What exactly determines the length of a metal expansion joint?
Many people ask "how long is the length of the expansion joint" as soon as they come up. It seems that the longer it is, the more it can compensate. Alas, this is really a big misunderstanding. The length regulation of metal expansion joint is not set by patting the head, and there is no such thing as "the longer the better". Once the length is exceeded, the stiffness, stability and fatigue life will all change, and may even make the bellows unstable. How on earth did that length come from? To put it bluntly, three things are pulling: bellows wavenumber, connection length, and end structure.
The wave number directly determines the compensation amount. The more waves, the greater the displacement that can be absorbed. However, as the wave number increases, the overall length of the bellows will increase, and the risk of column instability caused by internal pressure thrust will also increase. The length of the pipe is the straight pipe connecting the bellows and the pipe, which mainly leaves space for welding, but also affects the overall length. The end structure is better understood-flanges, tubes, flanges, tapered ends, each of which occupies a different axial dimension.
So you see, these three parameters are pinning each other like a triangle. If you want to compensate more, the wave number has to be increased, and the length will rise with it; But the length is too long, and the pipe bracket and layout don't agree. This is why metal expansion joint length regulations are never a single calculation, but the result of a comprehensive balance.
What are the hard requirements for length in GB/T 12777?
Really speaking of "regulations", the most authoritative standard for metal expansion joints in China at present is GB/T 12777 General Technical Conditions for Expansion Joints of Metal Bellows. This standard has mandatory requirements for design, manufacturing and inspection, and the length is naturally in it.
What does it stipulate? First, the wave number, wave pitch, effective length and total length of the bellows must be indicated on the drawing. Secondly, the manufacturing deviation is not allowed to mess around-the limit deviation of the total length is generally required by JS16 or higher, depending on the product grade. Furthermore, the standard also requires pre-stretching or pre-compression before leaving the factory, and this handling amount will directly affect the delivery length. The length of the expansion joint you get is probably not the same number as the length it actually works in the pipeline.
More critically, GB/T 12777 has requirements for the design fatigue life of bellows, and the length and fatigue life are directly linked. The bellows is too long, and the amount of deformation per wave is dispersed at the same displacement, which would have been the advantage; However, if it is too long, it will easily cause column instability, and there are restrictions on the aspect ratio in the standard. In other words, the length is not randomly determined, and technically it needs to pass the stability check.
How to calculate the length of common expansion joints?
Take the three most common. Universal corrugated expansion joint-the most common single axial type, the length is basically equal to the length of the end connector plus the length of the corrugated tube, and the thickness of the flanges at both ends. In other words, if the flange specifications are set, the wave number is set, and the total length is almost equal.
The external pressure single axial expansion joint is a little different. Its bellows is on the outside, and there is a guide sleeve inside, so there is an extra external pressure sleeve and sealing structure in the length structure, and the overall length is usually longer than the general-purpose type of the same diameter. This is also the reason why many design institutes are reluctant to choose it when space is tight.
What about straight pipe pressure balance expansion joints? There are two sets of working bellows and balanced bellows in this thing, and the length naturally doubles. It relies on balancing the bellows to eliminate the internal pressure thrust, so the force on the pipe support is small, but the cost is the large axial size. If you don't measure the installation space when you choose the model, and when you arrive, you find that a section grows and can't be installed, that's a headache.
What can go wrong when the installation length does not match the design length?
There is a common operation here called pre-stretching or pre-compression. If you think about it, the pipe expands when the operating temperature is high. If the expansion joint is installed at the free length in the cold state, the thermal displacement may exceed the design value. Therefore, when installing, the expansion joint is often pre-stretched for a section, so that the bellows can be "prepared in advance". However, if the on-site workers don't know the situation and hard-pull the pre-stretching back as an installation error, or simply install it in the wrong direction, the consequence will be that the actual deformation of the bellows exceeds the limit and the life will plummet.
Another type of misoperation is more common-the wrong length of the takeover at both ends. The manufacturing length of the expansion joint is according to the drawing, but after the pipeline is installed, the flange spacing does not match, so some people rely on forcibly screwing the bolts to make up. This is equivalent to adding an initial load to the bellows, which can vary from leakage to cracking of the bellows. The same is true for cold tightening. If the amount of cold tightening is wrong, the bellows may be pressed to the head all the time in the hot state, which can't absorb displacement at all.
Ask these length parameters when selecting to avoid not being installed
Therefore, when selecting a model, don't just throw a "how much DN, how much compensation" and be done. You have to ask these length parameters clearly:
- Effective length: The length of the bellows that actually participates in the deformation. This value affects the stiffness and compensation amount, and it is used in design calculations.
- Total length: Includes the entire dimension between the end connectors or flange faces. Pipe arrangement and bracket spacing must be reserved as such.
- Flange spacing: For flanged expansion joints, the distance between the two flange sealing surfaces is the mounting dimension. If it is a joint weld, it is the spacing on the outside of the grooves at both ends.
In addition, confirm whether it is the factory length or the installation length. Because some expansion joints have been pre-stretched when they leave the factory, they have to be adjusted during actual installation. If you don't ask clearly, directly leave the position according to the total length of the drawing, nine times out of ten will not match.
Two days ago, I met a customer, bought a general-purpose corrugated expansion joint, measured the pipeline spacing and placed an order. As a result, the thickness of the flange was not considered, and the goods could not be installed when they arrived, and he was in a hurry to catch the construction deadline. Finally, he had to take it over on the spot, which was time-consuming and labor-consuming. You say it was wrong or not? The length regulation of the metal expansion joint looks simple, but there are many doors inside. Next selection, ask the effective length, total length and flange spacing at once, which is better than anything else.