When a section of bellows isn't enough, do you think of lengthening it, or segmenting it? The first reaction of many engineers is "it's over if you lengthen it". That's really not what happened. Once the length of the bellows exceeds a certain critical value, the stability is a big problem, and the instability of the column caused by internal pressure can deform the whole pipe top. Therefore, there are segmented metal expansion joints-the long bellows is disassembled into several sections, and the middle is connected with a connector (intermediate pipe), and each section bears a part of the displacement.
When one section of corrugated pipe is not enough, it needs to be segmented-first, explain the structural logic of segmented metal expansion joint
To put it bluntly, the "segments" of segmented metal expansion joints are divided into the number and arrangement of bellows units. Single bellows can only absorb axial displacement, and it is difficult to deflect one millimeter laterally. If you want the pipeline to swing tens of millimeters in the horizontal direction and still work normally, one bellows can't do it. You have to rely on the combination of more than two bellows and the intermediate pipe to "disassemble" the lateral displacement into the angular displacement and axial displacement of each section to absorb it.
This logic is the same thing as the hinge mechanism in mechanical design. We think of each section of bellows as a hinge, with the middle pipe as a rigid link. The pipe should be offset laterally, with the left bellows opening and the right bellows closing, and the displacement is distributed between these two sections. When you look at the compound hinge transverse expansion joint and the compound straight pipe bypass pressure balance expansion joint, the structural essence is this idea.
Segmentation is not divided if you want: How to realize axial, transverse and angular displacements by segmentation cooperation
Displacement is not how you want to divide it. Axial displacement is mainly carried by single bellows, such as external pressure single axial expansion joint. The bellows is externally compressed, and the axial compensation is large, which is suitable for straight pipe section. However, if the pipeline has lateral displacement at the corner, it has to use a compound structure-two sections of bellows cooperate, and the middle pipe converts axial displacement into lateral deflection.
Angular displacement is more particular. A bellows itself cannot be angularly deflected, so two bellows must appear in pairs, and the middle is constrained by a tie rod or hinge to form a "universal joint". Don't ignore the role of the tie rod. It controls the maximum deflection angle of the bellows. Beyond the design angle, the bellows will become unstable. The most common problem in actual working conditions is: the angular displacement calculated on the design drawings, and the bellows will be wasted because the settlement of the support is deviated by two degrees. Therefore, when selecting the model, it is recommended to check the maximum allowable angle and leave sufficient margin.
The three most easily overlooked issues in model selection: stiffness, fatigue life and guide tube configuration
Many engineers will look at stiffness when selecting a model, but it is easy to see it wrong. The total stiffness of the segmented metal expansion joint is not a simple addition of the stiffness of each section, but an equivalent stiffness after being connected in series, which is calculated to be lower than that of a single section bellows. Low stiffness means small elastic reaction force, which is advantageous for the thrust of the fixed bracket. At the same time, however, the natural frequency of the system will change when the stiffness is low. If there is pulsating flow or reciprocating compressor in the pipeline, check whether resonance will occur.
Not to mention fatigue life. Bellows are flexible elements whose life depends on the number of displacement cycles. The displacement of each section of the corrugated pipe in the segmented structure is different, and the fatigue life is determined by the section that bears the maximum displacement. It's like the strength of the chain depends on the weakest link-you just look at the total displacement selection, not how much displacement is assigned to each segment, and sooner or later something goes wrong.
The deflector configuration is the most overlooked. The role of the guide tube is not only to divert the flow, it protects the inner wall of the bellows from high-speed medium scour, and can also reduce the fluid excitation. In particular, the intermediate tube position of the segment needs to be equipped with a guide tube, otherwise the turbulent zone when the medium flows from the previous section to the next section will directly flush the bellows trough. At the end of the day, the money of the deflector can't be saved.
From general-purpose type to pressure-balanced type, how to fit the seat of segmented metal expansion joint in actual working conditions
The general-purpose corrugated expansion joint is suitable for most conventional thermal pipes, mainly with axial compensation, and the price is also cheap. However, when the pressure is large and the pipe diameter is large, the internal pressure thrust is a big trouble. At this time, you have to look at the pressure balance type.Straight pipe pressure balanced expansion jointThrough the combination of working bellows and balanced bellows, the internal pressure thrust is self-balanced, and the fixed bracket does not have to bear huge blind plate force, so there is no problem in choosing it for large-diameter pipes.
In scenarios with bypass pipe requirements, such as the main steam pipeline of power plants, the double straight pipe bypass pressure balanced expansion joint is more suitable. It makes the bypass pipe and the expansion joint into one, saving the on-site piping. In another case, the pipe itself has a curved section, and the pressure balance expansion joint of the curved pipe can be directly arranged at the elbow position to absorb multi-dimensional displacement while maintaining pressure balance. What about high temperature conditions? The heat insulation layer is added to the high-temperature axial expansion joint, and the temperature resistance level has gone up, but the material of the corrugated pipe has to be changed accordingly. The fatigue life of Inconel 625 and 316L is one order of magnitude different.
Installation and operation and maintenance reminders: pre-tension, tie rod nut, liner barrel direction, don't wait for accidents to regret these details
Finally, talk about installation. Pre-stretching (cold tightening) is not done at all in many scenes. The amount of thermal expansion at the design temperature is fixed. If the bellows is pre-stretched by half of the thermal displacement during installation, the bellows can work in a more reasonable strain range in both cold and hot states, and the life may be doubled. Without pre-stretching, the bellows directly pushes to the maximum compression amount in the hot state, and the fatigue life drops linearly.
The adjustment of the tie rod nut is also particular. Tie rods are subjected to internal pressure thrust and are not intended to forcibly correct mounting deviations. Both nuts must be tightened at the same time and locked after adjustment in place. In reality, I have seen someone tighten only one side of the nut, which caused the bellows to be unevenly stressed and cracked after a few months of operation.
Not to mention the direction of the liner tube-the direction of the guide tube must be consistent with the flow direction of the medium, and the arrow is clearly marked. If the bellows is installed backwards, it will be directly washed and worn out by the medium. Two days ago, I met a customer who said that the expansion joint leaked after less than three months. When I disassembled it, I found that the guide tube was installed backwards and the weld was all punched through. Alas, this kind of low-level mistake really shouldn't have been made.
The segmented metal expansion joint is no mystery. Its structure is nothing more than the combination of "bellows + intermediate pipe + tie rod/hinge", but the displacement distribution, stiffness matching and guide tube configuration of each section need to be carefully calculated according to the actual working conditions. When it comes to model selection, I would rather spend half a day checking the calculation book than waiting for the equipment to be on the scene before regretting it.