Section 1: The "Safety Illusion" of Overchoosing-Why do many people think bigger is better?
When choosing expansion joints in the project, many people hold the mentality of "rather big than small". I think with a larger caliber and more wavenumber, the compensation ability will definitely be stronger and the system will be safer. Doesn't that sound reasonable? But the actual failure case slapped the face. Two days ago, I talked to a master who did power plant maintenance. He said that several of the corrugated expansion joints replaced on their pipelines were fatigue cracked because of the large selection. The principle is actually very simple: the larger the size of the expansion joint, the greater the stress generated when it is subjected to internal pressure, especially the position of wave peaks and troughs. Moreover, after the selection is large, the actual displacement of the system may be much smaller than the design value, and the corrugation is in a state of "not having enough food" for a long time, so the creep and stress concentration of the material are more obvious. Tsk, is it the opposite of intuition?
Section 2: What trouble will you cause if you elect big? — From instability to air leakage, don't try to hide from any
Let's start with instability. The expansion section is large, especially for large-diameter thick-walled expansion joints or external pressure single axial expansion joints. If the nominal diameter exceeds the actual pipeline by a lot, and the end constraints are not adjusted accordingly, plane instability or columnar instability will easily occur. Once the bellows becomes unstable, it is only a matter of time before the seal fails. Let's talk about fatigue life. The life of the expansion joint depends on the number of cycles and stress levels. If a large expansion joint is selected, the actual stress value of the bellows may exceed the design standard by more than 30%-the original designed life of 100,000 times will crack after 20,000 times. There is also the problem of installation space: the expansion joint is selected to occupy more axial space, and the layout of pipe bracket and guide bracket has to be changed accordingly, or it may interfere with adjacent equipment. Not to mention the cost. Stainless steel materials cost twice as much, and flanges, tie rods and guide tubes are all upgraded, which is pure waste.
Section 3: Under different working conditions, the "lethality" of excessive selection has its own emphasis
The corrugated expansion joint used in the power station industry is often on high-temperature and high-pressure steam pipelines. If it is selected large, the wall thickness of the corrugated pipe will be relatively thin and the pressure resistance will decrease. The metal corrugated expansion joint used in the cement industry contains more dust in the medium. If the flow rate is larger, the dust is easier to accumulate in the trough and the corrosion is aggravated. If a non-metallic expansion joint (fabric fiber expansion joint) is used, the consequence of choosing a large one is more direct-the tension bearing by the fabric layer is uneven, and it will bulge and tear in the short term. For the rubber compensator, choosing a large one will also make the rubber body crack by fatigue. Let's take a look at the rotary compensator and sleeve-type pipe expansion joint. Such products are sealed by packing. If the casing gap is larger, the sealing packing can't be pressed tightly, and air leakage is common. After all, every type of expansion joint has its best working size range, and blindly choosing large is digging a hole for yourself.
Section 4: What exactly is called "choosing the right"? Don't slap your head
Many people get the pipeline layout drawing, and as soon as they see the thermal displacement of 200mm, they directly choose an expansion joint with a rated compensation of 250mm. This approach is only one-third correct. The correct selection process should be: first clarify the medium, temperature, pressure and nominal diameter of the pipeline, then calculate the thermal elongation and cold tightness, and also consider the installation error and additional displacement of the end point. For general-purpose corrugated expansion joints, a safety margin of 10% ~15% should be left when selecting the model, but it should never exceed 20%. If it is a high-temperature axial expansion joint, the allowable stress of the material at the highest operating temperature should also be checked. There is also one that is easy to overlook-the deflector. For example, the diameter of the selected expansion joint is larger than that of the pipeline, and the size of the guide tube has to change accordingly. Otherwise, the medium will directly wash the inner wall of the bellows and wear it out in a few months. Therefore, the selection is not done by looking at the numerical value, but the guide tube, the tie rod and the limit structure have to be taken into account.
Section 5: What if I have already installed it? Can you remedy it if you elect it big?
This problem is case by case. If the expansion joint is only a small amount too large (say one gauge larger than the actual pipe) and space allows, you can limit the amount of compression of the bellows by adjusting the tie rod nut, or add pre-tension/pre-compression to match the actual displacement. For structures with guide barrels, the guide barrels can also be replaced to improve flow path matching. However, if you choose more than two specifications, or there are signs of instability, then don't hesitate to change it directly. The cost of forcibly making do is far greater than that of replacing a new one-the loss of shutdown and production and the risk of safety accidents can't be afforded. In addition, a reminder: the screw and tie rod of the expansion joint are not decorations, and they must be adjusted in place according to the actual displacement before installation. For details, you can check the question and answer "How to adjust the tie rod nut of the expansion joint" we wrote before.
Section 6: The most afraid thing about model selection is "taking it for granted"
The expansion joint is not the bigger the better, nor the smaller the better, it is "just right" the best. Different working conditions, different media and different pipeline layout correspond to different models and sizes of expansion joints. For example, high-temperature axial type is used for high-temperature pipelines, straight pipe pressure balance type is used for large temperature differences, compound hinge transverse type is used for transverse displacement, and directly buried (fully buried) expansion joint is used for buried in soil. Each product has its own "temper". Before the selection, it is best to communicate with the technicians of the manufacturer and give all the working condition parameters. Remember: Does inflated excerpts have an impact when they are big? The answer is clear — not only impactful, but potentially fatal.