1.What exactly is the built-in metal expansion joint? What is the difference between it and ordinary expansion joint?
No one in the dry plumbing business can escape thermal expansion and cold contraction. Everyone has seen ordinary expansion joints many times-a bellows is sleeved in the middle of the pipe, and flanges are welded at both ends, which is exposed and shiny. But there is a variant, called a built-in metal expansion joint, whose core logic is:Plug the bellows into the pipeInstead of hanging out.
In human terms, the ordinary expansion joint is "plug-in", the pipe is disconnected for a section, the expansion joint is embedded, and the bellows is exposed to the environment. While the built-in metal expansion joint is "inline", it is mounted directly on the inner wall of the pipe or hidden inside as part of the pipe, and the outside is still a continuous pipe surface. From the outside, you might not even notice that there's a compensator.
Then why does it exist? For one reason:Space constrained。 There are places where there is no place for you to install an exposed expansion joint, such as in pipe trenches, through walls, equipment-intensive areas, or where you need to keep the appearance of the pipe clean. Ordinary expansion joints occupy space, and bellows are prone to dust accumulation, corrosion and mechanical bruising. It is built-in, hidden, saving space and worry.
Of course, there are costs – the hassle of repairs. We'll talk later.
2. Structural disassembly: Why can it "work" inside the pipeline?
Structurally, built-in metal expansion joints and our siteDirect buried (fully buried) type expansion jointKind of related, but not exactly the same. Its typical construction is divided into three pieces:
- Bellows assembly: This is the core, usually pressed from austenitic stainless steel (304 or 316L), with wall thickness determined according to pressure. Corrugated pipe is directly subjected to displacement and pressure, and its wave shape, number of layers and wall thickness determine the compensation ability.
- Inner sleeve (guide tube): In the built-in structure, the guide tube is not optional. Its one end is fixed at the inlet of the bellows, and the other end is freely suspended in the pipe, which functions as a flow diversion medium to avoid high-speed fluid directly washing the bellows. By the way, ourexpansion joint guide tubeThe specific function is explained in detail in the FAQ, and the principle is the same.
- Outer Guard/Sealing Structure: The outer wall of the built-in expansion joint is the pipe itself, and it needs to be sealed and positioned between the bellows and the inner wall of the pipe. Commonly used are annular seals or welded structures. In some working conditions, the outside of the bellows is filled with insulation material to prevent heat loss.
So how does it absorb displacement? By the elastic deformation of the bellows. Axial tension compression, lateral deflection, angular displacement, as long as it is within the design range, it can eat in. However, because it is hidden inside, you have to see its "overall dimensions" clearly in the product information-actually refers to the length of the installation section and the required pipe diameter space.
3. What working conditions must be built-in metal expansion joints? Don't wait for a leak before replacing
Two days ago, I met a customer. The steam pipe passed through the wall, and the distance between the equipment on both sides of the wall was only 10cm. The ordinary expansion joint couldn't fit in, and the flange couldn't be screwed in place if it was hard installed. Finally, the built-in metal expansion joint was replaced, which was directly welded inside the pipe, and the wall opening did not need to be enlarged. In this scenario, it is not an option, it is the only solution.
Another example is buried pipelines, especially directly buried heating pipelines. OurDirect buried (fully buried) type expansion jointThat's what it does, but in fact, the built-in metal expansion joint is more thorough in the direct burial scene-the bellows doesn't touch the soil, and the outer wall has an anticorrosive layer for a longer life. There are also high-temperature and high-pressure steam pipes. If the pipe itself needs to be insulated, the built-in expansion joint can keep the insulation layer continuous and avoid cold bridges.
- Extremely limited space (pipe trench, wall penetration, equipment crack)
- Continuous outer surface of pipeline is required (heat preservation, anti-corrosion, aesthetic)
- The medium is high speed and contains particles, so it is necessary to avoid direct flushing by bellows (built-in guide tube is more effective)
Don't wait until the bellows breaks and the media spews out to regret it. When it's time to use the built-in, don't save that thousand dollars.
4. The three parameters that are most prone to rollover during model selection-pressure, displacement and medium
When choosing the built-in metal expansion joint, the most error-prone thing is not the structural form, but the parameter matching. Let's talk about three pits:
Number one, stress.Because the built-in expansion joint is hidden inside the pipe, it is subjected to internal pressure, but its end seal structure is often more fragile than the external one. The design pressure must be allowed with a margin of at least 1.25 times the working pressure. Refer to this site'sExternal pressure single axial expansion jointThat external pressure structure pressure level can be made higher, while the built-in structure is limited by installation space and is generally used for medium and low pressure (below PN16).
Second, displacement.Many people only look at the axial compensation amount and ignore the lateral and angular displacement. Because of the existence of the guide tube, the lateral displacement ability of the built-in expansion joint is inherently worse than that of the external expansion joint. If your tube has large lateral displacement (such as irregular thermal expansion), it is recommended to give priorityUniversal corrugated expansion jointOrCompound hinge transverse expansion jointDon't push on the built-in.
Third, the medium.This thing is most afraid of corrosive media. Once the built-in bellows leaks, the medium goes directly into the pipe interlayer, which is difficult to find. If the medium is acidic or alkaline, the bellows material should be upgraded to 316L or even 254SMO, or usePTFE compensatorMake the lining. Remember, the cost of overhauling space can only be made up by the design phase.
5. Installation and maintenance: Don't let a bolt destroy the entire compensator
Finally, talk about installation. The biggest difference between the installation of built-in metal expansion joints and ordinary expansion joints is:It has no flanges。 All fixed by welding or clamps. The most common mistake made during welding is to beat the expansion joint body as a force-bearing member in order to align the pipe.
-First spot welding positioning, confirm that the direction of the guide tube is consistent with the flow direction of the medium (the direction of the arrow is the flow direction of the medium, it's a cliche).
-Symmetric segment welding is used during welding to prevent the deformation of bellows caused by local overheating. The bellows has thin walls, and the welding current is controlled below 80A.
-Pressure testing must be done after installation. Because once the built-in structure is buried in the wall or pipe ditch, it can only be demolished if it leaks in the later period.
Maintenance-wise, the built-in expansion joint is almost impossible to check online. Therefore, material selection and welding quality are the first line of defense. It is recommended to reserve leak detection holes or vacuum interfaces on the outside of the bellows (pipe interlayer). This site'sSpecial hose for vacuumIt can come in handy in leak detection scenarios.
The tie rod nut of the expansion joint must be removed or loosened after installation (see FAQ Article 12 for details). Although the tie rod cannot be seen in the built-in structure, if it is a design with limit bolts, don't forget to adjust it to the design displacement value. Don't let a bolt jam the entire compensation package.