How much lateral displacement can the metal expansion joint hold when it expands radially?
When many people mention metal expansion joints, their first reaction is to "absorb the axial expansion and contraction of the pipe". Yes, it is true that the tube will grow or shorten when the temperature changes, but this is just the appearance. If the pipe only expands and contracts on the axis, then choose an axial expansion joint. But in reality, the pipe is alive-equipment vibration, foundation settlement, loose brackets, all of which can push the pipe sideways. This displacement perpendicular to the axis of the tube is what we callRadial expansion of metal expansion joint(also called lateral displacement or angular displacement). Let's break it up and talk about it today.
1. What exactly is radial expansion?
Expansion joints are not only responsible for "elongation" and "compression". It is normal for a steam pipe to rise by 30℃ and extend tens of millimeters axially. But have you ever thought that if the fixing bracket is slightly loosened, the pipe may deflect several millimeters in the corner? Even the vibration can make the tube sway back and forth like a bobblehead fan. These displacements away from the axis are radial expansions. If the expansion joint is designed only with axial compensation, it will leak after two months of installation-the bellows will be twisted unbearable.
How exactly did the radial expansion come about? To put it bluntly, thermal expansion and contraction are only inducements, and force is the pusher. For example, at the corner of the pipeline, the axial thrust will decompose into a lateral component; Another example is the impact when the equipment is started, and the uneven settlement of the foundation leads to the displacement of the support-these will cause the pipeline to deviate laterally. SoRadial expansion of metal expansion joint"Bearing" actually refers to the ability of the bellows not to be damaged under the action of lateral forces.
2. Several fist products to realize radial compensation
Different scenarios have to use different structures. Don't expect a universal corrugated expansion joint to conquer the world.
- Compound hinge transverse expansion jointLateral displacement is absorbed by two sets of hinges and an intermediate tube. The hinge constrains the axial elongation but allows the tube to swing from side to side like a joint. This structure can hold large lateral offsets, such as 8 mm, 10 mm or even larger. It is particularly suitable for the vibration and settlement conditions of the kiln tail pipeline of cement plant.
- Curved tube pressure balance expansion joint: The "all-rounder" in the high-pressure pipeline. It can absorb both axial displacement and radial displacement. The key is that it has a balance structure inside, which can offset the pressure thrust and prevent it from transmitting to the fixed bracket. It is often used in steam main pipes of power stations and high-pressure chemical pipelines.
- Universal corrugated expansion joint: The design itself is mainly axial, but withtie rodCan bear a small amount of lateral displacement. The function of the tie rod is to limit the axial elongation, so that the bellows can deform freely under the action of lateral forces. However, note that the tie rod nut must be adjusted in place, and it cannot be screwed to death or too loose (for specific adjustment methods, please see the question and answer "How to adjust the tie rod nut of the expansion joint" on this site).
When selecting, don't be cheap and take the axial type to top the large lateral displacement-the bellows will soon fatigue and crack, and the cost of replacing it once will be enough to buy several right models.
3. How to calculate the radial expansion? These are the key parameters
calculateRadial expansion of metal expansion jointQuantity, the core is to look at the lateral stiffness of the bellows. The transverse stiffness of single wave of corrugated pipe is related to wave height, wave pitch, wall thickness and material. For example: a DN300 corrugated expansion joint has a wave height of 60mm, a wall thickness of 1.5mm, and a single wave transverse stiffness of about 50N/mm. If the lateral displacement of the pipe needs to be compensated by 10mm, the reaction force is 500N. Add the wave number, and the total reaction force should be multiplied by the wave number. In addition, pressure also affects the effective area-the higher the pressure, the nonlinear increase of transverse stiffness. The specific formula is deduced in detail in the question and answer "Stiffness and Calculation Formula of Bellows" of this site. In actual engineering, the stiffness value provided by the manufacturer is generally directly used.
Note, however, that the amount of radial compensation is limited by the maximum allowable lateral displacement of the bellows. Exceeding the limit causes the column to become unstable-the bellows bends off like a spring, which is no joke.
4. Three pits that are most easily overlooked in model selection
Alas, I have been in this business for more than ten years, and I have seen too many rollover cases. The first pit: Think of the axial type as omnipotent. Some designers see the displacement of the pipeline and go directly to the general-purpose corrugated expansion joint. As a result, the lateral displacement is large, the guide tube is stuck or the bellows is twisted. Pit 2: Ignore the role of tie rods and hinges. Many forget to adjust the tie rod nut after on-site installation, resulting in the expansion joint being unable to freely deform radially. The third pit: limitation of radial displacement by the guide tube. The guide tube itself does not bear pressure, but if the radial displacement is too large, the guide tube will rub against the inner wall of the bellows or even desolder. As said in the question and answer "Specific Function of Expansion Joint Guide Tube" of this site, the guide tube protects the bellows from being eroded by high-speed media, but it will occupy radial space. When selecting, it is necessary to match the radial displacement, the clearance of the guide tube and the stroke of the tie rod well.
5. A real case of radial expansion
Last year, I helped a cement plant deal with the kiln end pipeline problem. The temperature of the pipeline rises from normal temperature to 350℃, and the axial expansion is actually not large (only 25mm). However, because it is the pipeline next to the rotary kiln, the equipment vibrates and the foundation slightly settles, and the measured lateral displacement reaches 8mm. Initially, they installed a universal axial expansion joint, but it leaked after three months of use. Later replaced withCompound hinge transverse expansion jointEquipped with two sets of hinges and intermediate connectors, the lateral compensation capacity is directly pulled to 20mm. During installation, the tie rod nut is specially adjusted to make the expansion joint pre-offset, so as to ensure the best working condition during operation. More than two years, no problem at all. So radial expansion seems unremarkable, but the actual destructive power cannot be underestimated.
6. Give a truth at the end
Radial compensation does not rely on patting the head to select the model, but depends on the specific working conditions. Don't be superstitious about "a bigger size is safe"-the stiffness of bellows changes nonlinearly with pressure, and sometimes a bigger size has greater lateral stiffness and worse compensation ability. It is best to provide the data of pipe direction, fixed bracket position and thermal displacement direction to the manufacturer, and let professionals help you match it. In the products of this siteCorrugated expansion joint for power station industry、Metal Corrugated Expansion Joints in Cement IndustryFor these two categories, the radial displacement condition is considered in the design, and the allowable lateral displacement value is also indicated in the technical parameter table, so it is okay to look up the table directly if you are too lazy.