First, find out what it is-what is the difference between non-metallic fiber corrugated compensator and metal expansion joint
"What is your non-metallic fiber corrugated compensator better than the metal one?” That's a real question. When many people hear the word "non-metal", they feel that it is definitely not as strong as metal. However, in actual working conditions, metal really can't bear it in some places.
Non-metallic fiber corrugated compensator, to put it bluntly, is what we often sayNon-metallic expansion joints (fabric fiber expansion joints)。 It does not need stainless steel bellows, but uses multi-layer fabric fiber cloth, insulating cotton, sealing film combined into a flexible connecting section. And metal expansion joints, likeUniversal corrugated expansion jointOrMetal rectangular expansion jointThe core is to absorb the displacement by the metal bellows.
What's the difference? Metal depends on its own elastic deformation, and has the advantages of high pressure and compact structure; However, when encountering high-temperature acidic flue gas, the metal surface will be oxidized, corroded, and even intergranular cracking. Non-metallic fiber corrugated compensator is just the opposite-it can't withstand high pressure (usually used below 0.5MPa), but its temperature resistance and corrosion resistance are a few blocks behind metal. In many desulfurization flues of power plants and hot air ducts of steel plants, metal expansion joints leak in one or two years, and non-metallic ones can last for five or six years. Is that the truth?
Second, structural disassembly: fabric fiber layer, thermal insulation cotton, sealing film... How can these materials be combined together to withstand high temperature and corrosion
Don't look like a "cloth", but there are actually several layers of "black technology" in it.
The outermost layer is usuallySilicone clothOrFluorine rubber clothThis layer is responsible for acid and alkali resistance and anti-aging. Inside isInsulating cotton-Ceramic fiber or rock wool, the thickness is designed according to the temperature, can block the high temperature of thousands of degrees. Further inside isSealing membranePolytetrafluoroethylene (PTFE) or silicone rubber is commonly used to ensure that the gas does not leak. These layers of material are fixed to the flange by means of a mechanical platen to form a flexible interlayer.
For example, the cement industry usesRectangular non-metallic expansion jointThe working temperature is often above 400℃, and there are dust and acidic gases in the pipeline. Metal expansion joint In this environment, the surface of bellows will be corroded by sulfide to make holes; As for the non-metallic fiber compensator, the insulating cotton blocks most of the heat, and the fluororubber film is acid-resistant. As long as you choose the right combination of materials, you can use it for three to five years without any problem.
In addition, some non-metallic expansion joints will also be equipped with guide liners (similar to the guide tube function of metal expansion joints) to avoid high-speed airflow directly washing the sealing film. This liner is typically stainless steel or ceramic fiberboard, which both protects the fabric layer and reduces flow resistance — andSpecific Function of Expansion Joint Guide TubeSimilar, but different materials.
3. What working conditions must it be? — — Real cases of flue gas pipelines, hot air systems, desulfurization and denitrification scenarios
In some working conditions, the metal expansion joint really doesn't want to be used, but it can't be used.
Flue gas pipeline:The sintering machine flue gas of power plants and steel plants, with a temperature of 150~350℃, contains strong acidic gases such as SO₂, HCl and HF. In the import and export of desulfurization tower in a power plant, the stainless steel metal expansion joint used before began to pit corrosion in half a year, and smoke leaked after perforation a year later. Replace withNon-metallic fiber corrugated compensator(The material is fluororubber + ceramic fiber), and it is still intact after three years of operation. Why? Metals are "chronic suicide" in wet acidic environments, while non-metallic PTFE membranes do not react with acids at all.
Hot Air System:The temperature of cement kiln head and boiler hot air duct can rush to above 800℃. The metal expansion joint must be made of superalloy (such as Incoloy 800) at this temperature, which is not only expensive, but also prone to stress cracking at the weld. However, the non-metallic expansion joint uses ceramic fiber insulation layer, the outer wall temperature can drop below 60℃, and the fabric layer itself can withstand the instantaneous high temperature of 1000℃, and the cost is less than 1/3 of that of superalloy.
Desulfurization and denitrification scenario:The net flue gas after wet desulfurization contains a large amount of water vapor and acidic droplets, which is extremely corrosive. Many projects are directly selectedRectangular non-metallic expansion jointOrNon-metallic expansion joint (fabric fiber expansion joint)With double seal structure (refer to this siteDouble-sealed single-axis circular baffle doorSealing ideas), basically prevent leakage.
4. Don't pat your head when selecting: temperature, pressure, medium composition and displacement. If a parameter is wrong, it is easy to cause problems
The non-metallic fiber corrugated compensator looks "solid", but the selection parameters are a little worse, and scrapping is a matter of minutes.
Temperature:The upper temperature resistance limit of non-metals depends on the insulation material and the fabric surface coating. Ordinary silicone cloth is resistant to 200℃, fluororubber cloth is resistant to 280℃, and ceramic fiber layer can reduce the inner wall temperature by 1000℃, but the outer wall temperature must be lower than the coating tolerance value. If you use a compensator resistant to 200℃ in 400℃ flue gas, no matter how thick the insulation layer is, it can't bear it, and the surface coating will be carbonized, cracked and directly scrapped.
Pressure:Non-metallic expansion joints usually only bear low pressure (0.1~0.5MPa). If it exceeds 0.5MPa, it is necessary to consider strengthening the frame or winding the pad with metal. Some clients putUniversal corrugated expansion jointAndNon-metallic fiber compensatorConfused, used on 1.0MPa steam pipeline, the result is compensator bulging and bursting.
Media composition:Acid gas fluorine rubber or PTFE membrane; oily steam-selected silicone rubber; Wear-resistant lining should be installed for those containing dust, otherwise high-speed particles will wear through the fabric layer. For example, a blast furnace gas pipeline in a steel mill, the medium contains tar and dust, and the non-metallic expansion joint of ordinary silica gel cloth is used, and a hole is ground in the inner layer in half a year. Later, the problem was solved when it was changed to the model reinforced by thickened ceramic fiber + stainless steel wire mesh.
Displacement Amount:Non-metallic expansion joints are characterized by absorbing large displacements (up to 50mm axially and 30mm transversely), but require precise calculations of pre-compression at installation. If the actual displacement exceeds the designed amount, the fabric layer can be pulled and torn. Therefore, stress analysis must be done for the pipeline when selecting the type, so don't be too troublesome.
5. The three easiest pits to step on in installation and daily maintenance, and how to avoid them
No matter how good the product is, it will be useless if it is not installed correctly. I've seen too many projects roll over because of installation details.
Pit 1: Not enough displacement space is left during installation.Someone screwed the flange of the non-metallic expansion joint directly to death, and the pipeline deadpulled the fabric layer after thermal expansion. As a result, the fiber layer at the edge of the flange was torn. Correct practice: According to the design displacement, pre-press or pre-pull with the adjusting screw during installation, and loosen the screw after operation (refer toHow to adjust the tie rod nut of expansion jointBut non-metals sometimes use limit plates instead of tie rods).
Pit 2: The medium flow direction is reversed.If there is a guide liner inside the non-metallic expansion joint, it must be installed in the direction of arrow, otherwise the airflow will directly flush the sealing film, and the life will be greatly reduced. I saw a desulfurization project where the construction team putDesulfurization flue gas baffle doorAnd the expansion joints were installed backwards, and smoke leaked in two weeks.
Pit 3: Daily inspection only looks at the appearance.It is normal to have dust or condensation on the surface of non-metallic expansion joints, but if local bulging, abnormal wrinkles, hard surface or cracks are found, the machine should be stopped immediately for inspection. I once encountered a power plant. The inspector found that there was a bulge in the compensator and didn't take it seriously. As a result, three months later, the bulge broke, and the smoke leaked, resulting in a shutdown for emergency repair. The correct maintenance cycle is-check the surface condition of the fabric layer once a quarter, check the thickness of the insulation layer once a year (evaluate the insulation effect with infrared temperature measurement), and repair or replace it in time if any damage is found.
In short, the non-metallic fiber corrugated compensator is a good thing, but it has to be "prescribed the right medicine". You use it in high-temperature corrosion flue gas pipes, and it is an artifact; You use it on a high pressure steam line, and it's a ticking time bomb. Only by choosing the right, installing the right, and maintaining the right can it give full play to its true value.