Starting with thermal expansion and contraction: Why do pipelines need to "catch their breath"?
Think about it. A steam pipe, whose temperature soars from normal temperature to 300℃, can stretch by 3-4 mm per meter. One hundred meters of pipe, light thermal expansion can eject you more than 30 centimeters. If held firmly in place, the stress in the pipe wall can crack the flange and push the bracket down.
A while ago, a buddy from a thermal power plant told me that a section of their main steam pipe was not equipped with an expansion joint, and the pipe burst directly during the test run, and the steam spewed out like a train whistle. That scene — tsk, it took three days to repair and lost hundreds of thousands. So the pipe has to "catch its breath", and the metal expansion joint is the one that leaves room for the pipe to expand and contract.
Core principle: elastic deformation of bellows
The elastic deformation of the bellows is used to absorb the displacement of the pipe。 Bellows are circles of thin-walled metal tubes that resemble accordion folds. When axially stretched, the corrugations are straightened, when compressed, the corrugations are squeezed together, when transversely deflected, one side is stretched and the other side is compressed, and the angular direction is used as a hinge.
We have a large number of products on our website that are this idea-such asUniversal corrugated expansion jointSuitable for conventional working conditions;In order to carry the flue gas above 800℃, the bellows material has been replaced with Inconel 625, and the stiffness has been specially designed. The stiffness thing is very critical: the lower the stiffness, the greater the compensation, but the pressure resistance also decreases. Using the calculation formula of bellows stiffness (which is deduced in detail in the question and answer of this site), it can be calculated that the same wall thickness and wave height, the more wave numbers, the axial stiffness almost decreases inversely. Therefore, when selecting the model, it is not a pat on the head, but the result of engineering calculation.
Then how does it count? Simply put, the stiffness of bellows is related to wave height, wall thickness, wave number and elastic modulus. For example, a 304 stainless steel bellows has a wave height of 50mm, a wall thickness of 1.5mm and a single wave stiffness of over 100 N; The wall thickness is increased to 2mm, and the stiffness is doubled directly. When designing, we must find a balance between the compensation amount and the bearing pressure.
How Critical Design Parameters Affect Compensation Capability
Let's take apart a few hard parameters:
- wavenumber: The more waves, the greater the total compensation amount (each wave contributes a little), but the smaller the axial stiffness, and the easy instability. For example, the same bellows of DN300, 8 waves can compensate 40mm, and 16 waves can reach 80mm.
- wall thickness: Thick pressure resistance, but the corrugation becomes hard, and the compensation amount becomes smaller. The 1.0 mm 304 bellows is 4 times more stiff than the 0.5 mm, but the pressure resistance increases from 0.6 MPa to 1.6 MPa.
- Material: The elastic modulus of 304 and 316L is similar, but 316L is corrosion resistant and suitable for chemical pipelines. If it's high temperature, you have to use superalloys-our website'sCorrugated expansion joint for power station industryThe 316L or 321 is used in large quantities.
- Wave height: The higher the wave, the better the flexibility (the distance between the trough and the crest is large), but the outer diameter of the bellows becomes larger, and the installation space is limited. Usually the wave height is 10~30 times the wall thickness.
These parameters need to be carefully calculated during model selection. I have seen people greed for cheapness and used thin-walled multi-wave to carry high-pressure steam. As a result, the bellows was deflated-deformed and unstable, and directly scrapped. So, don't try to save trouble, useLarge diameter thick wall expansion jointDon't make do with the general-purpose one.
Directions of "partial subject" with different structures
No matter how elastic the bellows is, it can only absorb the displacement related to the direction of the corrugation. Different types of products are actually "partial science":
- Axial type(e.g.External pressure single axial expansion joint): Specially absorb axial expansion and contraction, and add a layer of outer sleeve outside the bellows to prevent instability. Suitable for straight pipe sections, cannot withstand lateral offset.
- Compound hinge transverse expansion joint: With two bellows and hinge structure, it can absorb lateral displacement. For example, the steam pipeline of the air-cooled island needs to move up and down, left and right, so it has to rely on this.
- Curved tube pressure balance expansion joint: There is a balanced bellows inside, which can offset the thrust generated by internal pressure and reduce the impact on the fixed bracket. Suitable for high pressure large diameter pipes, such as main steam.
- Straight pipe pressure balanced expansion jointAndCompound straight pipe bypass pressure balance type: It is also used for occasions where balanced thrust is needed. The applicable scenarios of each structure are different, so it cannot be changed randomly.
- AndRotary compensator, absorbs displacement with rotational motion, suitable for large rotation angles.
Look at the direction of the pipe, look at the type of displacement, look at the pressure and temperature-there is no universal product. For example, the volume of the flue gas pipeline is large and the temperature is high.non-metallic expansion joints (fabric fiber expansion joints); However, when there is dust washing, it should be addedguide tube(Article 7 of Q&A on this site is very clear).
Pits installed and used
After the principle is finished, let's talk about the places where it is easy to stumble in practice. Many people think that the expansion joint is finished when it is installed, but it cracks in a few months.
First,guide tubeThe function is to prevent scour. When the medium carries particles (such as smoke), the guide tube guides the airflow to the inner wall of the bellows to avoid direct washing of the corrugations. However, in order to save costs, some people didn't add it, and the bellows were worn out in half a year.
Second,Do you want to remove the tie rod nut?The answer is: if the tie rod is used for transportation limit, it must be loosened or removed after installation (refer to Article 13 of Q&A on this site "Does the screw of expansion joint need to be removed"). Otherwise, the expansion joint can't move at all, which is no different from a rigid connection. If the tie rod is a designed load-bearing member (such as the tie rod of a hinged expansion joint), it will not move. Judgment method: Read the instructions or consult the manufacturer.
Third,Arrow Direction。 There is an arrow on the expansion joint pointing to the direction of the medium flow. If this installation is not followed, the direction of the guide tube is reversed, and the medium will form vortex at the inlet of the guide tube, which will aggravate vibration and erosion. Article 10 of the question and answer on this site explains it in detail.
And guess what? If one of the above three is committed, the life of the expansion joint will be directly discounted in half. It is much less troublesome to spend more time figuring out the installation details than to replace them later.
The principle of metal expansion joint is not mysterious-it is to use the elastic deformation of bellows to "overcome rigidity with softness". The key is to understand the principle thoroughly, choose the right model, and install it in the right direction. Next time you select a model, you can tell the way to the parameter table.