The skin is not a piece of cloth-it is only to understand its multi-layered structure to know where the weight comes from
Two days ago, I met a customer who asked me with a drawing, "How much does the skin of the non-metallic compensator weigh? Help me estimate it." I asked him about the working conditions, the number of layers and the material, but I couldn't answer them all. How is this a matter of estimation? The skin is not a piece of cloth, but a composite of several layers of functional materials. If you don't even know this, the calculated weight is a slap on the head.
Temperature resistant layer (e.g. silicone cloth, fluorine tape), insulation layer (ceramic fiber blanket or glass fiber wool), vapor barrier layer (polytetrafluoroethylene film), outer protective layer (fluorine rubber coated cloth or stainless steel wire mesh cloth). Some high temperature conditions will also be reinforced by wire mesh. Each layer of material density and thickness is different, can the weight be the same?
Several key parameters for weight calculation: number of layers, material density, thickness, lap allowance
Calculating the skin weight of non-metallic compensator is not so mysterious, and the core is four parameters: the number of layers, the material density of each layer, the thickness of each layer and the overlap margin. The formula looks like this:
Single layer weight (kg/m²) = density (kg/m³) × thickness (m).
Total weight = weight of each single layer plus x (1+ percentage overlap allowance).
Take an example. The density of silicone cloth is about 1400 kg/m³ and the thickness is 2mm, so the single layer is 2.8 kg/m². Ceramic fiber blanket density 200 kg/m³, thickness 10mm, single layer 2 kg/m². If three layers are stacked, plus 10% overlap allowance, it is almost 5.3 kg per square meter. This is only a rough estimate, and it depends on the specific formula and tolerances.
The easiest thing to overlook here is the overlap margin. The skin is not cut out in a whole piece, and the joints have to be overlapped and bonded, so this part of the area has to be counted. The balance is usually between 5% and 15%, depending on the skin width and size. Take a rectangular non-metallic expansion joint as an example. Multiply the perimeter by the width to calculate the total area, and then multiply the overlap coefficient to calculate it accurately.
How big is the difference in skin weight under different working conditions? The selection of power plants and cement plants is completely different
If you have worked in both a power plant and a cement plant, you know that the requirements for skin in these two conditions are very different. The flue gas temperature of power plants is relatively stable, generally not exceeding 400℃, with emphasis on anti-corrosion and anti-leakage, with few skin layers, thin thickness and light weight. Cement plants are different. There may be a lot of dust, high-temperature gas and even sparks in the air ducts. The skin has to be thickened with insulation layer and reinforced with wire mesh, and the weight is at least 30% ~50% heavier than that of the power plant.
I have seen the rotary kiln export of a cement plant, which used 4 layers of ceramic fiber blanket plus double layers of stainless steel wire mesh, and the weight per square meter was more than 15 kilograms. The skin of the boiler flue, of the same size, is only 8 kilograms per square meter. So you said, can the selection be copied?
The weight of non-metallic compensator skin directly affects the load and installation hours of flange bolts. It's too heavy. The flanges have to be reinforced, and the bolts have to be calculated as shear. If the manufacturer does not calculate the weight accurately when quoting, there will be a lot of trouble in the later period.
Measured case: How to calculate the skin weight of an 800×600 rectangular non-metallic expansion joint
Some time ago, I helped a desulfurization project to calculate an 800mm ×600mm rectangular non-metallic expansion joint (corresponding to the rectangular non-metallic expansion joint products of this station), and the diameter circumference = (800+600) ×2=2800mm =2.8m. The width of the skin is designed to be 350mm, and the expansion area is 2.8m ×0.35m =0.98 m². Operating conditions required by customers: flue gas temperature 350℃, pressure ±3kPa, weak acidic environment.
The inner layer of silicone cloth is 2mm (density 1400), the middle layer of ceramic fiber blanket is 10mm (density 200), the outer layer of fluororubber coated cloth is 1.5mm (density 1600), and a layer of PTFE film is 0.5mm (density 2200). The lap allowance is calculated at 10%.
Silicone cloth: 1400×0.002=2.8 kg/m²
Ceramic fiber blanket : 200×0.01=2.0 kg/m²
Fluorine rubber coated cloth : 1600×0.0015=2.4 kg/m²
PTFE film: 2200×0.0005=1.1 kg/m²
Total : 2.8+2.0+2.4+1.1=8.3 kg/m²
Multiply by an overlap factor of 1.1 to give 9.13 kg/m².
Total weight : 0.98 m² ×9.13 kg/m² ≈ 8.95 kg.
The actual production weigh is 9.2 kg, the deviation is very small. Therefore, don't estimate by feeling, follow this process, and it will be close to ten.
Common myth: The heavier the skin, the more durable it is? Talk about the true relationship between weight and life
"Is your skin heavy enough? Is it heavy?" I told him directly: This is only half true. The relationship between skin weight and life of non-metallic compensator is not linear. If you blindly thicken the insulation layer, the weight goes up, but it causes the flange to bear extra stress, and the interface is easy to tear. The password of life lies in the temperature resistance grade, anti-aging performance and process accuracy of each layer of materials, not simply the stack weight.
For example, if you use ordinary silicone cloth to weigh it to 3mm with the same temperature resistance of 400℃, it is better to use high-performance fluororubber coated cloth 1.5mm plus a layer of PTFE film, which has a lighter total weight but a longer life. Therefore, when choosing a skin, you should pursue "just right", not "the heavier the better".
Reference standard JB/T 12235-2015, and our practical advice to customers in model selection
The domestic non-metallic expansion joint has the national standard JB/T 12235-2015, which has regulations on the material, thickness and test method of the skin. But the standard doesn't give the weight calculation formula directly. Therefore, when we make selection for customers, we generally suggest: first determine the working temperature, pressure and corrosiveness of the medium, and then let the manufacturer provide the details and density of the multi-layer structure, and calculate it according to the above method. Then let the manufacturer measure the weight on the sample, and compare it with your calculation. If the deviation exceeds 5%, you have to find the reason.
In addition, it is best to mark the weight and direction on the skin of the non-metallic compensator before installation to facilitate on-site verification. After all, the weight written on the drawing is so far from the actual thing that no one wants to pay for the rework cost.
Calculate the skin weight of non-metallic compensator clearly, which is not only responsible for yourself, but also saves money for the project. Next time you encounter a similar problem, don't pat your head, just push it again according to this logic, you can't go wrong.