Where is the "freedom" of the free metal expansion joint? — — Selection, installation and common misunderstandings
A few days ago, a customer who made steam pipes called, and when he came up, he asked, "Where is the freedom of your free metal expansion joint? Do you not have to worry about it when you install it?" Tsk, this question is quite real, but it also exposes a common misconception-free metal expansion joints are not "free and loose" pipe fittings, and its word "freedom" is precisely the precise definition in technology.
The free metal expansion joint is essentially a compensator that absorbs the axial displacement of the pipe by the elasticity of the bellows itself. The biggest difference between it and the general-purpose corrugated expansion joint is that the free type has no external constraint structure (such as tie rods or hinges), and the corrugated pipe directly bears the thrust generated by internal pressure, so it can only absorb axial displacement, but can't bear lateral or angular displacement. And what about external pressure single axial expansion joint? Although it is also axially compensated, its bellows is compressed on the outside, and the structure is more stable, which is suitable for high-pressure scenarios. The free bellows is pressurized internally, so the pressure level generally does not exceed 1.6MPa, and it is easy to become unstable if it is higher.
Then why is it called "freedom"? To put it bluntly, the bellows can be freely expanded and contracted in the axial direction, and is not limited by the external tie rod. But note that this "freedom" has a limit-it can only move along the pipe axis, not laterally. Many on-site accidents are because the free-form is installed near the elbow that needs to withstand lateral displacement, and as a result, the bellows are twisted into twists. Alas, saying too much is tears.
Avoid these 3 pits when selecting
The first pit: medium temperature exceeds 350°C is also selected from the group consisting of free-form.The corrugated pipe of free metal expansion joint is usually made of 304 or 316L stainless steel, and the yield strength drops sharply at high temperature. It can be used normally within 300℃, and the high-temperature axial expansion joint (such as the one on this site) must be replaced above 350℃. Last year, there was a pipeline in a cement plant, with a medium temperature of 420℃. Free type was used, and it was fatigued and cracked in two months.
The second pit: the pressure exceeds 1.0MPa and it is cheap.Although the free type can support up to 1.6MPa in theory, it is recommended to leave a margin in actual use-the pressure level is within 0.6MPa, and if it is higher, it is recommended to use external pressure single axial expansion joint or straight pipe pressure balance expansion joint. The straight pipe pressure balance type can offset the internal pressure thrust, the thrust on the pipe support is much smaller, and the installation is convenient.
The third pit: the amount of axial displacement is considered small.The displacement compensation ability of the free type depends on the wave number, but the more wave number, the lower the overall stiffness and easy instability. Generally, the single-wave compensation amount is safer at 10-15mm. When you calculate the thermal elongation of pipes, you must take the installation temperature into account-the difference in pre-stretching can be twice as high between winter installation and summer installation in the north.
Installation Pit Avoidance Guide: Is the tie rod nut removed or not?
This question is asked repeatedly in the FAQ. Free metal expansion joints typically leave the factory with transport tie rods (aka temporary screws) that act to prevent the bellows from being stretched or compressed to deform during transportation and hoisting. Once installed in place, these screwsMust be removedOtherwise, the bellows can't expand and contract freely, which is equivalent to installing a dead knot. I have seen a case: a power plant removed half of the screw, and as a result, one side of the bellows was pressurized and the other side was tensioned, and it leaked in less than two years.
As for the direction of the guide tube-the free-type guide tube is generally designed as a single direction, and the arrow direction must be consistent with the flow direction of the medium. The function of the guide tube is to guide the medium to flow through the inner wall of the bellows, so as to avoid the high-speed airflow directly washing the corrugations. If installed backwards, the deflector will direct the medium toward the corrugation, aggravating wear and vibration. There was a desulfurization flue gas pipeline before, and the guide tube was installed backwards, and the ripples were worn out in three months.
How to adjust the tie rod nut? The free type does not have a permanent tie rod, only a temporary screw. If it is a large tie rod expansion joint (such as a double hinge type), the nut is adjusted to control the amount of pre-tension. However, the free type does not need to adjust the nut-just loosen and remove the temporary screw, and then check whether the bellows is stuck to ensure that the flanges at both ends are aligned.
Free vs Hinged vs Pressure Balanced: Who is more "free"?
The lateral displacement compensation capacity of free metal expansion joints is almost zero. If you need to absorb both axial and lateral displacement, instead of choosing the free type, you have to use the double hinge transverse expansion joint-it uses two sets of hinges and an intermediate connector to make the bellows angular deflection to achieve lateral compensation. Or use the curved tube pressure balanced expansion joint, which is also specially designed to absorb lateral displacement while also balancing pressure thrust.
Then why can the straight pipe pressure balance expansion joint replace the free type? Because the former cancels the internal pressure thrust internally through the two sections of bellows and the balance ring structure, the thrust on the fixed bracket is very small. The free type, on the other hand, must rely on the pipe's fixed brackets to withstand the thrust — if the brackets are not designed to be strong enough, the pipe will twist like a snake. Therefore, many large pipelines (such as the main steam pipeline of power stations) prefer to use straight pipe pressure balance type rather than free type, because the cost of support is too high.
How to calculate fatigue life? Actual data for power stations and cement industries
The fatigue life of free metal expansion joints depends on the number of cycles and stress levels. According to GB/T 12777 standard, the design fatigue life is usually 1000 times (safety factor 15), but in actual use,Corrugated expansion joint for power plant industryThe replacement cycle is generally 5-8 years, mainly depending on the number of starts and stops. In the cement industry, due to the high dust content of media and serious wear, the replacement cycle is shorter-it has to be replaced in 3-5 years. Inspection points: See whether there are cracks and corrosion pits on the corrugated surface, whether the guide tube has fallen off, and whether the flange sealing surface is damaged. If the wall thickness of the bellows is found to be thinned by more than 30%, replace it directly.
The vacuum degree is high, and the bellows is easily unstable by external pressure. In this condition, free type cannot be used, but double hinge expansion joint of air-cooled island vacuum pipe or special vacuum hose must be used. Both products are available on this site, specially designed for negative pressure environment, with thicker bellows wall thickness and reinforcing rings.
Industry Example: Which working conditions choose free type and which must be changed?
The flue gas temperature is generally 60-80℃, the pressure is low, the axial displacement is large, and the free type is completely OK. However, it should be noted that the flue gas contains SO₂, and it is best to choose 316L or higher grade corrosion-resistant alloy as the material of corrugated pipe. If you use 304, it will rust out in two years.
Free type can be used for small diameter pipelines with parameter pressure of 1.0MPa and temperature within 250℃, but high-temperature axial expansion joint or straight pipe pressure balance type must be used for large diameter (above DN500) or high temperature and high pressure (e.g. 4.0MPa/400℃).
As mentioned earlier, you must use a special model. There was once a project that used the free type in order to save money. As a result, when the vacuum was pumped to-80kPa at the start, the bellows was directly deflated. Lesson.
The Free Metal Expansion Joint is a good thing, but don't take it as a panacea. Before selecting the model, honestly calculate the displacement, pressure and temperature, and don't make do with other models. And guess what? Many accidents are caused by greed for cheap and saving trouble.