1. Find out first: Why does the non-metallic compensator have to be "high temperature resistant" and "wear resistant"?
"How many degrees can your non-metallic expansion joint (fabric fiber expansion joint) withstand?" Too general a question. High-temperature working conditions and wear-resistant working conditions often appear in bundles-such as the smoke duct next to the metal corrugated expansion joint in the cement industry, or the pipeline system behind the desulfurization flue gas baffle door of the power station. The temperature of the gas is always three to four hundred degrees Celsius, with hard dust particles entrained. The fabric layer of ordinary non-metallic compensators will embrittle at high temperatures, and the dust will break as soon as it is washed. Therefore, high temperature resistance and wear resistance must be met at the same time, and both are indispensable. We have actually measured that the life of the conventional silica gel cloth composite layer is less than 3 months under the continuous dust-containing gas flow of 450℃. After replacing it with high-temperature-resistant ceramic fiber + wear-resistant silicon carbide coating, it has not leaked for one and a half years. This is not metaphysics, it is material selection logic.
If you think about it, if you patronize high temperature resistance and ignore the scour of dust, the compensator will be like paper paste and won't last long; On the other hand, if you only do wear resistance but not high temperature resistance, the coating softens and falls off at high temperature, which is just as useless. Therefore, before the customer asks about the temperature, he has to talk about whether there is ash in the medium, how big the particles are and how much the flow rate is. This is the first step in the selection.
2. Structural disassembly: What does the high-temperature and wear-resistant non-metallic compensator look like?
Compared with ordinary non-metallic expansion joints (fabric fiber expansion joints), it has an extra layer of "body armor". From the inside out, it is roughly: stainless steel wire mesh skeleton → high temperature resistant ceramic fiber felt → PTFE film (anti-corrosion) → wear resistant coating (silicon carbide or alumina spray) → outer fluororubber cloth. Note that the "wear resistance" here is not by thickening the fabric, but by doing a hard coat on the windward side. You touch it with your hands, and the surface is rough like sandpaper-yes, that layer is used to resist dust scour.
In addition, the air duct of rectangular structure is used the most, so we also have rectangular non-metallic expansion joints for special adaptation. However, it should be noted that this kind of compensator cannot be made into a large tie rod, because the non-metallic compensator itself has good flexibility, relies on fabric deformation, and does not need the guide structure of metal bellows. Have you seen a big tie rod expansion joint? That's for metal bellows, stiff. The flexibility of non-metal depends on the stretching and twisting of the fabric itself. If a pull rod is added hard, it will limit its displacement, which is not worth the loss.
3. Three elements of selection: temperature, medium and installation space, one cannot be leaked
"I want your high-temperature resistant non-metallic compensator, with a diameter of 1.2 meters and a pressure of 0.1MPa." I asked him how much dust was in the medium? All he said was "a little grey". That's the pit. When selecting the type, three things must be confirmed first: ① continuous working temperature (not instantaneous peak temperature), ② whether the medium contains hard particles, particle size and flow rate, ③ compensation direction (axial or angular displacement). For example: the double hinge expansion joint of the vacuum pipe of the air-cooled island of the power plant is made of metal, but if you change it to a non-metal one, the pressure will not be able to withstand it. In turn, the low-pressure large-size air duct behind the flue gas baffle door, non-metal is the best solution.
We have also encountered customers who used non-metallic compensators on negative pressure pipes, and the result was deflated-because the fabric has limited negative pressure resistance, extra wire supporting rings have to be added when selecting the model. What about that? If there is negative pressure inside the pipeline, be sure to add stainless steel wire support rings to the structure, or simply choose the reinforced type with skeleton. In addition, the installation space is also critical: for example, if there is not enough space at the bottom of the horizontal pipe, the drooping of the compensator will cause local stress on the coating, which will crack after a long time. Therefore, it is best to provide pipe layout drawings when selecting, so that we can give accurate suggestions.
4. Installation and routine maintenance: Don't let a screw destroy the entire compensator
The biggest fear of non-metallic compensators is that they are pulled too tightly when installed. It doesn't have an elastic limit like metal bellows, where once the fabric is forcibly stretched to deformation, the coating creates micro-cracks and then leaks in a short time. Correct practice: Install according to the amount of pre-tension provided by us, and loosen the limit screw after installation (see "Does the screw of the expansion joint need to be removed" in the previous Q&A). In addition, the wear-resistant coating, although hard, does not withstand local scratching. For example: A rectangular non-metallic expansion joint in a steel mill was installed at the bottom of a horizontal pipe. As a result, workers stepped on it when setting up scaffolding, cracking the coating and leaking air three months later.
① Regularly check whether the coating is peeling, ② clean up the dust-after the dust clumps, the weight will pull the fabric, resulting in the failure of the compensator. You might ask: How often do you check? If the working conditions are stable, once every six months is sufficient; Once a quarter is recommended if the dust concentration is high or the temperature fluctuates greatly. When inspecting, use a bright flashlight to shine on the windward side to see if the coating is bulging or falling off, and then touch the edge with your hand to see if there are any burrs. In addition, don't forget to check the limit screw-be sure to loosen it after installation, otherwise the compensator will not be able to expand and contract freely, which means that it is not installed. Is that the truth?
If you choose the right high-temperature and wear-resistant non-metallic compensator, and then install and maintain it in the right way, it is really not a blow to last for ten years. Before the next purchase, first clarify these three questions: the temperature, what components of the medium, and how to arrange the pipeline. Leave the rest to us.