by Erik Reinhardt, F.L. Smidth Products Division, Kiln Systems
Heat correction of kiln cranks is cost seffective A kiln crank represents a serious threat to the availability of the production line. It is a threat that must be quantified and, where necessary, counteracted with appropriate measures. Quantification of cranks, by the way, is part of the F.L.Smidth type E kiln inspection programme.
What is a crank? A crank exists if the kiln is deformed in such a way that during its rotation the load on the supports varies periodically as a function of this rotation. A kiln could have similar deformation to a crank, but without this causing variations in the support loads. In such cases, the kiln is said to have a banana.
Kiln cranks are dangerous! Normally invisible and therefore unnoticed, they are capable of creating large extra loads between supporting rollers and tyres - loads so substantial that the risk of fatigue cracking of the supporting roller shafts and of the contact surfaces is greatly increased. Only in the extreme situation when a crank has become big enough to lift the tyre from the roller does it become visible and immediately noticeable.
Thermal cranks There are two types of crank. One, the thermal crank, results from circumferential variation in kiln shell temperature. If, for example, the material is distributed in the satellite cooler in such a way that a number of consecutive cooler tubes carry most of the material and the rest of the tubes most of the air, the tubes with most of the material will become much warmer than the rest of the tubes. Correction of a cranked kiln by the FLS heat correction method is being prepared. Good access is essential both for employee safety and for protection of the kiln in case unexpected process variation occur during the correction that may necessitate quick removal of insulation material.
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Due to the radiation, the kiln shell close to the warmer tubes will also become warmer than the other parts of the shell, its axial developers will become longer than those of the remaining part and the kiln will tend to bend. However, as such bending is hindered by the supporting rollers the loads on them will increase accordingly. Thermal cranks result from the process parameters and they
vary with changes in these process parameters. To some extent there is always a thermal crank in a kiln. Consequently, kilns are designed to endure cranks that are generated from what is considered normal process variations.
Mechanical cranks
Unlike thermal cranks, the other type, mechanical cranks, vary little with changing process parameters. They are caused by specific geometric inaccuracies in the kiln shell and are, therefore, constant in time and
position. Typically, such inaccuracies arise when kiln sections are welded together without their axes merging and, likewise, as a result of poor methodology during repair weldings of the kiln shell. A mechanical crank also occurs as the final consequence of hot spot formation. A mechanical crank can be corrected to restore the desired availability of the kiln. Traditionally, such correction takes place by cutting and rewelding the kiln at various places depending on the results of the crank inspection. This whole correction operation is rather time-consuming as it includes kiln stoppage, supporting the kiln at the cutting positions, removing the refractory, cutting, adjustment, rewelding and, finally, re-fitting refractory lining at the same positions. Most of all, however, it means a substantial loss of production.
Heat correction method In some cases, however, the loss of production can be reduced to an absolute minimum by using the FLS heat correction method. This method is based on the assumption that the kiln continues to operate normally, i.e. produce clinker, during almost the whole of the correction period, with only a few interruptions needed for fitting instruments, etc. The correction is achieved through a combination of controlled and simultaneous pretensioning and lowering of the yield limit in a specific area of the kiln shell. The kiln shell is pretensioned through suitable and provisional adjustments of the kiln axis and lowering of the yield limit is performed through likewise suitable and provisional increase of the kiln shell temperature to approximately 600 degrees
Celsius. The latter is a highly risky affair as it might melt the lining and cripple the shell. Constant monitoring by experienced and knowledgeable personnel is necessary.
The F.L.Smidth engineer helps with fastening insulation material to the kiln shell. Because of the potential risk involved, 24-hour uninter rupted monitoring is necessary. This factor and the very hot environment make heat correction an extremely demanding job.
Successful results The maintenance of a normal production rate during almost the entire correction process makes the FLS heat correction method an extremely cost effective tool which should be considered whenever the question of a crank correction arises. Unfortunately, not all cranks are suitable for correction by this method. The decision whether to apply the heat correction method or the traditional method depends upon the specific crank geometry as measured or described by the crank inspection.
Due to the risk involved, there is no guarantee of complete success. However, of the 16 heat corrections performed by F.L.Smidth service teams in latter years, the most recent one having taken place in Jamaica, only one necessitated additional correction. Pleased with not having had to lose two to four weeks of production and satisfied at having reestablished kiln availability, the remaining 15 cement producers, i.e. 94%, could look back on a successful crank correction effort.
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The heat correction process
If the supporting rollers deflect cyclically as a function of the rotation of the kiln, this is a clear indication of a kiln crank. As a rule of thumb, cyclical deflection in excess of 0.3 mm during one revolution of the kiln requires serious attention. Figure 4 shows a typical heat correction process. The dashed curve shows an initial cyclical deflection of the supporting
rollers amounting to 1 mm, which leaves no doubt that there is a serious kiln crank. When following the curve along the time axis it becomes obvious that at a certain point and in step with the continuously increasing temperature of the insulated section of the kiln shell, the supporting roller deflection begins to decline. Eventually, the supporting roller deflection approaches what appears to be a minimum
level of 0.2 mm per kiln revolution. Having reached this point, continuation of the heat correction process seems to cause no further reduction in roller deflection. It is therefore decided to discontinue the process by removing the insulation material from the kiln shell. The figure shows that the kiln shell temperature drops steeply as soon as the insulation is removed.
F.L. Smidth & Co. A/S, Vigerslev Allé 77, DK-2500 Valby, Denmark Tel : +45 36 18 10 00, fax : +45 36 44 11 19, e-mail :
[email protected], website: www.flsmidth.com
