Two days ago, I met a customer, and when I came up, I asked, "Is the metal wound expansion joint a corrugated expansion joint?" Alas, this problem is actually quite common, but it really can't simply draw the equal sign. Today, let's break it up and talk about it, from structure to selection, to the pits stepped on at the scene, all at once.
What exactly is a metal wound expansion joint? What does it have to do with the corrugated expansion joint?
Metal wound expansion joint, to put it bluntly, is a special form of corrugated expansion joint. You go to the catalogue on our site,Universal corrugated expansion joint、High temperature axial expansion jointIn essence, the elastic deformation of the bellows is used to absorb the displacement of the pipe. However, the word "winding" refers to its manufacturing process-spiral winding with multilayer metal strips, then rolling or hydroforming. Compared with the traditional integrally formed bellows, its wall thickness is more uniform and its pressure resistance is stronger, especially suitable for large diameter or high pressure working conditions.
Metal winding expansion joints belong to the family of corrugated expansion joints, but they focus more on the technical feature of "winding process". On our stationLarge diameter thick wall expansion joint、External pressure single axial expansion jointMany of them are made by this process.
Core three-piece set: bellows, guide tube and tie rod perform their respective duties
Take apart a metal-wound expansion joint, and you will find that there are only a few key parts in it, but each one is indispensable.
Bellows-the master of the real job
All displacement compensation depends on it. The number of layers, wave height and wave pitch of bellows directly determine how much axial displacement or transverse displacement it can absorb. Such asStraight pipe pressure balanced expansion jointYou have to rely on two sets of bellows to balance the pressure and thrust. When selecting the corrugated pipe, the material is very critical. When the temperature is high, stainless steel 304 or 316L should be used, and ultra-low carbon or duplex steel should be considered for corrosive media.
Deflector-Don't underestimate this sleeve
Our question and answer specifically explained the function of the guide tube. It is installed on the inner wall of the bellows, and it mainly does three things: first, divert the flow, let the medium flow smoothly, and avoid eddy current washing the ripples; The second is heat insulation. The guide tube in the high-temperature pipeline can block the radiant heat and protect the bellows; The third is to prevent the direct impact of solid particles. You seeMetal Corrugated Expansion Joints in Cement IndustryThe guide tube is almost standard, otherwise the dust can wear the bellows through.
Pull rod-not a display, a limiting bodyguard
Many people at the scene asked "how to adjust the tie rod nut of the expansion joint", and even thought that it could be removed after installation. The function of the tie rod is to constrain the bellows to deform in a predetermined direction, and at the same time bear the thrust generated by the internal pressure of the pipe. Such asLarge tie rod expansion jointIts tie rod can withstand several hundred kilograms of axial force, and the nut cannot be screwed to death during installation, so a compensation margin must be left. If it is installed backwards or the nut is locked, the bellows can't move when it is time to move, and the stress is all held on the wall of the pipe-it is strange that it is not bad.
Different working conditions, the slightest difference in type selection is a thousand miles
How to choose? Look at the three core elements: temperature, pressure and displacement type.
Look at the temperature first.Normal temperature pipelineUniversal corrugated expansion jointIt is enough, low cost and simple structure. However, if the temperature of steam pipeline or flue gas pipeline exceeds 400℃, the ordinary stainless steel bellows will creep and its life will drop by a cliff. That's the time to goHigh temperature axial expansion joint, it will add temperature-resistant gaskets and guide tubes inside, and replace the corrugated pipe material with Incoloy 800H or more heat-resistant alloy, otherwise it will crack for you in two or three months.
Look at the pressure again.Low pressure condition (≤1.6MPa) single layer corrugated pipe is enough to cope. For high-pressure working conditions (such as the main steam pipeline of the power station, the pressure is above 10MPa), it is necessary to use multi-layer winding structure orLarge diameter thick wall expansion joint。 And guess what? The higher the pressure, the thicker the wall thickness of the bellows, but the wave number will decrease instead-because the flexibility of thick-walled bellows is poor, too many waves will concentrate the stress.
Finally, look at the displacement type.Pure axial displacement, menu axial type. If it is a lateral displacement (like the angular displacement at the bend of the pipe), then you have to useCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 There was once a power plant project, and the pipeline had a lateral displacement of 20mm. As a result, the construction plan saved trouble and installed an ordinary axial type. After one month's operation, the side of the bellows bulged-the displacement direction was wrong, and the bellows was twisted.
Pipe diameter. Pipes below DN100, compact equipment, recommendedMetal hoseor small expansion joints; Large caliber above DN500, you have to considerDirect buried (fully buried) type expansion jointOrSleeve type pipe expansion jointYes, the installation space and compensation amount are different.
The three most common failures on site-how to avoid them in advance?
In this business for more than ten years, I have seen more failure cases than successful cases. It can be summarized in three words: tired, corroded and crooked.
Fatigue-the killer of low frequency and high amplitude
The bellows work under cyclic stress, and with each expansion or contraction, the material loses a little. Fatigue cracks usually start at the trough, because there is the greatest plastic deformation. How to prevent it? First, calculate the fatigue life when selecting the type. The design cycle number of bellows is generally 1000~10000 times. Don't choose thin walls to save money. Second, avoid the resonance frequency during installation, especially in pipelines with fans and pumps. If the vibration acceleration exceeds 0.5g, it is easy to accelerate fatigue. Third, if it really doesn't work, userubber compensatorOrRotary compensatorInstead, they are more suitable for large displacements at low frequencies.
Corrosion-a covert chronic disease
Chloride ion stress corrosion cracking (SCC) is the number one killer of expansion joints. Ordinary 304 stainless steel may have an accident above 60℃ in a chlorine-containing environment. The solution is simple: change the material. 316L, duplex steel, orPTFE-lined hoseevenPTFE compensator。 In addition, the weld of the guide tube must be passivated by pickling, otherwise the corrosion resistance of the weld area will decrease, and it will rot through there first.
Instability-the nightmare of instant collapse
Planar instability (crest and trough distortion) and columnar instability (overall bending). The main reasons are too high pressure, insufficient wall thickness, or improper adjustment of the tie rod during installation. How to avoid it? Select the type strictly according to the design pressure, and do not run under overpressure. In addition, make sure that the pre-tightening force of the tie rod nut is consistent during installation-as we said in Q&A, the tie rod nut has to loosen the stroke scale first, and then adjust it according to the cold tightness of the pipeline, even if it is tightened on one side. If the pipeline foundation settles and causes additional bending moments, additional installations will be requiredguide bracket。
The expansion joint is purely "it's better to maintain it in advance if you are sick". It takes five more minutes to calculate the temperature, pressure and displacement when selecting the model, which is much more cost-effective than shutting down the machine and changing the pipe afterwards. What do you think?