What the hell does internal pressure matter? — — Core parameters of expansion joint design
Many people choose metal expansion joints, just staring at the pipe diameter and displacement, but the internal pressure is regarded as a "secondary parameter". And the result? Within two months of installing the equipment, the bellows was directly wrinkled and torn, and even the whole pipeline system was blown away-guess what? Nine times out of ten, the internal pressure level was chosen low.
The pressure inside the metal expansion joint is essentially the "outward force" acting on the bellows wall. If you think about it, there is steam or water or gas running through the pipe, and once the pressure comes up, the bellows will bulge like a balloon. If this force is not accurately calculated in the design, the bellows will either be propped up to plastic deformation or fatigue cracking under cyclic stress. To put it bluntly, internal pressure is the bottom line that the expansion joint can "bear". Without this bottom line, all other performances are castles in the air.
How exactly does internal pressure affect the design? Three things are directly related: the wall thickness, the number of layers and the waveform of the bellows. Let's dismantle them one by one.
How to set the pressure level? — — Selection logic of bellows wall thickness, number of layers and waveform
The higher the pressure, the thicker the wall thickness. However, the thicker the wall thickness, the better-the thicker the stiffness, the ability to compensate for displacement will decrease. Therefore, the design will play a flower: replace a single layer of thick wall with multiple layers of thin wall. For example, the pressure resistance of 0.5mm three-layer bellows may exceed that of 1.5mm single layer, and the flexibility is better. You look like the guy in our stationCorrugated expansion joint for power station industryMultilayer structures are commonly used in high-pressure steam pipelines.
Let's look at the waveform again. Common U-shaped, ω-shaped and S-shaped, the voltage resistance performance is very different. The U-shape is the simplest and suitable for low pressure; The ω-shaped cross section is smoother, the stress concentration is small, and it can carry medium and high pressure; What about the S-shape? Generally, it is the choice in high-pressure or vacuum scenarios. For example,Special hose for vacuumIt is just the opposite of the internal pressure scenario, but the design logic is in the same vein-the waveform determines the pressure distribution.
Therefore, when selecting a model, don't just look at the "nominal pressure" written on the manufacturer's sample. You have to do the math: What's the stress at work? Is there any shock, pulsation? How much is the safety factor left? A true expert will check the columnar instability pressure by EJMA standards, rather than slapping the head to determine the wall thickness.
Which expansion joint should I choose for different pressure scenarios? - -From general purpose to pressure balance, don't get confused
Low pressure scenarios (0.1~1.6MPa), such as HVAC, general industrial water pipes, directly onUniversal corrugated expansion jointThat's enough. It has a simple structure, low cost, and the internal pressure is borne by the bellows itself, but note-it will generate a lot of thrust (pressure × cross-sectional area), so the pipe frame must be reinforced.
Medium pressure scenario (1.6~4.0MPa), such as steam pipeline of power station and chemical thermal pipe network. At this timeExternal pressure single axial expansion jointOrCompound hinge transverse expansion jointMore appropriate. The external pressure type structure can lead the internal pressure to the outer shell, reduce the direct force of the bellows, and have a longer service life.
What about high pressure and large diameter (above 4.0MPa)? Then you have to usePressure balanced expansion jointYeah. Such asStraight pipe pressure balanced expansion joint、Curved tube pressure balance expansion jointThey cancel out the blind plate forces generated by internal pressure through internal hinges or pull rods, so that the thrust of the pipe on the fixed bracket is almost zero. Hey, many engineering accidents on the scene are due to the indiscriminate use of general-purpose types. As a result, the bracket is pushed down and the pipeline is deformed. Think about it, tens of tons of thrust acting on the bracket, it is weird that it doesn't collapse.
Invisible requirements of internal pressure for installation: guide tube, pull rod and fixed bracket, one must not be missing
Choose the right product, and you have to demine it when you install it. The first is the deflector. Under the action of internal pressure, the high-speed fluid will impact the inner wall of the bellows, causing flow-induced vibration. Install aguide tube(We specifically talked about it in our Q&A) can guide fluid smoothly and reduce erosion. Especially for steam pipes, there is no guide tube, and the life of bellows is cut in half.
Then there are the tie rods and the retaining brackets. If you bought itLarge tie rod expansion jointOrCompound straight pipe bypass pressure balanced expansion jointThe tie rod is mainly used to bear the thrust generated by the internal pressure while limiting the displacement direction. Don't screw the tie rod nut to death when installing-leave some clearance to allow the bellows to work properly. Many on-site workers screwed all nuts to death, and as a result, the expansion joint became a rigid connection, which couldn't compensate for the displacement at all.
Not to mention the fixed bracket. How much thrust does the internal pressure generate? Simple formula: thrust ≈ pressure × effective area of bellows. A DN400, 2.5MPa universal expansion joint, thrust easily exceeds 100kN. If the fixed bracket can't carry it, the pipe will twist like a snake. What about that? It is the correct solution to choose the pressure balance type or install the limit rod.
Typical pressure-related failures: instability, fatigue cracking, how to avoid them from the selection stage?
Instability and fatigue cracking.
Let's start with instability. Under high pressure, bellows suddenly flattened like a can-this is called columnar instability. The reason is either that the wall thickness is too thin or the wave distance is too large. When selecting the model, be sure to check the instability pressure and leave at least 1.5 times the safety factor. weLarge diameter thick wall expansion jointIt is designed to cope with this scene, with thickened wall thickness, controlled wave height and greatly reduced instability risk.
Let's talk about fatigue cracking. The internal pressure fluctuation will lead to the repeated expansion and contraction of the bellows, resulting in stress concentration at the crest and trough, and finally crack initiation. The way to avoid it is simple: choose products with optimized waveforms, such as Ω shape; At the same time, the actual working pressure is controlled to not exceed 80% of the design pressure. If the field pulses frequently, it is necessary to install dampers or use them insteadMetal hoseTo flexible connection.
The pressure inside the metal expansion joint is not a parameter that can be "roughly estimated". We have seen too many customers take low-voltage parts to run high-voltage conditions, and as a result, they are replaced with new parts in less than half a year, and the cost has doubled several times. So, spend half an hour to calculate when selecting a model, and you can save tens of thousands of maintenance costs later. Have you encountered a similar pit?