Let's tell you what it is
When many people hear about non-metallic expansion joints, what comes out of their minds is "a piece of cloth". Not really. You will understand when you take it apart-skin, insulation cotton, stainless steel wire mesh, flange, guide tube, one layer at a time. The skin is not an ordinary cloth, but a glass fiber cloth coated with silicone, fluororubber or polytetrafluoroethylene, which relies on it for temperature resistance and corrosion resistance. Insulation cotton is responsible for heat insulation, stainless steel wire mesh gives the overall structure tensile strength, flange is responsible for connection, and the flow direction of the medium of the guide bobbin. This set of multi-layer flexible structure determines its biggest feature: it can absorb multi-dimensional displacement. What about metal bellows? It mainly relies on ripple deformation to carry it hard, and it is too much to bear as soon as the direction is too much. Non-metallic expansion joints are different. The skin is flexible and can be eaten in lateral, angular and combined displacements.
Compare it head-on with a metal expansion joint
On axial pressure, non-metallic expansion joints really can't beat metal bellows. This one is not an argument. But it wins in three places: First, it has a much greater ability to compensate for lateral and angular displacements, especially for large-sized sections such as rectangular flues, which metal bellows can't do at all-you let a metal bellows compensate for a rectangular flue of 1m ×2m? Cost, weight, and installation difficulty are all disasters. Second, the elastic reaction force is small, and the thrust on the pipe support can be an order of magnitude lower. To put it bluntly, metal expansion joints hard top, non-metal expansion joints soft let, brackets and foundations save a lot of things. Third, the skin itself is the insulation layer, so there is no need to pack extra insulation cotton. Don't forget, it can also withstand acid-alkali corrosion and wet smoke, which is the most headache for metal expansion joints-as soon as chloride ions come up, stainless steel will pitting and perforating you.
Which working conditions must it be?
The metal corrugated expansion joint of cement industry is used in high-temperature dust environment without fault; However, when it comes to low temperature and high humidity corrosion occasions such as desulfurization flue gas baffle door and power station flue, non-metallic expansion joint is almost the only choice. Why? The flue gas from the desulfurization tower is not high in temperature, but the acidic water vapor is particularly heavy. The metal bellows can't last in this environment for several years. The non-metallic expansion joint is watertight, resistant to chloride ion corrosion, and the rectangular cross section can be made into non-metallic. Power plant flue, dust collector inlet and outlet, fan connection section, you go and look for it, how many of them are made of metal bellows? It's basically a non-metal world.
I have to dig through the pits of type selection and installation for you
When selecting, skin layering must be asked clearly: How many layers? What material per layer? Don't just look at total thickness. Some manufacturers make thicker skin, and the actual cheap fiber cotton sandwiched in the middle is powdered when baked at high temperature. As for installation, the protective tie rod cannot be used as a permanent support-that thing protects the skin when transported and hoisted. After the pipe is in place and the flange is tightened, the screw for transport protection must be removed. And guess what? I've seen people forget to dismantle it, and the compensator can't do any displacement at all. They pull hard and finally tear the skin. There is also the direction of the guide tube, which must be opposed to the direction of the medium flow. Install backwards? The skin was directly washed away by the airflow, and its life was shortened by more than half.
Finally, maintenance and longevity
The most common problem of non-metallic expansion joints is the aging and cracking of the skin. Especially in the areas where ultraviolet light is directly exposed and acid splashes, the surface cracks first, then leaks, and finally tears. Judge whether to change it or not, without any advanced instruments: see if there are cracks and leakages on the surface, and feel if there is any local hardening-hardening indicates that the coating is aging. Usually, take a flashlight to shine on the flange connection to see if there are any abnormal deformation or leakage traces, which is more useful than measuring any pressure. The cost of replacing the skin is much lower than replacing the entire metal structure. A large-diameter rectangular non-metallic expansion joint, changing the skin once may only be 20% ~30% of the price of the whole set of equipment. This is also its biggest advantage in engineering: it can be repaired when it is broken and can afford to be repaired.
So if someone tells you that the non-metallic expansion joint is a piece of cloth, ask him: Can your cloth withstand the acid corrosion of wet smoke? Can compensate for three-dimensional displacement? Can be made into a rectangular flue interface with a height of 2 meters? Obviously not.