Why is the non-metal expansion joint of dust collector import and export more reliable than the metal? — — Talk about working conditions and pain points
Anyone who has worked in the dust removal system knows that the inlet and outlet positions are the "heart bridge" of the whole pipeline-the temperature fluctuates high and low, the dust particles erode, and the flue has some acidic corrosion. In the past, many project drawings saved trouble, and directly used metal rectangular expansion joints or general-purpose corrugated expansion joints. What was the result? In less than two or three years, the bellows cracked, the guide sleeve wore out, and even the tie rod nut loosened, resulting in overall failure. The root of the problem lies in the fact that metal parts can't withstand frequent thermal cycles and particle impacts, and the fatigue life is greatly reduced.
At this time, the advantages of non-metallic expansion joints (fabric fiber expansion joints) come out. It does not have the weak point of stress concentration of metal corrugation, absorbs displacement by flexible structure, and has better corrosion and wear resistance. Think about it, there is some sulfur oxide in the flue gas at the inlet and outlet of the dust collector. Metal parts are easily corroded, while fabric fibers and rubber layers are naturally acid-resistant. Is that the truth?
More importantly, the non-metallic expansion joint also isolates vibration and noise. Metal parts guide vibration, non-metals absorb vibration directly. Two days ago, I met a customer of a cement factory. It used to be made of metal, and it rumbled every day. After replacing it with non-metal, the noise dropped by 8 decibels. Don't underestimate this difference. Environmental acceptance often gets stuck in noise.
The core material of non-metallic expansion joint: fabric fiber, rubber, PTFE, which one is suitable for you?
Fabric fibers, rubber, polytetrafluoroethylene (commonly known as PTFE). Let's break it apart.
Fabric fibers(That is, we often call non-metallic expansion joint/fabric fiber expansion joint): The main body is a composite layer such as glass fiber cloth, silicone cloth and fluorine tape, with thermal insulation cotton in the middle. Advantages: Wide temperature range, from-40℃ to 1200℃ can be covered (according to the specific number of layers). Disadvantages: Air tightness is average, and additional sealing layer is required. It is suitable for working conditions with large temperature difference and dust, such as flue of power plant and cement kiln tail.
rubber compensator(Rubber): EPDM, neoprene rubber, silicone rubber, etc. Advantages: Good elasticity, strong ability to absorb displacement and high air tightness. Disadvantages: The upper limit of temperature resistance is low, generally not more than 150℃. Suitable for room temperature dust collection systems, such as bag dust collector air inlet.
Tetrafluorine compensator(PTFE Compensator): King of corrosion resistance, hardly afraid of any acid and alkali. Disadvantages: High price, less flexible. It is suitable for desulfurization system or chemical plant with strong corrosive medium. However, note that the temperature resistance of PTFE is usually-20℃ ~250℃, and if it exceeds, it has to be lined.
So when choosing, ask yourself three questions first: What is the flue gas temperature? What corrosive ingredients does it contain? What is the direction of displacement? If you are not sure, rectangular non-metallic expansion joints are a versatile choice, combining fabric fibers and rubber composite layers, a compromise solution.
Three parameters that are easily overlooked in model selection: displacement, temperature and guide tube design
Many people choose non-metallic expansion joints, only looking at the caliber and material. As a result, something went wrong when I installed it. These three parameters are the key.
First, the displacement amount.Non-metallic expansion energy saving absorbs axial, transverse and angular displacements. However, the amount of displacement allowed by different structures is different. For example, the axial compensation amount of single-layer fabric fiber may only be ±30mm, while the rectangular non-metallic expansion energy saving with rubber layer may be ±50mm. How to calculate? According to the pipeline thermal expansion formula Δ L = α L Δ T, combined with the on-site pipeline layout. Never slap your head.
Second, temperature.This one is the easiest to underestimate. The actual flue gas temperature may be 20℃-30℃ higher than the design. Once a steel mill was overhauled, the smoke temperature instantly rushed to 450℃, and the ordinary silica gel cloth was directly carbonized. Therefore, leave a margin when selecting the model, and consider whether to install insulation layer. A high temperature axial type expansion joint can be used as a reference here, but a non-metallic structure is more suitable.
Third, the guide tube design.Many people ask the specific function of the expansion joint guide tube? Simply put, it is to protect the non-metallic layer from direct washing by high-speed dusty airflow. If the size of the guide tube is wrong and the installation direction is reversed, the dust will hit the fabric fiber directly and wear out in three months. The thickness, material and opening direction of the guide tube are all particular. The standard requires that according to JB/T 12235-2015, especially when used for the inlet and outlet of dust collector, the guide tube must extend to the inside of the non-metallic layer at least 50mm.
Installation and commissioning: Tie rod, screw rod, arrow direction, one step wrong may be reworked
Installing non-metal expansion joints looks simple, but there are many actual pits. Let's start with the pull rod. The role of the expansion joint tie rod is to limit excessive displacement and protect the corrugation (or fabric) from being pulled. But many people forget to adjust it after installation-if the tie rod nut is locked, the expansion joint loses its compensation function. Correct practice: The tie rod is in a free state during installation, and after the pipe is butted in place, loosen the tie rod nut to the designed pre-tension/pre-compression value. How exactly to adjust? How to adjust the expansion joint tie rod nut, there is a standard step: first diagonally loosen, then gradually adjust, and finally tighten the lock nut.
Let's talk about the direction of the arrow. Does the direction of the arrow of the expansion joint refer to the direction of media flow or the direction of displacement? Many brands mark the direction of media flow. The biggest fear is playing backwards. The arrow faces outward, the opening of the guide tube faces inward, and the airflow directly impacts the non-metallic layer. We have encountered customers who installed it backwards, and the air leaked after three days of electricity. When removed, the fabric fibers were torn open by the airflow. So be sure to check the drawings and product markings before installation.
Does the screw need to be removed? For factory pre-installed transport screws (not adjustment screws), they must be removed after installation is complete, otherwise they will limit displacement. Alas, this error comes out too much.
Routine Maintenance and Failure Cases: The Three Most Common Dead Ways of Non-Metallic Expansion Joints
The deflector is worn out. As mentioned earlier, the deflector is improperly designed or installed, or the material is selected wrong. It took half a year for a waste incineration plant to wear through the guide tube, and the flue gas was directly injected into the non-metallic layer, burning out a big hole. Solution: Add wear-resistant liners when replacing, or choose thick-walled guide tubes.
Rubber aging and cracking. The rubber compensator is exposed to ultraviolet light for a long time, or it is exposed to oil mist, and it will harden and crack in three or four months. In fact, many rubber compensators are reinforced with short fibers, but protective covers must be added for outdoor use.
Fabric fiber layers layered. The reason is that the temperature exceeds the limit or the insulation cotton loses its insulation effect after being damp. Typical scene: steam condensate seeps into the insulation cotton during shutdown for maintenance, and the moisture vaporizes and expands during re-operation, bulging the fabric layer. Precautionary measures: Pay attention to rain and moisture protection during installation, and regularly check whether the insulation cotton is damp.
See, touch, hear. See whether the guide tube is worn, feel whether the non-metallic surface temperature is abnormal, and listen to whether there is any airflow howling sound. If any abnormality is found, stop the machine for inspection in time. Don't hold it hard. Although non-metallic expansion joints are durable, they are not indestructible.