The non-metal expansion joint, whose name sounds like a piece of cloth, is actually an assembly piece. Many people will subconsciously reach out to touch the real object when they see it for the first time-it is soft, indeed like a thick canvas. But if you really look at it as a piece of cloth, you would be very wrong. When disassembled, it contains at least a skin (loop band), a frame, a deflector, a thermal insulation layer and fasteners. Each part does something completely different, and no one can do without it.
Skin: Multi-layer composite, not one layer of fiber cloth
Let's start with the skin. This layer is not a random piece of fiber cloth, but a multi-layer composite structure. The common ones are silicone rubber, fluororubber, polytetrafluoroethylene, ceramic fiber cloth, etc. Each layer is responsible for one item: the outer layer is corrosion-resistant and aging-resistant, the middle layer is pressure-sealed, and the inner layer is high-temperature resistant. The specific layers to use depend on the medium temperature, corrosiveness, and whether it is round or rectangular-for example, rectangular non-metallic expansion joints, and the cutting and splicing methods of skin are different from those of round shapes. The four corners of the rectangle are the stress concentration points, and the skin has to be specially reinforced when splicing, so it can't be wrapped in a circle like a circle. Think about it, with the same material, the circular one bears uniform force, and the rectangular corner is easy to tear. Can the design be the same?
When choosing skin materials, don't just look at the highest tolerable temperature. If there is sulfide in the flue gas, the surface of silicone rubber will be hard and cracked; If there is fly ash, an abrasion-resistant PTFE coating has to be added to the outer layer. In order to save costs, some manufacturers replace the middle layer with ordinary polyester cloth, and as a result, it bulges and leaks in less than half a year. Every layer of skin had its usefulness. If one layer was missing, the entire lifespan would fall like a cliff.
Frames and Deflectors: Skeletons and Shields
Look at the frame and deflectors again. The frame is metal, which supports the skeleton of the whole expansion joint and withstands the internal pressure and thrust of the pipe. Without a frame, the skin is a soft cloth that bulges when the wind blows. The frame is usually bent and welded with steel plate, with flange holes on four sides, used to connect with the pipe flange. Don't underestimate this frame. It has to bear the blind plate force, especially for large-diameter pipelines. The thrust generated by internal pressure can reach tens of tons, and the frame will deform directly if its stiffness is not enough.
The guide plate is arranged in the direction of medium flow, on the one hand, the air flow is guided smoothly, and the vortex and wear are reduced; On the other hand, it blocks the high-temperature dust from directly washing the skin. Some working conditions will also leave a gap between the deflector and the skin to form a heat insulation buffer zone. The thickness and material of the deflector should also follow the medium. The deflector in the desulfurization flue gas baffle door generally has to be made of alloy steel, and ordinary carbon steel can't last long in wet acidic gas.
Thermal insulation: the invisible shield of the skin
Then say insulation. Many people ignore this layer, in fact, it determines how long the skin will last. The temperature of flue gas is often several hundred degrees, and the heat is directly transmitted to the skin, so silicone rubber can't bear it. Therefore, the inside of the frame and the back of the deflector are usually filled with heat insulation materials such as ceramic fiber blankets. In this way, the actual temperature felt by the skin is much lower than the medium temperature, and the life can be prolonged.
I have seen a customer who reduced the insulation layer from two layers to one layer in order to save hundreds of dollars. As a result, after three months of operation, the inner layer of the skin was coked. Later, I replaced it with a new one and honestly followed the drawings. The calculation of the thickness of the insulation layer is not determined by patting the head, but depends on the maximum temperature of the medium, the temperature rise rate, and the tolerance limit of the skin material. Generally, the flue gas temperature is 600℃, and the surface temperature of the skin is best controlled below 200℃, which depends on the insulation layer to carry down the temperature difference.
Fasteners: Air leakage mostly occurs on bead bolts
Finally, let's talk about fasteners such as beads, bolts and flanges. The skin is not glued on by itself, it is pressed to the frame flange in circles with strips and bolts. Problems are most likely here: uneven spacing of bolts, deformation of strips, or the wrong position of skin openings will leak. Some on-site workers try to save trouble when installing, and screw the bolts one by one. Guess what? Before long, the edges of the skin were blown by the air, leaking like a sieve. The correct way is to tighten the bolts symmetrically from the middle to both sides, and reach the specified torque two to three times to ensure that the pressure bar is evenly stressed.
In addition, it is also a common question whether to loosen the tie rod nut during installation-in fact, like the metal expansion joint, the tie rod for transportation should be loosened after it is in place, otherwise the expansion joint can't move. This tie rod is not a permanent fixture, it just prevents the skin from being pulled and deformed during transport. Many people forgot to dismantle it. As a result, as soon as the pipeline heated up, the expansion joint did not displace, and the frame was forcibly cracked.
Therefore, the composition of non-metallic expansion joint is not as simple as a layer of cloth. You go and look at the expansion joints that are still working normally after ten years of use. When you take them apart, the skin, frame, deflector, insulation layer, and fasteners are clean and perform their duties. Whichever piece goes wrong is a chain reaction. When selecting a model, it is much more useful to list the requirements of these components clearly than just looking at a brand.