1. What exactly is the U-shaped multilayer metal bellows expansion joint? — — Start with structural disassembly
To be honest, many pipeline engineers who have worked for ten years may not be able to explain the structure of expansion joints thoroughly. Let's talk today, get oneU-shaped multilayer metal bellows expansion jointTake it apart and see-it's essentially a retractable metal "spring tube", but it's not as simple as you think.
The core part is the bellows, which is formed by hydraulic or rolling of thin-walled stainless steel plates and has a U-shaped cross section. If a single layer is not enough, there are multiple layers, and the layers are closely fitted, not like matryoshka dolls with gaps. And guess what? One more layer is not intended to add thickness, but to share stress.
In addition to bellows, there are end tubes, deflectors, tie rods, and nuts. The role of the guide tube is always neglected-it is installed on the inner wall of the bellows, which directly blocks the erosion of high-speed fluids, especially high-temperature dusty media. About this siteSpecific Function of Expansion Joint Guide TubeIt is clear in that question and answer that the guide tube also reduces flow resistance and prevents eddy vibration.
Let's talk about the tie rod-this is something many people think is just a transport protector. Wrong! The tie rod is the core to withstand the thrust of internal pressure. Without a tie rod, the multilayer bellows will bulge like a balloon under high pressure. When adjusting the tie rod nut, pay attention to symmetrical screwing, otherwise the offset will lead to the instability of the bellows. The specific adjustment method can be seen in this siteHow to adjust the tie rod nut of expansion jointQ&A.
Second, what is good about multi-layer design? Triangulation of pressure, fatigue, and stiffness
Single-layer corrugated pipe does not bear enough pressure, then thicken? Don't worry, if it is too thick and stiff, the compensation amount will be gone. The multi-layer design is like layers of tissue paper stacked together, which is easier to bend than a thick sheet of paper, but instead has higher tensile strength. It sounds awkward, but that's how the mechanics of materials play.
In terms of pressure, each layer of the multilayer structure shares a part of the stress. According to the standard GB/T 12777, the burst pressure of the multilayer bellows is approximately more than 1.5 times of the same total wall thickness of a single layer. Fatigue life is more pronounced-you think about it, if a single layer bends repeatedly, it cracks quickly; However, slip can be achieved between multiple layers, friction energy dissipation, and the number of fatigue cycles can be increased by 2~3 times. Therefore, used in the power station industryCorrugated expansion joint for power station industryAnd cement industryMetal Corrugated Expansion Joints in Cement IndustryHigh-pressure and high-temperature working conditions are almost all multi-layer designs.
Stiffness? Because each layer deforms independently, the overall stiffness of multi-layer bellows is 20% ~30% lower than that of single layer with equal wall thickness. This means that under the premise of meeting the pressure requirements, the same amount of compensation can be achieved with fewer corrugations-if the length of the equipment is shorter, will the cost also come down?
Third, the three details that are easiest to overturn during model selection. Have you been recruited?
Two days ago, I met a customer and chose a general-purpose corrugated expansion joint to install it on the steam pipe. As a result, it leaked in less than three months. Check, the temperature is 500 degrees, and the ordinary 304 is used for model selection. Hey, I've seen too many of these pots.
The reduction of the permissible stress of the material by temperature is ignored. The yield strength of stainless steel drops at high temperature, and the allowable stress of 304 at 300℃ is only about 60% of that at normal temperature. The cement industry should pay special attention to the fact that the flue gas temperature is always above 600℃. At this time, heat-resistant alloy steel or high-temperature expansion joint must be used. This station has a special product line of high temperature axial expansion joint.
Only the axial compensation is counted, regardless of the lateral displacement. Although U-shaped multilayer bellows can absorb axial displacement, if your pipe has lateral offset or angular displacement, you must choose a tie rod or hinge structure. For example, the air-cooled island vacuum pipe uses double hinge expansion joints, otherwise the bellows will be twisted into twists.
Ignore deflector length. The guide tube must extend to the inner wall of the pipe, otherwise the medium will flush directly from the bellows trough and perforate within a year. Many engineering sites cut corners, and only half of the guide tube is done, which has endless troubles.
4. Installation and debugging: draw rod, guide tube and arrow direction, none of them can be wrong
Let's start with the arrow-that arrow on the expansion joint housing is not a decoration, it represents the direction of media flow. When installed backwards, the guide tube becomes a baffle, and the medium flows back into the bellows trough, and the pressure fluctuation can directly crack the weld. Q&A on this siteThe direction of the arrow of the expansion joint refers toThere are pictures and truth in it.
How to adjust the tie rod? When leaving the factory, the tie rod nut is generally loose, and the transportation fixing rod should be removed or loosened. Once installed in place, allow the pipe to be naturally aligned before pre-stretching or pre-compressing. The specific stretching amount is generally equal to 1/2 of the compensation amount according to the design drawings. Don't forget to lock the nut after adjustment-otherwise, when the pressure is pushed during pressure test, the pull rod will follow, and the bellows will be directly crushed to death.
When installing the guide tube, pay attention to the direction, and the open end faces the direction of the medium. If the pipe is bidirectional flow (such as a thermal circulation system), you need to install guide tubes at both ends, or choose a double seal structure-like this siteDouble-sealed single-axis circular baffle doorThat line of thinking.
In addition, for the installation of the expansion joint of the large tie rod, the pipe support must not be set directly below the expansion joint, but should be set on both sides of the end pipe. Otherwise, the sinking of the pipe will bend the expansion joint. Refer to this site for the correct installation methodCorrect installation method of expansion joint of large tie rod。
V. Disassembly of typical application scenarios: how to choose between power station, cement and chemical industry?
Power station boiler smoke air duct-the main problem is large diameter, high temperature, ash containing. It is recommended to use large diameter thick wall expansion joint or double hinge transverse expansion joint. Pay attention to wear-resistant guide tubes, and choose large wave height multi-layer design to absorb thermal displacement. There are also matching flue gas baffle doors and desulfurization flue gas baffle doors in this station, and the combined use effect is better.
Cement industry-The kiln tail flue gas temperature is high (900 ℃ +) and the dust concentration is high. Here, conventional metal bellows cannot be used, but non-metallic expansion joints (fabric fiber expansion joints) or high-temperature metal expansion joints must be used. If you have to choose U-shaped multilayer metal bellows, the outer layer must be sprayed with high-temperature resistant ceramic coating, and the inner lining guide tube should be thickened. The corrugated expansion joints used in the cement industry are all customized parts, so don't try to save trouble and buy general models.
Chemical industry-media is highly corrosive and may have pressure fluctuations. Here a PTFE-lined hose or PTFE compensator is often a better choice. However, if the pressure is high and the temperature is between-20~200℃, U-shaped multilayer metal bellows can also be used, provided that the material is 316L or duplex steel. In addition, the vibration of chemical pipeline is large, so it is recommended to match rubber compensator for vibration reduction.
One more word, don't confuse the expansion joint with the compensator, this site question and answerIs the expansion joint the same as the compensatorIt is one thing to say clearly that they are. However, in the specific selection, the four parameters of pressure, temperature, displacement and medium should be clearly sorted out according to the working conditions, and then determined according to the product data. If you are really unsure, you can check this siteMetal expansion joint weight tableTo estimate the specifications, but in the end, it has to be calculated by the manufacturer.