Why do you have to add castable to the flue expansion joint?
High-temperature smoke is not vegetarian. The flue temperature of power plants and cement plants is always five or six hundred degrees, and some even thousands. Ordinary metal bellows are directly exposed to this working condition, and it takes less than a few months to oxidize and deform. The castable is actually a layer of refractory armor for the expansion joint, which can both insulate heat and resist erosion. Two days ago, a customer asked me: Can you add more insulation cotton without castables? I said that insulation cotton is a soft bag, and the smoke will fall apart when it blows, so it can't hold it at all.
Three hard indexes of castable selection: temperature resistance, wear resistance and thermal shock resistance
Temperature resistance depends on the working conditions. It is enough to select corundum mullite below 900℃, and zircon or silicon carbide should be added above 1000℃. The wear resistance depends on the content of flue gas particles. There is less fly ash in the desulfurization flue, but the dust at the tail of the cement kiln is like sandpaper, and the wear resistance index must be ≥12. Thermal shock stability is most easily overlooked-the expansion joint itself is thermally expanding and contracting, and if the castable also cracks, it is better not to pour it. Tell me a case: a steel mill used a batch of high-aluminum castables, which were broken after only 20 thermal shocks. After replacing them with low-cement corundum castables, they lasted two overhaul cycles.
In this part of construction, 80% of the problems lie in this place
Before pouring, the inside of the expansion joint must be cleaned, and there must be no floating rust and oil stain, otherwise the joint surface will peel. In addition, castables have to leave shrinkage joints-some workers try to save trouble and pour them all at once, but they burst directly when drying. It is recommended to pour in two times, the thickness of each layer should not exceed 80mm, and the vibration should be even but not over-vibration. The curing is more critical. After 24 hours of natural curing, the temperature can be heated and baked, and the heating rate is controlled below 20℃/h. Many factories are in a hurry to put into production, and the baking time is cut in half. Finally, the surface of the castable looks fine, but the water vapor inside is not discharged clean, and it collapses as soon as it is put into production.
The cooperation between castable and expansion joint body is also particularly important
Non-metallic expansion joints (fabric fiber expansion joints) are flexible, so castables are generally only lined, and the thickness is controlled at 30-50mm. Too thick will limit the displacement of expansion joints. For high-temperature axial expansion joints or metal corrugated expansion joints, castables can be directly filled between corrugations, but reasonable anchors must be designed. The material of the anchor should be consistent with the expansion joint, and it is best to use stainless steel 304 or 310S. The welding should be full welding, not spot welding. In addition, don't forget the function of the guide tube-as mentioned in the FAQ of this site, the guide tube can reduce the direct scouring of castables by high-temperature flue gas and prolong the life.
Common Rollover Scenes and Remedies
Most prevalently, castables fall off for no more than two reasons: the anchors are not properly designed, or the baking is not in place. If the shedding area is small, it can be repaired locally, but the loose part must be chisel off and the anchor nail must be replanted. If the whole piece falls, you can only disassemble the expansion joint and redo it. There is also a pit-some people mix castables with insulation cotton in order to save money. As a result, the insulation cotton shrinks when heated, and the castables collapse. Remember: castable is the structural layer, insulation cotton is the insulation layer, the division of labor between the two is different, don't confuse. In terms of maintenance, it is recommended to take a look at the endoscope every time the machine is shut down for maintenance, and if any cracks exceed 2mm, they should be treated.