What are fabric expansion joints? I'll let you know the truth first
In the dry pipeline industry for more than ten years, every once in a while, someone asks with drawings: What is this thing? Why does it look like a canvas pocket? Guess what, that's what we're talking aboutfabric expansion joint, also called non-metallic expansion joint, fabric fiber expansion joint. To put it bluntly, it is a device that absorbs the thermal expansion, contraction and vibration displacement of the pipe by a ring of flexible fabric. You think of it as a "cartilage head" added in the middle of the pipe, and the pipe can twist wherever it wants, but the stress can't be transmitted, and the equipment is safe.
You don't look at the word "fabric" in its name, you think it's a rag. This set of things is used in high-temperature air ducts of power plants and cement plants, and it is more useful than metal bellows at critical moments. Don't believe it? Look down.
Take it apart and see: What is wrapped in this "cartilage head"?
Take a standard rectangular non-metallic expansion joint, for example. It is usually a four-layer structure, and each layer does a different job.
- Fabric band (skin)— The outermost layer was also the core. It is generally made of fluororubber, silicone rubber coated glass fiber cloth or polytetrafluoroethylene material, which is responsible for sealing and absorbing displacement. If this layer breaks, the whole expansion joint will be wasted.
- Thermal insulation-Close to the inside of the loop band and fill it with ceramic fiber felt or glass wool. The flue gas pipe is often four to five hundred degrees. Without its insulation, the rubber coating on the outside will melt in a few minutes.
- guide tube— — This thing is made of metal. It is inserted into the pipe to divert the airflow and prevent the high-temperature smoke from directly washing the fabric band. Don't underestimate it. Without the guide tube, the life of the loop can be shortened by half.
- Frame (flange)— — That is, the upper, lower, left and right four angle steel or channel steel welded frames, and the pipe flange is screwed to death with bolts. Frame strength determines how much pressure the expansion energy saving is subjected to.
Each floor is coordinated with the work. You asked if the insulation can be saved? I advise you not to save it, save the circle belt burning through, and change it once enough for you to buy ten layers of insulation cotton.
Compared with the metal corrugated expansion joint, what is the difference?
Since metal expansion joints are so strong, why do you have to use fabric expansion joints in flue gas ducts and air ducts? There's a deep doorway here.
The metal corrugated expansion joint absorbs displacement by the deformation of the bellows, but it has a fault: it is sensitive to fatigue life. The temperature fluctuates greatly in the flue gas pipeline, and there is acid-alkali corrosion. Once the metal corrugated pipe is cracked by fatigue, air leakage and ash leakage is a trivial matter, and shutdown for maintenance is a big trouble. Moreover, the displacement of metal pipes is limited, and metal bellows with large cross-sections such as rectangular air ducts are heavy and expensive to make.
The same cannot be said for fabric expansion joints. It compensates the displacement by the deformation of the flexible ring belt, which can move in three dimensions and absorb in axial and transverse directions. Moreover, it has great damping, can reduce vibration and noise, and its price is much cheaper than that of metal bellows of the same caliber. The disadvantage is that the pressure bearing capacity is poor, and it is generally used in air ducts and flues from several kPa to more than ten kPa. High pressure occasions? Then you should honestly use metal.
Which industries are using it? Tell me a few real scenarios
In the past few years, I ran the scene, and I saw these three places the most:
Boiler air duct of power plant。 The rectangular air duct that comes out of the boiler blower has high temperature and big vibration. What does the thermal displacement rely on to absorb? Commonly used non-metallic expansion joints. Especially the corrugated expansion joint used in the power station industry has been seen in both low-temperature and high-temperature sections, but the air duct inlet section is almost all fabric type.
Cement plant kiln head and kiln tail.The temperature of the flue gas from the rotary kiln can reach above 350℃, and the dust content is large. There are many metal corrugated expansion joints in the cement industry, but the section between the kiln tail and the dust collector is basically rectangular non-metallic expansion joints. The reason is simple: too much dust accumulates on the metal ripples, which will jam the ripples. The fabric band is not afraid of this, and the slight dust accumulation will fall off as soon as it is shaken.
Desulfurization system.The flue gas at the inlet and outlet of the desulfurization tower has high humidity, contains sulfide, and is extremely corrosive. At this time, candidate metal bellows, you have to keep an eye on the material, and the cost will rise. With fabric expansion joints, the outer layer of fluororubber and PTFE coating itself is corrosion-resistant, and the cost performance will come up.
There are also those flue gas baffle doors and desulfurization flue gas baffle doors next to, and fabric expansion joints are also liked at pipe connections. Why? When the baffle door expands and contracts, the pipes on both sides pull each other, and no soft connection has ripped the bolts early.
Focus on these five parameters in model selection, and don't be fooled by sales
Some sales come up and blow "our family has five layers of skin sandwich, the thickest in the world". You would be naive to listen to him. When choosing fabric expansion joints, just keep an eye on these five numbers:
- Diameter dimensions and profile.Rectangular or circular? What is the flange port size? This is not accurate, and the rest is all useless.
- Design displacement amount.Axial stretch, lateral offset, angular displacement, how many millimeters are each? How much displacement you give, how wide cloth you have to use for the loop belt, which directly determines the price.
- Medium temperature.The materials used at 150℃ and 500℃ are completely different things. Don't just look at the upper limit of "temperature resistance 800℃", but ask what is the long-term continuous working temperature.
- Pressure rating.The air duct is generally slightly positive pressure, but what in case there is negative pressure? If the negative pressure is large, the ring belt will deflate, and it will fail.
- Media composition.Does it contain sulfur? Does it contain dust? Are there any corrosive gases? This has the greatest impact on coating material selection.
These five parameters are not clear. After listening to the sales speech, you have a high probability of buying expensive or wrong.
Why do you have to change it after a few years of use? How to check daily?
To be honest, fabric expansion joints are consumables. Metal bellows can be used for more than ten years. In the environment of high temperature, dust and acid-alkali corrosion, the life span of fabric belts is generally three to five years. They run with high load every day, and they have been changed in two years. Don't expect it to be done once and for all. You have to leave room for replacement when designing it.
For daily inspection, we focus on three places: first, whether there are bulges, cracks and exposed fibers on the surface of the ring belt, second, whether the flange bolts are loose, and third, whether there is any abnormal noise and air leakage. You take a flashlight to shine on the inside of the ring belt. If you see that the color of the insulation layer is black and brittle, it means that it is running warm, and you have to arrange a replacement quickly.
There is also particular attention when changing. The old loop is removed, and the frame should be cleaned without burrs. When the new ring belt is installed, the bolts should be twisted diagonally and the force should be even, otherwise the force will deviate to one side, and the ring belt should be torn again.
In the final analysis, fabric expansion joints are not advanced technology, but they rely on those few layers of cloth to chew hard bones that metal expansion joints can't handle. The next time someone asks you, "What's a fabric expansion joint?", you tell him: That's the guy in the plumbing system who knows the best "make the sea brighter".