Wave nodes are not decorations, they are the "heart" of expansion joints
Many people think that the metal expansion joint node is just a circle of ripples, nothing more than to look good or lose weight. And guess what? That's a bad idea. The node is the only component in the whole compensator that can produce elastic deformation. The thermal expansion and contraction of the pipeline, mechanical vibration and installation deviation-wherever the displacement needs to be absorbed, the ripples of the node all depend on it. To put it bluntly, the node is not designed well, and the expansion joint is a pile of scrap metal.
How exactly does that wave knot work? When the pipe is elongated by heat, the node is compressed and the distance between the crest and trough is narrowed; When the pipe cools and shrinks, the nodes are stretched and the corrugation spacing is pulled apart. This repeated deformation is the "life" of nodes. If the design is not accurate, the crack will come to your door after hundreds of cycles.
The three parameters of waveform: height, distance and thickness, none of which can be ambiguous
Wave height, wave pitch and wall thickness-these three values are the lifeblood of wave node design. A larger wave height means a larger amount of displacement each wave can absorb, but at what cost? The voltage withstand capacity of the node plummets. Imagine a balloon, the bigger it blows, the easier it is to burst. If the wave distance is too small, the stress will be concentrated at the root of the trough, and the fatigue life will be greatly reduced; The wave distance is too large? Not to mention material waste, the overall stiffness will also be soft.
For example, the general-purpose corrugated expansion joint of the same DN300 has a wall thickness of 1.5mm in a normal temperature steam pipeline, but once the medium temperature exceeds 400℃, the wall thickness must be increased to 2.5mm or even a multilayer structure must be adopted. The general-purpose corrugated expansion joint and the high-temperature axial expansion joint in this station are shaped based on these parameter differences. Which do you think should you choose under high temperature conditions? Got it.
By the way, there is another point that is easily overlooked: multiple layers of nodes do not mean simply layering a few layers of skin. Leave gaps between layers, otherwise friction heat generation will accelerate failure. In the real case, some people welded the two layers to death in order to save costs, but it leaked after half a year.
Select materials to pick details, don't let stainless steel "back the pot"
304, 316L, 321, Incoloy 825…Every stainless steel has a different temper. 304 is cheap, and the temperature resistance range is about-20℃ to 800℃, but it will be finished when it encounters an environment containing chloride ions-pitting and stress corrosion will crack sooner or later. The expansion joint matched by the desulfurization flue gas baffle door is acidic flue gas after wet desulfurization, with high humidity and high chloride ion concentration. At this time, 316L is the starting line. If it meets high sulfur coal, Incoloy 825 is reliable.
The cement industry is even more ruthless. Working in high-temperature dusty smoke for a long time, the surface of the node should not only be corrosion-resistant, but also be able to withstand dust erosion. The metal corrugated expansion joint of cement industry in this station has made a wear-resistant protective layer on the outer surface of the node, which is not a gimmick, but the experience of repeatedly eating ash. If you don't do it, the trough will wear out in half a year.
Three ways of dying: fatigue, corrosion, instability
Designers have to know the three most common failure modes of nodes. The first is fatigue fracture-the steam pipeline of the power station starts and stops every day, and when the number of cycles of thermal expansion and contraction is enough, micro-cracks appear at the root of the trough, and then they crack through. The second is stress corrosion cracking, and chloride and hydrogen sulfide are invisible killers. If you don't know the composition of the medium when you choose materials, it will be too late to replace it if it leaks. The third is instability deformation-the wall thickness is too thin or the pressure exceeds the limit, the trough collapses, the peak bulges, and the whole node becomes a twist. There is only one core to solve these problems: explain the working parameters (temperature, pressure, displacement, number of cycles) clearly before selection, and don't hide them.
Two days ago, I met a customer who said that their pipeline pressure fluctuated greatly but didn't provide peak data. As a result, it became unstable after two months of installation. Who do you think the blame should be thrown on?
Type selection is not picking nodes, it should be looked at together with accessories
Just staring at the node parameters is not enough. For example, the external pressure single axial expansion joint, the node design is used to absorb pure axial displacement, and the internal guide tube is provided to reduce medium erosion. And straight pipe pressure balance type expansion joint? People use the double node structure to eliminate the blind plate force, so that the pipe support does not have to bear extra thrust. If you mount the axial type on a pipe section with lateral displacement, the node will be wasted if you twist it a few times.
Look at the node in the expansion joint of the large tie rod again-the function of the tie rod is to constrain the lateral displacement, so that the node only bears axial deformation. Pay attention when adjusting the tie rod nut (for details, please refer to the FAQ "How to adjust the tie rod nut of the expansion joint" on this site). If you tighten it too tightly, the node will be locked and the compensation ability will be lost. There are also hinges, double hinge transverse expansion joints, curved tube pressure balance types... Each model of node has its own specific "partner", and choosing the wrong one is equal to wasting money.
Installation and maintenance: The details make or break
The insulation layer in the node area should not be pressed too tightly. Some people think that the thicker the insulation layer is, the better, and they strangle it with iron wire. As a result, the node is caught and can't deform freely, and the displacement that should be absorbed is transmitted to the pipeline. Also, be sure to prevent nodes from bumping when transporting and storing – even a small pit can be the starting point for fatigue cracks. During regular inspection, focus on whether there are corrosion pits or cracks on the surface of the node, especially the heat-affected zone of the weld. Don't wait for a leak to cry.
Hey, the longer I work in this business, the more I feel that the design of metal expansion joints is an art of balance. Wave height, wave pitch, wall thickness, materials, accessories, installation... Which link goes wrong will put a question mark on the safety of the whole pipeline. It takes ten more minutes to explain the working conditions clearly when selecting the model, and the equipment can be used for several years. Is that the case?