What exactly is a wave crest? Don't be fooled by the name
The crest of the metal expansion joint, to put it bluntly, is the raised "fold" on the bellows. The flexibility of the whole expansion joint depends on it-there is no wave crest, and the metal shell is a dead pipe, which collapses directly when the temperature expands. Look at the general-purpose corrugated expansion joints and high-temperature axial expansion joints in our station. Their core components that can absorb displacement are the circles of wave peaks. To put it bluntly, the crest design is smashed, and the whole expansion joint is just a decoration.
Can the peak parameters be determined casually? Of course not. The three numbers of wave height, wave pitch and wall thickness contain each other. The higher the wave peak, the greater the compensation amount, but the pressure resistance decreases; The denser the wave pitch, the greater the stiffness, but the fatigue life may not be good-there is a balance here. Two days ago, a customer asked about the large-diameter thick-walled expansion joint of DN800, saying that the wave peak was too shallow, which led to insufficient compensation. This is because the parameters were not counted correctly.
How much is the difference of wave peak morphology under different working conditions?
The corrugated expansion joint used in the power station industry usually has a multi-layer thin-wall design, which can carry high temperature and high pressure by the number of layers. Think about it, the working conditions of several hundred degrees in the steam pipeline can't be held by a single thin wall at all. What about metal corrugated expansion joints used in the cement industry? The crest has to consider the problems of wear resistance and dust accumulation-once there is too much dust, dust will get stuck in the crest groove, and it will fail after a long time. There is also a hose lined with PTFE metal, and the outside of the wave peak is covered with fluoroplastic, which has good anti-corrosion performance but cannot bear too high pressure. When selecting the model, don't take the peak parameters of the general type to the corrosion condition. That is not to save money, but to lay mines.
In turn, some working conditions require the crest to be "soft" instead. For example, the double hinge expansion joint used in air-cooled island vacuum pipeline has a very "soft" wave peak design, which mainly absorbs angular displacement. Do you think it's okay to have a rigid crest?
What is most likely to go wrong with the crest?
Crack and instability, these two old enemies. Cracks mostly occur in trough transition zones, especially in scenarios with frequent hot and cold cycles, such as steam pipes. And guess what? The question and answer on "Service Life of Expansion Joint" in our station clearly states that the fatigue life is directly linked to the peak stress concentration factor. Want to live longer? Optimize the radius of the arc at the root of the wave crest, it's as simple as that.
Instability is when the crest is flattened or bulged, usually because the internal pressure exceeds the design value, or because the cold pull is excessive during installation. Some on-site masters pulled the expansion joint over in order to force the pipeline alignment, and as a result, the wave peak directly "collapsed". This is not a problem with the expansion joint, it is a man-made disaster.
Talking back to type selection-how to pick the crest combination according to the media and pipeline layout?
For corrosive ones, preference is given to PTFE-lined hoses or PTFE compensators, with a protective layer on the outside of the crest. Look at the pipeline layout again: if the axial displacement is large, use straight pipe pressure balance expansion joint (the one with balance ring, the wave peak only bears axial displacement and does not hold pressure); Large-diameter low-pressure pipeline, with external pressure single axial expansion joint, outside the wave crest is protected by an outer sleeve, not easy to unstable.
If the working conditions are particularly complicated, such as desulfurization flue gas system, where temperature, pressure and corrosion are all accounted for, then we have to use desulfurization flue gas baffle door with non-metallic expansion joint? No, the metal rectangular expansion joint will also work, and the wave peak must be recalculated according to wear resistance and corrosion protection. Really unsure? Throw the working condition parameters over, and we will run them for you with the stiffness formula. Anyway, the products in our station range from high-temperature axial type to double hinge transverse type, from direct burial type to rotary compensator, and there is always a peak parameter that meets your needs.
Tsk, at the end of the day, the more complex the metal expansion joint crest is, the better, the more matched the better.