Metal expansion joints and corrugated expansion joints, different names but the same thing? Stop mixing it up
To be honest, I have been in this business for ten years. Every time a new purchaser asks, "What is the difference between metal expansion joints and corrugated expansion joints?", I want to throw an encyclopedia directly. In fact, these two are talking about the same thing-the core structure is all corrugated pipe, which absorbs the thermal displacement of the pipe by corrugation deformation. The reason why the names are different is because of the industry habit: people who make equipment like to call them "corrugated expansion joints", while those who engage in pipeline design are more inclined to "metal expansion joints". The product list on our site is very clear,Universal corrugated expansion joint、High temperature axial expansion joint、Large diameter thick wall expansion joint, are essentially metal bellows structures. If you see anyone opposing these two nouns again, you can basically judge that it is a layman.
But don't worry, this is not absolute. In some special scenarios, such as vacuum lines or high-pressure steam systems, you don't use ordinary bellows- -likeSpecial hose for vacuumAndExternal pressure single axial expansion jointAlthough there is no "corrugation" directly in the name, the internal compensation element is still a bellows. To put it bluntly, the expansion joint is a functional description, and the bellows is a structural description. Many times, it is okay to draw the equal sign.
Stiffness, compensation and pressure rating: Three core parameters determine whether you can worry about it
chattingCharacteristics of metal expansion jointsThese three numbers cannot be avoided: stiffness, compensation amount and pressure level. Many pipeline accidents can be traced back to the source, either because the stiffness is too large, which leads to excessive binding force and cracks the weld joint, or because the compensation amount is too small, the bellows is forcibly broken.
Stiffness, simply understood, is the ability of bellows to resist deformation. The greater the stiffness, the more force you have to use to make it stretch or contract. However, the thermal expansion force of the pipeline is fixed, and when the stiffness is high, the reaction force on the pipeline and equipment will be greater-you thought that the expansion joint was welded firmly, but the next valve flange collapsed first. Therefore, the stiffness must match the allowable thrust of the equipment during model selection. Like ourStraight pipe pressure balanced expansion jointIts stiffness design takes into account the elastic coupling of the intermediate pipe and the bellows, which can effectively reduce the thrust of the main fixed bracket.
How much displacement does the expansion joint need to "eat in" the thermal expansion and contraction of the pipe. The steam pipe with a diameter of 500mm has a temperature rise of 200℃, and the thermal elongation per meter is calculated to be about 2.4mm. If the pipe section is 10 meters long, the total compensation amount will be 24mm. Not enough? Then wait for the bellows to deform plastically. In our stationCompound hinge transverse expansion jointAndCompound straight pipe bypass pressure balanced expansion joint, are designed for large compensation scenarios, and the compensation ability is amplified by multi-section bellows series or hinge structure.
The pressure rating is easy to understand-the wall thickness and number of layers of the bellows directly determine how much pressure it can hold. Single-layer bellows is enough for low-pressure systems of 0.25MPa, but high-pressure steam above 4.0MPa must be topped with multi-layer bellows or reinforcing rings. And guess what? Many people will ignore the reduction coefficient of medium temperature to pressure grade: at 300℃, the allowable stress of stainless steel is more than 30% lower than that of normal temperature, so the actual pressure bearing capacity will be discounted under high temperature conditions. That's why we specialize inHigh temperature axial expansion jointThis product line uses heat-resistant alloys and special corrugation parameters.
High temperature, large diameter, vacuum-when the working conditions change, the selection logic completely changes
The aforementioned is a general rule, but when it comes to special working conditions, the conventional experience fails directly. One of the most common examples: high temperatures. The expansion joint of ordinary austenitic stainless steel begins to be sensitized above 450℃, and the risk of intergranular corrosion increases sharply. At this time, either change to a nickel-based alloy such as Inconel 625 or increase the wall thickness. However, the increase of wall thickness will lead to greater stiffness and decreased compensation capacity-this is the seesaw problem in engineering. So likeMetal Corrugated Expansion Joints in Cement IndustryAndCorrugated expansion joint for power station industryAll are developed for flue gas at 800℃ ~1100℃, and the bellows are designed to be long and shallow, with high-temperature-resistant guide tubes and heat-insulating packing layers.
Large caliber is another matter. Expansion joints above DN3000 (such asLarge diameter thick wall expansion jointAndMetal rectangular expansion joint), bellows forming is extremely difficult, and often requires a combination process of pressing + welding. Moreover, the medium flow rate in the large-diameter pipeline is low, and it is easier to accumulate dust or liquid. The drainage hole must be considered in the design of the guide tube. Last year, there was a power plant project. Because the rectangular expansion joint guide tube did not open the drainage hole, the condensed water accumulated in it to accelerate corrosion, and it leaked in half a year.
The vacuum condition is more pitty. Under negative pressure environment, bellows are most prone to instability-that is, they are "deflated" by atmospheric pressure. The solution is to add a liner sleeve or optionallySpecial hose for vacuumThis kind of hose is supported by spiral steel wire inside, and the external pressure resistance can reach more than 0.1MPa. As forDouble hinge expansion joint for air-cooled island vacuum pipeline, which is specially designed for the condenser vacuum pipeline, the double hinge structure can both absorb axial displacement and resist internal pressure thrust.
Guide tube, tie rod, screw: these accessories are not installed correctly, no matter how good the expansion joint is, it will be useless
The expansion joint body is cow, and the auxiliary parts are installed wrong. Speaking of the guide tube, many people think that it is just "the lining of the inner wall of the pipeline", and its actual effect is great. The guide tube can prevent the high-speed medium from directly washing the bellows, and also reduce the flow resistance. However, the installation direction cannot be reversed-the arrow direction must be consistent with the flow direction of the medium, otherwise the guide tube will be washed away, the bellows will be directly exposed to the dusty airflow, and the wear speed will be more than 5 times the original. We specifically talked about it in the FAQ on our siteSpecific Function of Expansion Joint Guide TubeThe accompanying picture is very clear.
Tie rods and screws are the hardest hit areas again. Large tie rod expansion joints (such asLarge tie rod expansion jointThe type used in) tie rods, essentially limiting devices, allow the bellows to move freely in one direction but limit deflection in other directions. The nuts at both ends of the tie rod must be adjusted to reserve a clearance during installation-this clearance is half the design compensation amount. If it is screwed to death and the bellows becomes a rigid body, how can it be compensated? Not to mention not dismantling the transportation screw: for the convenience of transportation when leaving the factory, several screws will be temporarily welded to the expansion joint to lock the bellows, which must be dismantled after installation at the site. Not dismantling? The expansion joint of the pipe is directly broken when it is heated. OurDo you need to remove the screw of the expansion jointIt is clearly stated in the question and answer: 100% to be demolished.
If a step of installation goes wrong, the life will be cut in half directly. How to adjust the big tie rod and should the screw be disassembled?
I met a customer two days ago and boughtCurved tube pressure balance expansion joint, it leaked after less than three months of use. In the past, it was not cold tightened according to the drawings at all during installation-to put it bluntly, there was no compensation amount for pre-stretching/compressing the bellows to the installation position. For example, the design compensation amount is +25mm, and the pipeline temperature is low during installation. You have to pre-pull 12.5mm first, and the bellows can return to the middle position after the operation heats up. Good for the customer, directly welded to death. After heating up, the bellows has been pressed at the limit, so it is strange that the fatigue life is not cut in half.
How to adjust the big tie rod? It is divided into three steps: in the first step, loosen the nut and let the pull rod move freely; In the second step, according to the pipe drawing, adjust the nut position so that the tie rod is at the design length (usually equal to the free length of the expansion joint); In the third step, tighten the locking nut according to the design value with a torque wrench. Note that the locking nut is not to prevent the tie rod from expanding or contracting, but to prevent the tie rod from rotating and loosening. The function of the expansion joint tie rod is to bear the internal pressure thrust, so that the bellows can absorb the displacement with peace of mind. As forHow to adjust the tie rod nut of expansion joint, the answer is simple-if there is a gap compensation amount, leave a displacement space; If there is no gap (such as pressure balance type), just position it according to the drawing.
Once the installation is complete, be sure to check that all transport fixings are removed, including limit screws, support blocks, temporary welds. Even if one remaining screw is not removed, it may cause the expansion joint to fail to expand and contract normally. Since you paid for expansion joints, don't let the installation details ruin everything.