What are the "upper displacement" and "lower displacement" on the drawing talking about?
To put it bluntly, the up and down displacement of the metal expansion joint is the "nod" or "head up" of the pipe in the vertical direction. The technical term is lateral displacement or vertical displacement. You take a design drawing of a steam pipe, which is marked "upward displacement 30mm", which means that the pipe will arch 30mm upward when heated. In turn, cold shrinkage or the underlying bracket settles, and the pipe sinks, that is, downward displacement.
When the temperature of the steam main pipe from the boiler rushes above 400℃, the pipeline will naturally expand. A straight pipe may extend axially, but when it is restricted by an elbow or a fixed bracket, the force runs vertically-the middle of the pipe bulges, and the expansion joint has to "swallow" this upward displacement. If one of the underlying brackets sinks, the direction of displacement is directly reversed. If you can't figure this out, the selection is blind.
Where does the up and down displacement come from? Temperature is the main cause, but not the only one
Temperature changes are the biggest drivers. Everyone knows the truth of thermal expansion and contraction, but there are still a bunch of "hidden players" in actual working conditions. The self-weight of the pipeline will push the bellows down, and the long-term settlement of the support will bring continuous downward displacement. Wind load and even earthquake will make the pipeline swing suddenly. Especially in power stations and cement industries-the diameter of the pipe is always one or two meters, and the temperature is high (Corrugated expansion joint for power station industryOften working above 400℃), the amount of up and down displacement easily reaches tens of millimeters.
Installation error. Two days ago, I met a customer. During on-site installation, I found that the pipeline was out of alignment. I just used a crowbar to push the expansion joint up, and the bolts crackled. And the result? It leaked after two months of running. Why? The bellows has already been subjected to the initial displacement in the installation stage, and the thermal displacement is superimposed during operation, which directly exceeds the limit. Therefore, you must include the "initial offset" when designing, and don't just stare at the theoretical value.
Up and down displacement vs axial displacement vs angular displacement-don't mix it up!
Many novices put the three displacement parameters together to calculate the general ledger, which is a big taboo. The axial displacement is the expansion and contraction of the pipe in its own direction, the upward and downward displacement is the swing in the vertical direction, and the angular displacement is the rotation at the joint. The mechanisms of the three are completely different, and they have to be seen separately when selecting the type.
Let's say,Compound hinge transverse expansion jointIt is specially used to absorb lateral (including up and down) displacement. Its bellows are arranged in pairs, and the direction is guided by hinges. WhileStraight pipe pressure balanced expansion jointMainly cope with axial displacement, you take it to carry it up and down, and the bellows will quickly twist and fatigue. Is the piping arrangement L-type or Z-type? The up and down displacement is often concentrated near the elbow, and it is useless to install the expansion joint on the straight pipe section. The correct way is to do the pipe system stress analysis first, run it through with Caesar II or similar software, and accurately calculate the displacement direction and magnitude of each point.
How to determine the value of up and down displacement? Formula or software?
Don't slap your head to estimate, and don't bother. In a simple case, the axial elongation is calculated by the coefficient of thermal expansion × pipe length × temperature difference, and then it is converted into vertical offset according to the geometric constraints of the pipe system. For example, a 10-meter-long pipeline has a temperature rise of 200℃, the linear expansion coefficient of carbon steel is about 12×10⁻⁶/℃, and the axial elongation is 24mm. If the pipe is fixed at both ends with an elbow in the middle, this 24mm will translate into up and down displacement-by how much depends on the angle of the elbow and the length of the arm.
Forget about complicated piping, use Caesar II or AutoPIPE. Especially those with smokeDesulfurization flue gas baffle doorPipes, temperature fluctuations are large, the medium is corrosive, and the displacement path may be strange. In addition, the "compensation amount" marked in the product sample is conditional-such asUniversal corrugated expansion jointUsually three values are given: axial, transverse and angular, but they are independent and cannot be used at the same time. If you use 30mm for up and down displacement, the axial compensation amount will have to be discounted. See the curve chart in the product data sheet for details.
When selecting, displacement direction and constraint structure are vital gates
In the case of large up and down displacement, the expansion joint with tie rod or hinge is the preferred choice. The function of the tie rod (refer to the question and answer in our station) is to bear the thrust generated by the medium pressure, so that the bellows can only concentrate on absorbing displacement and will not be broken by internal pressure. Such asLarge tie rod expansion joint, the tie rod limits the axial displacement and specifically allows the lateral (up and down) displacement to be effectively absorbed. On the other hand, if the up and down displacement is large but the ordinary axial type is used, the bellows will twist in the vertical direction, and the cracks will come particularly quickly.
When installing, don't forget to look at the direction of the arrow on the expansion joint-that arrow indicates the direction of media flow, and it must not be installed backwards. Only when the guide tube is guided in the correct flow direction can it protect the bellows from high-speed medium erosion. Installing the guide tube backwards is not only useless, but also will aggravate the wear. In addition,How to adjust the tie rod nut of expansion joint？ Generally, the tie rod nut is in a pre-tensioned or pre-compressed state when it leaves the factory. After installation, it needs to be loosened or tightened according to the actual displacement direction. The specific methods are available in our product description.
Too much up and down displacement? Don't expect an expansion joint to carry it
If the displacement exceeds 50mm or even hundreds of millimeters, it is difficult for a single metal expansion joint to balance the fatigue life and pressure bearing capacity. At this time, the combination scheme is more reliable. For example, install a firstCompound hinge transverse expansion jointAbsorb most of the up and down displacement, and connect one in seriesHigh temperature axial expansion jointProcessing temperature expansion and contraction. Or withNon-metallic expansion joints (fabric fiber expansion joints)With metal expansion joint-the flexibility of non-metal is better adaptable to multi-dimensional displacement, but it is not as resistant to pressure and temperature as metal. LikeRectangular non-metallic expansion jointIt is used a lot in flue gas pipes because it can absorb the three-dimensional displacement of up and down, left and right and axial at the same time.
Another way to think is to useRotary compensatorWith the tie rod structure, the rotation angle is used instead of linear displacement, which is suitable for scenes with limited space. Of course, how to build it needs to be combined with pipeline layout, media parameters and cost tradeoffs. In short, understanding "up and down displacement" is not a back definition-calculating the data accurately, selecting the right product, installing the position, and missing one link will cause problems.