1. "Flexible shield" in high-temperature environment: What does the nonmetallic compensator rely on to withstand hundreds of degrees?
When it comes to high-temperature pipeline compensation, many people's first reaction is metal expansion joints. But there's something calledNon-metallic expansion joint(Also called fabric fiber expansion joint), it relies not on metal bellows, but on a multi-layer composite structure-silicone cloth, fluororubber, ceramic fiber felt and glass fiber cloth are stacked together to form a "flexible shield". askWhat does a high temperature non-metallic compensator do?To put it bluntly, in hundreds of degrees of smoke, hot air and dust pipes, it can both seal and absorb displacement, and is not afraid of corrosion.
Temperature resistance limits depend on the innermost insulation layer. The conventional silicone cloth is used as the inner lining, which can withstand 250 DEG C to 350 DEG C; Replace it with the structure of fluororubber or polytetrafluoroethylene compensator, which can reach 400℃; If ceramic fiber + heat-resistant alloy wire mesh is used, the long-term operating temperature can reach 600℃ or even 1000℃ for a short time. And guess what? In the desulfurization flue of the power plant, the flue gas temperature is only 150℃ ~200℃, but occasionally, high-temperature flue gas will rush into it. Because the non-metallic compensator is multilayered and flexible, it can withstand this temperature shock better than metal-the metal bellows may directly deform plastically when it encounters local overheating, while the non-metallic fabric layer can save its life by ablating the outer layer.
Not that warmer temperatures are better, of course. The maximum continuous service temperature of each material has clear data, and the selection depends on whether the working condition is "continuous high temperature" or "instantaneous peak". Two days ago, I met a customer who used an ordinary silicone cloth compensator on the normally open flue at 350℃. As a result, it was brittle and cracked in three months-that was because the material selection was not clear.
Second, absorbing multi-directional displacement and reducing noise and vibration: How to solve the most difficult problem of "thermal expansion and contraction" in high-temperature pipelines?
The trouble of high-temperature pipelines is not only the temperature itself, but also the displacement caused by thermal expansion and contraction. It is normal for a flue gas pipe tens of meters long, from cold to hot, to extend several centimeters axially and deflect one or two millimeters laterally. Metal expansion joints are usually only good at absorbing axial displacement, and complicated hinge or tie rod structures have to be added when they encounter lateral or angular displacement. WhileNon-metallic expansion jointBorn as a "cartilage head"-its fabric loop band can easily absorb displacement in three directions: axial, transverse and angular, especially suitable for rectangular pipes.
The outlet flue of the induced draft fan of a thermal power unit has a cross section of 3m ×2m and a temperature of 200℃. If installedMetal rectangular expansion jointFor manufacturing, thick steel plates are pressed, weighing several tons, and can only absorb a small angular displacement. modifyRectangular non-metallic expansion joint, which weighs only 1/5 of metal, can even be adjusted by hand during installation. During operation, the low-frequency vibration is almost completely absorbed by the fabric layer-this is its hard work of noise reduction and vibration reduction.
So how to prevent leaks? The structure is generally equipped with two seals: the outer layer is a temperature-resistant flexible skin (such as silicone cloth), the inner layer is a corrosion-resistant fluororubber or polytetrafluoroethylene compensator layer, and the middle is filled with ceramic fiber insulation cotton. Under the triple protection, even if the outer layer burns through, the inner layer can still stand for a while. So in principle,What does a high temperature non-metallic compensator do?It is to use a multi-layer flexible "pocket" to hold the displacement and stress of the high-temperature medium, and by the way, the vibration noise is also eaten.
3. Comparison of metal expansion joints: Which scenes must use non-metals? Pits not to step on when selecting a model
The metal expansion joint relies on the elastic deformation of the bellows, which can withstand high pressure (tens of MPa) and has a long life (tens of thousands of theoretical fatigue times), but its shortcomings are terrible-it is afraid of corrosion and crystallization, and it requires extremely high installation alignment. The bearing pressure of the non-metal expansion joint is generally 0.1~0.3MPa (suitable for low-pressure smoke duct), but the corrosion resistance is extremely strong. For example, the flue gas after wet desulfurization contains a large amount of SO₂, SO₃ and condensed water, and the metal bellows will be perforated in a few months.rubber compensatorOrRubber PTFE compensatorBut it will last two or three years.
Which scenes must use non-metals?
- Low pressure large section rectangular pipeline: The desulfurization flue of power plant and the exhaust gas pipeline at the tail of cement kiln are all non-standard cross-sections with a width of several meters. The metal expansion joint can't make such a large elastic element at all, so only rectangular non-metallic expansion joint can be used.
- Pipes containing corrosive media: Acid flue gas of waste incineration plant and blast furnace gas of iron and steel plant must be made of composite structure of fabric fiber and fluororubber.
- Multidirectional displacement and space constraint: For example, at the corner of the pipeline, the metal expansion joint needs to be set with multiple hinge combinations, and the non-metal one can handle three directions.
The pits not to step on when selecting are mainly the following points. First, don't putNon-metallic expansion joint (fabric fiber expansion joint)When used with metal-it cannot withstand high pressure, if the internal pressure of the pipe exceeds 0.5MPa, it must be used with metal or with a limit tie rod. Second, pay attention to the abrasion of medium particles. If there are hard particles (such as cement raw dust) in the flue gas, wear-resistant lining plate or guide tube should be added to the non-metallic inner layer (you can refer to the function of expansion joint guide tube). Third, the installation must be in the direction of the arrow, and don't install it backwards, otherwise the skin will tear in advance under tension.
IV. Real cases used in power plants, cement and flue gas systems-what media can high-temperature nonmetallic compensators hold?
Talk about a few actual working conditions and you will have an idea.
Case 1: Power Plant Desulfurization FlueIn a 2×300MW unit, the net flue gas temperature after wet desulfurization is 50℃ ~90℃, but it contains a large amount of sulfuric acid condensate. Previously usedUniversal corrugated expansion joint, leakage in half a year. Replace withNon-metallic expansion joint (lined with PTFE compensator), has been running for 4 years without replacement. Medium: SO₂, SO₃, water vapor, acidic condensate. Can carry it.
Case 2: Kiln tail waste gas of cement production lineThe temperature is 350℃ ~400℃, and the dust concentration is as high as 30g/Nm³. useMetal corrugated expansion joint Dust can get stuck in the corrugations and cause fatigue failure. modifyRectangular non-metallic expansion joint, with wear-resistant guide tube inside, dust flows away along the inclined plate, and there is no problem for three years. Medium: High temperature flue gas, cement raw material dust.
Case 3: Primary air duct of waste incineration power plantThe air is preheated to 250 °C, and the pipe has a large cross-section that needs to absorb thermal expansion and vibration. Directly onrubber compensator？ Rubber can't stand 250℃. Must useNon-metallic expansion joint (fabric fiber expansion joint), inner layer of fluororubber + outer layer of silicone cloth + intermediate ceramic fiber, life design of 15 years. Medium: Hot air, a small amount of corrosive gas.
See?What does a high temperature non-metallic compensator do?It is far more than the word "temperature". It has to face four enemies: high temperature, corrosion, dust and vibration at the same time. Choose right, it is the "flexible shield" of the pipeline; If you choose the wrong choice, it will be an accident danger.
5. Life and installation points: If you want it to last for a few more years, these details engineers must keep an eye on
Theoretically, the life of non-metallic compensators can reach 5~8 years, but in fact, many of them are scrapped after two or three years, which is rooted in installation and maintenance.
1. Pipe alignment must be checked before installationThe non-metallic skin has no self-centering ability, and the pipe flange is misaligned by more than 3mm. Forced fixation will lead to skin distortion and local stress concentration. It is best to temporarily fix it with a tie rod, and then tighten the bolts after adjusting it in place.
2. Bolt torque cannot be messed aroundGenerally, flange pressure plate is adopted for screw joint surface, and the bolt tightening torque should be carried out according to the product instructions. Screwing too tight will crush the edges of the skin, and it will be too loose and air leaking. It is recommended to pre-tighten diagonally in three times, and the final torque is controlled at 80~120N·m (depending on the model).
3. Check the guide tube regularlyIf there is a deflector inside, it depends on whether it is worn out. Especially in the cement industry, after the guide tube is worn out, the high-temperature particles directly scour the inner layer of the skin, and the hole can be broken in a few days. You can take a look at it with an endoscope every year when the machine is down.
4. Be careful not to press the insulation layer against the compensatorIn some sites, the thermal insulation cotton is outpaced in the pipeline, and as a result, the thermal insulation cotton is directly pressed on the non-metallic surface, resulting in the heat being unable to dissipate and the local temperature exceeding the standard. The outer surface of the compensator must leave a heat dissipation void of at least 50mm.
The national standard JB/T 12235-2015 clearly states the technical requirements of non-metallic expansion joints, and the selection, installation and inspection should be followed. Don't try to save trouble, and don't be superstitious about imported brands-domestic onesRubber PTFE compensatorIt tends to be more flexible in adapting to complex working conditions.
At the end of the day, find outWhat does a high temperature non-metallic compensator do?It is not to back the definition, but to understand the underlying logic of its "flexibility", "multilayer" and "corrosion resistance". The next time you encounter high temperature and low pressure, corrosion, and large-section pipes, you will know what to do.