In recent years, composite metal expansion joints have appeared more and more in power stations, cement and chemical projects. However, many people regard it as an "enhanced version" of ordinary corrugated expansion joint, thinking that it is nothing more than wrapping an extra layer of iron sheet. This understanding deviation is quite terrible. If you choose the wrong choice, the life will be cut in half, or if the pipeline will crack and the medium will leak, the maintenance period will be delayed for several months.
1. What is the composite metal expansion joint? What is the difference between it and ordinary corrugated expansion joint
The bellows of ordinary corrugated expansion joints are usually formed integrally with a layer of stainless steel (e.g. 304, 316L) or more temperature-resistant INCONEL. The so-called "composite" means that the bellows is made of two or more layers of different metal materials bonded and laminated, and the layers may be metallurgically bonded or mechanically stacked. Common combinations include stainless steel + corrosion-resistant alloy (such as Hastelloy), carbon steel + stainless steel, superalloy + heat-resistant steel, etc.
Why bother doing multiple layers? Because a single material often misses one thing in complex working conditions. Take the flue gas pipeline of the power station as an example. The flue gas side should be resistant to SO₂ and NO corrosion, and the outside should be subjected to high-temperature oxidation and sometimes abrasion. It is difficult for a material to carry these at the same time. The composite structure is to let each layer perform its own duty: the inner layer blocks media corrosion, and the outer layer bears structural strength and environmental oxidation.
2. What problems do the composite structures solve: high temperature, corrosion and fatigue, and the three working conditions are in the right seat
Composite metal expansion joint is not a panacea, it has to solve three kinds of problems.
High temperatures.Ordinary 304 stainless steel bellows will obviously oxidize and creep above 600°C. Replace it with high-temperature alloys (such as GH3030 and INCONEL 625), and the cost is so high that it hurts. The composite structure can be lined with thinner superalloy, and the outer layer is pressurized with slightly thicker heat-resistant steel, which not only controls the cost but also ensures the life.
Corrosion.In the flue behind the desulfurization system and wet dust removal, the condensed acid can pit 316L out of the pit. At this time, the inner layer is made of corrosion-resistant alloy (such as C276, titanium plate), and the outer layer is made of duplex steel or stainless steel, so the corrosion resistance and pressure bearing capacity are taken into account.
Fatigue.The pipeline starts and stops and vibrates repeatedly, and the bellows is stressed every minute and every second. Multi-layer thin-walled structures have better flexibility than single-layer thick-walled structures-under the same displacement, the single-layer stress of multi-layer structures is lower, and the fatigue life is naturally longer. This is the same as a stack of paper bends better than a thick piece of cardboard.
3. Parameters that must be clarified before selection: displacement, pressure, temperature and medium. Don't place an order if one is missing
Two days ago, a customer walked to the sketch with the pipeline and came to ask for a quote. He asked him how much displacement he was, looking blank. This is not an exception. In the selection of composite metal expansion joint, four parameters are indispensable.
displacement amountIncluding axial, transverse and angular directions, it must be the calculated result of thermal expansion of the pipeline, not estimated. Thermal elongation at design temperature, installation temperature correction, and settlement difference of equipment foundation are all included.
pressureDecide the number of layers of bellows and the wall thickness of each layer. High pressure does not mean that there are more layers, but may be shared by thick-walled single-layer or multi-layer superposition. Stress calculations need to be done here, not patting your head to determine.
TemperatureIt depends not only on the medium temperature, but also on the actual working temperature of the bellows. The temperature difference between the bellows with and without the insulated guide tube is 200°C. The creep and fatigue curves of materials under high temperature working conditions are completely different from those at normal temperature.
MediumDetermines what material to choose for the inner layer. The corrosion mechanism of flue gas, steam, acid-alkali liquid and dust-containing gas flow is different. The material selection logic of the dry flue gas at the inlet of the desulfurization tower, the wet flue gas in the clean flue, and the chloride ion environment in the slurry pipeline are different.
These four parameters are like "four columns". Fill in one less, and the design scheme is crooked. Don't believe the words "almost usable". It is only a matter of time before the expansion excerpt is small for deadlifting, and the fatigue cracking of the bellows is only a matter of time.
4. Common composite forms and applicable scenarios: From power station smoke duct to cement industry, don't use the structure in the wrong place
The most common compound forms are those, and each one has its own suitable territory.
The corrugated expansion joints used in power station industry mostly take the route of "corrosion-resistant alloy lining + stainless steel outer layer", especially the clean flue after desulfurization system. The high-temperature section (such as the outlet of induced draft fan) uses the combination of "superalloy inner layer + heat-resistant steel outer layer", which can not only withstand the smoke temperature above 450°C, but also withstand the thermal shock at start and stop. One more thing here-most of the smoke ducts of power stations have rectangular cross-sections, so metal rectangular expansion joints are often made into composite structures, and the reason is the same.
Metal corrugated expansion joints in cement industry are more faced with abrasion problems. The dusty hot air at the head and tail of the kiln has high wind speed and hard particles, and the inner layer is easy to wear out. In composite structures, the inner layer is often made into a replaceable wear-resistant bushing, or a harder material is directly used as a sacrificial layer.
High temperature axial expansion joints are used on pipe sections with lateral displacement. No matter how strong the composite structure is, it can't change the displacement characteristics of the product itself. The high-temperature axial expansion joint only absorbs axial displacement, and forcibly applies lateral misalignment to it, so the risk of lateral instability of bellows is extremely high.
5. Details that are easy to overlook in installation and operation and maintenance: guide tube, tie rod, pre-deformation, these pits should be avoided
The product is fine, but there are many cases where the installation goes wrong.
Install the guide tube in reverse direction.The function of the guide tube of the expansion joint is to let the medium pass smoothly and avoid the high-speed fluid directly washing the bellows. The open end of the guide tube must be facing the direction of medium flow. If it is installed backwards, it will not only fail to protect the medium, but also lead the medium into the dead zone between the bellows and the guide tube, accelerating corrosion. Some round pipes are marked with arrows for the flow of media, and they don't even look at them when they are installed, purely laying mines for themselves.
The tie rod nut is locked and locked.The role of the tie rod is to constrain the bellows from absorbing displacements beyond the design range, while transmitting pipe thrust. Some people think "tightening is stronger" and screw the tie rod nut tight-wouldn't the expansion joint become a rigid connection? The forces of thermal expansion and contraction are all suppressed on the pipes and equipment flanges. Generally, the tie rod nut will be pre-adjusted to the design position before leaving the factory. After installation, it will be loosened to the specified gap according to the drawings. The bigger the better, the tighter the better.
Ignore pre-deformation.When the pipeline is installed at normal temperature and operated at high temperature, the expansion joint should be pre-stretched or pre-compressed according to the actual displacement direction. For example, the bellows is compressed after operation, and it should be stretched for a section first when installing. Without pre-deformation, the bellows may directly cross the design displacement limit during operation, and the service life is greatly reduced. Nowadays, many products have been pre-deformed before leaving the factory, and they can be installed directly according to the marks at the site-the problem is that many people don't pay attention to those marking lines at all.
Composite metal expansion joints are not something to buy back and screw on. The selection data is accurate, the structural form is selected correctly, and the installation details are in place. These three points are taken care of, and it is a reliable elastic element. One less, it is the weakest link in the pipeline system. Alas, I have been in this business for a long time, and I have seen too many accidents in which bellows are torn and tie rods fall off. Nine times out of ten, these links have made mistakes.