1. What exactly do metal expansion joints do? -Don't think of it as a regular pipe fitting
Many people come into contact with metal expansion joints for the first time, and the thought that pops into their minds is: Isn't this just a reinforced pipe joint? Alas, that's really not the case. Ordinary tube joints only connect and seal, but metal expansion joints have only one core task – to absorb displacement. What is the force of thermal expansion and contraction after running high-temperature steam in the pipeline, or after cold winter? Have you done the math? When the temperature of a 100-meter carbon steel pipe rises by 100℃, the length will expand to 120mm. If there is no expansion joint, the thrust of the pipeline can directly deform the top of the fixed bracket, and even crack the flange of the valve and pump body. Metal expansion joints are specifically designed to swallow these displacements.
Of course, it can also reduce vibration and noise by the way. For example, installing one at the outlet of an industrial fan can obviously reduce the pipeline jitter. But remember one thing: Its job will always be to deal with thermal and mechanical displacement, not as an ordinary connector.
2. Disassemble and see the core: bellows, guide tube, tie rod, who is working?
Take a typicalUniversal corrugated expansion jointTo disassemble. The core component is the bellows – a sheet of stainless steel pressed into a wavy shape, the equivalent of a soft but strong spring. The number of layers, wall thickness and wave number of bellows directly determine how much pressure it can bear and how much displacement it can absorb. Materials, 304 and 316L are the most common, and Incoloy 825 or even Hastelloy are available in high temperature conditions.
Allow the medium to flow smoothly and avoid high-speed fluid directly washing the inner wall of the bellows. And guess what? In the hot air duct, without the guide tube, the bellows can be worn out in a few months. There are special articles on this siteSpecific Function of Expansion Joint Guide Tube, worth a closer look.
Then there's the pull rod. Some expansion joints come with tie rods, others do not. What are pull rods for? Limits excessive stretching or compression of the bellows while withstanding the internal pressure thrust of the pipe. For example, the transverse expansion joint of the large tie rod can transmit the axial force to the pipe support when the two bolts are tightened. But the tie rod is not mounted and not moved-we will talk about it in point five below.
3. There are so many types, what is the difference between axial, transverse and pressure balance types?
The easiest place to stumble in engineering selection is that the displacement direction is not clear. Expansion joints are divided into three categories according to the mode of absorbing displacement:
- Axial type: The bellows expands and contracts along the pipe axis. Such asExternal pressure single axial expansion joint, suitable for main displacement absorption of straight pipe sections. Its axial stiffness is marked in the parameter table-this data determines how big the thrust is, which directly affects the bracket design.
- Transverse type: The bellows bends and absorbs the vertical or horizontal offset of the tube. Such asCompound hinge transverse expansion jointTwo bellows and hinge structure are used to deal with the displacement of the corner of the L-shaped pipe. The degree of angular displacement is much larger than the axial displacement, but it works well when space is constrained.
- Pressure balance type: This structure is a little more complicated, typicallyStraight pipe pressure balanced expansion jointAndCurved tube pressure balance expansion joint。 They have balanced bellows inside, which counteract the blind plate forces generated by internal pressure, so that the plumbing equipment is not subjected to extra thrust. Steam turbine inlet and outlet, pump inlet and outlet, such as fear of thrust, it must be.
And there's anotherRotary compensatorRelying on rotation to absorb displacement, suitable for long-distance steam pipeline, high pressure resistance, large compensation. However, the seal structure of the rotary compensator needs regular inspection and is not maintenance-free.
4. Power station, cement, desulfurization... How to choose the right model under different working conditions?
Two days ago, I met a client.Universal corrugated expansion jointIt was installed directly at the entrance of the desulfurization tower, and it wore out in three months. Why? The desulfurized flue gas contains a large number of condensed sulfuric acid droplets, and the pH value is as low as 2, which stainless steel 304 can't bear at all. That occasion should usePTFE-lined hoseOr expansion joints lined with PTFE. This site has a specialDesulfurization flue gas baffle doorAndPTFE compensator, these are designed for corrosive flue gases.
The outlet temperature of the power station boiler is close to 600℃, and the pressure is more than ten kilograms. It must be usedHigh temperature axial expansion jointThe corrugated pipe material is at least 316L plus heat-resistant coating. Don't forget to configure the deflector and insulation layer. What about the cement industry? The air duct is full of dust, and the bellows are easily worn, so they have to be usedMetal Corrugated Expansion Joints in Cement IndustryWith a thickened guide tube inside and a wear-resistant bushing outside.
Air-cooled island vacuum pipe is another set of gameplay. The diameter of the pipe is large, the pressure is low but the vacuum degree is high, and the bellows are easy to be sucked flat. At this timeDouble hinge expansion joint for air-cooled island vacuum pipelineIt comes in handy, and it has reinforcing rings designed inside.
Design pressure, design temperature, displacement amount. Then compare the product informationMetal expansion joint weight tableAndMetal hose size comparison tableDetermine flange standards and connection lengths. The lazy way is to ask the manufacturer directly about technology, but you have to have a spectrum in your own heart.
5. Those pits in installation and maintenance: Is the tie rod removed or not? Where does the arrow go? How to adjust the screw?
When installing the expansion joint, the most likely problem is the tie rod. When the manufacturer delivers the goods, for the convenience of transportation, the nut will be tightened on the tie rod to press the bellows to the minimum length. When they arrived at the construction site, some workers tried to save trouble and welded them directly without removing the tie rod. As a result, as soon as the pipeline heated up, the bellows couldn't stretch at all, and it collapsed within two days.
Confirm the direction of the arrow on the expansion joint before installation. The arrow points in the direction of the medium flow, and if it is installed backwards, the guide tube will not only not protect the bellows, but will scratch. And then? Loosen the adjustment nut on the tie rod to return the bellows to free length. However, note that some expansion joints with tie rods (such as large tie rods transverse type) tie rods are permanent parts and can't be disassembled. Just loosen the nut to allow the bellows to move freely. How to judge? Look at the "pre-stretched/pre-compressed" logo on the product description or label. As forDo you need to remove the screw of the expansion jointThe FAQ of this site has a specific answer: if it is a transportation screw, it must be dismantled; If it is a working rod, adjust it to the design position and lock it.
After the installation is completed, the test must be carried out by water pressure or air pressure. The pressure is generally 1.5 times of the design pressure, and the pressure is kept for 10 minutes without leakage. If the site space is limited and it is impossible to test the pressure as a whole, you can test it in sections. When testing the pressure, remember to reinforce the fixed brackets at both ends of the expansion joint to prevent the thrust from pushing the tube out.
During routine maintenance, check the outer surface of the bellows regularly for cracks and corrosion pits. Especially forDirect buried (fully buried) type expansion jointIf you can't see it directly after being buried, you have to rely on the leak detection hole in the catheter to check. Hear a noticeable hissing sound? Then take care of it quickly.