Find out one thing first: what does it really look like?
Many people will be stunned when they see the external pressure metal corrugated expansion joint for the first time-why does this thing look different from the general-purpose corrugated compensator that we usually see? Simply put, the general-purpose bellows is exposed in the middle of the pipe, and the medium rushes directly into the bellows. As for the external pressure single axial expansion joint, the bellows is installed outside the shell-to be precise, it is in a steel outer sleeve. The medium goes through the inner pipe, and the pressure acts on the inner pipe wall and the shell. The bellows is only responsible for deformation compensation.
You know, this structure is naturally designed for high-pressure and large-displacement scenes. In our product library, it is called "external pressure single axial expansion joint". The name is a little twisted, but the principle is very straightforward: the bellows is placed outside the pressure chamber, and the internal thrust is balanced by external pressure. To put it bluntly, the shell carries the pressure, and the bellows carries the displacement, each performing its duty.
The first hardcore role: carrying high pressure without destabilizing
What are ordinary expansion joints most afraid of under high pressure? Columnar instability. The bellows bulge and burst like a balloon, and anyone who has seen it has a shadow in his heart. The external pressure type is different. The bellows is wrapped and protected by the external shell, and the medium pressure is completely shared by the shell. The bellows is only responsible for expanding and contracting and deforming honestly.
"The pressure of the steam pipeline is 10MPa, the temperature is 500℃, and the general-purpose expansion joint can't be used at all. What should I do if it collapses once I try?" I said you go to see the external pressure single axial expansion joint, which is what it does. Then he tried it and never mentioned the general-purpose thing again.
Another example. In the hot air duct of the cement industry, the temperature of seven or eight hundred degrees is common, and the pressure is not low. Ordinary expansion joints are installed, and they leak after one or two thermal shocks. What about the external pressure type? The shell separates the high-temperature medium from the bellows, and cooperates with the guide tube to guide the medium through smoothly (the function of the guide tube has been specifically discussed in our Q&A), so that the flow-induced vibration is greatly reduced, and the life of the bellows is directly doubled.
The second function: eliminate the blind plate force, and save enough money to buy several expansion joints
This role is easy to overlook, but everyone who engages in pipeline design knows it-blind plate force. Think about it, when the end of the pipe or the valve is suddenly closed, the high-pressure medium will generate a huge thrust at the closed end. This force is called blind plate force. It can directly push the fixing bracket down and tear the pipe weld.
Ordinary expansion joints need to add additional main fixing brackets to deal with blind plate force, and the cost is not small. The structural design of external pressure expansion joint is very clever: the medium pressure acts on the shell, and the inside of the bellows is almost unpressurized, and the whole thrust is offset inside the expansion joint itself. There is no need for additional main fixing brackets, which saves the cost of the whole pipe frame project.
To put it bluntly, the extra money spent on buying external pressure expansion joints has been made up for in the bracket project, and there is still a surplus.
The third function: anti-fatigue, long life-not to change for ten years is not to brag
What are bellows most afraid of? Media scour, mechanical damage, flow-induced vibration. The external pressure type hides the bellows in the housing so that the medium can't touch it at all. In addition, the guide tube helps to straighten out the flow direction of the medium, and turbulence and vortex are greatly reduced. Hot air pipelines in cement industry and steam pipelines in power stations start and stop all the year round, and ordinary expansion joints may leak in two or three years. External pressure type, easy to run for ten years.
Of course, it's not a panacea either. For example, if the pipeline needs to absorb lateral displacement or angular displacement, you have to find a compound hinge transverse expansion joint or a curved tube pressure balance type. External pressure single type axial type only handles axial displacement, don't be difficult for it.
Installation and use: Three pits must be avoided
First, look at the direction of the arrow. There must be an arrow on the external pressure expansion joint pointing to the direction of media flow. If the direction is reversed, the guide tube becomes a resistance piece, and the wear accelerates, and it will be scrapped in two or three months. Refer to the item "What is the direction of the arrow of the expansion joint" in our question and answer.
Second, the transport bolts must be removed. In order to protect the bellows when leaving the factory, there will be tie rod nuts to fix it. These transport bolts must be removed after the pipeline is installed and before the pressure test (refer to "Do you need to remove the screw of the expansion joint"). Not removing is equivalent to welding the expansion joint to death, it won't work.
Third, don't let it do horizontal work. The external pressure type only absorbs axial displacement. If the pipeline has angular displacement or lateral offset, the bellows will be twisted and the shell will be stuck. Before selection, calculate the pipe stress clearly, don't be lazy.
Is it expensive? Calculating the general ledger is cost-effective
Looking at the purchase price alone, the external pressure single axial expansion joint is indeed 30%-50% more expensive than the general-purpose type. The structure is complicated, and there are many materials used. The shell, guide tube and seal are all costs. But what about the general ledger? The fixed bracket is saved, the frequency of maintenance is reduced, and the overhaul period is prolonged. Combined, it is more cost-effective.