steam or smoke? Is axial displacement larger or lateral displacement larger? He was stunned for a moment and said, "Does it matter?" It mattered a lot. Metal expansion joint performance description this matter, if you don't even understand the problem it wants to solve, the rest is all busy.
The expansion joint is installed on the pipeline. To put it bluntly, it absorbs the displacement caused by thermal expansion and contraction, and reduces vibration and noise by the way. However, the displacement types of different pipelines are completely different: high-temperature steam pipelines are mainly axial expansion and contraction, flue gas pipelines may have lateral deflection, and large air ducts may also have angular displacement. If you take a general-purpose corrugated expansion joint to cope with high-temperature axial large displacement, isn't it difficult for it? So, before describing the performance, answer three questions: What is the medium? How hot is the temperature? Which direction is the displacement? The answer is different, and the selection logic is completely different.
2. Displacement, stiffness, temperature resistance, pressure resistance and fatigue life: five hard indexes for the performance description of metal expansion joints
After understanding the working conditions, look at the indicators. The performance description of metal expansion joint cannot avoid five parameters: displacement, stiffness, temperature resistance, pressure resistance and fatigue life. These five are not a parallel relationship, but a chain that bites into each other.
The amount of displacement is easy to understand, that is, how much expansion energy saving "extends out", but note that the larger the amount of displacement, the better. You ask for expansion energy saving to absorb 50mm displacement. As a result, the manufacturer gives you one that can absorb 80mm, and the stiffness is often low, which may cause instability. The stiffness directly determines the thrust of the expansion joint on the pipe support. The stiffness is too great for the pipe bracket to stand; Too small, the bellows is prone to instability. This balance depends on calculations.
Temperature and pressure resistance are a pair of combinations. When the temperature rises, the allowable stress of the material will be discounted; When the pressure rises, the wall thickness of the bellows has to be thickened, but once the wall thickness increases, the stiffness goes up again, and the displacement capacity decreases instead. Is that the truth? So you will see that the same expansion joint of DN300, the material and number of layers of bellows used under high temperature and high pressure conditions, are completely different from those at normal temperature and low pressure. As for fatigue life, it is not as simple as "can last for several years". It refers to the absence of leakage or rupture within a specified number of displacement cycles. For example, the design life is 1,000 times, and your actual working condition is to start and stop once a day, which is a matter of three years. If you look at one less of these five indicators, there will be problems sooner or later.
3. How big is the performance difference under different working conditions? Take high-temperature axial, general-purpose and pressure-balanced pendulums together
For example, the same DN500 steam pipe has a temperature of 450 °C, a pressure of 1.6 MPa, and an axial displacement of 40 mm. You use a general-purpose corrugated expansion joint, the wave is U-shaped, the material is 304, and the wall thickness is 1.5mm double layer-guess what? The lifespan may not even be 2,000 times. Replace with high-temperature axial expansion joint, replace the material with 316L or more temperature-resistant alloy, and change the waveform to multi-layer thin-walled structure. With the same displacement and pressure, the fatigue life can be doubled several times. That's the structural difference.
Another example is the large-diameter hot air duct, where the thrust generated by internal pressure is very large. At this time, when the ordinary expansion joint absorbs the displacement, the blind plate force will push the pipes at both ends open, ranging from the deformation of the bracket to the cracking of the weld. Pressure balance expansion joints (such as straight pipe pressure balance expansion joints and curved pipe pressure balance expansion joints) do this. They are equipped with balance bellows inside to eat the internal pressure thrust by themselves. It is not impossible to put it hard on the general-purpose type, but the cost of pipe fixing brackets may be more expensive than expansion joints. Under different working conditions, the angles of performance description are completely different: high-temperature axial type talks about temperature creep resistance, general-purpose type talks about cost performance, and pressure balance type talks about balance ability.
4. Material of guide tube, tie rod and bellows-how do these details affect the real performance of metal expansion joint
Many purchase orders only write "Expansion joint DN500 pressure 1.6MPa", and then ask the manufacturer to quote the lowest price. This is equivalent to buying tires without looking at the road surface. Can you use them as off-road tires on the sunny runway? Details determine performance, especially these.
The guide tube sounds like a "redundant" part, but its function is very great: first, it reduces the erosion and wear inside the bellows when the medium flows, and second, it reduces the flow resistance. The flue gas pipeline containing particles can be worn out in half a year without the guide tube. As for the tie rod, many people think that the tie rod is used for positioning. Wrong, the main function of the tie rod is to restrain the internal pressure thrust and prevent the expansion joint from excessively elongating under the action of pressure. You use it as a fixing bracket, then wait for the tie rod to break. The material of corrugated pipe is even more critical: 304 is suitable for general corrosive environment, 316L is resistant to chloride ions, and superalloy can carry above 800℃. The material has changed, and all the data of temperature resistance, pressure resistance and fatigue life have to be recalculated. Therefore, the performance description of metal expansion joint is not just four numbers, but all these "supporting roles" factors have to be counted in to truly describe its performance under this working condition.
V. The easiest pit to step on when selecting: Are metal expansion joints and corrugated expansion joints the same thing? Is the thicker the better?
"What is the difference between a metal expansion joint and a corrugated expansion joint?" I told him directly: a corrugated expansion joint is a type of metal expansion joint, but metal expansion joints are not all corrugated. There are also metal rectangular expansion joints and sleeve-type pipe expansion joints in this station, which are not bellows structures. As soon as you say you want "metal expansion joint", it defaults to bellows, and it is easy to buy the wrong type.
Another pit is "the thicker the better". These words hurt a lot of people. The bellows is thickened, and the pressure resistance does go up, but the stiffness soars, and the displacement compensation ability plummets. What's more troublesome is that the stress concentration of thick-walled bellows is more serious under alternating stress, and the fatigue life is shorter than that of thin-walled multilayer bellows. There was a chemical plant at the front end. Originally, it used 0.8mm three layers. They felt uncomfortable, so they replaced it with 2.0mm single layer. As a result, it cracked from the trough in less than a year. Alas, the selection depends on data matching, not material stacking.
So, don't just focus on the bellows. The performance description of metal expansion joint, in the final analysis, is a set of logic that strings together working conditions, structure, materials and calculations. Next time, ask clearly: temperature, pressure, medium, displacement direction and number of cycles. These five questions are much more useful than asking "how thick". If you want to know the specific structure in depth, you can go to the column of high-temperature axial expansion joint, general-purpose corrugated expansion joint and pressure balance expansion joint of this site. The parameters and materials are clearly written.