What exactly is an S-shaped metal expansion joint? How is it different from ordinary corrugated expansion joint?
Many engineers heard this name for the first time and thought it was some special shape of bellows. In fact, it belongs toUniversal corrugated expansion jointA subdivision of, the core difference is the geometric design of the waveform-the cross-sections of the crests and troughs are S-shaped transitions instead of the traditional U-shape or omega-shape. The advantages of this structure are obvious: under the same wall thickness, the S-shaped wave can withstand higher internal pressure and greater axial compliance. Two days ago, a customer who made steam pipes asked me why his U-shaped expansion joint cracked after two years of use, and it was okay after changing to S-shaped? The reason is here: the stress of the U-shaped wave is concentrated at the root of the wave peak, and it is easy to get fatigued when the internal pressure comes; The S-shape uses a smooth curve to disperse the stress, so it can naturally bear it.
From the perspective of mechanical properties, the stress distribution of S-shaped metal expansion joint is more uniform
The traditional U-shaped wave tends to generate stress concentration at the root of the crest and trough, while the S-shaped wave reduces the peak stress by a smooth curve transition. According to the empirical data mentioned in "Bellows Stiffness and Calculation Formula" on the station, the fatigue life of S-shaped bellows is more than 30% higher than that of U-shaped bellows under the same displacement. Of course this is not absolute either-you have to look at the actual conditions. If it is a high temperature and high pressure power station pipeline, it is recommended to matchHigh temperature axial expansion jointOrExternal pressure single axial expansion jointBecause their multilayer corrugated design can further disperse thermal stress. Don't underestimate this detail. Many on-site failure cases are because the waveform selection is not done correctly.
In addition, there is a special formula for calculating the stiffness of S-shaped corrugation, which can't directly set the empirical value of U-shaped corrugation. If you do your own calculation, remember to check the correction coefficient in "Bellows Stiffness and Calculation Formula". Of course, software is used now, but the principle must be understood: under the same displacement, the thrust required by S-shape is 15% ~20% smaller than that of U-shape, which is good news for the design of pipe supports.
The biggest headache in model selection is how to balance displacement compensation ability and stability
The S-shaped metal expansion joint, although flexible, is prone to destabilization when subjected to lateral displacement – especially in large-diameter models with lengths over 1.5 meters. At this time, you have to rely on the guide tube and the tie rod. stationaryCorrect installation method of expansion joint of large tie rodIt is clearly written in: the transport screw must be removed before installation, but the leveling nut cannot be locked at the beginning, and it must be fine-tuned according to the actual deviation after the pipeline is in place.
The guide tube of the S-shaped expansion joint is not optional. Under high-speed air flow or slurry scour, the guide tube can protect the inner wall of the corrugation and avoid direct impact of particulate matter. Look atMetal Corrugated Expansion Joints in Cement IndustryAndDesulfurization flue gas baffle doorIn all the wear cases, nine times out of ten are the consequences of saving the guide tube. The thickness and material of the guide tube should be selected according to the medium velocity-when the gas flow rate exceeds 30m/s or contains solid particles, it is recommended to use stainless steel to thicken the guide tube, and it should be matchedPTFE-lined hoseOrPTFE compensatorDo antiseptic treatment.
Talk about the application scenario: What working conditions are the most suitable for S-shaped metal expansion joints?
- First, thermal pipelineMedium and high temperature steam or heat transfer oil, pressure 1.0-2.5MPa, temperature below 350℃-this kind of scenario is better thanSleeve type pipe expansion jointCheap and more resistant to pressure than non-metallic expansion joints.
- Second, chemical plantIf there is slightly corrosive medium, use it with PTFE-lined hose or PTFE compensator. Note that the thickness of the lining layer should be adjusted according to the concentration of the medium.
- Third, the air-cooled island vacuum pipe systemIt is necessary to absorb both axial displacement and angular displacement. At this time, you can considerDouble hinge expansion joint for air-cooled island vacuum pipeline, it is actually equivalent to two S-shaped bellows plus hinge construction. Don't just replace it if you think the functions overlap. Each product has its best adaptation range.
If you are usingdirectly buried expansion jointOrCompound hinge transverse expansion jointS-shaped waveform can also be used, but attention should be paid to the corrosion prevention of the outer protective layer-the soil in the directly buried environment is much more corrosive than in the air.
Final key reminder: You must look at the direction of the arrow when installing the S-shaped metal expansion joint
This arrow is not decorative-it points in the direction of media flow and represents the mounting orientation of the deflector. If it is installed backwards, the guide tube will become a throttle, which not only produces pressure drop, but also easily causes vibration. I have seen a power plant install the arrow of the expansion joint in the wrong direction, and the pipe howls when the unit is running. Finally, it is found out that this is the minor problem.
In addition, does the screw of the expansion joint need to be removed? The answer is: the transportation screw must be disassembled, but the tie rod nut for adjustment cannot be completely disassembled, and the pre-compression space should be left according to the design displacement. If the operation of this step is not standardized, the life of the expansion joint will be directly halved. Whether it is a directly buried expansion joint or a double hinge transverse expansion joint, this principle is universal as long as it involves tie rod adjustment. How exactly to adjust? Stand onHow to adjust the tie rod nut of expansion jointThere are diagrams in the question and answer. I suggest you turn it out and have a look before installing it.
After all, the S-shaped metal expansion joint is not a panacea
If your piping system has severe vibration (such as reciprocating compressor inlet and outlet), it is recommended not to saverubber compensatorOrRubber PTFE compensator； If the flue gas temperature exceeds 600℃, be honest and practicalNon-metallic expansion joints (fabric fiber expansion joints)Add insulation. There is no shortcut in the project. Before the selection, the conditions of medium temperature, pressure, displacement direction, corrosiveness and installation space are clearly listed, which correspond to the parameter tables in the product information on the station, so there is basically no mistake.
Tsk, at the end of the writing, I found that there was another thing I didn't mention-the S-shaped metal expansion joint andMetal rectangular expansion jointIt's not the same thing. Rectangular ones are usually used in flue gas pipes and air duct systems. The waveforms are generally U-shaped or ω-shaped, and S-shaped ones are rarely used, because rectangular sections are inconvenient for smooth curve transition. Don't get mixed up.