Why do non-metallic expansion joints have temperature limits? -Find out its "skeleton" and "skin" first
Non-metallic expansion joints, also called fabric fiber expansion joints, are often called non-metallic compensators in the industry, which are actually "assembled goods". Its skeleton is a metal frame or steel wire mesh, which can carry high temperatures; But the real sealing and compensation function depends on that "skin"-non-metallic material. This skin determines its upper temperature limit.
You may ask, why can't non-metallic expansion joints withstand thousands of degrees of heat like metallic expansion joints? Very simply, metal is a solid rigid body, while non-metallic materials are essentially polymers or fiber composites. When the temperature is high, the molecular chain breaks or the fiber sinters, and the performance collapses. Just like a person wearing a down jacket, the Arctic can carry it at minus 50 degrees, but it will turn to ash in a second when thrown into a steelmaking furnace. Therefore, figuring out the ability of this layer of "skin" is the key to selection.
Upper temperature tolerance limit of common non-metallic materials
- Silica gel: The temperature of continuous use is about 200°C, and it can reach about 250°C in an instant. Good elasticity, but afraid of high temperature aging and acid-alkali corrosion. Commonly used in cryogenic flue gas ducts or air conditioning systems.
- fluororubber: The temperature resistance is higher than that of silica gel, and it is no problem at 250°C continuously, but it can reach 300°C in a short time. It has good oil and chemical resistance, but it is expensive, has poor elasticity at low temperatures, and is easy to harden in northern winter.
- Polytetrafluoroethylene (PTFE): It is what we often call tetrafluorine, which is used continuously at 260°C and has first-class corrosion resistance. This site'sPTFE-lined hoseAndPTFE compensatorIt is a typical representative, but PTFE itself has little elasticity, so it has to rely on corrugated structure or composite fabric to compensate for the displacement.
- Ceramic fiber: This is the handle of the high-temperature section. Ceramic fiber itself can withstand more than 1200°C, but it usually needs to be compounded with glass fiber and silica gel coating when making non-metallic expansion joints. According to the structural design, the overall temperature resistance is generally between 600°C and 1000°C. For example, this site'sNon-metallic expansion joint (fabric fiber expansion joint)AndHigh temperature axial expansion jointThe ceramic fiber layer is used.
Another common material- -Rubber, such asrubber compensatorAndRubber PTFE compensatorThe temperature resistance of natural rubber generally does not exceed 100°C, EPDM is about 150°C, and neoprene is about 120°C. Therefore, don't make rubber products into high-temperature working conditions at all, that is to dig a hole for yourself.
Comparison of applicable temperatures under different working conditions
For the same non-metallic expansion joint, you may have to change the material if you change the working condition. Let's compare three typical scenarios:
- flue gas pipe: The flue gas temperature span of power station boilers is extremely large, from the low temperature section (120°C ~180°C) behind the economizer to the high temperature section (400°C ~600°C). Silicone or fluororubber fabric is sufficient for the low temperature section, but ceramic fiber composite structures must be applied to the high temperature section, such as the one of this stationCorrugated expansion joint for power station industryIn the high temperature section, non-metal + metal composite scheme is often used.
- Hot air duct: Hot air ducts in cement industry and steel industry, the temperature is usually 300°C ~800°C. Below 300°C, the fabric can be coated with PTFE, and above 300°C, it must be reinforced with ceramic fiber + stainless steel wire mesh. This site'sMetal Corrugated Expansion Joints in Cement IndustryAlthough biased towards metallic structures, the non-metallic segments also follow this selection principle.
- Desulfurization system: The flue gas temperature after wet desulfurization is only 50°C ~80°C, but it is extremely corrosive (containing sulfuric acid and chloride ions). Temperature is not a problem at this time, corrosion is. Must choose corrosion resistantRubber PTFE compensatorOrLined with PTFE metal hose, although the temperature is low, the material can't be wrong.
So you see, the temperature span can range from tens of degrees to hundreds of degrees, and each working condition has its own "optimal solution".
Real Case of Temperature Selection Rollover
Two days ago, I met a customer. The design temperature of the hot air duct was 300°C, and the silicone fabric expansion joint was chosen for cheap. And the result? In less than three months, the silica gel layer was brittle and cracked, and the air leakage was a mess. Why? The upper limit of long-term continuous use of silica gel is 200°C, and 300°C is already a devastating temperature. This is the typical high temperature embrittlement rollover.
In turn, cryogenic hardening is not uncommon. At minus 30°C in winter in the north, some fluororubber or neoprene compensators become harder than stones, and they can't absorb displacement at all. The pipeline stress is directly transmitted to the supports and hangers, which damages the equipment. What about that? Either choose low-temperature resistant silica gel (silica gel can still remain elastic at-50°C), or use a composite structure to protect the low-temperature zone separately.
Plus, there's one point that's easy to miss: Temperature isn't static. Many pipelines will have temperature shock in the start-up stage. For example, the flue gas temperature instantly rises from normal temperature to 500°C in the initial stage of boiler ignition. If the thermal expansion coefficient and thermal conductivity of the expansion joint material can't keep up, local overheating will lead to burn-through. Therefore, when selecting the model, don't only look at the steady-state temperature, but also consider the temperature change rate and peak value.
What else should I look at when selecting a model besides temperature? Pressure, medium, displacement, don't miss a single one
Temperature is the boss, but not the biological father. If any of the three brothers, pressure, medium and displacement, lose their temper, the project will be in vain.
- pressure: The pressure bearing capacity of non-metallic expansion joints is generally low, generally ≤0.1MPa (i.e. 1kg). If the pipe purge pressure or system holding pressure exceeds this value, the non-metallic layer can be torn. At this time, you can considerUniversal corrugated expansion jointOrMetal rectangular expansion jointTo share the stress.
- Medium: In addition to the temperature, does the medium contain dust, acid, and viscous substances? For example, desulfurization system slurry pipeline, the medium is gypsum slurry, selectRubber PTFE compensatorOrPTFE-lined hoseIt's more reliable than ordinary rubber.
- displacement amount: The strength of non-metallic expansion joints is multi-dimensional displacement compensation, which can be absorbed in axial, transverse and angular directions. But the larger the amount of displacement, the more fabric layers and structural lengths are required. If you only want axial compensation, then chooseAxial expansion jointMore appropriate; If you want lateral and angular directions, thenCompound hinge transverse expansion jointOr the non-metallic rectangular expansion joint is more on the way.
Don't be superstitious about "high-temperature universal scheme". There is no universal material, only accurate matching. Throw the four parameters of temperature, pressure, medium and displacement to the supplier, and ask them to produce the design calculation book and material selection table. This is what engineers should do.