Zeki Özek, Özek Makina, offers insight into the processes involved in the inspection and maintenance of rotary kilns.
A rotary kiln is a tool for calcining thousands of tons of raw mix at temperatures of 1450°C every day. This tool, of which almost all parts are made of steel, can reach a total weight of 1000 t when the refractory and the product inside are taken into account. Rotating such a large mass on relatively small rollers and protecting it from such high temperatures is one of the greatest challenges facing cement production.
The rotary kiln has quite a simple geometry. Each pier, consisting of a tyre and two rollers, is positioned to form an equilateral triangle between the tyre and roller centres. Each station is positioned to share the total load in a balanced way. The shell, which is made of a thick steel plate and covered with refractory bricks, passes through the tyre centres of these piers, and ensures the transformation of the raw material into clinker while transferring it. The whole unit, which is installed with a slope that facilitates material transfer, works by rotating a girth gear mounted to the shell. So, the whole problem becomes simply keeping few equilateral triangles aligned in space. It really is that simple on paper. But consider that the object in question is a steel tube with a diameter of 4 – 5 m and a length of 60 – 70 m, through which thousands of tons of material is transferred while reaching a temperature of 1450°C.
The chemical reaction inside the kiln, the elastic deformation (ovality) of the shell, and the friction of the material passing through it seriously erodes and thins both the shell plate and the refractory. The temperature rising to different levels throughout the kiln causes the shell to bend temporarily or permanently. The thermal or mechanical crank-form shell rotation that occurs in this way creates a dynamic load on the rollers of each pier which can reach up to several times the estimated design value. The girth gear starts to move closer and further away from the pinion by rotating with a significant run-out, and even hits the pinion gear tooth at advanced levels, causing serious vibration. Meanwhile the rollers, which are skewed to push the kiln uphill, against the kiln slope, wear out in a conical surface form along with the tyres. The contact surfaces of tyres and rollers become narrower, the load and stress per unit area increase, and thus, surface scratches and cracks, as well as uneven, wavy abrasions on these surfaces are encountered frequently.
The axial movement cycle of the kiln, which loses its designed tyre/roller relation, is impaired. The pressure applied to the thrust roller increases, and eventually these surfaces cannot be freed from deformation. On the other hand, the wobble of the tyres causes dynamic contact with the rollers and directly alters the rollers’ effect on the kiln’s axial movement. The rollers move axially inside the bearings and warm up due to the friction of the thrust disc and the bearing.
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Read the article online at: https://www.worldcement.com/special-reports/08022023/refining-kiln-alignment/
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