Practical Guide to Maintenance of Non-Metallic Expansion Joints: From Fault Diagnosis to Repair
Non-metallic expansion joints (fabric fiber expansion joints) are used too much in the smoke and air ducts of power plants, cement plants and chemical plants, and there are rectangular and round types. Compared with metal expansion joint, its advantages are large compensation, corrosion resistance and good heat insulation, but its weakness is also obvious-the fabric layer is easy to age and break. Many on-site personnel are eager to replace a new one as soon as they find a leak. In fact, most of the faults can be repaired, which saves time and money than replacing a new one. The premise is that you have to understand how it is broken and how to fix it.
Getting the Fault in Place First: The Three Most Common Situations
Bulging or tearing on the surface of the fabric loop. This is usually caused by too much fluctuation in the temperature of the medium, or by long-term scour of fly ash particles. It is especially common in the desulfurization flue of power plants. The flue gas temperature suddenly rises from 80℃ to 150℃, and the fabric will bulge if it can't stand it. The second type: leakage at the flange connection. The bolts are loose? Seal Gasket Aging? The preload force during installation is not well controlled? It's all possible. The third type: the inner insulation cotton (aluminum silicate fiber cotton) collapses or drains, causing the outer skin to burn-you can feel the uneven temperature when you touch it with your hand.
Then how to judge quickly? Don't rush to dismantle the furnace after it is shut down. Touch the temperature distribution along the outer surface with your hand. Which piece is abnormally high, which means that the nearby insulation layer has failed. Then spray the flange seam with soapy water, bubbling is air leak. This method is simple and crude, but it works, and it can be done in one step.
Hands-on repair: one step of disassembly and assembly cannot be missed
After confirming the fault point, support the pipes on both sides of the non-metallic expansion joint before starting-this step can't be saved! Otherwise, the pipe will sink after removing the flange, and it will be difficult to put it back then. Then remove the flange bolts and remove the old fabric loop strap and platen. Pay attention to the protection of the flange surface. Don't smash it hard with a hammer. Once the flange surface is damaged, the seal will be over.
Next remove old sealant and rust from the flange face. Check the metal frame for corrosion deformation-this is easily overlooked. The structure of metal rectangular expansion joint in this station is similar to the metal parts of non-metal expansion joint. If the skeleton members are found to be seriously corroded, they need to be welded or replaced first. Don't save this step, otherwise it will be useless to install the new circle belt.
Once cleaned, replace with a new fabric loop strap and ceramic fiber gasket. Note that the fabric should be installed along the flow direction of the medium, and the wrong direction will aggravate the wear. The pressure plate bolts should be gradually tightened diagonally, and the torque should be controlled at the recommended value of the manufacturer (generally 80-120N·m). Don't screw too tightly, otherwise the flange will be easily fractured.
Installation is not finished: Sealability test must be done
It is not finished when it is installed, and the sealability test must be done. Small pipelines can be pressurized by 0.05MPa with compressed air, and large pipelines can be directly passed through flue gas for half an hour before retesting. Acceptance criteria reference JB/T 12235-2015: leakage rate not exceeding 1.5% of design value, surface temperature not exceeding ambient temperature +40℃.
Also, don't forget to check the tie rod or limit device. The expansion joint tie rod nut, if improperly adjusted, can limit the amount of compensation and cause the fabric to tear ahead of time. How to tune it? Let the tie rod be in a free state in the cold state, observe the displacement direction in the hot state, and then fine-tune the nut to leave a margin. For specific methods, you can see the FAQ on this site about "How to adjust the tie rod nut of the expansion joint", which is very detailed.
Want to use it long after repair? Daily maintenance to keep up
If you want to use it for a long time after maintenance, you have to be diligent at ordinary times. Monthly inspection: Focus on whether there are cracks on the fabric surface, whether the flange bolts are loose, and whether the guide tube (if any) has fallen off. Open the insulation layer every six months to check the internal cotton felt. If you find hardening or the thickness is reduced by more than 1/3, replace it in advance and don't wait for it to collapse.
In addition, the corrosion environment of the non-metallic expansion joint near the desulfurization flue gas baffle door is more harsh. It is recommended to choose the anti-corrosion layer made of polytetrafluoroethylene compensator, or directly replace it with a rubber compensator-the same as the rubber compensator of this site, which is acid and alkali resistant, and the price is suitable. If you already have a desulfurization flue gas baffle door at your station, you should pay more attention to the matching non-metallic expansion joint, and the corrosion rate is several times faster than that of ordinary flue.
Alas, in the final analysis, the core of the craft of non-metallic expansion joint maintenance is "judgment first, then hands-on, and maintenance later". Don't change it as soon as it leaks. You can't afford to wait for the construction period if you can afford it. When this job is done, it can save a lot of money in the factory a year.