How many hundreds can a piece of cloth carry? Talk about the underlying logic of the temperature range of non-metallic expansion joints
When many people come into contact with non-metallic expansion joints for the first time, they will think that this thing is just a piece of "cloth"? If you can hold a hundred or two hundred degrees, it will top the sky. But the reality is that in places such as cement kiln tail and boiler flue, it has to work in flue gas at 300 to 400 degrees Celsius or even higher. Isn't it a little counter-intuitive?
In fact, the truth is very simple: non-metallic expansion joints do not rely on a single layer of fabric to carry the temperature at all. Its temperature capability depends on the synergy of the entire composite structure-fabric, insulation, sealing layer, each with its own division of labor. But the outermost layer of cloth determines "whether you can carry it", and the hidden insulation layer inside determines "how long you can carry it". If you don't understand this logic, the selection is blind.
Silicone cloth, fluorine tape, polytetrafluoroethylene-where is the temperature ceiling of different fabric layers
Let's go straight to the hard goods. There are only a few kinds of fabrics most commonly used for non-metallic expansion joints, and the upper temperature limit is basically the industry consensus:
- Silicone cloth: The continuous working temperature is about 200~250 DEG C. The advantages are good flexibility and aging resistance, but once it exceeds 250 ℃, the silicone coating begins to decompose and the fabric becomes brittle. Therefore, silicone cloth is suitable for compensators from room temperature to medium temperature, such as air conditioning air duct and drying equipment.
- Fluorine tape: Can carry it to the beginning of 300℃. The high temperature resistance of fluororubber is stronger than that of silica gel, and it is also resistant to oil, acid and alkali. It is often used for the front flue of the desulfurization tower, but it won't work further up.
- Polytetrafluoroethylene (PTFE) film or coated cloth: This is the "carrying handle" in non-metallic fabrics at present. The long-term service temperature of pure PTFE can reach about 260℃, but the glass fiber cloth + PTFE composite layer after special impregnation treatment can reach 350℃ or even higher in a short time. Note, it is short-term. When continuous operation exceeds 300℃, PTFE will slowly soften and creep, resulting in leakage.
Many people look at this table and run to choose PTFE cloth, thinking that everything will be fine. Hey, don't worry-the fabric is only the first line of defense, and the real trick is in it.
Don't just look at the fabric: insulation and sealing structures are the real protagonists of high-temperature conditions
Two days ago, I met a customer, saying that his non-metallic expansion joint at the tail of the cement kiln leaked after less than half a year. On disassembly, the fabric layer is intact, but the insulation layer in the middle — ceramic fiber felt — has been sintered to powder by high temperatures. This is the typical "strong outside but dry inside".
The function of the insulation layer is to block the heat flow inside through porous materials (ceramic fibers, aluminum silicate wool, glass fiber felt). If the insulation layer is not thick and dense enough, the heat penetrates directly into the inner surface of the fabric, and the fabric soon becomes unbearable. A standard non-metallic expansion joint such as oursNon-metallic expansion joint (fabric fiber expansion joint)It is usually designed in 3~5 layers: an outer layer of fluorine tape or PTFE cloth, 2~3 layers of heat insulation felt in the middle, and an inner layer of corrosion-resistant sealing film (such as F46 film).
More critical is the sealing structure. At high temperature, the pressing force between the metal flange and the cloth surface will be loosened due to thermal expansion, so it is generally necessary to match stainless steel pressing strips and high-temperature sealing gaskets. Don't underestimate this. The root cause of many leaks is the failure of the seal at the flange, not the broken fabric itself.
Temperature and pressure are husband and wife, who has the final say when fighting? Synergistic Relationship Must Be Considered in Model Selection
When the temperature goes up, the pressure goes down. These two parameters are like a couple fighting-one is always pressing the other. For example, the same PTFE fabric compensator can withstand 0.3MPa at 200℃, and it may not even be able to withstand 0.1MPa at 300℃. Why? Because the strength of the fabric decreases with the increase of temperature, and at the same time the aging rate of the seal ring accelerates.
Therefore, when selecting the model, we must get the "temperature-pressure" combination of working conditions, instead of simply asking "how many degrees can this expansion joint withstand". For example:rubber compensatorGenerally used for normal temperature and low pressure (0.1~0.2MPa), andRectangular non-metallic expansion joint350 ℃ and slightly positive pressure are common in the boiler flue. If you throw the rubber compensator into the 250℃ flue, it will be wasted in less than a month. Conversely, if high-temperature resistant non-metallic expansion joints are used on high-pressure pipes, the flange connections will also be pushed open.
Cement kiln tail, boiler flue, desulfurization tower-three real cases teach you to calculate the applicable temperature
It's boring to talk about theory. Let's look at the case directly:
- Cement kiln tail: The typical temperature is 350~400 ℃, and the flue gas contains dust and alkaline substances. The selection scheme isNon-metallic expansion joint (fabric fiber expansion joint), PTFE impregnated glass fiber cloth for the fabric, three layers of ceramic fiber felt in the middle (20mm each), and an inner F46 film for corrosion protection. The sealing structure adopts stainless steel corrugated press strip + graphite gasket. The actual operating temperature is 380℃, the pressure is-3kPa, and the life can reach more than 2 years.
- Boiler flue: The temperature is generally 150~250℃, but an instantaneous peak of 300℃ may occur when starting and stopping. At this time, silicone cloth or fluorine tape is enough, and the price is also cheap. However, it should be noted that the boiler flue gas has a large moisture content, so waterproof fabrics should be selected, such asRubber PTFE compensator(lined with PTFE) can be solved well.
- Desulfurization tower import and export: The temperature is 80-120 DEG C, but the medium contains dilute sulfuric acid, which is highly corrosive. Conventional non-metallic expansion joint fabrics are prone to corrosion, so it is recommendedPTFE compensatorOr rubber compensator lined with PTFE, although the temperature is low, anti-corrosion is the first priority.
Do you see the doorway? Each working condition should be calculated separately: first set the temperature section, then look at the medium and pressure, and finally combine the number of layers.
Three-step self-examination method: You can also judge by yourself that non-metallic expansion energy saving can't be used in your working condition
Well, after talking about so many dry goods earlier, finally give me a grounded self-examination method, so as to save you from being fooled by sales:
- Step 1: Find out the working condition parameters-Maximum continuous operating temperature, instantaneous peak temperature, medium (whether containing acid, alkali, particles), working pressure (positive or negative pressure?). These data are indispensable.
- Step 2: Check fabric and insulation— — Take a product sample and ask what material the outer cloth is made of (silica gel? Fluorine glue? PTFE?), which kinds of felt are used for the insulation layer, and how thick each layer is. If the total thickness is less than 50mm, it basically can't bear long-term operation above 250℃.
- Step 3: Look at the seal and connection structure-Is the flange surface flat or with stops? Stainless steel 304 or 316 for beading? Is the gasket asbestos rubber or expanded graphite? If the seal is not good, no matter how good the fabric is, it will be for nothing.
After these three steps, he felt confident in his heart. Finally, the national standard for non-metallic expansion joints is JB/T 12235-2015, which can be used as a reference when selecting. However, the practical application depends on practical experience. After all, the standard is only the lower limit.