I met a customer two days ago. The non-metallic expansion joint on the flue gas pipe was used for less than a year, and the skin leaked. When it was removed, the surface of the silicone cloth was cracked in a large area, and the reinforcing layer was exposed. After asking around, I realized that when I selected the model, I looked at the temperature resistance data and ignored the small amount of SO₂ in the medium. This thing turns into dilute sulfuric acid when it encounters condensed water, and ordinary silicone cloth can't hold it at all.
So,Material of non-metallic compensator skin?This problem is by no means just flipping through the samples and picking the one with the highest temperature resistance. If the skin is chosen wrong, it can be replaced in advance at least, and production can be stopped at worst. Today, break up these five questions and explain them clearly to help you avoid detours.
1. The skin is not a layer of cloth: the structure determines the lower limit of performance
Many people think that the skin is just a piece of cloth, and it is done by holding the medium. If it is so simple, why divide it into non-metallic expansion joints (fabric fiber expansion joints) and rectangular non-metallic expansion joints?
Temperature resistant layer, reinforcing layer, heat insulating layer, outer protective layer。 The temperature-resistant layer directly contacts the medium and is responsible for blocking high temperature and corrosion; The reinforcement layer is a skeleton, usually made of glass fiber or steel wire mesh, which bears the pressure tension; The heat insulation layer is made of ceramic fiber felt or the like to prevent heat from transmitting outside; The outer protective layer shields the whole structure from wind and rain and prevents mechanical damage.
Each of these four levels does its own work, and one is indispensable. The inner layer burns through, and the enhancement layer follows; The outer layer is damaged and watered, and the inside is no matter how good it is, it is useless. Therefore, when choosing skin, don't just stare at a certain layer, but look at the overall structural design and matching between layers.
2. Silicone cloth, fluororubber cloth and polytetrafluoroethylene film: how to compare the three mainstream materials
The most common skin surface materials in the market are these three kinds, each with its own temper.
Silicone clothTemperature resistance-60℃ to 250℃, good flexibility, cheap price, it is a cost-effective choice for general working conditions. However, it is afraid of strong acid and alkali, oil, especially corrosive flue gas containing sulfur. The client mentioned above is planted on this.
Fluorine rubber clothTemperature resistance-20℃ to 200℃, chemical corrosion resistance is one grade higher than silica gel cloth, and it has good resistance to acid, alkali and oil. The disadvantage is that it is easy to harden at low temperatures, has poor elasticity and is more expensive. It is much more reliable than silica gel cloth when used in desulfurization flue gas and chemical medium pipelines.
Polytetrafluoroethylene membrane (PTFE)It is almost corrosion-resistant ceiling, can carry almost all strong acids and alkalis, extremely low friction coefficient, and does not adhere to dust. However, the temperature resistance is generally about 180℃, the mechanical strength is poor, and it is easy to tear, so it is usually used on glass fiber cloth or fluororubber cloth, rather than as a skin alone.
Is there anything else? Yes. LikePTFE compensatorIt is all-PTFE structure itself, but that belongs to another class of products, so I won't go into it here. When choosing the skin of the non-metallic compensator, directly throw the working condition parameters to the manufacturers, and let them give the scheme according to the matching of these three types of materials, which is less trouble than yourself.
3. Select the skin according to the working conditions: high-temperature flue gas, corrosive medium and high-temperature dust are different
Not long-winded, just say the conclusion.
Flue gas pipeline at the tail of boiler and cement kiln in power stationThe temperature is often above 200℃, the dust is large, and there may be sulfur corrosion. In this working condition, the inner layer of the skin must be laminated with fluororubber cloth or PTFE glass fiber cloth, the reinforcing layer must be made of stainless steel wire mesh, and the heat insulation layer must be thickened to more than 80mm. Mentioned in the information on the siteCorrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustryAlthough the main body is a metal bellows, the matching non-metal skin is the same routine.
Desulfurization system, wet dust removalThis kind of corrosive medium environment, the temperature is not high but the pH is very fierce. PTFE or fluororubber cloth is the first choice for the skin layer, and attention should be paid to the problem of condensate accumulation-even if the medium temperature is low, the inner material must be able to withstand the long-term soaking of acid and alkali liquid.
Dry dust removal, pneumatic conveyingIn high-temperature dust conditions, wear is more deadly than corrosion. The inner layer of the skin should be made of fluororubber cloth with good wear resistance, and a layer of wear-resistant protective layer should be added between the heat insulation layer and the reinforcing layer. Otherwise, the dust will wear the skin out in a few months like sandpaper.
Selection is not single-point optimal, but system matching. You go to the manufacturer with the working condition and ask: Which skin structure do you recommend for this working condition? Those who know the business naturally know which plan to use.
4. Composite lamination process: why is the life of the same material several times worse
It is also silicone cloth + glass fiber cloth + ceramic fiber felt. The products of Factory A will be used for 5 years, and the products of Factory B will leak for 2 years. What's the difference? Craft.
Instead of layers of cloth simply sewn together, the skin is created byHigh temperature vulcanization, rolling composite, interlayer bondingWait for the process to be made into a whole. The bond strength between layers is not enough, and it will delaminate, bulge and crack after several times of thermal expansion and contraction. The vulcanization temperature and time control are not in place, and the temperature resistance of silicone cloth is directly discounted.
When you look at the skins that have problems, nine times out of ten, it is not the wrong material choice, but the rough composite process. The most typical is to use ordinary glue to stick between layers, which is not used. Genuine manufacturers will use special high-temperature adhesives to control the process parameters in the whole process of internal mixing, calendering and vulcanization, and each batch has a test record.
So don't just compare the thickness and weight of the sample, and ask one more question: What glue do you use for composite? Is there a temperature resistance aging test report? This is more real than anything else.
5. Installation and routine maintenance: Don't wait for a leak to regret it
No matter how good the skin is, it will be for nothing if it is not installed properly.
First, the pipe flange must be flat, and the bolts should be evenly tightened diagonally. If the local pressure is too tight, the skin will be pressed out of marks or even fractured. Second, do not stretch or twist the skin when it is installed. The reserved displacement is adjusted from the factory, so do not move on the spot. Third, sealant and gasket should be added to the place of riveting or bolting, which is the most easily overlooked leak point.
Routine maintenance is not so mysterious, just regular inspections. Check the skin surface every quarter for any cracks, aging, bulging, and dust accumulation that causes local overheating. Don't wait until the smoke comes out before shutting down the machine. That will cost you a lot.
To put it hard, in order to save money in some projects, the material of the skin of the non-metallic compensator is kept to the lowest level, and the installation and maintenance are not paid attention to. In the end, if something goes wrong, the product is blamed. This account is actually not cost-effective at all.
When choosing a skin, remember three sentences:Material matches working conditions, process determines life, installation and maintenance guarantee bottom line。 These three are achieved, and the non-metallic compensator can be used for more than ten years. If you can't do it, then wait for it to be changed every two years.