Why do you want to think about it?
Two days ago, I met a buddy from a power station, complaining that the corrugated expansion joint on their set of high-temperature flue gas pipes, the stress at the weld is concentrated, and it cracks every three months. Every time it is shut down and replaced, the maintenance cost alone is 20,000 yuan less, and with the loss of shutdown, the boss's face is green. The cement industry was even worse. Under the attack of dust and moisture, the corrugated metal expansion joint corroded twice as fast as expected. At this time, someone wondered: Can you change the metal expansion joint to non-metal?
Don't jump to conclusions yet, we have to look at the work conditions. Non-metallic expansion joints (also known as fabric fiber expansion joints) do save the day in some scenarios, but if they are used in the wrong place, they will dig themselves.
What is the difference between metal and non-metal expansion joints?
Metal expansion joints, such as corrugated expansion joints in power station industry or metal corrugated expansion joints in cement industry, rely on the elastic deformation of the bellows wall to absorb displacement. The design pressure is high, and it can withstand more than ten MPa. However, the shortcomings are also obvious: the welding process is slightly worse, or there are corrosive components in the medium, and the fatigue life will fall off a cliff. If you think about it, the stress concentration at the weld seam, when it hits sulfide-containing flue gas, it will perforate in a few months.
The non-metallic expansion joint (rectangular non-metallic expansion joint) is made of multi-layer composite fabric and silicone coating. The amount of compensation is large, one can be several times that of metal, and the vibration and sound insulation effect is outstanding. The most important thing is corrosion resistance-the environment of desulfurization flue gas and acid-alkali medium can't be held by metals, but non-metals can be used for two or three years. However, the pressure-bearing capacity is a flaw. Don't touch the high-pressure steam pipeline. The rubber compensator and rubber PTFE compensator of this station all belong to this family, each with its own emphasis.
Which occasions should really be changed?
Desulfurization flue gas baffle door rear section pipe. The flue gas temperature is 80~150℃, the humidity is high, and it contains sulfide. The metal expansion joint is quickly pitted. Replace it with non-metallic expansion joints (such as the rubber compensator or non-metallic expansion joints of this station), the cost can be reduced by 30%, and the life span can be doubled. The same is true for the kiln tail pipeline in the cement industry. The low-frequency vibration is large, and the metal is prone to fatigue fracture, while the rectangular non-metal expansion energy-saving absorbs multi-directional displacement, which is perfectly adapted.
But be warned, not everywhere can be changed. High-temperature axial expansion joints, corrugated expansion joints for power station industry and other high-pressure and high-temperature occasions, don't blindly change them. It's an accident. There are also high-pressure scenes such as directly buried (fully buried) expansion joints and external pressure single axial expansion joints, which are honest and practical.
How to avoid pits in model selection?
You think there is only one kind of non-metallic expansion joint? Thinking too simple. The upper temperature resistance limit of the fabric fibers is about 400°C, but the wear resistance is poor; Rubber PTFE compensator has good corrosion resistance, but low pressure. The selection must be according to the working conditions: rectangular non-metallic expansion joint for flue, equipped with guide tube to prevent dust accumulation-note that the flow direction arrow of guide tube must be consistent with the medium as mentioned in the Q&A of this site. How to determine the size of rubber compensator for vibration reduction at fan outlet? Referring to the national standard JB/T 12235-2015 for non-metallic expansion joints in this station, the flange surface and displacement must match the original pipeline.
Installation and operation and maintenance, I have seen many of these pits
The flange bolts are tightened too tight. The compressive stress of the non-metallic ring belt is excessive and it is easy to tear. Second: No cold tight size reserved. As soon as the pipe shrinks in winter, the loop belt is directly pulled and cracked. Third: Forgot to add the deflector. The high-temperature flue gas directly washes the fabric layer, and the life is cut in half. Also, the direction of the arrow of the expansion joint must be consistent with the flow direction of the medium, and the waste will be quickly installed backwards. Daily inspection depends on whether the coating is bulging and the fibers are delaminated. If you find signs, change them quickly. Don't wait until you leak before stopping.
It is not a panacea to change metal expansion joints from non-metals, but in scenes with high low-temperature corrosion vibration such as desulfurization, cement and fan export, it will save a lot of money if you use it right. Every link of selection, installation, operation and maintenance has to be kept an eye on, otherwise the money saved will fill the pit of rework.