First distinguish: the skin of metal expansion joint and non-metal expansion joint is not the same thing
When many people see the word "skin", they think that all expansion joints rely on this layer of cloth to absorb displacement. That's a big misunderstanding.
The body of the metal expansion joint is corrugated pipe-stainless steel, which absorbs the thermal expansion and contraction of the pipe by corrugation deformation. The skin on the outside is, to put it bluntly, a protective layer and does not participate in displacement compensation. What truly absorbs displacement by the skin (fabric band) isNon-metallic expansion joint(also called fabric fiber expansion joint). If these two are mixed up, it is easy to go wrong when choosing a model. You take the working condition of the metal expansion joint and ask the non-metal manufacturer for skin, and people will report you a silicone cloth scheme, and it will burn through as soon as the temperature rises.
What is the picture of adding skin to the outside of the metal expansion joint?
Since the bellows itself can compensate for the displacement, what is the picture of putting a layer of skin on the outside? Three words: heat insulation, wear resistance and corrosion resistance.
Take the steam pipe as an example. The surface temperature of the bellows can reach three or four hundred degrees, and the heat radiation is very strong when exposed to the outside, so the outer guard plate is too hot to touch. Adding a layer of skin can block the heat radiation and reduce the temperature of the outer guard plate, which is also safe for maintenance personnel. Another example is the dust removal pipe. There are dust particles in the flue gas, and the bellows can be easily worn down under high-speed washing. The skin is equivalent to wearing a layer of "bulletproof vest" on the bellows.
Don't use the skin as a seal, it doesn't bear pressure.The seal of the metal expansion joint depends on the bellows itself. If the skin is broken, the medium will not leak immediately, but the bellows will lose its protection, and its life will be greatly reduced.
Material composition of skin: not one layer of cloth, but multi-layer composite structure
Common combinations areSilicone coated fiberglass clothMake the outer layer,Polytetrafluoroethylene filmMake an isolation layer,Ceramic fiber clothMake insulation. Silicone cloth is enough for low temperature (below 200 ℃); If the temperature is high, you have to add ceramic fiber and stainless steel wire mesh, otherwise you can't hold it.
The skin is multi-layer composite, and each layer has its own division of labor. The outermost layer should be weather-resistant and aging-resistant, the middle layer should be insulated, and the inner layer should be resistant to medium corrosion. Some manufacturers report the thickness to you according to the standard configuration, but if there is sulfur or acid and alkali in your working condition, the inner layer of the standard configuration can't hold it at all, so it must be replaced with PTFE as the isolation layer.
What to look for when choosing a skin? Three key parameters
The first isDesign temperature。 This determines the temperature resistance rating of the skin material. Silicone cloth below 200℃ can be done, and ceramic fiber can be obtained at 300-400℃. No matter how high it is, stainless steel wire mesh reinforcement must be considered.
The second isMedium corrosivity。 There are sulfur, acid and alkali media in the smoke, and the inner layer must be isolated with polytetrafluoroethylene film, otherwise the skin will rot in a few months.
The third isCompensation amount。 In the working condition of large compensation amount, the skin should leave sufficient wrinkle margin. Don't underestimate this-the skin itself does not absorb displacement, but it has to deform with the bellows, and there is not enough margin to tear after a few pulls.
Therefore, when choosing the skin, don't just look at the thickness and material. These three parameters must be clearly explained to the manufacturer.
If the skin is broken, don't rush to replace it as a whole
If the skin is broken, don't rush to replace it as a whole. Remove it and see the damaged location before determining it.
If the edge is opened, it is because the strip is not pressed tightly or the adhesive is aging, so you can continue to use it if you make it partially. If it is an intermediate tear, it is that the compensation amount is underestimated, or the skin is chosen too hard, and the deformation can't keep up with the bellows. At this time, it is useless to make up light, so you have to recalculate the compensation amount and change it to a skin with softer or greater fold margin.
Check that the skin is tightly attached to the metal frame bead. Many times the air leakage is either broken skin or loose beading. Just fasten it, and you don't have to change the skin at all.
Let's just say that skin is a protective umbrella when used correctly, and a consumable when used incorrectly. Understanding its positioning and selection logic can save a lot of maintenance costs.