Are metal expansion joints and expansion joints the same thing or not?
Before talking about selection, make the concept clear. Many people have asked me: Are metal expansion joints and expansion joints two kinds of things? When you open the product catalog, you will find that some are called corrugated expansion joints, others are called metal compensators, and others are called expansion joints. In fact, they are all the same kind of equipment-flexible components used to absorb thermal displacement of pipelines and reduce vibration and noise. In technical terms, it is more often called "expansion joint" or "compensator", while "expansion joint" is just a common name for the engineering site. To put it bluntly, just like the relationship between "potato" and "potato", they are called differently and have the same function. Therefore, when you see the "universal corrugated expansion joint", "high-temperature axial expansion joint" and "straight pipe pressure balance expansion joint" in the product list of this site, they are essentially a form of metal expansion joint expansion joint.
Open the catalog: What kind of metal extension joint do you need?
Dozens of models in the catalog, how to pick? Two days ago, I met a customer, and the opening required a "metal expansion joint". As a result, when I asked about the working conditions-the high-temperature flue gas pipe was dusty and the space was limited. I told him directly: Don't deal with the general-purpose type, but "corrugated expansion joint for power station industry" or "metal corrugated expansion joint for cement industry". Why? Because there are corrosive media in the smoke and the temperature is high, ordinary bellows can't bear it.
The core of selection is one: seat according to the displacement direction. The pipe only does axial expansion and contraction? Select Axial Expansion Joint or External Pressure Single Axial Expansion Joint. Need to absorb lateral displacement? Use "double hinge transverse type expansion joint" or "large tie rod expansion joint". Want a large amount of compensation and are afraid of pressure thrust? On "Straight Pipe Pressure Balanced Expansion Joint" or "Curved Pipe Pressure Balanced Expansion Joint". There are also special scenes: "directly buried (fully buried) expansion joint" for buried pipelines, and "double hinge expansion joint for air-cooled island vacuum pipelines" for air-cooled island vacuum pipelines. Don't be too troublesome. If you spend 10 minutes more looking at the working conditions, the equipment will be used for 10 years less.
Another reminder: Don't ignore the non-metallic ones. For example, "non-metallic expansion joint (fabric fiber expansion joint)" has good corrosion resistance and vibration isolation, but it can't bear pressure; While the "rubber compensator" is suitable for low pressure water systems. Metals and non-metals have their own jobs, so don't mix them.
Three parameters that must be determined in model selection-temperature, pressure and displacement
Speaking of Breaking Heaven, just keep an eye on these three numbers in the end. First, temperature. It determines the bellows material. Within 200℃, 304 stainless steel is used, and above 400℃, 316L or Incoloy 825 is obtained. If it is higher, it will be superalloy. If the material is not selected correctly, the bellows will creep and crack directly. Second, stress. The working pressure directly affects the wall thickness and wave height. Don't expect a general-purpose type for high-pressure pipes, at least use "large-diameter thick-walled expansion joints" or designs with reinforcing rings. Third, displacement – this one is most easily underestimated. The design drawing writes "axial compensation amount 50mm". Do you think any product can meet it? Wrong. The compensation ability of bellows is directly related to wave number, wave pitch and layer number. To check the "Metal Expansion Joint Weight Table" and "Bellows Stiffness and Calculation Formula", don't pat your head to estimate.
For example: a steam pipe, temperature 350 °C, pressure 1.6 MPa, axial displacement 80 mm, transverse displacement 20 mm. If you just buy a "universal corrugated expansion joint" and install it, the lateral displacement will make the bellows laterally unstable. The correct way is to use the "double hinge transverse expansion joint" with the "axial expansion joint", or directly use the "straight tube pressure balance expansion joint" to eat the thrust. And guess what? Many people are cheap and hassle-free, and there are abundant cases of pipeline rupture caused by mistaken selection.
Four Most Common Mistakes at the Installation Site (with Correct Operation)
Mistake 1: The transport screw is not disassembled.When the expansion joint leaves the factory in order to be transported and fixed, it will bring transport screws or tie rod nuts. Must be removed after installation! Otherwise, the expansion joint can't move, which is equivalent to installing a rigid pipe. How to dismantle? Refer to "How to adjust the tie rod nut of the expansion joint" and "Do you need to disassemble the screw of the expansion joint"-loosen them all, and the length of the screw should be greater than the expected displacement.
Error 2: The installation direction is reversed.Many metal expansion joints are marked with the direction of the arrow, which is the direction of the medium flow. Especially with a guide tube (such as "universal corrugated expansion joint"), the guide tube must be facing the direction of media flow, otherwise vortex will form to erode the bellows and leak in a few years. Specifically, see "Specific Function of Expansion Joint Guide Tube".
Mistake 3: The bracket is not set in place.Fixing brackets and guide brackets must be set on both sides of the expansion joint. The fixed bracket should be able to withstand the internal pressure thrust, and the guide bracket should ensure that the pipeline does not deflect. Some people directly use the expansion joint itself as support, which is looking for death.
Error 4: Pre-stretch/compression ignored.When the temperature difference between the installation temperature and the working temperature of the pipeline is large, the installation length needs to be adjusted according to the requirements of the manufacturer. For example, the thermal expansion of low-temperature pipelines, and a part of the expansion joint is pre-compressed during installation; High temperature pipe cold shrink, pre-stretch part. If this step is not done, the life of the expansion joint will be cut in half directly.
Maintenance and Life: How to Make Metal Expansion Joints Last Three to Five Years
Don't be a hands-off shopkeeper after the equipment is installed. Regular checkups are the last word. Check the bellows surface every six months for rust, cracks and depressions. Especially in corrosive environments, such as near the "desulfurization flue gas baffle door" and "flue gas baffle door" used on desulfurization flue gas pipelines, metal expansion joint welds are most prone to problems. If the patent leather is found to fall off, reapply anticorrosive paint in time.
Note also: Do not allow the expansion joint to work under conditions that exceed the design displacement. Pipeline settlement and foundation displacement all add additional burden. Make displacement records once a year and compare the design values. If you exceed it, find a way to adjust the support or replace it.
Finally, one thing that is easily overlooked: Don't press the insulation layer on the expansion joint. Many projects outsource the insulation of the pipeline, which directly wraps the expansion joint to death. That thing is limited when it moves, and the bellows quickly fatigue. Sufficient space must be left for expansion and retraction. To achieve the above points, it is normal for the metal expansion joint to last for three or five years.