What is the difference between air duct expansion joint and pipe expansion joint? Many friends who come into contact with the air duct system for the first time choose expansion joints according to the idea of metal pipes. And the result? I chose an expensive and heavy metal piece, and it went wrong every three days after installing it. In fact, the air ducts (especially the flue gas ducts of power plants and cement plants) have low pressure, large diameter and rapid temperature change. What is most needed is a compensator that is lightweight, corrosion-resistant and can absorb multi-dimensional displacement. This is the home field of non-metallic expansion joints (also known as fabric fiber expansion joints or rectangular non-metallic expansion joints).
Core structure: not by corrugation, but by fabric deformation
The core structure of the non-metallic duct expansion joint is actually not complicated-it is made of temperature-resistant fabrics (such as silicone cloth and fluororubber cloth) as flexible bands, matched with metal flanges or strips, and sandwiched with insulating cotton in the middle. It does not expand or contract by corrugations, but by the deformation of the fabric itself to absorb displacement. So it can simultaneously absorb axial, lateral, and angular displacements with little reaction force. Think about it, the air duct is so long, thermal expansion and cold contraction a few times, the metal expansion joint holds back its strength and does not release, and the weld will crack sooner or later. The non-metallic "soft connection" instead keeps the pipeline life.
What kinds of scenarios must it?
The first is the desulfurization flue gas system. The flue gas temperature is low but the humidity is high, and it is extremely corrosive. The metal expansion joint will perforate in less than two years. Our desulfurization flue gas baffle doors are often matched with non-metallic expansion joints, which are reliable in sealing and acid-and alkali-resistant. The second is the large-diameter air duct-such as the air duct of air-cooled island and power station boiler, which is often three or four meters wide. The metal expansion joint needs to be customized with such a large bellows, and the cost is sky-high, while the rectangular non-metal expansion joint is made into a rectangular section, which is convenient to install on the spot, and the price is only one-third of that of metal parts of the same size. The third is the place where vibration and noise reduction is needed, such as the inlet and outlet of the fan. The non-metallic ring belt itself can absorb vibration.
Not pressure resistant? This has wronged it
Are non-metallic expansion joints not pressure resistant? This is not wronged. It is really not suitable for high-pressure steam pipes (that requires high-temperature axial expansion joints or external pressure single axial expansion joints). However, in the air duct system, the working pressure is usually only a few to tens of kPa, and non-metals can completely withstand it. Moreover, it implements the national standard JB/T 12235-2015, which clearly stipulates the leakage rate, temperature resistance grade and fatigue life. Look for this standard when buying, don't buy three-no products.
There's a pit in installation and maintenance to warn you
Although non-metallic expansion joints are called "soft connections", they must never let the ring belt bear load during installation. Many construction sites save trouble. If the pipeline doesn't fit the mouth, the non-metallic expansion joint is used as the adjustment gasket for deadlifting. As a result, the ring belt is twisted and the life is directly cut in half. The correct way is to fix the pipe first, and then use the expansion joint to make the final connection. In addition, the ring belt is a wear part, but it is very convenient to replace it-just remove the press bar and replace it with a new ring belt, without moving the flange. In contrast, the metal expansion joint is scrapped once the bellows is cracked. Does this sentence speak to your heart? Two days ago, a cement factory purchaser complained to me that they used to use metal corrugated expansion joints, which broke once every six months, and they had to shut down the kiln for two days once they were replaced. Later, it was replaced with our rectangular non-metallic expansion joint, which was used for two years without any problems.
Four key parameters in model selection
- Sectional dimension of air duct: Round tube or rectangle? Rectangular non-metallic expansion joints are naturally adapted to rectangular air ducts, but metal parts are difficult to process.
- Maximum temperature of medium: Usually silicone cloth is resistant to 200℃, and fluororubber is resistant to 250℃. If you have high temperature resistance, you can add heat insulation layer.
- Design displacement: Especially for the air duct in the horizontal direction, the thermal expansion is estimated at about 1.2mm per meter, so don't forget to leave a margin.
- Is corrosion resistance required: The desulfurization system must choose acid-resistant coating belt, ordinary fabrics can't bear it.
Give these four parameters to the manufacturer, and basically match the appropriate non-metallic air duct expansion joint. Don't think about it by yourself. Find a professional for professional matters. Our factory engineers deal with these things every day, and one sentence can save you half an hour of searching information.