Find out what flue expansion joints do first-not only to absorb displacement, but also to withstand high temperatures and corrosion
You may think that the expansion joint is what is used to "catch your breath" when the pipe expands and contracts thermally? That's half true. The flue expansion joint, especially in the flue gas pipelines of power plants, cement plants and steel plants, faces more than just displacement. The high-temperature smoke can move hundreds of degrees, and there are sulfide and dust in it. The corrosion is not polite at all. If you only choose according to the ordinary corrugated expansion joint, in less than three months, the skin will rot and the frame will be a mess. Therefore, before disassembling the flue expansion joint structure, you have to remember that it is a "three-defense warrior"-anti-displacement, anti-high temperature and anti-corrosion. Moreover, these three tasks are not each doing their own things, they are fighting. For example, you want to make the skin soft to absorb displacement, but it is too soft to bear the high temperature; You want to make the metal frame strong, but too thick will add thermal stress. How to balance? Look at how each part is divided below.
Core component 1: skin (non-metallic layer) -how to select materials and how to overlap so as not to leak
The skin is the outermost layer of "cloth" of the flue expansion joint, which is actually pressed together by multiple layers of composite materials. Don't think it is simple. If you choose the wrong one, it will directly leak, smoke or even explode. Our common non-metallic expansion joints (such as our site'sNon-metallic expansion joint (fabric fiber expansion joint)), the skin is generally divided into two layers: the outer layer is silicone-coated glass fiber cloth, which is responsible for temperature resistance and UV resistance; The inner layer is polytetrafluoroethylene (PTFE) film or fluororubber coating, which is responsible for anti-corrosion and anti-penetration. There will also be a layer of ceramic fiber blanket or glass fiber needle felt sandwiched in the middle as a heat insulation buffer. How to lap? Let me tell you a key point of operation: the skin must be seamed with a lap seam, not a butt seam. The overlap width should be at least 50mm, and the overlap direction should be along the direction of airflow-pressing downstream from upstream, so that the flue gas will not drill directly into the gap. Apply a circle on the overlap with high-temperature sealant, press tightly and then apply stainless steel press strips. And guess what? Many installation teams save trouble, directly apply tape to the seams, and as a result, they leak as soon as they start up. So, keep an eye on the installation step.
In material selection, temperature is the key. If the flue gas temperature is lower than 200 °, ordinary silicone glass fiber cloth will do; If it reaches above 350°, it is necessary to use fluororubber coating or pure PTFE membrane. We have them in our stationPTFE compensatorAndRubber PTFE compensator, for different media, you can refer to. There is no universal material, only the matching of working conditions.
Core component 2: Metal frame and guide tube-how do they work together to allow flue to pass through smoothly
The skin is soft, but there must be a skeleton to hold it up, or it will collapse when it sucks. This skeleton is a metal frame-generally a rectangular or circular structure welded with angle steel or channel steel. The material is commonly Q235B or 304 stainless steel, depending on the corrosion component in the flue gas. The width of the frame determines how much lateral and axial displacement the skin can absorb. Don't underestimate the bolt holes on the frame. When installing, the skin pressing strip must be locked with anti-loosening nuts, otherwise it will be loose after a few vibrations.
The deflector is more critical. Have you seen that "small tube" facing inward inside the expansion joint? That's the deflector. Its function is to guide the flue gas through smoothly, avoid high-speed airflow directly washing the inner wall of the skin, and reduce vortex and resistance at the same time. Note:The direction of the arrow of the expansion joint refers toWhat? Is the installation direction of the guide tube. The arrow is pointing in the direction of media flow. Don't install it backwards. What happens if you put it backwards? For example, two days ago, there was a cement factory customer, and the direction of the guide tube was reversed. As a result, the flue gas poured back into the gap between the skin and the guide tube, and the high temperature directly burned through the skin, which leaked in three days. So before installation, be sure to see the arrows stamped on the housing clearly.
The connection between the guide tube and the frame is usually welded with several supporting ribs on the inner wall of the frame, and then the guide tube is spot welded or bolted. If you are usingRectangular non-metallic expansion jointThe guide tube is welded in sections, and an expansion gap of 3-5mm should be left between each section, otherwise the top of the guide tube will be deformed by thermal expansion and contraction.
Core component three: insulation layer and sealing structure-don't underestimate these layers, their life depends on them
As mentioned earlier, the smoke temperature is high, and it is definitely not OK to directly expose the skin. Therefore, the inside of the skin and between the outside of the guide tube must be filled with an insulation layer. Aluminum silicate ceramic fiber blanket is commonly used, with a thickness of generally 50-100mm and a density of about 128kg/m³. When installing, roll the fiber blanket into a tubular shape, stuff it in, compact it, and then fix it with stainless steel wire mesh or steel belt. Then the question comes: the insulation layer directly contacts the smoke, will it collapse after a long time? Yes. Therefore, a layer of stainless steel wire mesh or fiberglass cloth should be wrapped outside the insulation layer, and it should be covered. In addition, between the insulation layer and the skin, it is best to add a layer of aluminum foil reflective layer to push the heat back to the top, which can reduce the surface temperature of the skin by 30-50 °.
What about the sealed structure? It mainly looks at two places: one is the seal between the skin and the frame, which is embedded in the striping groove with high-temperature-resistant sealing strip (silicone rubber or fluororubber); The other is the seal between the frame and the pipe flange, with graphite gasket or metal wrapped gasket, not with ordinary asbestos gasket, it will shatter as soon as it burns. From our stationHigh temperature axial expansion jointIn the sealing design, special emphasis is placed on multi-layer sealing rings to prevent smoke leakage.
Common pits in actual construction-what happens if the guide tube is installed in the opposite direction? Do you want to remove the tie rod nut?
Just now, I talked about the direction of the deflector, and then I talked about a pit: the tie rod nut. Many flue expansion joints come with tie rods to limit excessive stretching or compression of the expansion joints, but the nuts are locked when transported. After installation,How to adjust the tie rod nut of expansion joint？ Remember: After the pipeline is fixed, the nut must be loosened and the gap of 3-5mm is kept (some manufacturers require finer, according to the instructions), otherwise the expansion joint can't move, and it will all be held on the pipeline. A buddy asked me: "Do the screw of the expansion joint need to be removed?" Depends. If the tie rod is used for transport protection, it must be removed; If it is a permanent limit rod, loosen the nut but keep the screw. Consequences of not tearing it down? In a project last year, the nut was not loosened, and the thermal expansion of the pipe directly broke the baffle door next door after startup. Oh, yeah,Electric plug-in insulation doorAndFlue gas baffle doorIt is also commonly equipped with flue system. Pay attention to the fitting clearance when installing.
How to deduce the structural requirements-temperature, pressure, medium and three-step locking scheme according to the working conditions
When you get a working condition, don't come up and flip through the sample. Follow the following three steps:
Step 1: Look at the temperature.The flue gas temperature determines the skin material and the thickness of the insulation layer. Silica gel glass fiber +30mm insulation below 200°; 200-400° with fluororubber coating +50mm thermal insulation; Above 400 °, all PTFE film +80mm insulation should be used, and even a cooling air jacket should be added.
Step 2: Look at the pressure.The flue system pressure is usually not high (0.1-0.5MPa), but pay attention to the difference between negative pressure and positive pressure. Under negative pressure conditions, the skin is easily sucked flat, so the frame should be strengthened, and the guide tube should be supported against backward suction. Pressure exceeding 0.3 MPa, preferredMetal rectangular expansion jointOrUniversal corrugated expansion jointThe upper limit of the withstand voltage of the non-metal is generally 0.1-0.2 MPa.
Step 3: Look at the media.For wet flue gas containing sulfur, chlorine and fluorine, the skin must be PTFE or F46; If it contains more dust, the outer surface of the skin is coated with wear-resistant coating. If it is wet flue gas after desulfurization,Desulfurization flue gas baffle doorAndPTFE-lined hoseIs a common configuration because it is too corrosive.
The flue expansion joint construction is actually not complicated, but every detail can destroy the whole system. Don't be too troublesome. After getting the drawings, check the overlapping direction of the skin, the arrow of the guide tube, the gap between the tie rod and the nut, and the compaction degree of the insulation layer one by one. The finer you do it, the lower the repair rate.