Corrugation effect of metal expansion joint: it is not just "telescopic"
Two days ago, I met a buddy who was doing pipeline design. When I talked about the expansion joint, he opened his mouth and said, "Isn't it just a telescopic bellows? It can compensate for the displacement when it expands and contracts." That's half true, but think simply about the corrugation of metal expansion joints. The ripples of the bellows look like the folds of an accordion, and the mechanical logic, material knowledge and matching of working conditions behind them can make laymen confused if they take one out.
The core function of bellows-absorbing displacement and reducing stress
Thermal expansion and contraction. The steam pipe is heated by 100℃, and the 100-meter-long pipe can be extended by more than 120 mm. If there is no expansion joint, it will be a matter of time before the weld tears, the bracket deforms, and even the flange collapses. Through axial expansion and contraction, transverse deflection and angular rotation, the corrugated structure resolves the thermal displacement and vibration energy of the pipeline one by one. Such asHigh temperature axial expansion jointSpecifically dealing with axial displacement;The lateral expansion joint of the compound hinge can handle the lateral swing. However, the absorption displacement of the corrugation is not just as simple as "squeezing"-when each peak and trough of the corrugation is deformed, the material will bend and stretch, and the concentrated stress will be dispersed on the whole corrugated surface through the cooperation of plastic deformation and elastic recovery.
Why is it the same ripples, some can resist tens of kilograms of pressure, and some deflate when pressed gently? The answer lies in the mechanical logic of the corrugated shape.
The Mechanical Logic Behind the Ripple Shape
Wave height, wave pitch and number of layers directly determine the stiffness, pressure bearing capacity and fatigue life of the expansion joint. The larger the wave height, the stronger the ability to compensate for displacement, but the pressure bearing capacity decreases, and instability is easy to occur; The smaller the wave distance, the more crests per unit length, the increased stiffness, and the weakened displacement ability. Single-layer thick-walled bellows usually have a wall thickness of more than 1.5mm, which is suitable for high-pressure applications-such asLarge diameter thick wall expansion jointIt is used for the main steam pipeline of power station, and the pressure can reach more than 10MPa. Each layer of multi-layer bellows (such as 3~8 layers) is as thin as 0.3mm, which reduces the overall stiffness through interlayer slip, has large compensation amount and long fatigue life, and is suitable for flue gas pipelines with low pressure and large displacement. Why not use multiple layers in high-pressure situations? Because interlaminar friction and stress concentration will shorten the life instead, and it is difficult to manufacture. Conversely, low-pressure large-diameter flueMetal rectangular expansion joint, the corrugated section is designed to be rectangular, and the wave height is larger. Sometimes, reinforcing rings have to be added to prevent internal pressure bulging.
How to select corrugated structure under different working conditions
Working conditions vary widely, so you can't pat your head when choosing models.
- High temperature environment:For example, at the boiler outlet of a power plant, the temperature is above 600℃, and the ordinary stainless steel bellows will be oxidized and brittle in a few days. This is the time to use itHigh temperature axial expansion jointThe material is Inconel 625 or heat-resistant alloy, and the solution treatment is done after corrugation molding to ensure high temperature strength. Thermal insulation layer and guide tube should be added to avoid direct erosion of corrugations by high-temperature medium.
- Corrosive media:The chemical industry often touches hydrochloric acid and sulfuric acid, and stainless steel is finished if it touches it. The solution isPTFE-lined hoses orPTFE compensator-The bellows is covered with a layer of polytetrafluoroethylene, or directly lined with fluoroplastic. Note that PTFE cannot bear high pressure, so when the pressure exceeds 1.6MPa, it is necessary to use metal bellows plus PTFE bushing, rely on metal to bear pressure, and PTFE is anti-corrosion.
- The large-diameter smoke said,Desulfurization system, flue gas pipeline at the tail of cement kiln, with a diameter of two or three meters, low working temperature but strong corrosion. At this timeMetal rectangular expansion joints andRectangular non-metallic expansion jointIt's mainstream. Non-metallic expansion joints (fabric fiber expansion joints) are made of composite materials such as silicone cloth and fluororubber, which can withstand the temperature of about 200℃, can compensate the three-dimensional displacement, and are cheap. However, pay attention to the tightness when installing, and don't let the fabric slack and bulge.
"Side Effects" of Ripple and Countermeasures
Corrugated construction is not a panacea. High-speed fluid washes the inner wall of the corrugation and can wear it out in a year. Soguide tubeIt is standard-it is installed on the inside of the corrugation, and the medium is diverted to the center of the pipe to avoid direct washing of the corrugation. The deflector also reduces flow resistance and reduces noise. Another side effect is over-stretching of the ripples: when pressure fluctuations or water hammers occur, the ripples can exceed the design displacement and tear directly. The solution is to addTie rod and limit nutLock the maximum expansion and contraction. You seeThe directly buried (fully buried) expansion joint can't be maintained when buried underground. The tie rod, anti-corrosion coating and insulation layer are indispensable. The corrugation has to be double-layer structure, and the outer layer is protected by sacrificial anode.
The "memory effect" of ripples. After many large displacements, the ripples will produce plastic accumulation and the stiffness will decrease, which is the beginning of fatigue aging. It is much more cost-effective to regularly check whether the wave crest has cracks and whether the wave pitch is uniform than to replace the tube with a new one.
Guide to Selection and Pit Avoidance
Tell me a few lessons of blood and tears that I have seen countless times.
First, stiffness and system matching.Some people calculated the stiffness calculation formula and found that the elastic reaction force of the expansion joint was too big, which pushed the pipe support askew. The smaller the stiffness, the better-if it is too soft, it will easily become unstable and bulge under the action of internal pressure; Too hard to absorb displacement. According to the pipeline thrust and the allowable load of the support, the wave height and the number of layers should be deduced by the stiffness formula. WeStiffness and Calculation Formula of BellowsThere are detailed derivations in the frequently asked questions and answers. It is best to let the manufacturer provide the stiffness curve when actually selecting the model.
Second, install pre-stretch/pre-compression.When the installation temperature is lower than the operating temperature, it should be pre-stretched; Otherwise, it is precompressed. The pre-stretching amount is generally half of the compensation amount, depending on the pipe material and temperature difference. Without pre-stretching, the corrugation is in a relaxed state in the cold state, and when it is heated during operation, the telescopic stroke directly eats half of its life. There was a project before, and the expansion joint of the big tie rod forgot to be pre-stretched, and it was leaked in three months.
Third, I forgot to dismantle the transportation tie rod.This problem is too common. The expansion joint will be locked with a tie rod or bolt when it leaves the factory to protect the corrugation. These temporary fixtures must be loosened after installation, otherwise the corrugation will not work and the pipe will still be hard connected. WeHow to adjust the tie rod nut of expansion jointAs mentioned in the question and answer, before dismantling, confirm whether the system has been fixed, and don't dismantle the pipe and fall off.
When selecting, don't just look at the "nominal diameter" and "compensation amount" on the sample, but also list the medium temperature, pressure, corrosiveness, cycle number and installation space clearly. When you can't make up your mind, go directly to the manufacturerModel and size of expansion jointTable, let the technology come out of the plan. Remember, the corrugation effect of metal expansion joints cannot be summarized by the words "telescopic"-it is the cross-product of machinery, materials and thermodynamics. If you choose the right one, you will worry about it for ten years, and if you choose the wrong one, you will rework it for three days.