Metal expansion joint insulation process: Don't let the bellows "stuffy" in the insulation layer
Metal expansion joint insulation technology sounds like an ancillary work of pipeline insulation. Only when you really do it can you realize that there are many doors inside. Pipeline insulation is to reduce heat loss. Besides heat insulation, the expansion joint insulation is more important to ensure that the bellows can deform normally, avoid condensate corrosion, and prevent external fires or personnel burns. Many project drawings save trouble, and the general-purpose corrugated expansion joint and pipeline are randomly wrapped with rock wool. As a result, the corrugated pipe is stuck and the compensation amount is completely wasted. The insulation layer hoops the bellows so tightly that it can't move even if it wants to expand or contract. What is the difference between installing the compensator and not installing it? This account has to be clearly settled.
Look at the material and structure first, then talk about insulation
The biggest difference between expansion joint insulation and pipeline insulation is that you have to find out what expansion joint is in your hand first. Universal corrugated expansion joint, high-temperature axial expansion joint, external pressure single axial expansion joint, the number of bellows layers, the presence or absence of guide tube, and the connection mode are all different. Can the insulation scheme be the same?
Take the expansion joint with the guide tube as an example. The temperature at the flow direction end of the medium is high, and the guide tube itself has an expansion gap. If the insulation layer blocks that gap, the guide tube will expand and directly break the insulation layer. There is also a directly buried (fully buried) expansion joint. This thing comes with an outer sheath when it leaves the factory, which is originally designed to resist compression and waterproof. If you wrap it with a layer of thermal insulation, the groundwater will seep in, and the bellows will be soaked in moisture, and the corrosion rate will be faster than without wrapping it. These pits will not be marked for you on the construction drawings.
Two days ago, I met a customer who insisted on adding rock wool outside the directly buried expansion joint, saying that he was afraid of freezing. I asked him: Your pipe is buried underground, and the temperature below the frozen soil layer is stable. Why do you add that layer of insulation? He thought for a moment and went silent.In this business, you can't just work according to the drawing, you have to use your brain.
How to choose insulation materials? All four indicators are indispensable
Select insulation materials, thermal conductivity, temperature resistance grade, bulk density and hydrophobic rate, all four indicators must be less. Aluminum silicate cotton is resistant to high temperature, but the water absorption rate is high, and the heat insulation performance drops linearly when water enters; Foam glass is water-repellent, but brittle, afraid of impact, and the bellows will crack when it moves; Composite silicate coating is suitable for special-shaped parts, but the thickness is not easy to control. If it is thick, it will affect the displacement, and if it is thin, the heat insulation will not be enough.
Many people just stare at the thermal conductivity and ignore that the expansion joint is a flexible body. Insulation materials must move with the bellows. For plates with too much rigidity, the bellows will crack as soon as it is stretched, and water will enter the cracks. When it freezes in winter, the stress on the surface of the bellows will concentrate, and fatigue cracks will come out. That's the real thing. So you choose the material and ask first: Can it deform with the bellows?
The construction sequence is wrong, and the rest is all for nothing
The easiest thing to roll over is "installing insulation first or installing expansion joint first". The correct sequence is: after the expansion joint is installed in place, the tie rod nut is adjusted, and the pressure test is qualified, then the heat preservation is started. You have to wrap the insulation first and then put on the expansion joint. How can you adjust the pull rod? How to check the weld? As soon as the expansion joint expands and contracts, the insulation layer tears itself first.
Thermal insulation construction itself is also particular. Clean the floating rust and oil stains on the surface, apply anti-rust paint, and then wrap flexible insulation felt in layers. Each layer is overlapped with staggered seams, fixed with stainless steel wire mesh, and the outermost layer is made of metal shell. The overlapping direction of the shell should follow the flow direction of the medium to prevent rainwater from flowing back. Many old masters know that this job looks simple, but every layer can't be sloppy. If the seams are not set up properly, the heat will run away from the gaps. If the outer shell is not done smoothly, the rain will be ruined.
Special working conditions, special templates
If you hit the metal expansion joint next to the desulfurization flue gas baffle door, you have to be extra careful. The medium contains acidic condensate, and the outside of the insulation layer must be waterproof sealed, otherwise the acid will penetrate into the root of the bellows along the insulation layer, and it will corrode and perforate in a few months. This is not to scare you. I have seen an expansion joint at the outlet of a desulfurization tower, that is, the insulation layer was not waterproof. After half a year, it leaked at the trough. When I disassembled it, the root of the bellows was corroded like a honeycomb.
High-temperature axial expansion joint is usually used on the main steam pipeline of power station. The thickness of insulation layer should be determined according to thermal calculation, but the trough of bellows cannot be filled, so thermal displacement space must be left. If you fill the trough, there will be no place for the thermal expansion and contraction of the bellows, and the stress will be held in the trough, so the fatigue life will be directly shortened.
As for non-metallic expansion joints and fabric fiber expansion joints, people's skin itself has the heat insulation function. If you wrap a metal shell, it is purely superfluous, but it hinders the exhaust heat dissipation of the skin. The water vapor inside the skin can't be discharged, and it ages faster.
If the heat insulation is not done well, the life of the expansion joint will be cut in half directly
Bellows will produce thermal stress under alternating temperature, and the uneven insulation layer will lead to local temperature gradient, which will accelerate the initiation of fatigue crack. I have seen a cement factory, the metal corrugated expansion joint on the rotary kiln flue gas pipeline, the insulation layer fell off for half a year and no one took care of it. As a result, penetrating cracks appeared at the trough, and production was stopped for two days for emergency repair, and the loss was enough to buy hundreds of expansion joints.
There are only tens of thousands of high-temperature axial expansion joints, and hundreds of thousands will be lost after two days of shutdown. The heat preservation process seems to be an auxiliary work, which is an umbrella when it is done well, and an accelerator when it is smashed. This truth is understood by the old masters who have done maintenance.
Therefore, the metal expansion joint insulation process, don't regard it as an accessory of pipeline insulation. From material selection to construction, every step has to be according to the temper of the expansion joint. Do you want the bellows to breathe freely in the insulation, or do you smother it alive? Weigh it for yourself.