Nine times out of ten people in the dry plumbing business have dealt with expansion joints. But if you ask what the process principle of non-metallic compensator is, many people will be stunned for a moment, and then say vaguely: Isn't it just a piece of cloth sewn in a circle? Alas, if this is heard by the old master of dry craftsmanship, he can laugh angrily.
Non-metal compensators and metal expansion joints, who does whose work? The metal expansion joint absorbs the displacement by the deformation of the bellows, has high pressure resistance and long life, but it is sensitive to fatigue and has nothing to do with corrosive smoke. Non-metallic compensator is different, it relies on multi-layer flexible fabric to bear displacement, corrosion resistance, high temperature resistance, large compensation amount, especially suitable for large-diameter low-pressure pipeline. The non-metallic compensator can just catch the large displacement and corrosive medium that the metal expansion joint can't carry. The two are not a substitution relationship, but a division of labor relationship-metal work, non-metal really can't do it; Nonmetal work, metal really can't catch it.
Skin is not a piece of cloth: What role does each layer play in a multi-layered composite structure
The skin is the core of a non-metallic compensator, but it is never a single-layer structure. You go and take a look at the non-metallic expansion joint (fabric fiber expansion joint) of a regular manufacturer. The skin is at least four or five layers from the inside to the outside, and each does its own thing.
The innermost layer is a temperature-resistant layer, which directly contacts the medium, either silicone rubber glass fiber cloth or fluororubber composite cloth, which is responsible for one thing: blocking high-temperature smoke. In the middle is the insulation layer, usually with ceramic fiber blanket or glass fiber wool, to trap the heat and prevent it from jumping out. Further outside is the structural reinforcement layer, high-strength glass fiber cloth or polyester canvas, which bears the tensile force and ensures that the whole skin is not torn apart. The outermost layer is a weather-resistant layer, fluororubber or silicone rubber coating, which is anti-UV, anti-aging and rainwater erosion.
How to fit between layers? It's not just brushing glue and sticking it. There is a parameter called peel strength in the composite process. What range will be controlled by regular enterprises? There are test records before leaving the factory. You take a piece of skin that has been used for a year and tear it apart. If the layers are cleanly separated, then the composite pressure is not enough at the beginning, and the temperature is not in place.
Skeletons and frames: What makes soft connections hold up to the pressure? There's something special about it here
The skin itself is soft, so what does it rely on to keep its shape? By the skeleton. The frames of non-metallic compensators are generally divided into two types: one is a metal frame, welded with angle steel and channel steel, wrapped around the skin, and bearing the internal pressure thrust and moment; The other is an internal guide tube, which is inserted inside the pipeline to guide the flow direction of the medium and protect the skin from being directly washed by the airflow.
There must be a gap between the guide tube and the skin. Some people don't understand and think that the tighter you lean against it, the better the seal. On the contrary, the skin deforms when it works, and if the guide tube presses against it, it will wear out in a few movements. The size of the gap is particular, and it is calculated according to the displacement and medium temperature during design, not determined by slapping the forehead.
How to connect frame and skin? Most are compressed with strips and bolts. The spacing between bolts will not be too large, and it is easy to leak if it is too large; It's also not too small, too small to install troublesome. If you look at the four corners of the rectangular non-metallic expansion joint, that position is most prone to problems-there are many skin folds at the corners, and the pressing force is uneven. If you steal a little work, it will start to leak after a year and a half. Therefore, a better manufacturer will thicken the strip at the corner, or use an arc transition, which is a real skill in technology.
From blanking to molding: What are the key steps of the process of non-metallic compensator?
The blanking of non-metallic compensators is not difficult or simple. The round one is easy to handle, just expand it and count it as a fan; The rectangular one is troublesome. The four corners have to be cut, and the incision angle and overlap length are all particular. The material is not cut correctly, and the skin is full of pleats when it is loaded, and it won't take long to crack from fatigue.
In the molding stage, the most critical process is the press-fitting of skin and flange. Before pressing, a sealing strip should be placed on the flange surface, and then the skin should be placed on it, and the tension around it should be adjusted to be uniform, and then it should be gradually pressed with pressing strips and bolts. This "step-by-step" is important-you have to twist the bolts diagonally alternately and press them into place three to four times. If you press them too fast or too hard, the internal fibers of the skin will be unevenly stressed, and the local stretch will be overstretched, which will greatly reduce the life span.
What about welding? To say welding is also a key step. Should frame welds be fully welded or intermittently welded? Most manufacturers do intermittent welding, because the frame will deform during use, and full welding is easy to crack. However, the splicing corners of the frame must be fully welded, where the force is concentrated, and intermittent welding can't bear it. These details, the drawings won't tell you at all.
By the way, there is also the pressure test before leaving the factory. Pressure testing is not as simple as hitting it with an air pump to see if it leaks. According to JB/T 12235-2015 standard, non-metallic expansion joints should be tested for airtightness, and there are clear requirements for pressure holding time and test pressure. Some factories try to save trouble. Even if there is any bulge on the surface of the skin with the naked eye, it is done. What can this find out? The needle eyes and tiny holes in the inner fibrous layer are simply invisible to the naked eye.
Principle: Why is non-metallic compensator the first choice in power plant desulfurization and cement flue?
Say response to use. The temperature of the flue gas in the desulfurization system of the power plant is not extremely high, but it contains sulfur, chlorine and a large number of slurry particles, which is highly corrosive and abrasive. Metal Expansion Joints In this environment, bellows are quickly corroded and perforated. The skin of the non-metallic compensator is fluororubber composite glass fiber cloth, which is naturally acid-and alkali-resistant. In addition, the guide tube separates the airflow, and the use environment is bad. It can still last for three to five years.
What about the cement flue? Mainly due to large dust and large temperature fluctuations. The kiln tail flue gas can reach 400℃, and the temperature changes drastically when the machine is started and shut down. Thermal fatigue cracks of metal expansion joints are common under this working condition. The non-metallic compensator itself is flexible, and the thermal stress can't be transmitted at all. Moreover, the insulation layer of the skin can effectively insulate the temperature, and the frame temperature will not be too high.
There are also those installed next to the desulfurization flue gas baffle door at the flue outlet, which have a narrow space and complicated pipelines. The non-metallic compensator can be made into a rectangular and non-standard shape, which takes up little installation space and is convenient for maintenance. Would you like to try a different metal expansion joint? For such a large bellows assembly, there is simply not enough maintenance space.
To put it bluntly, the process principle of non-metallic compensator is not so mysterious, that is, the word "flexibility" is made to the extreme: the temperature, corrosion and displacement are all digested by multilayer composite materials, and then the pressure, erosion and structural stability problems are covered by metal frames and guide tubes. However, the more simple things sound, the more details hide the door. Those invisible press-fit tensions, compound temperatures, and bolt tightening sequences are the watershed that determines whether it will last for three or ten years.
When you purchased the non-metallic compensator, did you ask the manufacturer about the thermal aging data of each layer of material of the skin? If you haven't asked, remember to ask next time. With this question, half of the suppliers who only know how to fight prices can be screened out.