1. What exactly is a non-metallic expansion joint? Why do you have to use it instead of metal?
Non-metallic expansion joints, also called fabric fiber expansion joints or non-metallic compensators-to put it bluntly, they are pipe compensators made of flexible materials (silicone cloth, fluororubber, polytetrafluoroethylene film). The biggest difference from metal expansion joints is that it does not deform by bellows, but by sliding and elasticity between fabric layers to absorb displacement. Then when do you have to use it? Two scenes are the most typical.
First, high-temperature flue gas pipeline. The flue gas temperature of power plants and cement plants is always five or six hundred degrees Celsius, and the metal bellows can't bear it at all. It won't take long to oxidize and crack. The cost of replacing it once is enough to buy several non-metals. Second, corrosive media. For example, the wet flue gas in the desulfurization system contains sulfur and acid, and the metal is quickly corroded, while the rubber PTFE compensator or PTFE compensator can hold up steadily for three to five years. In addition, non-metallic expansion energy-saving absorbs large displacements in multi-dimensional directions, and the installation space requirements are low, which is especially convenient for rectangular pipes-the "rectangular non-metallic expansion joint" in our products is specially designed for this kind of scene.
To be honest, many engineers subconsciously choose metal expansion joints as soon as they touch high-temperature pipelines. As a result, they cracked after long, and they changed non-metallic joints after tossing them back. Tsk, you really have to think about the working conditions before choosing a model. Otherwise? Rework fees are more expensive than equipment fees.
2. Structural disassembly: fabric, compartments, frames, each layer has said
Although the non-metallic expansion joint looks like a circle of cloth, its internal structure is very particular. The standard structure is generally divided into three layers: the outermost layer is fabric fiber (silicone cloth, glass fiber cloth, polytetrafluoroethylene film composite), which mainly plays the role of sealing and temperature resistance; In the middle is insulating cotton (ceramic fiber or rock wool), which is used to reduce heat transfer; The inner layer is a guide tube, which guides the airflow to avoid direct flushing. Some will also add multiple layers of compartments, such as the "non-metallic expansion joint (fabric fiber expansion joint)" product provided by us, which is specially aimed at high-temperature dusty flue gas, and the inner layer is reinforced with wear-resistant stainless steel wire mesh-which can withstand the erosion of dusty air flow and double its life.
Frame design is key. For rectangular non-metallic expansion joints, the frame is usually welded with angle or channel steel and is subject to the thrust at the end of the pipe. Under pressure conditions, the frame wall thickness and the arrangement of ribs directly affect the product life. Two days ago, I met a customer. The frame of the drawing I drew was too thin, and it was deformed and leaked half a year after installation-a waste of construction period and scolded. So don't just focus on flexible materials when designing, the rigid part is just as important. Is that the truth?
3. How to determine the design parameters? Temperature, pressure and displacement must not be less
The design basis of the non-metallic expansion joint is the national standard JB/T 12235-2015. There are only three core parameters:
- Temperature: The upper temperature resistance limit of the fabric material. Ordinary silicone cloth is resistant to 250℃, fluorine rubber cloth is resistant to 350℃, and ceramic fiber layer can be above 600℃. When selecting, leave a margin-for example, the maximum flue gas is 400℃, and at least the composite structure with resistance to 500℃ is selected.
- pressure: Non-metallic expansion joints have weak pressure bearing capacity, so they are generally only used for low-pressure pipelines (≤0.1MPa). If the pressure exceeds 0.05MPa, the frame should be strengthened or the tie rod limit should be installed. Our common question and answer mentions that "the function of the expansion joint tie rod" is to prevent the pipeline thrust from crushing the expansion joint.
- displacement amount: Divided into axial direction, transverse direction and angular direction. Non-metallic expansion energy saving absorbs the displacement in these three directions at the same time, but cannot be superimposed in design. For example, the axial compensation is 50mm and the transverse compensation is 20mm. In actual selection, the maximum working condition should be selected, and the fatigue life should be considered at the same time. By the way, don't ignore the role of the deflector-it reduces airflow impact and wear, and it also prevents dust accumulation. How to design it specifically? The length of the guide tube is generally 1/3 to 1/2 of the diameter of the pipe. Too short is useless, and too long increases resistance.
What about that? Calculate the thermal expansion, formula L × α × Δ T, and don't pat your head.
4. Actual selection: how to match power station desulfurization, cement kiln tail and chemical plant?
The working conditions of different industries vary widely, and the selection ideas are also different. Take three examples:
- Power station desulfurization system: The flue gas temperature is low (50~80℃), but the moisture contains sulfur and is extremely corrosive. At this time, the candidate "rubber PTFE compensator" or "polytetrafluoroethylene compensator" is more suitable, which is corrosion resistant and has good sealing performance. Used together with "desulfurization flue gas baffle door", it can completely isolate flue gas and avoid leakage.
- Kiln tail of cement industry: The flue gas temperature is high (300~500 ℃), and the dust content is large. Preference is given to "non-metallic expansion joints (fabric fiber expansion joints)", the inner layer is insulated with ceramic fibers, and the outer layer is silicone cloth. We also have a kind of "metal corrugated expansion joint in cement industry", but metal has a short life at the kiln tail, and non-metal is the mainstream.
- Hot air duct of chemical plant: The temperature fluctuates greatly, and explosion-proof should be considered. You can use the metal + non-metal composite structure of "high-temperature axial expansion joint", but there are also pure non-metal ones-when selecting the type, you should calculate the thermal expansion clearly, according to the formula L × α × Δ T, and don't pat your head.
There is no universal solution, and every design has to be weighed against media characteristics, temperature, pressure, and pipeline routing. You really don't think it's troublesome. If you choose the wrong one, it will cost tens of thousands of dollars.
5. Installation and operation and maintenance: several pits that are prone to rollover
The non-metal expansion joint looks simple in structure, but it still has problems if it is not installed carefully.
- Pre-stretching/pre-compression: When installing, the guide tube must be aligned with the flow direction of the medium, and the direction of the arrow cannot be reversed. In our question and answer, we specifically said that "the direction of the arrow of the expansion joint refers to the flow direction of the medium". If the guide tube is installed backwards, it will be overturned by the airflow, and the air leakage plug can't be blocked at that time.
- tie rod nut adjustment: The pull rod is generally adjusted to the installation length during delivery, and the site should be reviewed. If the pipe thrust is large, the nut must be tightened to the specified torque, otherwise the expansion joint will shake back and forth during operation. How exactly to adjust? Refer to our question and answer of "How to adjust the expansion joint tie rod nut", which contains the steps.
- Heat insulation problem: The non-metallic expansion joint itself has an insulation layer, but when insulating the external pipeline, pay attention to leaving room for activity, and don't stick with hard insulation materials. Otherwise, the expansion joint can't move, the displacement is fully pressed onto the pipeline, and the weld is easy to crack.
Finally, the life span is generally 3~5 years, depending on the working conditions. Regularly check the fabric surface for bulging and cracking, and replace it in time if it is aged. Don't wait for a leak to repair it, then the damage will be huge-a day out of production will be enough to buy several new expansion joints.