What is Unconstrained Metal Bellows Expansion Joint? To put it bluntly, it relies on the deformation of the bellows itself to absorb the axial displacement of thermal expansion and contraction of the pipeline, and there are no constraints such as tie rods and hinges.
When you take it apart, the structure is reassuringly simple: a section of bellows plus flanges at both ends is gone. With this site'sUniversal corrugated expansion jointAndExternal pressure single axial expansion jointIt's a kind of thing. You may ask: What's the difference between it and a constrained one? Don't worry, let's talk about it in the next section.
The biggest difference between it and constrained expansion joints (such as double hinge transverse type and straight tube pressure balance type) is that the unconstrained version can only absorb axial displacement and cannot withstand transverse or angular forces.
However, because of its simple structure, low cost and light weight, it is the most cost-effective choice in many straight pipe sections. For example, a 500-meter-long steam linear pipeline can solve the problem with unconstrained axial expansion joints. The double transverse type with hinges on the hard side not only costs more money, but also has more maintenance points. This site'sCompound hinge transverse expansion jointAndCurved tube pressure balance expansion jointThey are typical of constraints, which absorb lateral and angular displacements, but are at least twice as expensive. So the first thing in the selection: confirm whether there is lateral displacement in the pipe system? No? Then use the unconstrained one.
In practical application scenarios, unconstrained metal bellows expansion joints most often appear in straight sections such as steam pipelines of power stations, hot air pipelines of cement industry and desulfurization flue gas pipelines.
For example, this site hasCorrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustryThis is the kind of product that is used. Two days ago, I met a customer from a cement factory. The temperature of the hot air duct was 450℃. When he came up, he asked if there was a general type. I said that your temperature is over 400℃, you have to add insulation layer or use it insteadHigh temperature axial expansion jointOtherwise, the corrugated pipe material can't bear it. Pay special attention to the medium temperature and pressure when selecting the model-if the temperature exceeds 400℃, you have to consider adding heat insulation layer or switching to high-temperature axial type, which is a hard lever. In terms of pressure, it is generally fine below 0.25MPa; If it exceeds 1.0MPa, it will be counted as fatigue life.
There are four key parameters for selection: wave number (determining the compensation amount), wall thickness (determining the withstand pressure), guide tube (anti-scour) and fatigue life (number of cycles).
Anyone who knows the trade knows that the direction of the guide tube must follow the flow direction of the medium, otherwise the bellows will be washed out soon. How to calculate stiffness specifically? FAQs on this site includeStiffness and Calculation Formula of BellowsYou can set it directly. Let me talk more: some people only look at the wave number regardless of the stiffness. As a result, after installation, they find that the expansion joint is too hard and the pipe can't be pushed. Therefore, we have to check the thrust of the elastic reaction force on the pipe frame at the same time. And the wall thickness, don't choose too thin to save money-you thought you saved 50 yuan, but it cracked in two years, and the labor cost of replacing it once is enough to buy three new ones.
The easiest thing to be overlooked during installation is whether to remove the tie rod nut. Many people forget to loosen the tie rod nut for transportation after installation. As a result, the expansion joint can't work, and the pipe is still broken.
What about that? The correct method is: after installation in place, loosen the nut on the limit screw to the position corresponding to the design displacement. Refer to Q&AHow to adjust the tie rod nut of expansion joint。 In addition, the direction of the arrow of the guide tube must be facing the flow direction of the medium, which is inThe direction of the arrow of the expansion joint refers toIt is very clear in. One more point: the flange bolts should be twisted diagonally and preloaded to the specified torque in three times. Don't try to tighten it quickly. That's how the flange leaks in all likelihood.
Common faults are nothing more than bellows cracking, guide tube falling off, and flange leakage.
Cracking is mostly due to fatigue life or twisted during installation; The deflector tube falls off generally because of poor welding quality; A flange leak means that the bolts are not tightened evenly. Solution: Leave enough fatigue safety factor during type selection, ensure pipeline coaxiality during installation, and check regularly. Want to extend the service life? Check out the Q&AService life of expansion jointRegular maintenance points mentioned in. In the final analysis, it is normal for the unconstrained metal bellows expansion joint to have no problems for ten or eight years if you choose, install and use the right thing.