What exactly is a double-layer insulated metal expansion joint? Disassemble to see the structure
Let's talk about the actual scenario first. Two days ago, I met an engineer from a thermal company. When I came up, I asked: Is your expansion joint with thermal insulation just like wearing a cotton-padded jacket for the bellows? That's half true. The double insulated metal expansion joint is indeed like wearing clothes, but it is a three-layer structure: the innermost is a bellows, the middle is an insulation layer, and there is a metal protective shell on the outside. Each of the three layers does its own work, and one is indispensable.
The bellows is the core and is responsible for absorbing the thermal displacement of the pipe. The insulation layer is responsible for locking the heat of the medium and reducing the heat loss. The outer protective shell is not ordinary iron sheet. It has to withstand external environmental erosion and accidental bumps, and also to prevent the insulation layer from being damp and failing.
Compared with ordinary expansion joints, this structure has not only an insulation layer, but also a layer of prediction of working conditions: the medium temperature, the environmental humidity, and whether the pipeline is set indoors or outdoors. These factors directly determine how thick the insulation layer should be and what material the shell should be made of.
Insulation is not as simple as a 'cotton-padded jacket': materials and processes
The choice of insulation materials, many people think that the thicker the better. Wrong. There's a balance between thermal conductivity and bulk density. Commonly used insulation materials include aluminum silicate wool, rock wool and aerogel felt. The thermal conductivity of the three decreases in turn, but the price also doubles in turn. Aluminum silicate wool has high temperature resistance and can carry more than 1000℃; Rock wool is cheap, but its temperature resistance and waterproof performance are a bit worse; Aerogel has the best thermal insulation performance, and its thickness can reach 1/3 of that of aluminum silicate.
The construction process of the insulation layer is more important than the material itself. The same material, laid loosely and compacted to the design bulk density, the insulation effect can be worse than 30%.
When regular manufacturers make double-layer insulation metal expansion joints, the insulation layer will be laid in layers and overlapped in staggered seams to avoid the formation of thermal bridges through gaps. Some manufacturers try to save trouble, and a layer of cotton and a roll are done. On the surface, there is no problem. After half a year, the shell is partially hot, that is, the insulation layer collapses. This matter is difficult to find out during acceptance, and it will be late when it is put on the pipeline.
Don't just look at the caliber in the selection: temperature, pressure and displacement can't be missed
Many purchasers come to us for a quotation with the pipe caliber, saying that there are two sets of DN500. Every time I have to ask: What's the temperature? How much pressure? What is the displacement amount? If one of these three parameters is missing, the selected product will either waste money or have an accident.
Temperature determines the material of the bellows. Use 304 stainless steel below 350℃, and consider Incoloy 825 or higher high temperature resistant alloy above 400℃. Pressure determines the number of layers and wall thickness of the bellows. The high-pressure working condition is not solved by thickening single-layer bellows, but by making a multi-layer thin-walled structure-the number of layers increases, and the pressure resistance is improved, but the flexibility will not sacrifice too much.
The amount of displacement is another parameter that is easily underestimated. Axial displacement is easy to understand, but lateral and angular displacement are not understood by many people. When the pipeline turns a few corners, the direction of thermal expansion will change. At this time, the axial expansion joint alone is not enough, so we have to consider the transverse type of compound hinge or the universal hinge type. There was a cement factory project before. Because only the axial displacement was calculated but not the lateral displacement, the bellows was twisted after being installed.
Installation and operation and maintenance: Don't ignore the details of deflector, pull rod and drain port
There have been too many moths in the installation of this link. Let's talk about the guide tube first. Its function is to guide the flow direction of the medium and protect the inner wall of the bellows from being washed by high-speed fluids. However, the guide tube has directional requirements, and it must be installed along the flow direction of the medium. On the contrary, it not only loses the protection function, but also increases the flow resistance. There is generally an arrow mark on the product, so remember to check before installation.
Let's talk about the pull rod. Expansion joints with tie rods (such as double hinge transverse type), tie rod nuts are locked during transportation and installation to hold the bellows in place and prevent transportation damage. When the pipeline is installed and the system is ready for operation, these nuts should be adjusted in place according to the design requirements, rather than being locked all the time. Someone asked if the screw of the expansion joint needs to be disassembled-not disassembled, but adjusted. Loosen to the design value and tight to the position. This adjustment step directly determines whether the compensator can work normally.
Drain outlet. Double insulated metal expansion joint for outdoor installation with a drainage hole left at the bottom of the enclosure. The insulation layer is the most afraid of water. Once the shell is not tightly sealed and rainwater enters, the insulation performance is immediately discounted in half. The function of the drain outlet is to allow the seeped water to flow out and prevent long-term water accumulation from corroding the shell and bellows. When installing, be careful not to block the drain outlet, and don't let the insulation material plug it up.
Life and Failure: Pits of Bellows Fatigue and Insulation Aging
The service life of the expansion joint, here scores two systems to look at. Corrugated pipe is a metal piece, and its failure mode is mainly fatigue. Each wave of thermal expansion and contraction is a cycle, and the number of cycles is limited. There is a calculation formula for the fatigue life of bellows in EJMA standard, which is calculated according to 1500 or 10000 cycle life in design, and the results are very different. When selecting, you should confirm the number of design cycles with the manufacturer. Don't just ask "how many years can it be used"-the pipeline starts and stops once a day and once a week, which are completely two working conditions for bellows.
The aging problem of the insulation layer is more concealed. Aluminum silicate wool will gradually fibre and powder under high temperature for a long time, the original fluffy structure will slowly collapse, and the heat insulation effect will decline year by year. The process is not as sudden as the bellows breaking, it is boiling frogs in warm water. After the shell is exposed to sun and rain for several years, the weld may rust or even crack, rain seeps in, and the thermal conductivity of the insulation layer rises sharply after being damp, which is equivalent to white installation.
Therefore, don't just stare at the bellows during regular inspection. Feel whether the temperature of the shell is uniform, check whether the drainage outlet is smooth, and see whether the shell has bulge deformation. The bulge indicates that the internal insulation may have collapsed or flooded, and it is time to disassemble it for inspection.
In the final analysis, the choice of double-layer insulation metal expansion joint is not only the product, but also the combination of selection computing power, manufacturing technology level and after-sales response speed.