First distinguish "which length"-installation length ≠ bellows length
When it comes to the length requirement of metal expansion joint, the first step is to make the concept clear. The length parameter on the manufacturer's sample usually refers to the installation length-that is, the total length of the expansion joint with flanges at both ends, which can be directly loaded into the pipeline, including the structural parts such as end pipe, flange and connector. The effective length of the real working bellows is determined by the wave number and wave pitch: wave number × wave pitch = bellows length. The compensation amount depends on the length of the bellows, not the installation length.
When selecting general-purpose corrugated expansion joint or high-temperature axial expansion joint, the compensation amount is calculated first, and the compensation amount determines the wave number, and the length of the bellows is determined only after the wave number is determined. Finally, the size of the structural parts at both ends is added, which is the installation length. When you get the drawings, ask yourself first: Is this L from the flange end face to the opposite flange end face, or is it the length of the structural parts at both ends of the bellows? Don't underestimate this problem. If the length of a guide tube is different, it may not fit in the site.
In another compensation form, the length algorithm is totally variable
If the pipeline goes transverse displacement, choose the compound hinge transverse type expansion joint, the length algorithm is completely different from the axial type. The transverse type of compound hinge depends on the length of the intermediate connector and the hinge deflection angle, and the transverse displacement = the length of the intermediate connector × tan deflection angle. That is, the bellows at both ends only provide angular flexibility, and what really determines the displacement ability is the length of the middle section of the pipe. That section of the pipe is not randomly set. If it is shorter, no matter how big the deflection angle is, it can't be repaired.
Rotary compensator is another set of logic, which absorbs displacement by rotation angle, and the installation length follows the direction of pipeline. There is no unified formula to set. The length occupied by the pressure balancing structure must be included in the curved tube pressure balancing expansion joint and the straight tube pressure balancing expansion joint. For the same pipe system, choose axial type or transverse type, and the length requirement is not a set of algorithms at all. This is why some people are planted on the bellows, others on the middle pipe-the structural form changes, the computational logic changes completely.
The longer the bellows is, the better, and it will become unstable to a certain extent
The more wavenumber of bellows is not the better. To a certain extent, the stability will go wrong. There is a critical length of column instability in the bellows under pressure. Beyond this length, the column instability of the pipe will occur first, instead of being pulled first. This is why there are external pressure single axial expansion joints-the bellows can be made longer by placing the bellows on the external pressure side to avoid the column instability problem under internal pressure.
Large diameter thick wall expansion joints and directly buried (fully buried) expansion joints should additionally consider the limitation of stiffness and soil load on the length direction. The soil friction and covering pressure of buried pipeline will all act on the expansion joint. The longer the length is designed, the greater the external constraint. When designing, the upper limit of length is often not the amount of compensation, but the stability. This order cannot be reversed.
Between the length of the drawing and the length of the field installation, there are also pre-tension and tie rods
The length on the drawing is the theoretical value, the length actually installed on the site, and it is also affected by the cold tightening and the adjustment of the tie rod nut. When the pipe system is cold tightened (pre-stretched), the installation length of the expansion joint should be reserved according to the amount of cold tightening, and the tie rod nut should be retracted inward for a few turns or loosened outward for a few turns to directly change the initial position in the length direction.
The installation length should be checked on the basis of the measured spacing after the determination of the two fixed brackets. Don't just stare at the design value. Two days ago, I met a customer, and the construction was installed according to the drawing. As a result, the flange didn't match-a measure, and the installation length was 4 cm shorter than the actual spacing. Most of these problems are because no one counts the length of the tie rod nut and the guide tube into the mounting dimensions. The guide tube is usually a section longer than the bellows, sometimes with a tapered mouth, and the length of this part is easy to miss.
When you select the type, click these 4 items once, and you will basically not be wrong
How to determine the length requirement of metal expansion joint? In the following order, you can basically avoid most pits:
- Confirm the type of pipe displacement first— Axial, transverse or angular? The displacement direction determines the structure form of the expansion joint. For axial displacement, general corrugated expansion joint or high-temperature axial expansion joint should be selected, and for transverse displacement, compound hinge transverse expansion joint should be selected. Both axial and transverse directions have to consider pressure balance type.
- Then calculate the wave number and bellows length according to the displacement— — What is the single wave compensation amount? As soon as the total displacement is divided, the wave number will come out.
- Then add intermediate pipe, end pipe, flange and so on to obtain the total length— — The length of the bellows is only the middle section, and there are end pipes, flanges and guide tubes at the front and rear, and the horizontal type is also considered the middle pipe.
- Finally check stability and field flange spacing— — See whether the bellows exceeds the critical length of column instability, and whether the actual distance between the two fixed brackets on site can accommodate the installation length of the expansion joint.
If you are not sure, find the manufacturer to give the length size according to the actual working conditions. Don't copy the installation length of similar projects. General type, double hinge transverse type, external pressure single axial type, direct buried (fully buried) type, each type of length logic is different, but this check order is general. First calibrate the displacement type, and the subsequent length calculation is meaningful, otherwise no matter how accurate the formula is, it will be useless.
In the final analysis, the length requirement of metal expansion joint is a logical chain derived from the compensation amount to the structural size. Which link is missing, the scene will show you the color.