Don't use the small caliber as a smaller version, it's a lot more pretentious than you think
Instrument pipelines in chemical plants, clean pipelines in pharmaceutical workshops, and low-temperature pipelines in air separation equipment-these places have thin pipes and small pressure fluctuations, but the working conditions are trickier than the last. You can just take a large-diameter expansion joint and install it in proportion, and it will leak in less than two months. Why? Because the rigidity ratio of small-diameter bellows has changed, the stress concentration point is different. With the same wall thickness, the radius of curvature from peak to trough of small diameter bellows is smaller, and the local stress can be amplified several times. This is not my nonsense. Anyone who has done fatigue tests knows that the fatigue life of bellows of DN25 may only be 40% of that of DN200 with the same material and the same wave height.
Structural details: Do you want to add the guide tube? It's a pit
Small diameter metal bellows expansion joints usually have few layers, single or double layers, dense wave pitch and thin wall thickness. But whether to add a deflector or not depends on the medium. The medium flow rate is low and the cleanliness is high (such as the instrument gas source), so it can be omitted to save some costs. However, if it is steam, corrosive gas, or medium with particles, the guide tube must be added. Without the guide tube, the high-speed fluid will directly wash the corrugated root, and the life will be cut in half. The one in our stationSpecific Function of Expansion Joint Guide TubeThere is a picture in the question and answer, which is clear at a glance-the weld at the root of the corrugation is the stress concentration zone, and it is here that the crack starts when it is washed away again.
This site'sUniversal corrugated expansion jointAndHigh temperature axial expansion jointAvailable in small diameter models, these details are structurally considered: the guide tube thickness is calculated by flow rate, and the welding process is individually calibrated. To put it bluntly, small-caliber things have higher processing accuracy requirements.
Selection parameters: Pressure and temperature cannot be seen separately
Many people only look at the rated pressure and temperature on the sample when selecting the model, and think that 1.6MPa and 300℃ are no problem. But have you calculated the conversion stress? When 304 material is at 300℃, the allowable stress drops directly from 137MPa at normal temperature to 88MPa, which drops by more than 35%. At this time, you use 304, and the external pressure stability of the bellows is fine, but the circumferential stress caused by the internal pressure may have exceeded the limit. You can still carry it if you change it to 316L. If the working condition still has a little chloride ion corrosion, you have to go directly to Incoloy 825.
The amount of compensation is also not blind to the rated value on the sample. The single-wave compensation amount of small-diameter bellows is usually only 60%-70% of that of large-diameter bellows, because the diameter of bellows is smaller, and the geometric nonlinearity is more sensitive. Multiple waves can be connected in series to reach the stroke, but the stability decreases when the wave number is more than 4, and the transverse stiffness may be so low that the pipe will become unstable when it is slightly disturbed. At this time, you have to add a pull rod or hinge. This site'sCompound hinge transverse expansion jointDesigned for this scenario, the hinge structure controls lateral displacement and the tie rod limits axial overstretching.
Installation: most afraid of rotation, one torque will destroy it
Small-diameter pipeline itself has little rigidity. If the bellows is twisted slightly for installation deviation, the bellows will become a "twist" and the internal stress will soar. I have seen a case: the scene was misaligned, and as soon as the flange bolts were tightened, the bellows was directly twisted and deformed, and the pressure test exploded. The fry is right at the trough, a typical torsional fatigue crack.
Pre-tighten half of the bolts first, weld with deviation calibration, and then tighten completely. The tie rod nut should keep the factory mark position in the cold state, and do not adjust it randomly. If you adjust too much, the bellows will be crushed to death or pulled over, and the compensation amount will be wasted. As for how to adjust, this site has a special answer- -How to adjust the tie rod nut of expansion jointWritten clearly.
Quality judgment: Don't just look at wall thickness and wave height, the key is R angle and weld
The common fault of domestic small-diameter bellows is that the R angle of the trough is too small. In order to save materials, the die is reluctant to open, which results in stress concentration cracking. You can measure it: Use R gauge to card the trough, and the radius should not be less than 15% of the wave distance. For example, the wave pitch is 10mm, and the wave trough R angle is at least 1.5mm. Below this number, fatigue life is directly discounted by 30%.
In addition, the weld must be 100% penetrated or radiographic inspected. Small-diameter tubes have short welds, but the microstructure changes in the heat-affected zone of the weld may be more severe than those of large-diameter tubes-because of the rapid heat dissipation and high cooling rate, it is easy to produce martensite or hard-brittle phases. Not to mention pores and slag inclusions, all of them have to be repaired. The large-diameter thick-walled expansion joint of our station is as strict in technology, while the small-diameter joint is more delicate. After all, the labor cost of replacing one if it is broken may be more expensive than the expansion joint itself.
Finally, don't try to be cheap, there are many small-caliber pits
In some factories on the market, the wave pitch of the bellows is uneven, and the wave height deviation can reach ±1mm. It doesn't take long to install and leak-not a weld crack, but a fatigue crack at the trough thinning. If you really want to choose, give priority to this siteUniversal corrugated expansion jointOrHigh temperature axial expansion joint。 They have mature wave design and fatigue test data in small-caliber segments, and each batch of water pressure and air tightness tests are done out of the factory, at least you know it.
The price is hundreds of dollars more expensive, but in exchange for a few years of stable operation. Do you say it's worth it?