First, take it apart first: What is the layer of cloth outside the non-metallic expansion joint?
You might think that the non-metallic expansion joint is just a normal layer of cloth, like canvas or umbrella cloth. If you really think so, you will stumble when choosing a model.
Take the fabric fiber expansion joint (that is, the "non-metallic expansion joint (fabric fiber expansion joint)" in the product list of this site), which is not a single-layer structure at all. The outermost layer is usually a coated fabric-in human terms, a layer of temperature-resistant, corrosion-resistant rubber or resin is coated on a high-strength fiberglass cloth or other fiber-based cloth. Common coatings include silicone rubber, fluororubber and polytetrafluoroethylene (PTFE). Why not just go to metal? Because non-metallic expansion joints need flexible compensation, vibration reduction and corrosion resistance, once the metal outer layer is put on, these advantages are all wasted, and thermal expansion and contraction are easy to crack.
Therefore, the outer layer of cloth is actually a "composite material", not an ordinary cloth. If you choose the wrong one, the equipment life will be directly discounted in half.
Second, what is the difference between silicone cloth, fluorine tape and PTFE cloth?
These three outer materials are the mainstream of the market, but their temper is completely different. Direct to dry goods:
- Silicone cloth: Wide temperature resistance range (-60℃ ~ +250℃, can carry 350℃ in a short time), good weather resistance and moderate price. Suitable for general hot air duct, air conditioning and ventilation system. If you use it to fight sulfur flue gas, it will be brittle in two months.
- Fluorine tape: The corrosion resistance is much stronger than that of silica gel cloth, and the resistance to acid, alkali and oil media is good. But the price is much more expensive, and the upper limit of temperature resistance is about 200℃. The flue gas of cement industry has a large dust content and frequent temperature fluctuations, and the outer layer is quickly worn out with ordinary silica gel cloth. At this time, fluorine tape has to be thickened, and even a layer of stainless steel wire mesh has to be pressed on the outer layer to strengthen it-just like the working conditions of metal corrugated expansion joints in our cement industry.
- PTFE coated cloth: It can resist almost all strong corrosive media (including sulfuric acid condensate such a killer), temperature resistance-200℃ ~ +260℃, and does not adhere. In the flue gas baffle door of power station desulfurization system (such as the desulfurization flue gas baffle door of this station), the medium contains sulfuric acid condensate, so multi-layer PTFE composite structure must be used. Ordinary silicone cloth will bulge and pierce in three months.
Think about it, if you choose the wrong outer layer, the expansion joint may be scrapped in a few months, and the labor cost of replacing it is more expensive than the material cost. Is it worth it?
Third, the on-site working conditions are different, and the outer layer selection is very different
Two days ago, I met a customer from a cement factory and asked me: Why did the non-metallic expansion joint they bought leak after half a year? I asked him to take a picture and come over to see that the outer silicone cloth was dusted like sandpaper, and the inside fabric was all exposed. Only after asking did I know that the flue gas temperature did not exceed 200℃, but the dust concentration was high and the particle hardness was high. In this working condition, ordinary silicone cloth is paper paste.
For example, the desulfurization system of the power station has sulfuric acid condensate in the flue gas, and the pH value is as low as 1~2. If you use fluorine adhesive tape, although it is acid-resistant, the fluctuation of temperature and repeated infiltration of condensate will also cause the fluorine adhesive to blister and delaminate. The PTFE composite structure must be applied, and the inner layer must be added with an anti-corrosion isolation layer.
On the other hand, if it is just room temperature air in the air conditioning ventilation duct, a layer of silicone cloth is more than enough, and there is no need to waste money on PTFE. Therefore, don't expect one material to dominate the world. The key is the three parameters of medium temperature, corrosiveness and particle concentration. If you don't understand, you can directly ask the manufacturer for the technical parameter table, which is much more reliable than guessing.
4. Can the outer layer be repaired if it is damaged? How to extend life?
The outside of the non-metal expansion joint is bulging. Can you stick a piece of adhesive tape to deal with it? My advice: Look at the breakage location. If the outer coating is partially peeled off and the inner fabric layer has not been injured, it can be temporarily treated with repair tape of the same material (such as silicone repair tape), and it will last one or two maintenance cycles. However, if the fabric layer has been exposed, or the bulge area is more than the size of a palm, then don't mend it, and directly replace it as a whole-because the outer layer is not only decoration, it bears the main task of sealing and isolating the medium. If it is broken, it is equivalent to pipeline leakage, and the corrosive medium directly erodes the internal structure, and it won't take long to be completely scrapped.
Want to live longer? When installing, be careful not to let external machinery scratch. During inspection, regularly check whether the coating has aged and cracked. If you find fine cracks, quickly treat them with special repair glue. In addition, pay attention to whether the displacement of the pipe support exceeds the design value of the expansion joint-we have a case in which the customer regarded the expansion joint as a universal compensator, and the actual displacement exceeded the design value by 30%, resulting in long-term fatigue cracking of the outer layer.
5. Now you should know how to choose the outer material, right?
- Look at the temperature: PTFE or silica gel is preferred when it exceeds 200℃ (silica gel is more stable for short-term high temperature);
- Look at the corrosion: there are strong acids and alkali, and the condensate is directly applied to PTFE;
- Look at wear: for media containing high concentration of particles, choose fluorine tape + stainless steel wire mesh reinforcement layer;
- National standard requirements: JB/T 12235-2015 has clear suggestions on the use conditions of different materials. Please refer to it when designing and selecting, and don't pat your head.
Don't just look at the price. The cheap outer layer often can't last a maintenance cycle. At that time, the shutdown loss and replacement labor cost will add up to more than ten times the saved material cost. If you are not sure, go directly to our manufacturer for a detailed technical parameter table-we have a corresponding recommended scheme for each working condition, which is much more reliable than your own blindness.