The frame is the skeleton, the skin is the skin: what is the use of a non-metallic expansion joint frame
The non-metallic expansion joint is composed of skin, insulation layer, frame and platen. Many people pay full attention to the skin. After all, the skin directly contacts the medium and looks the most "conspicuous". But experts know that the framework is the real role. It is like a human skeleton, and the skin is nothing but skin.
The frame should bear the thrust generated by the medium pressure, the reaction force generated by the thermal expansion of the pipeline, the alternating load caused by the vibration of the equipment, and the skin should be tightened to maintain the sealed shape. Once the stiffness of the frame is not enough or the material is selected wrong, the skin has not reached its life, the frame is deformed first, and the whole expansion joint will be wasted. You say it was wrong or not?
Wrong material selection, rust wear in three years: Q235, 304 or heat-resistant steel?
The choice of frame material directly determines the expansion energy saving for several years. We have seen too many-the non-metallic expansion joint on the desulfurization flue must be framed with Q235 carbon steel. It looked fine for the first two years, but in less than three years, the surface of the angle steel was rusty like shortcake, and the slag fell when the hand touched it. In this kind of working condition, with high chloride ion concentration, high humidity and condensing acid, how can Q235 bear it?
Generally, 304 stainless steel is used for corrosive environment, and 316L is obtained for desulfurization slurry and chloride-containing environment. For real high-temperature flue gas pipes, such as hot air duct of power station and tail flue of boiler, the frame must be made of heat-resistant steel, such as Q345R, 15CrMo or higher grade heat-resistant alloy. When the temperature exceeds 400℃, the allowable stress of ordinary carbon steel drops linearly, and the creep deformation can't run.
When you choose the frame material, do you choose it according to "can be used" or "working conditions have been calculated"? These two standards, the result can be several times the difference of life.
Profiles alone are not enough, but structural design can be seen: stiffness and reinforcing ribs of large-size rectangular frame
Non-metallic expansion joints are most commonly used in power plants and cement plants with rectangular cross sections, which are one or two meters wide at every turn. Such a rectangular frame with a large opening, after bearing internal pressure, the side plates will bulge outward, the four angular stresses will be concentrated, and the middle deflection will be the largest. If only two angle steel pieces are selected and welded according to experience during design, the frame itself will bend first without waiting for the skin to leak.
Large-size rectangular frames must be stiffened. The layout spacing of horizontal bars, vertical bars and diagonal braces should be calculated according to finite element method, instead of patting your head and welding a few casually. Web height, flange thickness, weld form, each item must match the stress model. Two years ago, there was a project. The flue section was 3m ×2m, the frame was welded with No.10 channel steel, and the spacing of reinforcing ribs was 800mm. As soon as the machine was started, the negative pressure sucked in the side plate by more than 20mm, and the skin was torn on the spot. Later, the No.12 channel steel was changed, and the reinforcing ribs were encrypted to 400mm, which was considered stable.
To put it bluntly, the frame is not as simple as "profiles put together a frame". If the stiffness is not enough, the skin is equivalent to rubbing on the blade, and no matter how good the non-metallic skin is, it will be useless.
Just stick the skin on? Pressing plates, bolts and insulation coatings can not be accommodated
Some people think that when the frame is welded, the skin is laid up, the pressure plate is pressed, the screws are tightened, and it's done. It's so simple, and there won't be so many leaky expansion joints on the market.
The width, thickness and bolt spacing of the pressure plate directly affect the sealing pressure at the edge of the skin. The pressure plate is too narrow, the skin can't be held down, and the wind blows and lifts the edges; The spacing between the bolts is too large, and the skin in the middle of the two bolts is bulging, which will wear out over time. Besides, the bolts themselves, in an acidic environment, ordinary galvanized bolts will rust to death in half a year, and they can't be screwed off even if they want to maintain them. At this time, you have to use stainless steel bolts, and the preloading force should be uniform. It is best to tighten them three times in sequence with a torque wrench.
And thermal insulation. If the inside of the skin is in direct contact with high-temperature smoke, the temperature may exceed 200℃, and the skin material can't bear it at all. Whether the thermal insulation cotton is filled or not, whether the thickness of the fill is enough or not, and whether there is any misalignment directly determine the surface temperature of the skin. Some manufacturers try to save trouble, and the thermal insulation cotton plug is crooked. After a few months of operation, the skin is burnt from the inside, and the exterior looks quite new-is this money spent unjustly?
On-site installation rollover case: How did the anchor bolts come loose and the frame deformed
No matter how good the equipment is, if you don't pay attention to the installation link, there will still be problems. I saw a pipe in a chemical plant. After the non-metallic expansion joint was installed, the pipe flanges on both sides were fastened to the frame. When the screws were tightened, the frame was twisted into a parallelogram, and the skin was wrinkled like a rag. This is not a design problem, it is a deadlift without alignment during installation.
Another time, the anchor bolts weren't tightened. Frame fixed to concrete bearings, four anchor bolts, three screwed and one unscrewed. As soon as the system runs, as soon as the vibration comes up, the looser and looser the untightened bolt, the corner of the frame is suspended, the whole expansion joint is skewed, and the skin is excessively stressed on one side, which is torn apart in half a year. When I went to the scene, I saw that the bolts were all polished, and obviously they had been shaking since they were loose.
Transportation and hoisting can also deform the frame. The diagonal error of the rectangular frame exceeds 5mm, so you have to take a jack hard top when installing it. After the top, the internal stress is still there, and it is slowly released after running, and the frame is crooked again.
Therefore, the non-metallic expansion joint is not a good skin choice, and everything will be fine. Whether the frame material is correct, the stiffness is not calculated, whether the pressure plate bolts are matched, and whether the installation is according to the rules. Which ring leaks, and all the money spent in front will be wasted. The equipment manufacturer has given you a rectangular non-metallic expansion joint. Don't rush to install it when you get it. Check the frame size, material report and weld inspection record, and then go to the site. Save that half an hour, and possibly save an unplanned downtime later.