When many people hear the name "semi-circular metal expansion joint" for the first time, the first reaction in their minds is: What is the difference between this thing and the general-purpose corrugated expansion joint? Is it the wave height point and the round point?
In fact, the semicircular metal expansion joint is not defined by the wave height, but the wave section is close to semicircular, the crest radius and trough radius are not large, the middle connecting section is short, and there is even no straight edge section. The wave patterns of general-purpose corrugated expansion joints are mostly U-shaped or ω-shaped, with a transitional arc between the peaks and valleys, and the overall flexibility depends on the number of waves. The semicircular type is different. Each wave of it is buckled on the tube wall like a half circle. The flexibility of the wave is much greater than that of the U-shaped wave, and the amount of displacement that can be absorbed per unit wave number is much greater.
At the same diameter and pressure, the required amount of compensation can be achieved with fewer wavenumbers. In other words, the semi-circular metal expansion joint has lower axial stiffness and better elasticity. But you have to remember that the more "rounded" the radian, the more obvious the stress concentration at the root of the wave after being pressurized. It relies on the waveform itself to disperse the stress, so the requirements for material thickness and molding process are more critical than the general-purpose type.
Then again, since it's so "soft", where exactly is it used?
What kind of displacement does it mainly absorb? What problems does it solve in the power and cement industries
The main job of semi-circular metal expansion joint is to absorb axial displacement, that is, the thermal expansion and contraction of the pipe along the centerline direction. Its axial flexibility is large, and its lateral deflection ability is actually average, unlike the lateral expansion joint of compound hinge, which specializes in lateral displacement. If you used it as a hinge, it would be overqualified, and it might make the ripples unstable.
In the electric power industry, it often appears on the smoke duct system and steam and water pipeline of power station boilers. The smoke duct has a large diameter and high temperature, and the thermal displacement often runs along the pipeline axis. It is much more cost-effective to absorb this displacement with a semicircular metal expansion joint than a stack of general-purpose corrugated expansion joints-with the same compensation amount, the axial length of the semicircular type is shorter, and the arrangement does not take up space.
The situation in the cement industry is similar. The waste heat air duct and the circulation air duct of the raw mill at the head and tail of the kiln have high medium temperature and large dust content, and the thermal expansion and contraction of the pipeline are obvious. Many cement plants use metal corrugated expansion joints in cement industry, but when the space at the equipment interface is tight and the compensation requirement is large, semicircular metal expansion joints have advantages. It has less wavenumber, short total length, and easy installation space.
Type selection and pit avoidance: How to weigh the three parameters of pressure, temperature and fatigue life
Selection, to put it bluntly, is a "triangle transaction". Pressure, temperature, fatigue life, you can't want the best of all three.
If the pressure is higher, the wall thickness of the bellows will have to be increased, or the multi-layer structure will be used instead. But as soon as the wall thickness goes up, the flexibility of the wave goes down. With the same amount of displacement, the number of waves required becomes more, and the tube becomes longer. Temperature is also the same logic. The higher the temperature, the lower the allowable stress of the material. To maintain the same pressure resistance, it still has to be thickened.
What about that? Many people report the parameters as soon as they come up: pressure 1.6MPa, temperature 400℃, fatigue life requirement 10,000 times. If these three conditions are put together, the semicircular metal expansion joint may not be made directly, or it may be ridiculously expensive. You have to make trade-offs like negotiations: Can the fatigue life be reduced to 5,000 times? Does the temperature spike only occur when driving and stopping, and it is not that high in normal operation? Can the pressure be calculated according to the design pressure instead of the nominal pressure?
Fatigue life is calculated by "number of cycles under design conditions", not by calendar life. Some customers say "I want to use it for 20 years", which is two concepts different from fatigue life. You first find out how many times the process pipeline cycles a year, and then talk about the life span. Otherwise, the designed expansion joint will either be insufficient rigidity or waste of cost.
The most overlooked details during installation: guide tube direction, pre-stretching and cold tightening
There are more pits in the installation site than in the selection.
Let's talk about the direction of the guide tube first. If a guide tube is installed inside the semi-circular metal expansion joint, its direction must follow the flow direction of the medium. The function of the guide tube is to reduce the direct scouring and howling of the fluid to the corrugation. If the direction is reversed, the medium directly pours into the gap between the corrugation and the guide tube, which not only fails to protect, but also may cause vibration. Usually, the media flow arrow is marked on the receiver, which must be checked before installation.
Let's talk about pre-stretching and cold tightening. These two words speak of two formulations of the same thing-during installation, the expansion joint is first artificially elongated or compressed for a section to allow it to return to a free state or even compressive stress state under hot state. Why do you do this? Because when the pipe operates at a temperature higher than the installation temperature, the expansion joint is compressed and the corrugation is subjected to compressive stress. After pre-stretching, part of the compression amount during operation is "advanced", the actual working stress of the corrugation is reduced, and the fatigue life naturally goes up.
The amount of pre-stretch is not determined by slapping your head casually. It is related to the pipeline installation temperature, operating temperature and pipeline support layout. The cold tightness value must be issued by the designer, and the installer must do so. Some construction units save trouble, and the cold tightness value is directly calculated according to half of the compensation amount, which is risky-whether the insulation is done or not and the ambient temperature will affect the actual thermal displacement. Two days ago, I met a customer. The expansion joint was not cold and tight after installation, and the ripples cracked after three months of operation. When I removed it, I saw that the troughs were all fatigue cracks.
When non-standard customization, don't just focus on corrugated materials, these structural parts also determine the life
Nine times out of ten, semi-circular metal expansion joints are non-standard customized. When customizing, almost everyone will ask first, "What material is used for the bellows? 304 or 316L? Or Inconel?" Material is important, of course, but if you focus all on the corrugation, you ignore other fatal parts.
The material and wall thickness of the pipe and flange directly determine that the expansion energy saving cannot withstand the end thrust transmitted by the pipe. In order to save costs, some manufacturers take over thin-walled pipes. As a result, the corrugation is good, and the take-over pipes are first cracked by pipeline stress.
Don't be sloppy about the material of the deflector. In high-temperature dusty medium, if the guide tube wears out before the bellows, the bellows will be directly exposed to the dusty airflow. Some industry standards require that the material of the guide tube is not lower than that of the bellows, or even thickened. You can think about it, is it troublesome to change the deflector, or is it troublesome to change the bellows?
There are also structural reinforcement rings. Under the condition of large diameter and low pressure, reinforcing rings are often needed to improve the stability of semi-circular metal expansion joints. The spacing, thickness and material of the reinforcing ring have to be calculated, and it is not just welded twice. Some suppliers don't mention the reinforcement ring at all when they quote you, and then tell you to "add money" when the picture is drawn-you didn't even do this piece in your budget.
Let's put it this way, the wave shape of the semi-circular metal expansion joint is only the first step. What really determines its life is the comprehensive strength of the entire structural chain. When you select a model, you will review each structural part as the protagonist, so that you will not suddenly be called to the scene to "take the pot" after two or three years of operation.