Based on our experience we can distinguish six different concepts of rotary kiln maintenance. Aligning and inspecting the kiln with these knowledge the user can ensure long time of operation – unexpected stoppages reduced to minimum. Ignoring these 6 issues equals more damages of rotary equipment.
The components like support rollers are very often ignored by the users of rotary kiln. It is not possible to operate kiln without taking care of rollers and its foundations. Many people look differently to rotary equipment then housing, base plates or concrete support around. But truth is the solid supports is the key of maintenance-free operation. With a poor foundation the kiln failure will appear very quickly.
The rotary kilns are design to work for a very long time. Because rotation speed is quite slow all components needs some time to get damage, but want to ensure long service life of rotary kiln most of all the user has to take care of proper geometry of components.
First and a very important parameter is a base slope. If it is not correct can wear taper shape of rollers and tires. Irregular surface shape cause incorrect axial kiln balance or at least make it difficult to control this movement. Also the component wear is higher compared to align position of rollers (skewing, inclination etc.). High force of kiln thrust can increase the damage process of other components for example thrust roller, tire side surface, base plates etc.
Keeping kiln align is a very powerful tool of maintenance. Parallel position of tires and rollers running surfaces causes the same equal contact and stress distribution. Anyway after years of operation the contact surfaces will change into tapered or concave surface shape. This irregularities in kiln geometry should be corrected – grinding of all deviated parts should be performed. This procedure will improve contact and stress forces on running components. Grinding also has some disadvantage because tire reduces its thickness an higher stress and ovality appears. But anyway this step is more profitable then irregular surfaces of working components. Before the grinding procedure starts the proper base slope should be set (align).
Under-tire clearance of tires is another important parameter that should be monitored, controlled and optimizing for preventive maintenance procedure. The difference between the value of relative movement for hot and cold kiln have to be known. Kiln operation with under-tire clearance out of tolerance can create a serious damage like refractory failure, contraction of shell, cracks on the shell or tire surface, broken chair pads (shims). This is important for the user to control this parameter. Cement Plant crew should log the measurement value especially during start-up of the kiln after each stoppage.
Under-tire can be adjusted by shim changing. After many years of operation the tire inner surface and chair pads getting worn out. For each case proper thickness of shims has to be calculated and installed to get a tire migration within tolerance. Base on our experience it is always better to change all chair pads for thicker ones then install many thin shims. Thin shim wear very quickly and migration going back to previous value.
Important segment that should be included in maintenance schedule is a girth gear root clearance monitoring. Girth gear and pinion meshing is a very precise working components and is also the most expensive part and the time of delivery is long that’s why the user should pay a special attention to keep it align. The potential problems can be caused by wrong meshing, lubrication, misalignment, contamination, contact. The most serious, that can create catastrophic failure, is not correct contact of teeth. Damaged drive components (shaft, teeth, gear rim, pinion) can indicate long stoppage if there is no spare parts.
All gearing needs to have some parameters within tolerance to work correctly. This parameters is root clearance and backlash – describes correct alignment of gear and pinion teeth and also degree of wear. To monitor root clearance user can try to control pitch line separation, if not special measurements are necessary. Additional clearance is always included during erection to prevent excessive contact (interference). The tolerance of root clearance and backlash should be given by OEM.
Root clearance measurement should be taken at least at four points located equally on shell circumference. Correct meshing guaranty maximum service life of drive components.
Another part of preventive strategy is correct kiln thrust alignment. It means that all the supports, rollers have to be as close to the parallel line to kiln axis as possible. This geometrical position distribute equal reaction and load to each support. Of course during kiln operation axial thrust can change. Main causes are: temperature distribution, refractory failure, coating falls, shell bending (bow or “dog leg” crank formation), dust or oil on running surfaces, support base slope changes, kiln axis alignment changes, production process changes, weather etc. Some of this factors are within users control, some of them not. Keeping all geometrical parameter within tolerance helps to control kiln operation.
Kiln axis alignment both in horizontal and vertical plane is a significant point of maintenance. Straight axis reduces kiln shell stress and load distribution to the optimum and safe values. The maximum available deviation of kiln axis should never be exceeded. It is recommended to control kiln axis alignment at least once or twice a year or after main parts replacement.
Rotary kiln service method available today allow to control and monitor all of mention above parameters without affecting production, during normal kiln operation. The precise measurements are necessary to maintain high efficiency and long life of kiln operation.
In summary maintenance preventive strategy should include:
- Constant monitoring of kiln geometrical parameters and keeping them within design tolerances
- Alignment of kiln axis
- Alignment of kiln axial thrust (balance)
- Keep correct value of under-tire clearance parameters
- Alignment of girth gear and pinion meshing
- Alignment of roller position (horizontal skewing and inclination) in relation to kiln axis.
- Control the wear of components
Maintenance program that includes these parts will minimize significantly operating costs and reduce time of unplanned kiln stoppages.