Find out one thing first: The bellows in the expansion joint is not done by just pressing a few folds
Common corrugation shapes in the industry are U-shaped, ω-shaped, C-shaped, S-shaped, and rare V-shaped and rectangular waves. U-shape is the most versatile, simple to manufacture, moderate stiffness, suitable for most general-purpose corrugated expansion joints; The ω-shaped cross-section is like a Greek letter Ω, with short wave height, large wall thickness and strong pressure resistance. It is commonly used in high-temperature axial expansion joints and corrugated expansion joints used in power station industry; C-shape is actually similar to U-shape, but the trough is smoother, the stress distribution is more uniform, and the fatigue life is longer. It is mostly used in occasions that require frequent displacement such as transverse expansion joints of compound hinges. S-shape is born for large compensation, with a large ratio of wave height and wave pitch, but relatively weak withstand voltage, so it is generally used in low-voltage and large-diameter applications. And guess what? Many customers only care about the size when they get the product, and don't ask about the corrugated shape at all-this is precisely the first hurdle in selection.
Different corrugation shapes directly determine the mechanical properties of expansion joints
In the calculation of stiffness (refer to Article 1 of FAQ on this site), the stiffness formula of U-shaped corrugation is complicated, but the empirical value is easy to estimate; Because of the thick crest and low circumferential stress, ω-shaped wall thickness can be 20% thinner than U-shaped wall thickness at the same pressure, but at the cost of small compensation. Look at the fatigue life again-C-shaped corrugation has a large radius of curvature and a small stress concentration. The test data show that the life energy of C-shaped corrugation is more than 30% higher than that of U-shaped corrugation under the same displacement. But this is not to say that the U-shape is not good. The general-purpose corrugated expansion joint has been used for twenty years, and the technology is mature and the cost is low. Two days ago, I met a customer in the cement industry. I felt that the pressure of my own pipeline was not great, so I had to choose the omega shape. As a result, it was not only expensive, but also unable to be installed because the compensation was insufficient. Therefore, the selection of corrugation shape depends on the specific working conditions: Ω shape for high pressure and high temperature, C shape for frequent displacement, S shape for large compensation and low pressure.
Flip through the product information in the station, and you can directly check your seat
For example, most of the "universal corrugated expansion joints" are U-shaped corrugations, which have a wide range of applications; "High-temperature axial expansion joint" is clearly marked with ω-shaped corrugations, because it has to cope with steam above 500℃; "Double hinge transverse expansion joint" and "air-cooled island vacuum pipe double hinge expansion joint" need to absorb multi-directional displacement, and the internal bellows often adopt C-shape or multi-layer C-shape combination. In addition, "large diameter thick wall expansion joint" and "external pressure single axial expansion joint" will adopt U-shaped reinforcing ring or ω-shaped in order to balance pressure resistance and stability. For balanced structures such as "straight pipe pressure balanced expansion joint" and "curved pipe pressure balanced expansion joint", because the bellows has to bear the internal pressure thrust, ω-shaped or multi-layer U-shaped are usually used to improve the pressure bearing capacity. Mind you, don't take the analogy of "rubber compensators" or "non-metallic expansion joints", their corrugated construction is a whole other set of logic.
Then how to make a decision according to the working conditions?
When the working pressure exceeds 2.5MPa and the temperature exceeds 400℃, priority is given to ω-shaped corrugated expansion joints, such as "corrugated expansion joints for power station industry" and "metal corrugated expansion joints for cement industry" in this station are typical. If the displacement of the pipe is large, but the pressure is only 0.5 MPa or less, such as a flue gas pipe, a U-shaped or S-shaped pipe can be used, or even a "non-metallic expansion joint (fabric fiber expansion joint)" can be used instead. If the pipeline vibrates frequently or requires frequent maintenance, such as air-cooled islands or desulfurization systems, the life advantage of C-shaped corrugations is reflected. It needs to be reminded that the corrugated shape should not only look at the cross-sectional view, but also look at the wave height, wave pitch, number of layers and material thickness-if one of these parameters is wrong, no matter how good the shape is, it will be useless. For example, in order to resist external pressure, the wave height of the bellows in "vacuum special hose" will be very small, which is completely different from the conventional U-shape.
Finally, let's talk about a point that is easy to overlook: the binding of corrugation types and manufacturing processes
U-shaped and C-shaped mostly adopt hydroforming or mechanical rolling, which has low cost; Omega shape is usually molded by compression because of its narrow crest and large depth. The mold is expensive but has good consistency. S-shape is common in hydroforming and then spinning shaping. When selecting, don't just look at the shape name, but also ask the manufacturer's molding process, heat treatment parameters and interlayer fit rate. Accessories such as "expansion joint guide tube" and "expansion joint tie rod" in this station will also affect the actual working conditions of bellows-don't just focus on picking corrugations and neglect the overall structural matching. All in all, the corrugation type is the "gene" of the performance of the expansion joint. Understand it, and the selection is half successful.