How exactly do metal expansion joints expand? Physical processes from bellows deformation to displacement absorption
When many people come into contact with the metal expansion joint for the first time, they will stare at the circle of corrugated pipes in a daze: this thing looks like the drain pipe of a washing machine. How can it swallow the thermal expansion and contraction of tens of millimeters of pipes? In fact, the core principle is very simple-relying on the elastic deformation of the bellows itself to "eat" the displacement. Think of it as a retractable spring tube, except that the spring is made of stainless steel or alloy, and the shape of the corrugations has been specially calculated to bend repeatedly without fatigue fracture while withstanding internal pressure and temperature.
Each corrugation (also called crest and trough) of the bellows will bend and stretch when stressed, and multiple corrugations are accumulated together, and the compensation amount comes out. Take what we always sayUniversal corrugated expansion jointFor example, the single wave compensation amount is typically between 2 and 8 mm, depending on the wave height and wall thickness. A set of more than ten waves of expansion joints can easily handle tens of millimeters of axial expansion and contraction. To put it bluntly, this is to accumulate small deformations into macroscopic displacements-the same principle as stacking Arhats. One person doesn't have much height on tiptoe, but more than a dozen people can reach the roof after stacking them. Isn't this the principle?
The direction of displacement in the pipe is not only axial. The steam pipe heats up and the pipe elongates along the length, which is axial. However, sometimes the pipeline is deformed or turned due to the bracket, and there will be lateral misalignment or angular deflection. At this time, it is not easy to rely on the axial expansion joint alone. Hard installation is equivalent to letting the bellows twist to work, and it will crack in a few cycles.
What to do? Engineers added tie rods, hinges or universal rings outside the bellows to forcibly limit the deformation direction of the bellows. Such asCompound hinge transverse expansion jointTwo sets of bellows are used to cooperate with the hinge structure to absorb the lateral displacement;Curved tube pressure balance expansion jointThen the internal pressure of the bellows on both sides is self-balanced, and the angular displacement is absorbed, while the pressure thrust is not transmitted to the fixed bracket. You see, the same bellows, with different restraints, can make more than a dozen functions-the products listed on our website, likeStraight pipe pressure balance type、Double straight pipe bypass pressure balance type、External pressure single axial expansion jointThey are all derived from this idea.
The elastic limit of metal is just that little. Will it not crack when bent repeatedly? Good question. Here we have to talk about the fatigue life design of the expansion joint. The bellows will be pre-strained before leaving the factory, and the wall thickness at the trough is usually a little thicker than the peak, in order to transfer the stress concentration zone to the peak position. In addition, the guide tube is an accessory-the guide tube is sleeved on the inner wall of the bellows, which can not only guide the flow of the medium without washing the ripples, but also reduce the vibration caused by turbulence (for specific functions, please refer to the Q&A in our station).
The service life of the expansion joint is generally calculated according to 1,000 to 10,000 full cycles in design. Under actual working conditions (such as how many times a year the power plant pipeline starts and stops), it is no problem to use it for twenty or thirty years. Two days ago, a customer asked the gas bellows to be changed every few years. In fact, it was civil gas hoses, which are different from industrial metal expansion joints. Civil gas hoses have age requirements, and the type test used in industry is carried out according to the standard JB/T 12235. As long as it does not exceed the design displacement and pressure, it can be checked regularly, and there is no need to pat your head to change them.
When installing the expansion joint, what exactly is that arrow direction and tie rod nut for? The direction of the arrow refers to the direction of the flow of the medium, and its main function is to prevent the direct impact of the medium on the ripples caused by the reverse installation of the guide tube. The tie rod nut is locked in the transportation and installation stage, and the purpose is to pre-compress or pre-stretch the expansion joint to the middle position to avoid the on-site pipeline being misaligned; After all the pipes are connected and the fixing bracket is welded, loosen the nut to the specified position (usually the product description will give the distance between the two nuts), otherwise the expansion joint is locked and there is no room for expansion at all. You say it was wrong or not?
To put it bluntly, the principle of metal expansion joint expansion is not mysterious. The difficulty is to choose the right model and install it correctly according to whether it is axial, transverse or angular displacement in the pipeline system. Like those used in the power station industryCorrugated expansion joint for power station industryCement industryMetal Corrugated Expansion Joints in Cement Industry, the working temperature is high, the dust is heavy, and it will be equipped with heat insulation layer and wear-resistant lining-but in the final analysis, the corrugations of the bellows are still bending honestly, bearing all the thermal stress for the pipe. Therefore, the next time you see the bulging bellows on the pipe, you will know that it is not a beautiful decoration, but a hard-core part that actually carries displacement with your body.