1. Find out first: What exactly does the "rebound" of the metal expansion joint mean?
Many people hear the word "rebound" for the first time, thinking that it is the way the bellows bounces back by themselves after being squashed. That's pretty much what it means, but engineering is more detailed-rebound refers to the ability of a metal bellows to return to its original shape when external forces are removed after being subjected to axial compression or tension. This ability is directly linked to the rigidity of the bellows. The greater the stiffness, the harder the rebound, and the worse the displacement absorption capacity; The stiffness is too small, the rebound is soft, and it is easy to produce permanent deformation. Some of our common products, such as general-purpose corrugated expansion joints and high-temperature axial expansion joints, will be tested for stiffness before leaving the factory, so as to confirm whether their rebound characteristics are within a reasonable range.
2. What moth will happen if the rebound is not good?
Two days ago, I met an old brother who was engaged in power plant pipelines, saying that the external pressure single axial expansion joint used in their steam pipeline found that the flange leaked after half a year's operation. After removing it, it shows that the bellows has obvious uneven deformation, and the local wave distance is "squeezed to death". This is the typical rebound failure-the bellows does not spring back when it is time to spring back, resulting in the stress concentration in the weld and trough position, and long-term fatigue cracking. Insufficient rebound will also make the thermal displacement of the pipeline unable to be fully absorbed, and it is not uncommon to transmit the force to the fixed bracket, and the accident of deformation or even fracture of the bracket. On the other hand, too much rebound is not good, and it can't be pulled by pre-stretching during installation, or the adjacent equipment is broken during operation.
3. What factors are secretly controlling the rebound?
The amount of rebound is not metaphysics, but the core is several variables: wave number, wave height, wall thickness, material yield strength, and working temperature. The more wavenumber, the more "soft" the whole, and the rebound ability decreases; The wall thickness increases, the stiffness increases, the rebound becomes stronger but the displacement absorption capacity decreases. In terms of materials, austenitic stainless steel rebounds well at room temperature, but when it reaches above 400℃, the creep effect will greatly attenuate the rebound value. This is why high-temperature axial expansion joint must be selected for high-temperature working conditions. Its bellows material and heat treatment process are optimized for high-temperature rebound attenuation. In addition, the corrosive medium will also reduce the effective wall thickness, which indirectly affects the springback.
4. The "pre-stretching" and "pre-compression" during installation are essentially a game with rebound
Does the screw of the expansion joint need to be removed? How to adjust the expansion joint tie rod nut? There is no rebound behind these problems. When the manufacturer delivers goods, the expansion joint is usually pulled to the designed compression amount (or stretching amount) and locked with a tie rod, so as to keep the bellows in the best pre-strained state after installation. If the rebound prediction is inaccurate, the pre-pull is overdone, and the expansion joint of the pipeline has no room to compensate once it heats up; The pre-tension is not enough, the rebound abundance is too large, and the bellows bear excess stress in the cold state. Special attention should be paid to the installation of the expansion joint of the large tie rod. Before adjusting the nut, confirm the rebound direction, and generally push backwards according to the displacement direction of the pipeline. Real case: A cement factory used a metal corrugated expansion joint in the cement industry, but it was not pre-stretched according to the drawings during installation. Half a year later, the bellows bulged because the thermal displacement ate up the rebound margin.
5. How to "cure" rebound under different working conditions?
There is no panacea, you have to prescribe the right medicine.
- High temperature steam pipeline:The high-temperature axial expansion joint is preferred, and the material should contain molybdenum or niobium. At the same time, the rebound attenuation rate in hot state is calculated, which is generally 0.7~0.8 times of the cold stiffness.
- Large diameter low pressure system:For example, the corrugated expansion joint used in power station industry, the rebound problem is more due to the sagging deformation caused by its own weight. At this time, it is necessary to consider the support of the guide tube or increase the wavenumber to compensate.
- Corrosive media:Lined with PTFE metal hose or PTFE compensator, the rebound is mainly provided by metal bellows, and the PTFE layer does not bear force, but it is necessary to prevent the PTFE layer from rebounding after being squeezed during installation.
- Frequent start-and-stop systems:For example, the high-temperature pipeline next to the flue gas baffle door has fast temperature circulation and corrugated pipes are easy to fatigue. It is recommended to use compound hinge transverse expansion joint or straight pipe pressure balance type, which share the rebound stress through multi-section corrugated pipes.
6. How to "press to death" the rebound risk in advance when selecting a model?
The most direct way is to let the manufacturer provide the product stiffness curve and allowable rebound. Regular manufacturers, such as us, have complete data for each model (general type, external pressure single axial type, curved tube pressure balance type, etc.). When you select the model, you only need to give the medium temperature, pressure, displacement, and number of cycles, and the engineer can calculate the required wave number and wall thickness. If the space is constrained, a direct buried (fully buried) expansion joint can be considered, whose external structure strengthens the restraint of springback. And don't forget the deflector-it not only protects the bellows from being washed out, but also helps stabilize the flow direction and reduce the extra rebound fluctuations caused by turbulence. Bottom line: Don't pat your head and estimate, let the working condition data speak for yourself.