When it comes to expansion joints, most of the things that pop up in everyone's mind are stainless steel bellows, or those soft fabric fibers. But if you encounter a really high-temperature working condition with hard particles-such as the tail of cement kiln, pulverized coal pipeline of power plant, catalytic cracking unit of chemical plant-the ordinary expansion joint will be scrapped in two months, and anyone will feel distressed. At this time,Cermet expansion jointThis hardcore contestant should be on the stage. Don't be scared by the name. In fact, it "puts" a layer of ceramic armor on the metal bellows, which not only retains the flexibility of metal, but also borrows the ability of ceramics to resist high temperatures and erosion. To put it bluntly, it is specialized in treating all kinds of "hot and hard" pipeline displacement compensation problems.
1. What exactly is a cermet expansion joint? Don't be fooled by the name
The name sounds mysterious, but you can see it when you take it apart: metal structures are used as skeletons (usually austenitic stainless steel or heat-resistant alloys), and ceramic materials are used as liners or coatings. This ceramic is not the kind of ceramic tile in your home, but high-purity alumina, silicon carbide or zirconia, which can withstand the high temperature above 1500℃, and its hardness is so high that most particulate dust rebounds directly when it hits it. The application scenario is those pipelines with a temperature exceeding 600℃ and a large amount of dust or hard particles in the medium-typically such as the tertiary air duct of cement production line, the return pipe of circulating fluidized bed boiler of power plant, and the high-temperature catalyst-containing pipe of chemical plant.
Two days ago, I met the equipment minister of a cement plant, who told me with a sad face that the average life of the expansion joint at the outlet of their preheater is less than half a year, and if you change it once, you have to shut down the kiln for three days, resulting in a loss of hundreds of thousands. I recommended the cermet expansion joint to him, and his first reaction was, "Is this thing reliable?" As a result, it took eight months to install it. When I removed it, I saw that the ceramic layer on the inner wall was only slightly worn, and the bellows was intact. He said at the time, "If I had known that this thing would work so well, I would have wasted so much money in the past two years." Well, that's what happened.
Second, what is the essential difference between it and ordinary metal expansion joints and non-metal expansion joints?
The common metal expansion joints we sell at our station-such asUniversal corrugated expansion joint、High temperature axial expansion joint— — Stainless steel is all used, and the upper limit of temperature resistance is generally 300-400℃ (special alloys can reach about 600℃). When encountering higher temperatures or media containing hard particles, the surface of the bellows will accelerate oxidation and erosion, plus thermal fatigue cracks, which won't last long. WhileNon-metallic expansion joints (fabric fiber expansion joints)Although the temperature resistance can reach 500-800℃ (depending on the material), the fiber structure is naturally afraid of particle wear, and its pressure-bearing capacity is weak, so it is generally only used for low-pressure flue gas pipelines.
The outer metal bellows is responsible for absorbing displacement and bearing pressure; The inner ceramic lining is responsible for heat insulation and particle erosion. Think about it, the same dusty flue gas at 1000℃, ordinary metal expansion joints are perforated for two months, non-metal expansion joints are worn and leaked for three weeks, and cermet expansion joints can work for more than one and a half years. This difference was not a slightest.
The coefficients of thermal expansion of ceramic layers and metal layers are different. If the material selection is not well matched, the ceramic will collapse after alternating hot and cold several times. We have encountered customers who found someone to make it themselves. After three weeks of use, the ceramic pieces crackled and fell down, and finally came to us for repair. Alas, this is the consequence of being greedy for cheap and not looking for professional manufacturers.
3. What working conditions must it be? — — Real cases from the front lines of cement, electric power and chemical industries
Cement industry:The most typical ones are the kiln head and the kiln tail. Taking the 5000t/d production line as an example, the temperature of the tertiary air duct is about 950℃, which contains a large amount of clinker dust and alkali metal vapor. Originally using ordinaryMetal Corrugated Expansion Joints in Cement IndustryWith an average lifespan of 4-6 months. After switching to cermet expansion joints, the service life is extended to more than 18 months, and 2-3 times of shutdown for maintenance are reduced every year, and the maintenance cost of a single line is directly saved by more than 200,000.
Power industry:The return pipe and return valve outlet of a circulating fluidized bed boiler (CFB), temperature 850℃, contains high concentration fly ash. Previously a power plant usedCorrugated expansion joint for power station industryAs a result, the guide tube eroded and perforated within half a year. Later, we fitted expansion joints with ceramic liners to matchFlue gas baffle doorUsed together, no problems now for two years. Another scene is the original flue gas inlet of the desulfurization system. Although the temperature is only 160℃, it contains a large number of gypsum slurry particles, and the wear resistance of ceramic lining is better than that of rubber.
Chemical industry:Regenerator outlet flue of catalytic cracking unit (FCC), temperature 650-720℃, containing catalyst fine powder. Such pipelines usually also involve frequent start-and-stop thermal expansion and contraction, and common expansion joints are quickly fatigued. We useStraight pipe pressure balanced expansion jointThe scheme with ceramic lining solves the dual requirements of displacement compensation and wear resistance. Customers say that it saves 30% maintenance hours compared with other schemes.
Fourth, the three easiest pits to step on during model selection: material, structure and installation
Pit 1: The wrong choice of materials is equal to dry for nothing.Many people think that ceramics are the same, but in fact they are divided into three forms: coating, lining and integral ceramics. The coating is suitable for medium temperature and low wear conditions (≤800℃), the lining is suitable for high temperature and heavy wear (≤1200℃), and the integral ceramics (such as all-ceramic bellows) can withstand higher temperatures but are expensive and have poor flexibility. When selecting the type, it should be determined according to the pipeline temperature, particle hardness and flow rate. Don't listen to the supplier's bragging "we can use it all".
Pit 2: The structural design ignores the guide tube.Internal to the expansion jointguide tubeIt plays the role of diverting flow and protecting bellows. The guide tube of cermet expansion joint also has to be lined with ceramic, otherwise the guide tube will break first, and no matter how good the ceramic layer is, it will be useless. Moreover, the length and taper of the guide tube should match the pipeline, otherwise vortex will generate and wear will be aggravated. Some of our customers bought a bargain, and the guide tube was 50mm shorter. As a result, the airflow directly hit the bellows, which was scrapped in two months.
Pit 3: The cold tightness was not done correctly during installation.Cermet expansion joint because of the high rigidity of ceramic layer, when installedThe screw of the expansion joint needs adjustment(The cold tightness is calculated according to the design temperature). If it is blindly compressed or stretched like an ordinary expansion joint, the ceramic may fracture. We have specially issued installation guidance, emphasizing that the pre-compression amount should be controlled between 50%-70% of the bellows compensation amount. Also, the pipe support must be independent, so that the expansion joint cannot bear the pipe's own weight, which andUniversal corrugated expansion jointSame, but ceramic pieces are more delicate and must be strictly enforced.
Five, and other products of this site collocation ideas: Don't fight alone, to form a team
Cermet expansion joints are not a panacea, and many times need to be used in conjunction with other products to exert maximum effect. For example: in the cement kiln tail system, you use a cermet expansion joint to compensate the axial displacement of the kiln, but there are still secondary air ducts, grate cooler outlet and other positions in the system, and the temperature in these places is slightly lower (400-600℃). Use ourHigh temperature axial expansion jointOrMetal Corrugated Expansion Joints in Cement IndustryIt is enough, there is no need to put all ceramics on (saving costs).
Desulfurization systems in the power industry, with high inlet flue gas temperature and SO₂ corrosive gas, can be consideredPTFE-lined hoseMatching the segmented arrangement of ceramic expansion joints: ceramic is used in the high-temperature section, and tetrafluorine is used in the low-temperature section. In addition, when the pipe is switched or cut off, it is necessary to useElectric plug-in insulation doorOrDouble-sealed single-axis circular baffle doorTo cooperate, avoid hot air backward flow to burn the expansion joint. In chemical catalytic cracking units, we commonly useCompound hinge transverse expansion jointTo absorb multi-directional displacement, coupled with the ceramic lining, a set of combination punches down, the pipeline hardly needs overhaul.
Used in a vacuum pipe (such as an air-cooled island)Double hinge expansion joint for air-cooled island vacuum pipelineIf high-temperature steam is washed with water droplets, you can also add a ceramic sheath inside. We have done it for a power plant, and the effect is good. In short, don't always think of one product to conquer the world. There are more than 20 kinds of expansion joints and supporting valves in our station, which can be combined according to the four dimensions of temperature, medium, pressure and displacement, so that the system can truly live long. And guess what? A customer used to bid for each expansion joint separately. Later, we helped him do a system selection, using cermet expansion joints to matchRotary compensatorAndCompound straight pipe bypass pressure balanced expansion jointThe life of the entire pipeline has been increased by three times, saving millions in maintenance costs every year. It's called professionalism.