As soon as the "non-metallic expansion joint leaked" was reported on the spot, many people's first reaction was to remove it and replace it with a new one. Don't rush yet. Non-metallic expansion joints (fabric fiber expansion joints) are not cheap to replace, but more expensive to stop. You spend half a day figuring out what kind of leakage it is, and you may be able to solve it on the spot, without having to weld it at all.
Leak first distinguishes whether it is "seam leak" or "tear"
On the surface, the leakage of non-metallic expansion joints is air leakage, ash leakage and smoke leakage, but the failure mode is completely different, and the treatment method is completely opposite.
The so-called "seam leak" refers to the medium drilled out from the gap between the skin and the flange platen, or seeped out from the skin overlap seam and stitch eyelets. This kind of leak is generally not large, "hissing" like a whistle, or even just ash. Most of the seam leakage is not the fabric broken, but the seal is destroyed when the pressure plate bolts are loose and the flange surface is uneven. It is simple to handle: tighten bolts, clean flange surfaces, replace sealant, and many times don't have to stop the machine.
The "tear" is much more serious. There are cracks and cracks on the surface of the skin, the fiber layer is broken, and even the internal insulation cotton can be seen blown out. This leak is structural, indicating that the fabric has lost its overall strength, and the plug cannot be plugged, so it can only be replaced.
So the first step is to take a flashlight and light it clearly-whether it is a drain in the seam or a crack in the skin. Just drawing a conclusion without even distinguishing this is all busy afterwards.
The most common reasons for leaks at the scene: which is the main cause?
After distinguishing the leakage method, let's see why it leaks. According to our experience in running the site over the years, the reasons for the leakage of non-metallic expansion joints basically can't escape the following four:
1. Installation deviation
During installation, it was not installed according to the reserved displacement, or the pipeline was forced to match, and the expansion joint was hard broken. This is a "congenital deformity"-the fabric has already been stressed before it is run. As soon as the temperature rises, the local stress is concentrated, and it is torn from the corners first. Many new installations leak within a few months, 80% of which is the problem.
2. Dielectric Corrosion
The sulfide and acidic condensate in the flue gas pipe corrode the non-metallic skin much faster than expected. Especially the expansion joint at the inlet and outlet of the desulfurization tower, the flue gas humidity is high, the acid dew point is low, and once the fluororubber or silicone rubber coating on the skin surface has pinholes, the acid liquid penetrates into the fiber layer, and the fabric can rot through in a few months.
3. Temperature over-limit
The skin materials of non-metallic expansion joints have a clear upper limit of temperature resistance. For example, silicone rubber coated cloth generally resists about 200℃, fluororubber can reach around 250℃, and polytetrafluoroethylene film can reach 260℃. In actual working conditions, instantaneous overtemperature and local high-temperature flue gas scour will lead to carbonization and cracking of the surface coating, and then the fiber layer will lose protection, and the strength will drop by a cliff.
4. Fabric Aging
This is the most normal but the easiest to overlook. Any non-metallic material has a service life. Outdoor ultraviolet irradiation, alternating cold and hot, and repeated displacement and deflection, the fabric will slowly become hard, brittle, and even powdered. Expansion joints have appeared surface micro-cracks and leakage after more than five years of use, which is most likely the reason.
Among these four types of reasons, installation deviation and temperature exceeding limit belong to "hard defects" and appear the fastest; Media corrosion and fabric aging are "chronic diseases", but the consequences are more insidious. How to judge the main cause? Look at the damage location. If the leak is concentrated at the corners and edges of the expansion joint, preferential suspicion of mounting deviation; If the damaged surface is evenly distributed in the middle of the windward surface, it is mostly due to medium corrosion or temperature exceeding the limit.
Do not blindly replace after leakage: Judgment criteria for temporary leak plugging and shutdown for maintenance
Two days ago, I encountered a thermal power plant project. The non-metallic expansion joint at the outlet of the induced draft fan leaked, and the operators were so anxious that they had to change it overnight. We went over and looked-the sealant between the skin and the flange was aged and hot smoke was coming out of the gaps, but the fabric itself was intact. There is no downtime at all in this case. Tighten the bolts one by one, and re-apply the high-temperature sealant. It took two hours to get it done, and it hasn't leaked again after two or three months of operation.
But on the other hand, if there is a crack exceeding 50mm on the skin surface, or the multilayer structure has been separated in layers, then don't hesitate to apply for shutdown and replacement directly. Temporary plugging can't solve the structural damage, but will only make the break bigger and bigger, and even pump all the insulation cotton away. At that time, even the pipe bracket will be stressed.
Is the integrity of the fabric structure still。 Seam leaks can be treated temporarily, and tears must be shut down for maintenance. When you are not sure, use a probe to gently poke the broken edge. If the fiber is loose and shattered at a poke, don't think about it and change it.
Preventing Leakage from Material Selection and Structure: Key Points of Design and Installation
Instead of leaking and tossing again, it is better to strangle the hidden dangers at the source in terms of material selection and structure.
In terms of material selection, the skin of non-metallic expansion joints (fabric fiber expansion joints) is never a layer of cloth, but a composite of multiple functional layers-corrosion-resistant layer, airtight layer, reinforcement layer and heat insulation layer, each performing its own duties. When selecting the model, the working condition parameters must be clearly stated at once: medium temperature (including instantaneous overtemperature), pressure, medium composition (especially with or without acid gas), displacement (axial, transverse and angular directions respectively), and installation position (indoor or outdoor). These parameters are different by one gear, and the skin material is completely different. The flue gas has high sulfur content, so the outer fluororubber coating is essential; If the temperature exceeds 250℃ all the year round, the composite structure containing high silica fiber cloth must be considered.
Structurally, there are three major details that determine whether it will be missed:
- The deflector must be installed.The guide tube can block the high-temperature dusty smoke from directly washing the inner surface of the skin, and without the expansion joint of the guide tube, the life of the skin is shortened by at least half.
- The flange face must be flat.When leaving the factory, the flange surface should be machined and re-measured with a level ruler before installation. If the unevenness exceeds the standard, the gaps will still leak no matter how tight the bolts are.
- The pressure plate bolt spacing should not be too large.The standard practice is that the bolt spacing should not exceed 100mm, and it should be evenly tightened diagonally. The torque value should be according to the design requirements, so it is not possible to only screw a few points to save trouble.
In addition, when installing, don't pull the expansion joint into the pipeline for the convenience of the counterpart. Non-metallic expansion joints are flexible components and are not intended to compensate for installation errors. The correct way is to adjust the pipeline in place first, and then install the expansion joint to ensure that the expansion joint is at the initial length specified in the design.
Daily inspection checklist to prevent leakage: what to see, touch and measure
Finally, give a practical inspection list. It's not complicated. Just take ten minutes to walk around every time you shut down for maintenance.
- See:Whether the skin surface is discolored, blistered, cracked or whitened; Whether the pressure plate bolts are rusty or loose; Are there any traces of dust or smoke on the edge of the flange-that's a sign of early leakage.
- Touch:Wear gloves and touch the surface of the skin in a circle to feel any abnormal bulging or collapse. A bulge indicates that the internal layers are detached, and a collapse indicates that the insulation cotton may have been lost.
- Test:Sweep the surface temperature distribution of the skin with an infrared thermometer gun. If the temperature in a certain area is obviously high, it means that the internal heat insulation layer fails. In addition, check the gap between the pressure plate and the flange with a gauge. If it exceeds 0.5mm, it is time to re-tighten.
If possible, record the findings of each inspection and compare them with the last one. Fabric aging is a gradual process. With historical data, you can figure out how long it will last in advance, rather than waiting for a leak to react passively.
The non-metallic expansion joint leak, it sounds like a fault, but the equipment is talking to you. Figure out what it's saying, and you'll know whether to fix it or replace it.