Takeover is not a supporting role: Why does the performance of corrugated metal expansion joints tend to get stuck on the takeover?
People who do pipeline compensation design will most likely pay full attention to the bellows body-whether the wavenumber is enough, whether the wall thickness is good, and whether the material is corrosion resistant... These are of course important. But to put it bluntly, many problems on the spot are not the bellows cracking first, butCorrugated metal expansion joint connectionIt collapsed first. What is the takeover? It is the straight pipe section where both ends of the expansion joint are welded to the pipe. It is responsible for connecting, guiding, withstanding end thrust, and sometimes withstanding media erosion. No matter how accurate your bellows calculation is, if the wall thickness of the nozzle is 2mm less, or the length is short, resulting in stress concentration, the whole expansion joint will be installed for nothing. A few days ago, a friend from a power station project spat with me. They usedCorrugated expansion joint for power station industryThe factory tests are all good. As soon as the steam pipeline didn't run for two months, cracks appeared at the welding seam. After removing it, the reason is very direct: the wall thickness of the connection pipe is selected according to the general pipeline, and the additional bending moment that the end of the expansion joint needs to bear is not considered at all. So, stop taking over as a supporting role, it is that "invisibility threshold".
Know three things before choosing a model: medium, temperature and displacement direction-all are indispensable
I have encountered many purchases, and as soon as I came up, I asked, "Do you have an expansion joint for DN600? Give me a quote". And then what? When I ask the medium, I don't know; Ask the highest temperature, about 300℃; When asked about the direction of displacement, say, "Just absorb some heat expansion casually". I generally dare not accept this kind of list, because there is a high probability of fighting in the later period. None of the three elements of selection can be missing. Let's go through it:
- Medium: Determines the material of the receiver and bellows. Carbon steel for both steam ducts and hot air ducts? Don't make a fuss. The hot air of clinker in cement industry contains sulfur and dust, which is highly corrosive, so it is generally usefulMetal Corrugated Expansion Joints in Cement IndustryWear-resistant bushing or guide tube must be added to the tube. Gas, flue gas and chemical media have their own particularities.
- Temperature: Affect wall thickness calculation and fatigue life. The allowable stress difference between 200℃ and 400℃ is large. Does the pipe need heat treatment? Do expansion joints need to be provided with thermal insulation? These have to be ordered in advance.
- displacement direction: Axial, transverse or angular? Determines the structural form of the expansion joint-is usedUniversal corrugated expansion jointStillCompound hinge transverse expansion joint？ The length of the tube and the stress point are completely different. For example, if the lateral displacement is large, the pipe needs a longer rigid section to ensure that no additional bending stress is generated.
These three parameters are coupled to each other, one less, and the design is blind. Do you think that's the case?
How to determine the length and wall thickness of the takeover? Don't take the universal formula hard
Many engineers are used to using the empirical formula in GB/T 12777 or EJMA standard to calculate the wall thickness of the takeover, and then when they look at the calculation results, the wall thickness is 10mm, which feels enough. But the reality is often much more complicated. For example, the expansion joint you choose isExternal pressure single axial expansion jointIn addition to internal pressure, its end takeover is subjected to the thrust of the bellows due to pressure-this thrust can sometimes reach tens of tons. If the wall thickness of the joint is calculated only by internal pressure, the root of the weld will soon be fatigued and cracked.
The takeover length is even more of a pit. If the length is too short, the welding heat will be directly conducted to the bellows, which will affect the resilience performance of the bellows; The length is too long, on the one hand, it increases the cost, and on the other hand, it is easy to become unstable at high temperatures. Generally, there is a range of experience: the length of the nozzle is at least 1.5 times the wave pitch of the bellows, and not less than 8 times the wall thickness. But that's just the conservative floor. I have seen a cement plant hot air pipe project, Party A insisted on their "ancestral" 200mm length pipe to change, as a result after installation the vibration exceeded the standard, later replaced withLarge diameter thick wall expansion jointThe matching 350mm long connector solves the problem.
So, don't finish with the formula. It is best to let the manufacturer do finite element analysis according to your actual working conditions, especially in large diameter, high pressure and high temperature scenes-don't worry about the design and verification fee.
Two of the most common pits at the installation site: pre-tension and treatment of the limit screw
Let's start with pre-stretching. As soon as many on-site installers see that there are screws on the expansion joints, they habitually dismantle them all, thinking that they are in the way. You can dismantle it, but after dismantling it, you are dumbfounded when the pipe matches-because the length of the expansion joint has been fixed, and forcing the matching will cause the bellows to twist. The correct way is: the expansion joint usually comes with a limit screw (also called a transportation screw) when it leaves the factory. Before installation, confirm the cold tightness of the pipeline, pre-stretch (or pre-compress) the expansion joint to the specified length according to the design requirements, then lock the screw, and then remove the screw after the pipeline is welded. Note: It is the pipe that is well welded, not the pipe that is right. After welding, nondestructive testing should be done to confirm that the weld is qualified before the screw can be removed. Otherwise, if it is demolished early, the weld may be directly cracked once the bellows is stressed.
The second pit is that the limit screw was removed too late. Some masters have a strong sense of safety and feel that "it is safer to keep the screw", so they let it always carry it. As a result, after the pipeline runs, the thermal expansion is limited by the screw, and the bellows can't displace normally, which in turn transmits the force to the bracket, resulting in the deformation of the bracket and even the pipeline disengagement. You say it was wrong or not?How to adjust the tie rod nut of expansion jointIt is a technical job, and the principle is: adjust to the design length during installation, remove the limit device immediately after welding, but retain the limit nut function of the tie rod (if it is a tie rod expansion joint). Don't be confused by the loose, the tight.
Case Comparison: Power Station Steam Pipeline vs Cement Industry Hot Air Pipeline, Where is the Take-over Design Difference
Let's compare two real scenes. One is the main steam pipeline of the power station, which usesCorrugated expansion joint for power station industry。 The steam temperature is 540℃, the pressure is 10MPa, and the pipe diameter is about DN200. The material of the connection must be heat-resistant alloy steel (such as P91), with a wall thickness of at least 12mm, and the connection between the connection and the bellows should be rounded to avoid stress concentration. More importantly, the displacement of this scenario is mainly axial displacement, usually employingStraight pipe pressure balanced expansion jointOrFor external pressure single axial expansion joint, the length of the connection pipe should meet the requirement of offsetting the thrust at the end of the bellows, so the connection pipe is often 100~150mm longer than the conventional one, which is used to install the pressure balance ring.
The other is the hot air duct of kiln head in cement industry, which is usedMetal Corrugated Expansion Joints in Cement Industry。 The medium is dusty hot air, the temperature is 350~400 ℃, but the pressure is very low (slight positive pressure). The pipe diameter is large, common DN2000 to DN4000. Under this working condition, the wall thickness of the nozzle does not need to be too thick, but 6~8mm carbon steel is enough-because the pressure is small and the thrust is small. But the difficulty lies in dust scour and high-temperature oxidation. Therefore, a guide tube (also called an inner lining tube) needs to be added on the inner side of the tube, and the material of the guide tube is often made of wear-resistant stainless steel or ceramic coating. Moreover, because it is a large diameter pipe, the displacement is often a combination of transverse and angular directions, so it is often usedCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 The length of the tube mainly depends on the installation space of the hinge mechanism, which is generally shorter than that used in power stations, but requires higher coaxiality.
See the difference, right? The same part called "corrugated metal expansion joint takeover" is basically two design logics in two industries. If you take the set of standards of the power station to set cement industry projects, you will either overdesign and waste money, or ignore the erosion problem, which will lead to perforation in half a year.
Corrugated metal expansion joint connectionThis matter, it is simple to say, but it can write a book if it is complicated. The key is not to treat it as an accessory, but as a component of the system. Explain the medium, temperature and displacement direction clearly when selecting the type; Don't be lazy to set formulas when designing; Handle the pre-tension and limit screw during installation. Do these three, and your expansion energy saving will last several years longer.