What exactly does a metal wound expansion joint mean?
The "metal winding expansion joint" often called in the industry is actually not a standard product model, but a general term for metal expansion joint that uses bellows as an elastic element and enhances its pressure bearing capacity through multi-layer metal strip winding or lamination process. To put it bluntly, the general-purpose corrugated expansion joints, high-temperature axial expansion joints, and even the corrugated expansion joints used in power station industry in our factory can be classified into this category if the corrugated pipe has a multi-layer structure. This product name is not listed separately in the product information of this site, but its core component-bellows, is the kind used in metal hoses, but the expansion joint has to bear greater axial and lateral displacement.
What's the difference between that and ordinary corrugated expansion joint? The difference is in the word "winding". Multi-layer metal belt winding (or hydroforming) out of the corrugated pipe, the pressure capacity is stronger than single-layer corrugated pipe, suitable for high temperature and high pressure working conditions. In industries such as power stations, cement and chemical industries, steam, flue gas or corrosive media run in pipelines. If single-layer bellows can't hold it, they have to rely on multi-layer structures. So, don't be confused by the name-just take your seat when choosing a model.
Structural disassembly: bellows, guide tube and tie rod, all of which are indispensable
A typical metal-wound expansion joint, mainly composed of bellows, guide tube, end tube (flange) and tie rod assembly. The bellows is the "heart". Multi-layer metal strips are wound (or hydroformed) to create corrugations, and the elastic deformation of the corrugations absorbs the displacement. The role of the guide tube is particularly practical-to prevent high-speed media from washing the inner wall of the bellows and prolong the life, which is specifically mentioned in the FAQ. The tie rod is even more interesting, not only for fixing during transportation, but also for adjusting to the designed length after installation, otherwise the pressure thrust will collapse the pipe support.
Like straight pipe pressure balance expansion joint and compound hinge transverse expansion joint, the structure is more complicated, but it remains the same. For example, the double hinge transverse expansion joint is suitable for absorbing lateral displacement. It has two bellows plus hinges in the middle, and the tie rod system is also more complex. Don't just look at the name when selecting the model, you have to find out what direction of displacement the pipeline should absorb.
Three steps of type selection: displacement, pressure and temperature, one cannot be less
Selection is easy to roll over. The first step is to figure out the direction of thermal displacement of the pipeline system – axial, lateral or angular? For axial displacement, choose general type or straight tube pressure balance type, and for transverse displacement, use compound hinge or curved tube pressure balance type. Two days ago, I met a customer from the cement industry, who said that the pipeline only went in the axial direction. As a result, a general-purpose corrugated expansion joint was installed, and it leaked in three days. Check down and find that there is actually a 5mm lateral offset. Can the universal model absorb lateral displacement? Can, but is limited, and has no guiding structure. The correct way is to choose the metal corrugated expansion joint in the cement industry, or install a tie rod to limit the transverse direction.
The second step is to look at the medium temperature and pressure. High-temperature steam pipelines (such as power station industry) have to use high-temperature axial type, which can carry above 600℃; If the medium is highly corrosive, consider a PTFE-lined hose or PTFE compensator. Here is a detail: the higher the temperature, the lower the fatigue life of the bellows. When selecting the type, you have to check the stiffness calculation formula. The formula in the common questions and answers can be applied directly.
The third step is to check the guide tube and the structural stiffness. The deflector is not thick enough and wears out in a few months; If the stiffness is not accurate, the expansion joint itself will become unstable. How to calculate? Referring to the calculation formula of bellows stiffness, combined with the model and size table of expansion joint, the calculated data is reliable.
Don't step on pits: real cases of installation errors and failed selections
More pits on the installation. The direction of the arrow on the expansion joint represents the flow direction of the medium. If the guide tube is installed backward, it will fail directly. Think about it for yourself, how long will the medium, in turn, flush the back of the bellows? There is also the problem of tie rod nut adjustment-remove the transport screw and adjust the tie rod nut to free length after installation. Many people ignore this action, and as a result, the expansion joint is compressed at the beginning, and the displacement ability is all wasted. In the frequently asked questions and answers, the article "How to adjust the tie rod nut of the expansion joint" is clearly written. It is recommended that you turn it over before installation.
Another, deflector wear is an invisible killer. In the steam pipeline, if the medium flow rate is high and contains impurities, the guide tube may wear out in one year, and the bellows will be directly exposed to the scouring, which will leak quickly. Therefore, every time you inspect, take a flashlight to shine on the inlet end of the deflector to see if there is any perforation, which can't be saved.
Maintenance and replacement: Don't wait for a leak
The design life of metal-wound expansion joints is usually 10~15 years, but the actual life depends on the working conditions. The expansion joint in the steam pipe, if the guide tube is worn out and the bellows is fatigued by high-frequency vibration, may have to be replaced in three to five years. How to judge? See if the tie rod nut is loose and listen to the pipe for any abnormal noise. Non-metallic expansion joints (fabric fibers) and rubber compensators have shorter lives and require regular inspections.
The service life of the expansion joint is strongly correlated with the installation quality. If the tie rod nut is tightened too tightly during installation, the bellows is in a pre-compressed state, and the actual displacement that can be absorbed is discounted. To do it correctly, adjust the tie rod nut to the free length after installation to leave the bellows in its natural state. For specific operation, refer to the FAQ "How to adjust the expansion joint tie rod nut".
Summary: Return to product selection, don't be confused by the name
To put it bluntly, metal winding expansion joint is a name for metal bellows expansion joint. When selecting the type, it can directly correspond to the general corrugated expansion joint, high-temperature axial type, straight pipe pressure balance type and other products of this station. Power stations, cement, desulfurization and air-cooled islands have special models in different industries-for example, double-hinged expansion joints for vacuum pipelines in air-cooled islands, and desulfurization flue gas baffle doors with non-metallic expansion joints for desulfurization systems. Remember one thing: if the parameters are calculated accurately, installed in place, and checked regularly, it will be able to serve safely for more than ten years. If you still have questions, it is much better to flip through the "Service Life of Expansion Joints" and "Metal Expansion Joint Weight Table" in the FAQ than to pat your head.