Find out what the silicone cloth is for in a non-metallic compensator-not a simple layer of cloth
When many people hear "non-metallic compensator silicone cloth", their first reaction is: Oh, it's a layer of cloth, which is padded in the middle of the flange to prevent leakage. Tsk, thinking so, the first step is missing.
The role of silicone cloth in non-metallic expansion joints (also called fabric fiber expansion joints) goes far beyond sealing. It has to carry three things at the same time: temperature, pressure and displacement compensation. For example, in the high-temperature flue gas pipeline, the temperature always goes up to 300℃, and ordinary rubber has long been baked. The silicone cloth has been treated with special coating, and the temperature resistance range is usually-60℃ to +300℃. Some models with glass fiber or ceramic fiber layer can withstand more than 600℃ for a short time.
More importantly, the silicone cloth is the core of the flexible compensation layer – it is responsible for absorbing the axial expansion and contraction, lateral offset, and angular displacement of the pipe. If you think of it as a simple gasket, it is equivalent to turning the whole compensator into an iron plate, and all the elasticity that it should have is gone. Two days ago, I met a customer, who said that it had leaked after installing it for only three months. When I took it apart, I saw that the silicone cloth was pressed into a dead fold by the screw, which didn't compensate at all. It's not that the product is not working, it's the wrong usage.
Rollover site before installation: the flange surface is not treated, the size is not checked, and the screw direction is reversed
Before installing non-metallic compensator silicone cloth, three low-level errors can scrap a good set of products.
The first, the flange face is not treated.The flange removed from the old pipe, which is all welding slag, rust and greasy, is directly installed with silicone cloth? That is equivalent to laying carpet on sandpaper-no matter how tight the press is pressed, the leakage point will appear instantly as soon as the medium flushes. Standard practice: Clean it with a wire brush or angle grinder to ensure that the flange surface is flat and smooth without bumps. If there is a pit exceeding 1mm, it must be repair welded and polished.
Second, the size is not reviewed.Don't say "about the same". When installing non-metallic expansion joints, the diameter of flange bolt holes, hole spacing and outer diameter of flange must be measured against the drawings. 5mm more, the bolt can't pass through; It is 3mm less, and the press strip can't cover it. In a case of a power plant, the workers put the silicone cloth on without measuring the hole distance. As a result, the bolts were twisted diagonally, and the silicone cloth was torn directly at the bolt hole, and it was scrapped before it was put into production.
Third, the direction of the screw is reversed.This one is the most hidden. Many non-metallic compensators are equipped with pressing bars and fastening screws on both sides of the flange. The threaded end of the screw should face the outside of the pressing bars to facilitate screwing the nut. However, someone tried to save trouble and threw the screw in from the inside. As a result, the nut was blocked by the flange when it was twisted halfway, and it couldn't be pressed tightly at all. In the end, it could only be disassembled and reinstalled, and the construction period was delayed for two days.
Core four steps: line drawing and positioning → strip pressing → bolt pre-tightening → symmetrical tightening, one step wrong reduces the life by half
So how on earth do you pretend? Don't mess around, go in this order, and rework if the order is wrong.
Draw line positioning.First, lay the silicone cloth flat on the flange surface, and draw a circle along the inner diameter and outer diameter of the flange with a marker to ensure that the silicone cloth is centered and not skewed. Especially for rectangular non-metallic expansion joints, the four sides must be symmetrical. If the offset is 5mm, the compensation amount will be offset by 10%. When drawing the line, pay attention to the fiber direction of the silicone cloth-the warp direction (length direction) corresponds to the axial displacement of the pipeline, and the weft direction corresponds to the lateral displacement. If this direction is reversed, the compensator is equal to uninstalled.
Strip compacting.The strip (usually angle or flat steel) should fit the flange and completely enclose the edge of the silicone cloth. The bead joints are staggered as much as possible to avoid weak spots in the same position. The bolt holes on the press strip must be aligned with the flange holes, and cannot be pulled hard.
Bolt pre-tightening.Many people are fast in this step, so they can directly use the electric wrench to fight it to the end. SO WRONG! Pre-tightening means to first screw all the bolts with a wrench until they just touch and eliminate the gap, probably until they can't be screwed by hand. Don't rush on the torque, there is also symmetrical fastening in the back.
Symmetrical fastening.That's the key. Tighten three times from center outward, diagonally crossed. The first screw to 50% of the design torque, the second to 80%, and the third to 100%. For example, M16 bolts, the recommended torque is 80~100N·m. Do you have to use a torque wrench and screw it by feel? The error can exceed 30%. In this way, the silicone cloth is uniformly stressed, the sealing surface is flat, and the service life is at least doubled.
Should the pre-stretching of silicone cloth be done or not? Don't be led off by the experience of the old master
"You should tighten the silicone cloth first, otherwise it will be loose and baggy when used." Is this correct? According to the situation.
Silicone cloth itself is a flexible material. If it is intentionally stretched during installation, the advantage is that it can apply tension in advance when the pipeline is cold, and it is not easy to wrinkle after thermal expansion. But the disadvantages are more obvious: excessive stretching, early fatigue of internal fibers of silicone cloth, accelerated aging and fracture at high temperature. Especially when the non-metallic compensator is used in high-temperature flue gas or steam pipeline, the amount of thermal expansion is inherently large, and the pre-stretching makes the silicone gel cloth bear additional stress in the hot state.
I checked the installation manuals of several mainstream manufacturers (including the information of rectangular non-metallic expansion joints on this site), and generally recommended:Not pre-stretched。 When the silicone cloth is installed, it can be kept in a natural flat state, and a small margin (1% ~2%) is allowed to cope with fine adjustment. Only when the displacement of the pipe is extremely large and the elasticity of the silicone cloth itself is insufficient, it is necessary to pre-stretch at the stretch ratio given by the manufacturer (usually not more than 5%). Master's experience may apply to old-fashioned asbestos gaskets, and silicone cloth is different. Don't blindly follow the trend.
Three pits that frequently occur in use: media scouring, temperature exceeding limit, and loose beading
Once installed and set and forget? Thinking too much. Three common problems in the running phase, pay attention in advance.
Media flushing.If there is high-speed dusty flue gas in the pipeline (such as desulfurization flue of thermal power plant), and the particulate matter washes the surface of silicone cloth for a long time, the coating will gradually wear out, expose the fiber layer, and then leak. Solution: Add a guide tube or wear-resistant liner inside the non-metal expansion joint (such as the wear-resistant liner matched by the desulfurization flue gas baffle door of this station). If not, then you have to check the windward side of the silicone cloth regularly, and if you find wear, patch the sheet immediately.
Temperature exceeded limit.The temperature resistance of silicone cloth has an upper limit. Some process pipelines occasionally overheat by dozens of degrees during operation, such as 300℃ in design, but 350℃ in actual fact. It may be okay once or twice, but repeated overheat will cause the silicone cloth to become brittle, hard and cracked. Don't expect it to bear the design limit. Over-temperature 10% may reduce its life by 50%. It is recommended to install temperature monitoring points or choose higher grade composite fabrics (such as ceramic fiber cloth + silicone coating).
The bead is loose.The bolts will slowly loosen under high temperature and vibration, especially when non-metallic compensators are used for fan inlet and outlet pipes. Check once a week and tighten the nut with a wrench. Some customers ignore it after installation. After half a year, the strips fell off, and the silicone cloth was blown by the airflow and torn instantly. This is called a small loss.
When is it time to change silicone cloth? Just look at these three points, don't wait for a leak before dismantling
Silicone cloth is not a permanent piece, so you should change it. Don't be reluctant. Three judgment criteria:
- Cracked or hardened surfaces.Press by hand, if the silicone cloth loses its elasticity, cracks will appear when it is gently broken, indicating that the coating has aged and failed. If you don't change it at this time, the next step will be penetrating leakage.
- Local bulging or delamination.The silicone cloth and the reinforcement layer disengaged and bulged like blisters. The reason is the infiltration of the medium or the action of thermal stress, and the bulge will rupture if it is continuously used.
- There are leakage marks at the flange bead.Even if it's just a wet mark, don't wait for a drip. Disassemble and check. If the edge of the silicone cloth has been corroded by the medium and become brittle, it must be replaced.
There is no fixed standard for the replacement cycle, but according to the industry experience of this station, under normal working conditions (temperature ≤250℃, no strong corrosive medium), silica gel cloth can be used for 2~3 years without problem. If the working conditions are bad, such as high sulfur content in flue gas or large pipeline vibration, it is recommended to stop the machine for inspection once a year. Don't wait until it leaks before removing it-at that time, even the flange may be corroded by the medium, and instead of a piece of cloth, the whole expansion joint will be replaced.
How to use non-metallic compensator silicone cloth? Treat it as a precision part of the system, not as a consumable. It takes an extra half hour to install and saves half a year to run.