What exactly is the corrugated expansion joint of metal steel strip? First understand how the bellows came from
The corrugated expansion joint of metal strip, to put it bluntly, is to roll thin-walled metal plates into corrugated pipes, and then match them with pipes, flanges and guide tubes to make an elastic element that can absorb the thermal displacement of pipes. How did the bellows come from? Bottom line: Stainless steel strips or plates are stamped and formed into wavy folds. Don't underestimate these wrinkles, they give a straight tube the ability to "telescope springs" that can be digested by axial stretching, compression, and even lateral deflection.
This kind of expansion joint can be seen everywhere in the smoke duct of power plant, the tail of cement kiln and the chemical pipe gallery. The materials are mostly 304 and 316L, and Incoloy 825 or GH3030 have to be used in high temperature occasions. The thickness ranges from 0.5mm to 3mm, and the number of layers is not fixed. Single layer, double layer and multi-layer are available. Why do multiple layers? Because of the same amount of compensation, multi-layer thin sheets are more flexible and have much better fatigue life than single-layer thick sheets. This principle means the same thing as a stack of paper is easier to bend than a thick piece of cardboard.
Although they are all metal expansion joints, bellows, guide tubes and tie rod nuts have their own jobs
A complete metal steel strip corrugated expansion joint is by no means as simple as just corrugated pipe. The bellows is the core, responsible for deformation and compensation. What about the deflector? It is mounted on the inner wall of the bellows to guide the medium flow smoothly and prevent the high-flow gas from directly washing the corrugated root. You think about it, the flue gas pipes carry dust particles, and without the guide tube, the bellows will be worn out in less than two years.
Tie rods and nuts are another system. The tie rod does not absorb displacement, its function is to constrain the bellows, convert axial displacement into transverse displacement, or simply limit overstretching. When selecting, when the working pressure is high and the pipeline thrust is large, the bellows alone can't bear it by itself, so the thrust has to be transmitted to the pipeline support by the tie rod. How to adjust the nut? Before leaving the factory, the manufacturer will generally adjust the pre-deformation amount, and after on-site installation, if it is not necessary to absorb the lateral displacement, the tie rod nut should be loosened to allow the bellows to expand and contract freely; If the design requires the pull rod to be stressed, the nut must be screwed to the designated position according to the drawing. There are a lot of doors here, which will be explained later.
Don't just focus on caliber and pressure in model selection. These three issues are the key
Caliber DN500, pressure 1.6MPa, how much is it? Of course, caliber and pressure are important, but with these two numbers alone, it is impossible to choose the type at all. You have to answer three questions first.
The first, displacement direction and magnitude. Is it axial elongation or compression? Is there a lateral offset? Is the compensation amount ±50mm or ±100mm? The design compensation amount, wave number and layer number of bellows are directly linked to this. Second, medium and temperature. Flue gas, steam, corrosive gas, is the temperature 200℃ or 800℃? When the temperature is high, the allowable stress of the material drops straight down, and the fatigue life of the bellows also shrinks. Third, installation space and pipe support arrangement. Where should the expansion joint be installed? Are there any fixing brackets on either side? Is there enough space for a tie rod? If these problems are not clearly explained, the manufacturer can only report a "standard product" to you, which often can't be installed at the scene.
How to match different working conditions? Universal type, external pressure single type, straight pipe pressure balance type, one pipe each
In the product information in our station, the metal expansion joint is very fine. Commonly used is a general-purpose corrugated expansion joint, which is suitable for ordinary pipelines, absorbs axial displacement, has low cost and simple structure. What about external pressure single axial expansion joint? The bellows is under pressure outside, the medium does not directly contact the trough, and it is not easy to accumulate dust. It is suitable for occasions that need to be cleaned, such as food and pharmaceutical pipelines.
The name of straight pipe pressure balance expansion joint sounds complicated, but in fact its function is very clear: it cooperates with the balance bellows and the pressure balance bellows, so that the expansion joint does not generate blind plate force while absorbing displacement, that is to say, it will not transmit the thrust generated by fluid pressure to the pipe support. This is particularly suitable for use on high-pressure thick pipes, such as the main steam pipes of power stations. There are also compound hinge transverse type and curved tube pressure balance type, which are all variants made for specific angle or space limitations. Don't be greedy for cheap selection, and don't pile configuration, according to working conditions.
Can the tie rod nut be removed during installation? Where is the guide tube? Don't make mistakes with these details
Installing this step, there are quite a few people who flip over. Let's start with the tie rod nut. When the expansion joint leaves the factory, for the convenience of transportation and hoisting, the bellows are generally locked with pull rods and nuts to prevent deformation during transportation. After installation, if the design requires pure axial compensation, the tie rod nut must be loosened so that the bellows can expand and contract freely. If you forget to loosen it, the expansion joint becomes a rigid short pipe, and the thermal expansion and contraction of the pipe all runs to the bracket and equipment, ranging from the deformation of the bracket to the leakage of the flange.
If the draw rod is designed to bear the internal pressure thrust, the nut must be locked according to the torque given by the manufacturer, and cannot be screwed to death at will. Let's talk about the direction of the guide tube. Arrows are generally drawn on the guide tube, which refers to the flow direction of the medium. Install backwards, the medium directly washes the bellows gap, and the noise, wear and vibration all come. Another point is that the pipes at both ends of the expansion joint must be provided with guide brackets to ensure the correct displacement direction, otherwise the bellows will be twisted.
Two frequently stepped pits: the thicker the ripple, the better, and the bigger the compensation amount, the better it is
Some users feel that the thicker the wall thickness of the bellows, the safer it is, which is putting the cart before the horse. Corrugated pipe absorbs displacement by elastic deformation. When the wall thickness increases, the stiffness will be greater. The stress generated under the same displacement will be higher, and the fatigue life will decrease. When selecting the type, the material, the number of layers, the wave height and the wave distance are all calculated by design, not determined by patting the head. It's thick, the displacement that should be compensated can't be compensated, and the pipeline stress has not been eliminated. Do you think this is safe or dangerous?
The greater the amount of compensation, the greater the wavenumber of the bellows, and the length of the entire expansion joint becomes longer. However, the wave number is too large, the stability is poor, and the column instability is easy to occur. Moreover, the compensation amount and fatigue life are contradictory: if you let the bellows eat the maximum compensation amount every time, its life will be drastically shortened. The rated compensation amount given by the manufacturer in the sample is calculated based on a certain fatigue life (such as 1000 times). If the actual working conditions start and stop frequently, it has to be used in a lower proportion. To put it bluntly, you should leave a margin when selecting the compensation amount, but leaving a margin does not mean blindly increasing it.
Alas, in this business for more than ten years, I have seen many leakage accidents caused by careless selection and random installation. The corrugated expansion joint with metal strip looks obscure, but it is a silent role in the pipeline system. You take it seriously, and it works safely; If you don't take it seriously, it will give you a look at the overhaul. Therefore, it is much less worry-free to find out the working conditions before the selection, and to buckle the details when installing.