How harsh is the working environment of a high temperature expansion joint? The bellows itself can't carry it
Tell me the truth. Two days ago, the equipment supervisor of a cement plant called me and said that the expansion joint on their kiln tail flue cracked in less than four months. I asked him did you make castables? The other party was stunned for a moment. "You still need castables? Isn't the bellows able to withstand high temperatures?"
The flue gas temperature is often 800℃-1100℃, and some even rushes to 1300℃ instantaneously. There are also a lot of dust and corrosive gases (SO₂, NO₂) trapped in the medium. The stainless steel materials used in the bellows itself, such as 304 or 316L, will experience carbide precipitation, intergranular corrosion and cliff-like decrease in strength under long-term high temperature-this is called high-temperature failure. Even if you use a nickel-based alloy like Inconel 625, the surface temperature exceeds 650℃ and you have to stop.
Streaking bellows are consumables under high temperature conditions. What about that? It must be dressed with a piece of "armor"-castable.
The first layer of protection of castables: thermal insulation to prevent high temperature failure of corrugated pipes
Castables are first and foremost thermal insulation. Its thermal conductivity is typically between 0.3-0.8 W/m K, while the thermal conductivity of corrugated stainless steel is above 15 W/m K. What's the meaning? Castables can block most of the heat radiation and heat conduction. For example, we giveCorrugated expansion joint for power station industryWhen designing castables, the inner surface temperature should be controlled below 200℃, so that the yield strength of bellows can be maintained at the design value.
At the expansion joint at the outlet of the induced draft fan of a thermal power plant, the temperature of the outer wall of the bellows before castable was 480℃, and after 80mm thick lightweight mullite castable was placed, the outer wall dropped to 180℃. The lifespan has been pulled directly from 9 months to 5 years and still hasn't broken. Thermal insulation can't pass, and the rest is all nonsense.
The second layer of protection: diversion and anti-scour, blocking the wear of high-speed dusty airflow
You think only the heat is the killer? High-speed dusty airflow is the real "taking your life". The flow rate of the air duct on the cement production line is often 18-25m/s, which is mixed with clinker particles. When that thing hits the bellows, it is like sandblasting. The geometric structure of the peaks and valleys of the bellows itself is a "ash nest", and the dust accumulates in the valleys and rubs them repeatedly, and wears them out quickly.
After the castable is hit, a smooth flow channel will be formed on the inner side, without groove dead angles, and the airflow will pass smoothly. At the same time, the wear resistance of castable is 3-5 times that of ordinary carbon steel (high aluminum castable, Mohs hardness above 7). OurMetal Corrugated Expansion Joints in Cement IndustryBefore leaving the factory, the diversion design is done, but the diversion tube plus castable is the double insurance-the diversion tube blocks most of the frontal impact, and the castable fills the gap to prevent vortex erosion.
Think about it, if you don't hit castable, how fast will the bellows be worn out at the crest? We have measured that the unprotected bellows perforates in an average of 3 months at the outlet of the cement mill. After beating the castable, carry it for at least two years.
The third layer of protection: structural support and anti-corrosion, isolating corrosive media from the bellows
Acidic condensation in high-temperature smoke is a hidden killer. In some projects, the temperature drops below the dew point after the furnace is shut down, and SO₃ and water vapor combine to form sulfuric acid, which adheres to the inner wall of the bellows. Stainless steel is most afraid of chloride ions and concentrated sulfuric acid, pitting corrosion and stress corrosion cracking follow one after another. Castables are alkaline or neutral materials (such as corundum) that keep acidic media out of the bellows.
Additionally, the castable also provides structural support. Especially for large fluesNon-metallic expansion joint(Fabric fiber expansion joint), although it is temperature-resistant but has poor rigidity, the castable can fix the shape and prevent negative pressure deflation. InHigh temperature axial expansion jointThe castable layer can share a part of the axial pressure and reduce the stress concentration at the root of the bellows. It can be said that castable is not only a protective layer, but also a part of the load-bearing structure.
How to beat castables to be qualified? Construction details and material selection determine life
After discussing "why to fight", let's talk about "how to fight" next-there are more people stepping on pits here.
Material selection:Select different materials for different working conditions. High aluminum castable for flue gas temperature ≤800 DEG C; Corundum mullite for 900-1100 DEG C; High temperature resistant castables containing silicon carbide must be used above 1200℃. Don't listen to the manufacturers saying "general-purpose type", that's all fooling laymen. We giveDesulfurization flue gas baffle doorFor matching expansion joints, we should also consider acid resistance, and choose castables combined with low calcium aluminate cement.
Construction Details:First, the anchors must be welded firmly, 304 stainless steel anchor nails, with a spacing of less than 200mm, are distributed in a plum blossom shape. Second, before pouring, the bellows surface must be removed from rust and brushed with anti-oxidation coating. Third, curing-it can't be baked immediately after pouring. It must be naturally cured for 24 hours before baking according to the temperature rising curve (usually, the temperature is raised to 600℃ every 50℃/h for 4 hours). If the curing is not in place, the castable will crack and fall off, and dry for nothing.
Thickness control:Not that the thicker the better. Too thick will increase the weight and affect the compensation ability of the expansion joint. Experience value: 40-60mm in the center, and the edge can be as thin as 20mm. Use special molds to ensure uniform thickness. In addition, the end of the guide tube should leave an expansion gap, otherwise the castable will bend the top of the guide tube as soon as it expands.
To put it bluntly, playing castables looks simple, but in fact, it tests the understanding of working conditions and the seriousness of construction. We have seen too many customers buy castable paste by themselves, and the result is that slag falls in two months. I regret that I didn't find a professional manufacturer to mix itUniversal corrugated expansion jointThe castable scheme of.
Why Should High Temperature Expansion Joints Be Castable?Because the bellows is a "crispy skin" under the attack of high temperature, erosion and corrosion, the castable is the human shield that can carry and fight. No pouring, compulsory within six months; If you water it right, you don't have to worry about it for five years.