Why is the rectangular expansion joint a "big problem" in high temperature conditions?
Rectangular flues in power stations or cement plants, thermal displacement is much more complicated than circular pipes. Circular pipes are mainly axial expansion and contraction, but rectangular flues have transverse and angular compensation requirements in addition to axial direction. In addition, the flue gas temperature can often be five or six hundred degrees or even higher, and ordinary rubber compensators or non-metallic expansion joints (such asNon-metallic expansion joint (fabric fiber expansion joint)) Can't bear it at all-rubber begins to become brittle above 150℃, and even if silica fiber is used for non-metals, the upper limit is just over 400℃. At this time,Metal rectangular high temperature corrugated expansion jointIt becomes the only option that works. However, the products in the market are mixed, and there are many cases of model selection and rollover. Two days ago, I met the equipment supervisor of a cement plant. The rectangular expansion joint they bought at a large price leaked in less than half a year after installing it. When I removed it, the crack in the corner of the bellows was like a spider web. The reason? The material was chosen wrong, and the design did not do finite element analysis.
Structural "disassembly" of metal rectangular high-temperature corrugated expansion joint
Although it looks like an iron box, there are many internal doors. Bellows are typically made of austenitic stainless steel, such as 321, 316Ti, or even nickel-based alloys such as Incoloy 825. The wall thickness is determined according to the design pressure and temperature. If it is thin, it will be easy to fatigue, and if it is thick, the compensation ability will be discounted. There are two ways to join them: flanged and welded. The flange is suitable for frequent maintenance conditions, such as near the desulfurization flue gas baffle door; Welding is more seal-friendly, and is suitable for situations where you want to remain unmoved for ten years after one installation. The guide tube is essential-we have answered in detail in the FAQ. The guide tube guides the high-temperature flue gas to the inner wall of the bellows to avoid directly washing the trough. The service life of the expansion joint with the guide tube can be more than doubled. Ring plate, tie rod, limit bolt and these accessories are listed in FAQHow to adjust the tie rod nut of expansion jointAs Li also talked about, they control the expansion joint to not exceed the displacement and prevent the bellows from becoming unstable. Note that the role of the tie rod is not to bear the weight of the pipe, but to limit the direction of displacement.
"Baking" Test of Bellows Material by High Temperature
Many people think that as long as it is stainless steel, it can withstand high temperatures, but it is not. 304 starts to oxidize and peel at 400℃, and 316L barely sustains to 500℃, but the risk of intergranular corrosion is very high. If you really want to dry above 600℃, you have to use 321 (stabilization treatment) or Incoloy 800 series. For example: the flue gas temperature at the outlet of the rotary kiln of a cement plant is 700℃, and the original 304 bellows cracked in three months-not burned through, but high temperature fatigue and intergranular corrosion. Later, the 321 material was changed, and with a reasonable number of layers and wave height design, it ran for more than a year without any problem. By the way, the fatigue life of the material at high temperatures is discounted, which is directly related to the service life of the expansion joint-we have in FAQService life of expansion jointAs mentioned in, life is strongly related to temperature and the number of cycles. Therefore, when selecting the model, don't just look at the rated temperature on the sample. Ask the manufacturer: Under this specific working condition, how many cycles is the design life?
Design difficulties brought by "rectangle"
It is also a metal bellows. There is a mature formula for calculating the stress of the circle, but the rectangle is much more troublesome. Different length-to-width ratio of rectangular section, the stress distribution of bellows is extremely uneven, and the stress concentration coefficient at corner may be 2~3 times that of straight edge. Rectangular expansion joints made by many manufacturers either have a large wave height to reduce stress, or are thickened or reinforced at the corners or adopt combined corrugated structures. Another approach is to directlyMetal rectangular expansion jointThe corner uses double-layer waves, but the cost also goes up. In addition, the flow resistance in rectangular pipes is also a problem, and the shape and thickness of the guide tube directly affect the pressure loss. When selecting a model, be sure to ask the manufacturer to provide a finite element analysis report for your aspect ratio and compensation amount-don't just look at the parameters on the sample. And guess what? Some small factories can't even draw CAD drawings, so they directly take the formula of circular expansion joints to set rectangles. Do you dare to use this product?
"Pits" and "Dao" in installation and maintenance
When installing metal rectangular high temperature corrugated expansion joint, there is a very critical choice: cold tightening (pre-tensioning) or hot tightening? This depends on the operating temperature of the pipe and the direction of compensation. Generally, high-temperature pipelines are recommended to be cold and tight, and the pre-tension amount is designed according to about 50% of the compensation amount-but you have to consult the manufacturer for confirmation, because the best pre-tension value of bellows with different wave heights and layers is different. When installing, the direction of the arrow of the guide tube must follow the direction of the medium flow. This is in FAQThe direction of the arrow of the expansion joint refers toLi specifically explained, don't install it backwards, otherwise the smoke will directly wash the trough and penetrate in two or three months. In addition, the tie rod nut needs to be checked and adjusted in the early stage of operation-we found that many customers left it alone after installation. As a result, after a few months, the displacement exceeded the design range, and the bellows was pulled to death or squashed. The pipeline support cannot be lazy, the rectangular expansion joint itself does not bear the weight of the pipeline, and an independent pipeline support must be set. The most outrageous case we have ever seen: the customer used the expansion joint as a load-bearing part, and the whole flue collapsed. Alas, this kind of accident is completely avoidable.
When should you use a metal rectangle and when is it more cost-effective to use a non-metal one?
This question is often asked by clients. Non-metallic expansion joints (e.g.Rectangular non-metallic expansion joint) The upper limit of temperature resistance is about 400℃, and a few silica gel fibers can achieve 450℃, but its advantages are large compensation, low cost and convenient installation. When the medium temperature exceeds 400℃, or there are corrosive gases (such as flue gas after desulfurization), metal becomes the only option. However, metal rectangular high-temperature corrugated expansion joints are expensive, and a set of DN1000×600 rectangular parts may be tens of thousands. Therefore, if the temperature is controllable and the working conditions are not extreme, it is reasonable to try the water with a rectangular non-metallic expansion joint first. We have both metal rectangular expansion joints, rectangular non-metal expansion joints and fabric fiber expansion joints in our product library, which customers can match according to actual budget and parameters. However, one thing to remember: non-metallic expansion joints are sensitive to particle erosion in flue gas. If there is more fly ash in flue gas, metallic joints will be more resistant to manufacturing. In short, don't be cheap or overdesign-finding the most suitable balance point is the skill.