Let's talk about the conclusion first: caliber is only the threshold, not all
Many purchases report "two expansion joints of DN200" as soon as they come up, which actually only talks about the surface layer. The size of pipeline metal expansion joint is not just by looking at the caliber. The diameter, wave number, length and flange spacing are each connected with the compensation amount and pressure level. If you choose a small bellows to crack, if you choose a large flange, you won't match it. Rework is a lesson bought by money.
In the size of the expansion joint, the four internal parameters of wave number, wave height, wall thickness and layer number really determine the performance, plus two installation parameters of total length and flange end spacing. To put it bluntly, the caliber is just a framework, and how to fill it is a technical job.
What exactly do the numbers in the model represent?
DN100-J, DN200-Z, the following letters indicate the connection mode and structural form. Take the general corrugated expansion joint of this station as an example. The number in the model is the nominal diameter, which corresponds to the inner diameter of the pipe. But don't think that DN200 must be 200mm. DN is the nominal diameter, not the actual outer diameter. It is just a nominal value for convenient comparison. In actual installation, the outer diameter of the connecting pipe depends on the steel pipe standard, and the flange connection depends on the flange standard. If it is an external pressure single axial expansion joint, there may be an additional set of numbers in the model, such as "DN300-16". This 16 usually represents the pressure level of 1.6MPa. When selecting, don't just stare at the caliber. The pressure grade has a great influence on the wall thickness and wave height, while the wall thickness and wave height directly change the wave pitch, and the total length naturally changes when the wave pitch changes. Therefore, it is normal for the same DN300 to have different pressure levels and a length difference of 100mm.
The amount of compensation is the core of size
The amount of compensation is the soul of bellows expansion joint, which determines the wave number, and the wave number determines the length. For every 1mm increase in axial displacement, it usually increases by half to one wave. Lateral displacement and angular displacement are more troublesome. They not only affect the length, but also require a specific geometry of the bellows, which can't be solved by adding wave numbers alone.
Take the straight tube pressure balanced expansion joint, for example, it has two sets of bellows inside, one set to absorb axial displacement and one set to balance pressure thrust. The dimensions of these two sets of bellows are linked, and the length of the middle cylinder depends on the wave number distribution of the bellows at both ends. The compound hinge transverse expansion joint is different. It depends on the hinge structure to bear the pressure and thrust, and the bellows are only responsible for absorbing the transverse displacement. Therefore, its dimensional logic is: the larger the transverse displacement, the longer the distance between the two groups of bellows. This is completely different from the axial idea of "just add wave number".
Think about it, if a point needs 30mm lateral displacement, and your installation space is only 500mm, then the double hinge lateral type may not fit in at all. At this time, you have to switch to a combination of two hinges, or replace a non-metallic expansion joint. Therefore, the size is not determined by the manufacturer, but the working condition is determined first, and the size follows the compensation amount.
Takeover or Flange? The mode of connection determines that size of the last segment
The attachment method directly affects the installation length. The flange thickness and sealing surface height should be calculated for flange connection, and the overlap length of the pipe extending into the expansion joint should be calculated for pipe connection. The same expansion joint can be made into a flange and a pipe, and the total length can be 50 to 100mm difference.
The numbers in the metal hose size comparison table look like standard values, but they have to be converted when actually selecting the model: the length of the hose refers to the distance between the end faces of the looper flange at both ends, not the length of the hose body. Many scenes can't be installed, just confusing this. The same is true for corrugated expansion joints. When ordering, it should be clear whether it is "end-to-end" or "take-over outer port to outer port". In the product information of this site, the metal hose and expansion joint are marked with these two lengths, so it is not easy to make mistakes by copying them directly when raising capital.
High Temperature, High Pressure and Large Caliber: Additional Practice Classes in These Three Scenes
When the high temperature axial expansion joint is above 600℃, the corrugated pipe material must be replaced with Incoloy 800H or higher heat-resistant alloy. The material changes, the modulus of elasticity and the coefficient of thermal expansion change completely, and the geometric dimensions of the bellows have to be recalculated. You think just zoom in one size according to the room temperature model? That's impossible. Under high temperature, the fatigue life of bellows will drop by a cliff, and the wave height and pitch have to be increased to ensure the design life.
Large-diameter thick-walled expansion joints are more particular. For those above DN1000, every 1mm increase in wall thickness, the internal stress change of bellows is very sensitive, and the forming radius of bellows directly determines whether qualified waves can be pressed out. Air ducts in the cement industry and hot air ducts in power stations, rectangular or circular expansion joints above DN3000 are often seen, and transportation of this size is a problem. Therefore, in the industry, it is often made into a segmented structure and welded on the spot. How do you determine the position of the segment? It depends on the transportation height limit and hoisting capacity, which are stuck before the theoretical calculation in actual selection.
Choose the easiest pit to step on
Only look at the nominal size of the sample, and ignore the influence of the guide tube on the flow section. The guide tube is placed on the inner wall of the bellows to reduce the erosion of the bellows by the medium. It does not change the external dimensions, but has a direct effect on the medium flow rate and pressure loss. If your pipe diameter is DN200 and the medium flow rate is high, you want to choose the expansion joint of DN200. The installation size is correct, but the guide tube occupies the cross section, and the medium may not pass. At this time, it may be necessary to select the expansion joint of DN250, and connect the DN200 pipeline with reduced diameter at both ends. The size has changed, but many people don't know it.
Ignore the interference of tie rods and shipping status with mounting dimensions. Expansion joint with tie rod, the tie rod is tightened during transportation, and it should be loosened to the working position after installation. This adjustment may be several tens of millimeters. If you don't reserve this adjustment space when you install it, the pull rod will push against the bracket or other equipment. A few days ago, I met a customer and bought a double hinge transverse expansion joint. The installation time did not consider the range of movement of the hinge. As a result, the hinge just stuck on the beam during pressure test, so it could only be cut off and reloaded.
No or incomplete capital withdrawal. When ordering the size of pipe metal expansion joint with the manufacturer, at least provide: diameter, pressure, temperature, medium, displacement in each direction (axial/transverse/angular), connection mode and end face spacing, installation space limitation, whether a guide tube or liner is required. With one less of this information, the manufacturer can only estimate according to the default value. And guess what? The estimated size is often a bit different from your scene in a certain direction.
Therefore, filling in the parameter table completely, clearly marking the upper and lower limits of the installation space, and mentioning the height and width limit in the transportation route is much less expensive than changing the drawings afterwards.