Find out what it looks like first
The structure of non-metallic expansion joint of air duct is actually not complicated. Outside is a metal frame-flange, angle steel ring, used to hard connect with the pipe; Inside are layers of flexible material, stacked like a sandwich. The innermost layer is temperature-resistant and corrosion-resistant fluororubber or polytetrafluoroethylene, which directly contacts the medium; A thermal insulation layer such as ceramic fiber is sandwiched in the middle to prevent heat from conducting outward; The outer layer is a silicone rubber coated fiberglass cloth that protects the interior from mechanical scratches or UV aging. This composite structure allows it to not only bear high-temperature smoke, but also freely expand and contract like a spring. Compared with metal bellows, it has no short fatigue life plate-metal bellows may crack after repeated expansion and contraction for hundreds of times, and non-metallic fabric layer has no metal fatigue problem. Moreover, it can absorb multi-directional displacement. To put it bluntly, it is a lump of high-strength fabric that can be deformed, and you can twist it however you want.
The working principle is four words to put it bluntly: flexible compensation
When the hot air in the air duct blows, the duct will lengthen, and the fabric layer of the expansion joint will wrinkle or stretch, eating this length change. The key lies in its multi-layer structure: each layer has a different division of materials. The inner sealing medium does not leak; The middle layer is insulated to prevent heat from being transmitted to the outside, otherwise the outer metal frame will overheat and deform; The outer layer protects the whole from mechanical damage. If you think about it carefully, it absorbs not only axial expansion and contraction, but also lateral misalignment and angular deflection-for example, the pipe is crooked because of the foundation settlement, the non-metal expansion energy saving follows the twist, and the metal bellows is easy to crack if twisted too much. Therefore, the working principle of the non-metallic expansion joint of the air duct can be summarized as follows: using the free deformation of the flexible fabric layer in three-dimensional space, it compensates the displacement of the pipe due to thermal expansion and contraction, vibration or installation deviation, while keeping the seal.
To put it bluntly, it is a hose that can carry high temperatures and is corrosion-resistant, and acts as a "cushion" between pipelines and equipment.
When designing non-metallic expansion joints of air ducts, the most headache is not strength, but temperature
We have encountered a cement plant project. The flue gas temperature rushed to 650℃, and the ordinary silica gel coating was directly carbonized, which broke when touched. Finally, the combination of PTFE + ceramic fiber was used to carry it down. Don't worry too much about the pressure. The air duct is basically slightly positive or negative pressure, and the design pressure generally does not exceed 0.1MPa. When selecting the model, we mainly focus on three numbers: design temperature, displacement and diameter size. For example, rectangular non-metallic expansion joints are more suitable for square smoke ducts, and round ones can use the standard model of non-metallic expansion joints (fabric fiber expansion joints) in our station. The larger the size, the stronger the displacement compensation ability, but the installation space must also be left. Don't be greedy, otherwise it will be more troublesome if you can't fit it on the spot.
By the way, there is still a pit in temperature-some working conditions contain acidic gas in flue gas, such as wet flue gas after desulfurization. Although the temperature is only 50~80℃, it is extremely corrosive. The inner layer material has to be made of PTFE or highly corrosion-resistant fluororubber, otherwise it will be perforated in half a year.
The most asked question by customers: How many years will this thing last?
To be honest, life depends on the working conditions. In the wet flue gas environment behind the desulfurization flue gas baffle door, the acid corrosion is serious, and it has to be replaced in two years. But in clean air ducts, it's fine for seven or eight years. Alternatives are also available, such as rubber compensator temperature resistance is usually below 200℃, and PTFE compensator has good corrosion resistance but high cost. So don't expect it once and for all, it's business to regularly check the outer layer for cracks and the inner layer for bulges. How to check? See if the outer silicone rubber cloth is hard and brittle, and if it is elastic when pressed with your hand; If there is a bulge in the inner layer, it means that the layers are peeled off and must be replaced.
Installed with a pit easy to step-the direction of the guide tube
The specific function of the expansion joint guide tube is to guide the air flow smoothly, avoid the high-speed air flow directly washing the fabric layer, and prolong the service life. However, the guide tube must be installed along the flow direction of the medium. If it is installed backwards, it will drive, cause vortex vibration and even tear the fabric layer apart. There is usually an arrow on the expansion joint to indicate the flow direction of the medium. Don't install it backwards. In addition, be careful not to crush the flexible section to death during transportation, and remove the transportation tie rod before installation at the site. How to adjust the tie rod nut? Very simple, loosen the locking nut first, adjust it in place and tighten it to prevent loosening. Remember, does the screw of the expansion joint need to be removed? If it is a pull rod for limiting position, loosen the gap after installation to leave a telescopic space; If it is for pre-tensioned/pre-compressed screw, it is retained as required and cannot be disassembled.
"Why can't I twist your pull rod?" I asked him if he didn't loosen the lock nut first? He said oh, and then it was done smoothly. Such small details, one mistake is enough for you to toss for half a day.
Finally, let's talk about practical things: in the air duct of power station, which one should be chosen between metal corrugated expansion joint and non-metal expansion joint?
Metal has high pressure resistance and long life, but it does not carry large displacement, and once fatigue cracking is difficult to repair-welding repair is troublesome, so it can only be replaced as a whole. Non-metal displacement is large, vibration reduction is good, maintenance is simple, a section of fabric layer can be changed, the cost is much lower. The kiln head air duct of cement industry, the powder feeding duct of power station, and the non-metallic expansion joint are almost standard configurations. The non-metallic expansion joints (fabric fiber expansion joints), rectangular non-metallic expansion joints and rubber compensators in our station are all aimed at different scenarios. Next selection, don't just look at the price, first calculate the displacement and temperature before finalizing. For example, if the temperature exceeds 400 DEG C, select tetrafluorine fiber + ceramic fiber; Select non-metals if the displacement exceeds 50mm; Selecting non-metal as a vibration source can also reduce vibration by the way.
After all, the working principle of the non-metallic expansion joint of the air duct is to use the flexible layer to "eat" all kinds of displacements of the pipeline. The structure is simple but there are many design details. If you choose the right one, you don't have to worry about it for five years; If you choose the wrong one, you will have to change it in half a year.