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The rotary kiln is the heart of a cement plant. Its main characteristics are its large size, high operating temperature, thin, easily deformed steel shell, and a complex array of instrumentation, requiring a high level of operator expertise. Consequently, various accidents often occur due to equipment failures or operational errors. Rotary kiln downtime due to accidents generally results in significant economic losses. For example, a φ3.5×145m wet-process rotary kiln can lose clinker production by approximately 25 tons per hour for a single downtime, resulting in direct economic losses exceeding 10,000 yuan. Therefore, minimizing accidents and failures to minimize the frequency and duration of kiln downtime is crucial for stabilizing the kiln's thermal system, improving clinker production and quality, reducing maintenance workload, and increasing economic benefits. The following is a brief analysis of common accidents and failures associated with φ3.5×145m rotary kilns by someone with years of experience in cement plants.
During the process of burning raw materials into clinker, the air flow temperature in the kiln reaches above 1600℃, sometimes as high as 1800℃, the material temperature reaches 1450℃, and the surface temperature of the firing zone cylinder is 200℃ to 300℃. The refractory bricks built in the kiln and the kiln lining glued on play a role in protecting the cylinder, so that the cylinder is not burned by high temperature and prevent or reduce the temperature in the kiln from spreading outward from the cylinder.
However, when the kiln lining and refractory bricks are worn away due to long-term erosion, or the cylinder is squeezed due to excessive deformation, the cylinder steel plate will be directly affected by the high temperature or even directly burned by the flame, and the steel plate will burn red or dark red. This phenomenon is called "red kiln". In severe cases, the steel plate will soften, causing convex and concave deformation in local parts of the cylinder, or even the cylinder will be burned, causing a major "red kiln" accident. According to the "red kiln" accidents that occurred in our company's three φ3.5×145M rotary kilns and similar domestic kilns, the "red kiln" phenomenon generally occurs in the firing zone or high-temperature areas.
Based on our company's analysis of numerous "red kiln" accidents and data from other domestic cement plant rotary kiln accidents, we conclude that the causes of "red kiln" accidents are multifaceted.
These factors can be summarized as follows:
1) the selection and quality of refractory bricks;
2) the quality of refractory brick masonry;
3) the raw material formulation;
4) the firing process;
5) issues within the kiln itself, such as cylinder out-of-roundness, misaligned centerline, insufficient rigidity, and significant radial deformation.
During one rotation, the cylinder undergoes six alternating cycles of radial deformation, alternating between convex and concave, which is clearly the primary cause of kiln lining loosening and separation. Cylinder deformation is most severe at the tyre, and the gap between the tyre and the cylinder significantly impacts this deformation. Therefore, it is essential to monitor and control kiln deformation and the gap between the tyre and the cylinder to identify and address any issues promptly. Our approach to addressing these issues is to appropriately increase the thickness of the new backing plate based on the actual gap measurements and then replace it. After the replacement, the rotary kiln performed well under normal production operation. As for the misalignment of the kiln centerline, it was due to failure to align it during installation, sinking of the foundation, uneven wear of the wheel tyres and supporting rollers, and frequent irregular kiln adjustments.
To ensure cylinder rigidity and minimize deformation, welded cylinders should be used instead of riveted ones whenever possible. High-quality steel plates, such as 16Mn or QZ235C (A3), can be used. The cylinder plate can also be appropriately thickened. The typical thickness of the steel plate under the wheel belt is: for rotary kilns with diameters less than 4M, the relative thickness δ/D is 0.012-0.015 (δ is the cylinder plate, D is the kiln diameter); for kilns with diameters greater than 4Mn, δ/D can be 0.015-0.020. In actual production, to monitor the kiln cylinder (primarily the firing zone) for heating and temperature, fire watchers regularly touch the surface of the kiln in the firing zone (a water spray device is installed in this section) and use their sense of touch and experience to determine whether the temperature is normal.
The tyre is a large, solid steel ring, primarily mounted on the kiln. It transmits the full weight of the kiln to the supporting rollers, allowing the kiln to rotate on them for normal production. It also strengthens the kiln's rigidity. However, during normal production, damage to the tyre and supporting rollers, or tyre breakage, often impacts normal kiln operation. These problems can include severe wear, flaking, cracking, crushing, or the formation of steps on the supporting rollers.
The main causes include:
(a) misalignment of the kiln centerline;
(b) sinking of the supporting roller foundation;
(c) uneven wear resulting in excessive stress on a particular pair of tyres and supporting rollers; and
(d) poor casting quality, including pores, sand holes, and stress concentration areas.
Once these defective areas are subjected to excessive transient loads, they can lead to cracks or even fracture. Treatment and repair methods: To prevent excessive contact stress between the tyre and the supporting roller, the contact width between the tyre and the supporting roller should not be less than 60-70% of the tyre width. Both surfaces must be flat. If the supporting roller surface is uneven or tapered, the rotation of the supporting roller can be used to make a turning repair using a trolley tool holder. If there are defects on the supporting roller that do not affect its use, it can be repaired by welding, but it should be noted that the hardness of the welding performance must not exceed the hardness of the tyre surface, otherwise the tyre may crack.
Generally, 507 welding rods are used on the surface. As for problems such as misaligned center line and sinking of the supporting roller foundation, they can be handled by adjusting the method after measurement. In addition, to prevent the huge friction generated on the tyre and the supporting roller surface when the kiln body shrinks as the temperature drops when the kiln is stopped, which may damage the tyre or the supporting roller surface, a small amount of waste oil can be applied to the supporting roller surface when the kiln is stopped to reduce friction and avoid surface damage.
Based on the above brief analysis, it can be seen that the common faults and accidents of rotary kiln equipment are diverse and the causes are not the same. As long as we strengthen daily maintenance and operation, and use some advanced instruments and other modern detection methods, we can easily manage, use and maintain our kilns and keep the rotary kilns in normal operation.
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