Over the years of making expansion joints, the most common sentence that customers have heard is: thicker and strong. But the thickness of the node of the metal expansion joint is really not the thicker the iron sheet, the better. Find out one thing first: the node is the part of the expansion joint that really deforms and sucks, and the pressure, displacement and temperature are all pressed on it. How much thickness is chosen directly determines how many years this expansion energy saving will last and whether it can withstand the thrust and fatigue cycle of the pipe system.
Pressure, temperature and compensation amount, how do these three hard indicators affect the node thickness?
Let's start with stress. The higher the pressure, the greater the pressure thickness required by the node, which is easy to understand. However, it should be noted that the thickness mentioned here is "pressure-bearing thickness", not just thickening it casually. When calculating, it is usually checked according to EJMA standard or GB/T 12777, and the circumferential stress and meridional stress generated by internal pressure should be within the allowable range.
Temperature is more direct. The higher the temperature, the more obvious the material permissible stress decreases. Take 304 stainless steel as an example. The allowable stress can be more than 130MPa at room temperature, and it may be less than half at 600°C. Therefore, under high temperature conditions, the node thickness has to be increased accordingly, or more temperature-resistant materials, such as 316L and Inconel 625, have to be changed.
The amount of compensation is the easiest to miscalculate. The larger the compensation amount, the larger the node deformation amplitude. At this time, it is not the thickness of single layer, but the combination of multi-layer thin plates to share the strain. Why? Because the single layer is thick, the bending stress will increase sharply, and the fatigue life will drop sharply. Therefore, in the working conditions of large diameter and large compensation, almost all of them use multi-layer structures.
What about that? Three parameters such as pressure, temperature and compensation are put into the calculation model simultaneously, and the node thickness, layer number and wave number are checked cyclically. Adjust which one is not satisfied until the fatigue life and stability are all passed.
Thick but short lived? The Game of Fatigue Life and Stiffness
Many people think that thickening is durable. In fact, if the node is too thick, the bending stress will increase sharply and the fatigue life will decrease. It's not alarmism. Every time the node bends, a tensile and compressive strain occurs on the inner and outer surfaces. The larger the thickness, the larger the strain under the same amount of deformation, and the earlier the fatigue crack appears.
So you see, high-temperature axial expansion joints and large-diameter thick-walled expansion joints, these products almost all adopt multi-layer thin plate structure in design. Two layers of 3mm are not as good as four layers of 1.5mm to bear fatigue. With the same total thickness, the more layers, the smaller the strain of a single layer and the longer the life. The trade-off is that the stiffness increases a bit, but it can be adjusted by wave number and wave height.
Alas, I have seen too many cases where the expansion joint leaked in half a year because of "more thickening" on the spot. You really can't pat your head when you select a model.
How to implement the node thickness under different working conditions?
Corrugated expansion joints used in power station industry and metal corrugated expansion joints in cement industry have great differences in working conditions, and the treatment of node thickness is completely different when selecting the type.
The power station industry, especially the boiler flue duct and steam turbine pipeline, has low pressure but high temperature, large displacement, and frequent start-and-stop. At this time, the node thickness tends to be thinner and more layers, and multi-layer structure is used to carry heat fatigue. Like high-temperature axial expansion joints, 4 layers, 6 layers or even more layers are often superimposed, and the thickness of a single layer is controlled between 1mm and 1.5mm.
What about the cement industry? Dust wear is the worst enemy. In addition to compensating for displacement, metal corrugated expansion joints in cement industry have to be wear-proof. The node thickness should not be too thin, otherwise it will wear out in a few strokes. Usually wear-resistant bushings are added at the trough position, or thicker single-layer boards are selected, with fewer layers. Under wear conditions, thickness is life, but fatigue should also be taken into account. Generally, it will be made into two or three layers, plus guide tube protection.
High temperature flue gas pipes are another matter. Flue gas temperature fluctuates greatly, and there is sulfide corrosion. At this time, not only the thickness, but also the material. The selection of corrosion-resistant alloy is more critical than simply thickening. If you thicken the ordinary 304 to 3mm to carry 600°C sulfur-containing flue gas, it is better to use 1.5mm 316L to be reliable.
How to check the node thickness during acceptance? Don't be fooled by the numbers on the drawings
The thickness of the node of the metal expansion joint is not just marked by the nameplate. On-site acceptance, take a thickness gauge to measure the thickness of peak, trough and straight edge section, and all three positions have to be measured. The thinning ratio of crest and trough is different in the molding process, and the heat affected zone may be generated in the straight edge section due to welding.
Why measure three positions? Because the crest is stretched, the thickness becomes thinner; The trough is compressed and may be slightly thicker. The design drawings say "nominal thickness", which may be 15% to 20% thinner than the nominal thickness at the crest after actual molding. This amount of thinning is considered in the allowable value of the calculation, but only if you confirm that the actual thinning does not exceed the design margin.
And the number of layers. The expansion joint of multi-layer structure is useless to measure the total thickness by light, so it has to be disassembled to see the thickness of single layer. And guess what? Some manufacturers will make up the number of layers, but there are only 3 layers in the mark of 4 layers. The thickness of each layer, the fit between layers, and whether there are scratches have to be checked. Measurement methods, allowable deviations, and comparison with design documents are the keys to avoid "shrinking goods" entering the site.
There is no uniform "standard value" for node thickness, only "calculated value". You take the same expansion joint and ask three factories, and they may quote you three thicknesses. This is not a question of who is right or wrong, but the design margin and fatigue check of each are different. When reviewing the drawings, don't just stare at the thickness number. Only by comparing the five parameters of pressure, temperature, compensation, fatigue life and material can you see the doorway.