What exactly is an air duct metal expansion joint? What is the difference between it and ordinary bellows?
When many people hear the "expansion joint", they think of the round stainless steel bellows in the pipe gallery, and then directly put the air duct on it-tsk, nine times out of ten, it will overturn. What is the biggest difference between air duct system and process pipeline? The air volume is large and the pressure is low, but the temperature can instantly soar from normal temperature to several hundred degrees (such as flue gas pipeline and desulfurization flue), and there are dust and corrosive gases sandwiched in the medium. In our product line, likeMetal rectangular expansion joint、Corrugated expansion joint for power station industry、Metal Corrugated Expansion Joints in Cement IndustryThese are actually regarded as metal expansion joints of air ducts, but special structural optimization has been made for rectangular flue, circular flue and high-temperature dusty flue gas. In other words, think clearly before choosing a model: Is your tube square or round? Is the media dirty or not? What is the temperature exactly? Don't carry it hard with general-purpose bellows, you can't carry it.
Three parameters that cannot be avoided in model selection: temperature, pressure and displacement
Let's talk about the temperature first. If it exceeds 600℃, don't expect 304. It has to be above 321. Our familyHigh temperature axial expansion jointIt is prepared for this kind of scene. With I321 or above, the corrugated structure has to be thicker. For the pressure, the air duct pressure is generally not high (most are within 0.1MPa), but if you connect the desulfurization flue, you have to refer to itDesulfurization flue gas baffle doorThat set of corrosion margin design-leave a surplus in wall thickness, and the number of corrugated layers should not be too small, otherwise it will be corroded by acid dew point in a few years. The most pitiful thing is the amount of displacement. Many people only measure the axial thermal elongation, ignoring the lateral swing and angular displacement of the pipe. For example, a 20-meter flue, the temperature rises from 20℃ to 350℃, and the axial extension is more than 100mm. However, when the bracket layout is unreasonable, the pipe will tilt 20~30mm to the side. You only count the axial direction. Once you put it on and run it, the expansion joint is twisted into a twist. To put it bluntly, first draw a picture, clarify the direction of thermal expansion and contraction of the pipeline, the position of the fixed bracket and the guide bracket, and then calculate the displacement.
Different shapes and different working conditions, the products selected are too different
Rectangular air ducts (such as power station flue) are preferredMetal rectangular expansion jointThe square corrugation fits the structure better, and the installation is also hassle-free. The circular air duct can be made of a general-purpose orExternal pressure single type axial type expansion joint. If there is vibration in the pipe itself, or if the installation deviation is large, considerCompound hinge transverse expansion jointOrStraight pipe pressure balanced expansion joint-These two can absorb lateral displacement and angular displacement, and do not produce the blind plate force of the main fixing bracket. What about high-temperature dusty gas in cement industry?Metal Corrugated Expansion Joints in Cement IndustryThe interior is equipped with wear-resistant guide tubes. The role of the guide tube is explained very thoroughly in our Q&A-it not only diverts the flow, but also protects the bellows from being directly washed by dust. And guess what? Some people suspected that the deflector was in the way, so they dismantled it themselves. As a result, the bellows was worn out in half a year. Don't be superstitious that one model conquers the world. The working conditions of air ducts vary widely, so you have to prescribe the right medicine.
Installing Rollover Sites: The Three Most Common Pits
The transport tie rod is not removed. Many engineering teams finished installing the big tie rod expansion joint and tightened the tie rod nut for fixing it, thinking that the tighter the better. Then as soon as the system heats up, the expansion joint is locked, and the bellows is like a holding balloon-all the stress is piled on the weld, can it not crack? Remember, the transport tie rod is used to prevent deformation when leaving the factory, and must be removed after being installed in place (for details, how to remove and adjust the nut, refer to our Q&A"Does the screw of the expansion joint need to be removed?"）。 Second pit: Install the guide tube in reverse direction. There are arrows on the guide tubes pointing to the direction of the medium flow. If you install it backwards, the high-speed airflow will directly overturn the guide tube, and the bellows will be exposed to high-temperature smoke, which will be scrapped in a few minutes. The third pit: the spacing of brackets is not as designed. The distance between the fixed bracket and the guide bracket is not strictly controlled, the thermal displacement of the pipeline is all squeezed to the expansion joint, and a corrugated joint bears the force in three directions, so the fatigue life is directly discounted. Especially for the expansion joint of the large tie rod, there are detailed installation methods in our home station, which can save more than half of the trouble.
Maintenance is not about waiting for a leak before replacing
The service life of the metal expansion joint of the air duct is directly linked to the material of the bellows, the corrosiveness of the medium and the number of cycles. For example, the expansion joint next to the baffle door, take a look at the corrugated surface for cracks or corrosion pits every time you overhaul it-especially near the weld. The maintenance cycle of rubber compensator (non-metal) and metal expansion joint is different: rubber ages fast and may have to be replaced in two or three years; The fatigue life of the metal can be estimated by calculating the bellows stiffness (ref."Stiffness and Calculation Formula of Bellows"）。 Don't wait until the machine stops to discover that the expansion joint has been scrapped-the loss is not something that can be stopped by a single accessory money. Prepare replacement parts in advance and do preventive maintenance according to the number of cycles. This is the practice of old drivers.