The air duct is equipped with a metal expansion joint, which is not a good picture, but a "breath" for the pipe.
Thermal expansion and cold contraction, once the temperature of the steel air duct rises, the pipeline of tens of meters can be pushed out by several centimeters. The result of hard carrying is weld cracking and equipment deformation. Two days ago, I met a customer. The air duct at the inlet of their desulfurization tower was not equipped with a suitable expansion joint, and the inlet and outlet flanges of the fan were cracked in less than half a year-the maintenance cost was enough to buy several expansion joints. Then why do you have to use metal? Non-metallic expansion joints can indeed be used in many occasions, such as low-temperature and low-pressure flue gas pipelines. Fabric fiber expansion joints are low-cost and convenient to install. However, high temperature, high pressure or scenes requiring tight sealing, such as hot air duct of power station boiler and exhaust gas pipe at the tail of cement kiln, metal expansion joints are hard currency. The "high-temperature axial expansion joint" in this station's product is specially designed to deal with this kind of working conditions. Stainless steel 321 or 347 is made of corrugated pipe, which can carry above 600℃.
Expansion joint for air duct, rectangular or round, don't buy the wrong one
Rectangular air ducts (such as flues of power stations and ventilation ducts of cement plants) best match "metal rectangular expansion joints", which can directly fit the flange surface, with large compensation and space saving. For circular air duct, use "universal corrugated expansion joint" or "large-diameter thick-walled expansion joint"-note that if the air duct pressure exceeds 0.25MPa or the temperature exceeds 600℃, you have to consider "high-temperature axial expansion joint" or "external pressure single axial expansion joint". And guess what? In some projects, in order to save money, ordinary circular expansion joints are installed on rectangular air ducts. As a result, the stress at the nozzle is concentrated, and it leaks in three months. Don't be stupid, the air duct shape and the expansion joint shape must match, this is an iron rule.
Among the selection parameters, the temperature compensation and guide tube are the easiest to step into pits
If the air duct medium temperature exceeds 600℃, stainless steel 321 must be used as the corrugated pipe material, which cannot be supported by ordinary 304. Pressure: Most of the air ducts are slightly positive pressure or negative pressure, but the desulfurization flue gas pipeline may have positive pressure fluctuation. At this time, the "double-sealed single-axis circular baffle door" and the expansion joint can be used together to prevent the flue gas from flowing back. Compensation: The axial elongation of the air duct is absorbed by the wavenumber of the corrugation, but the lateral displacement and angular displacement have to be solved by "compound hinge transverse expansion joint" or "curved tube pressure balance expansion joint". There is another detail-the direction of the deflector. When there are particulate matter or high-flow gas in the air duct, the guide tube must flow toward the medium, otherwise the ripples will be directly scoured, and the life will be halved. The FAQ of this site specifically talks about "the specific function of the expansion joint guide tube". The direction of the arrow refers to the flow direction of the medium. If you install it backwards, wait for air leakage.
None of the pull rods, bolts and baffle doors should be careless during installation
Many on-site masters think that the expansion joint can be screwed on, which is wrong. When the metal expansion joint for air duct leaves the factory, the tie rod is usually pre-pulled (to prevent transportation deformation). Before the installation and formal operation, the tie rod nut must be loosened to the designed length, otherwise the expansion joint will not work-the corrugation will not be able to move if it is screwed to death, which is equivalent to white installation. In addition, if there is a "smoke baffle door" or "electric plug-in insulation door" in the air duct system, the position of the expansion joint should be placed between the baffle door and the fan, so as to avoid the thrust generated by the action of the baffle door being transmitted to the expansion joint. Don't ask me how I know-there is a power plant that welds the baffle door directly to the rear end of the expansion joint. As a result, when the baffle door is closed, the expansion joint corrugation is squashed and scrapped directly. Look at the "How to Adjust the Expansion Joint Tie Rod Nut" in the product manual before installation, and just follow it.
Life and maintenance, don't expect it once and for all
There is no fixed replacement cycle for metal expansion joints, which mainly depends on the fatigue life of bellows. Generally, the design cycle number is 1000-10000 times, but under actual working conditions, temperature fluctuation and corrosive medium will accelerate aging. It is recommended to check the corrugated surface every six months for cracks and corrosion pits, and to measure whether the height of the waveform is uniform with a gauge. Found that the guide tube fell off or the sealing surface leaked? Change it quickly, don't delay. If the air duct contains sulfur or chlorine (such as desulfurization flue gas from coal-fired power stations), preference is given to corrosion-resistant alloy bellows, such as 316L or duplex steel. Someone asked if it would be okay to line the PTFE hose? It is for pipelines, and the air duct temperature is high, so fluoroplastics can't bear it. It is necessary to use products specially designed for corrosion conditions, such as "desulfurization flue gas baffle door" and "corrugated expansion joint for power station industry".
In summary, the metal expansion joint of air duct is not a standard part, and it must be customized according to four data: temperature, pressure, medium and displacement
If you don't understand, ask the manufacturer to report the parameters. Don't pat your head yourself. I have listed more than a dozen related products here, such as "Metal Corrugated Expansion Joint in Cement Industry" and "Double Hinge Expansion Joint for Air-cooled Island Vacuum Pipeline", all of which are selection references in specific scenarios. Remember, if the air duct of the metal expansion joint is selected wrong, it can lead to air leakage and efficiency decrease, or equipment damage and production shutdown.