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The Direct Method


The Direct Method

The kiln must be aligned so that flexing and distortion of the kiln shell are minimized and that loads to the support bearings are properly shared . Flexing and distortion of the kiln shell vastly increases mechanical wear and tear and can severely reduce refractory brick life. Poor load sharing amongst the supports leads to roller and bearing problems as well. The Direct Method of alignment is used to find the shell centre points to be aligned without any measurement to the support components, namely the tyres or the rollers, and this is done with the kiln in full operation.

The advantages of being able to do this would be self evident. Such a method would have the directness and inherent accuracy promised by a bore sight alignment and also the advantages of an external procedure, that is measuring the position of the shell hot and running. By definition it would then be the ideal preventative maintenance tool since it can be scheduled any time the kiln is up and running. The fact that there is a rotating object, the shell, allows its centre of rotation to be found irrespective of the fact that it may not be circular and that it may have planetary motion. To do this it is only necessary to find three mean shell positions. Properly selected, these three positions will reveal the centre of rotation. The most important aspect to emphasize is, with the kiln in normal operation, the position of a series of support points have to be obtained by external measurement which can be aligned without knowing any details about the nature of the supports. This has been accomplished by finding centres of rotation as opposed to the physical centres of tyres and shell. Aligning these points then puts the shell in the lowest state of stress which can be achieved by positioning the support rollers.

1. Introduction
2. Historical preference for the internal bore sight alignment
3. Limitations to the internal bore sight alignment
4. Limitations of external procedures
5. The Direct Method
6. The apparatus
7. A note on accuracy
8. Final observations
9. Conclusion

1. Introduction

Naturally, the intention is to establish alignment with the kiln in operation. The advantages of doing so are compelling. What is the Direct Method of alignment? - The Direct Method of alignment is to find the shell centre points to be aligned without any measurement of the support components, namely the tyres or the rollers, and doing this with the kiln in full operation. The advantages of being able to do this would be self evident. Such a method would have the directness and inherent accuracy promised by a bore sight alignment (Fig. 1), and also the advantages of an eternal procedure, that is measuring the position of the shell hot and running. By definition it would then be the ideal preventative maintenance tool since it can be scheduled.

 

Fig. 1

2. Historical preference for the internal bore sight alignment

Bore sight alignment (Fig. 1) is almost always used in the erection of kiln shells when they are delivered to site in more than one section. For all but the shortest kilns this is usually the case. Subsequently, after the kiln has been lined with refractory and set into operation many users still prefer to check alignment using this internal technique This is the most direct method of alignment measurement because, unlike external procedures, it circumvents the need to measure the support component geometries. That is its great advantage. Unfortunately this advantage is more than offset by the need to shut the kiln down and have it 100% available for measurement. This is a shut down period where no other work can take place for the duration of the measurement work, and seriously detracts from using the alignment as a preventive maintenance tool. Working inside the kiln also presents physical challenges that compromise accuracy.

3. Limitations to the internal bore sight alignment

The internal bore sight alignment, in spite of the directness of shell position measurement, has some serious limitations. Each centre for a roller support station is found from a composition of several rotational kiln positions. As a minimum, 3 positions are necessary. More are preferred since the location of the centre, measured from the inside surface of the steel shell, will gain precision with the number of shell reference points. Each point, however, requires the coating on the brick, and the refractory brick itself, to be drilled. In addition, for each set of points the kiln has to be partly rotated. It is easy to understand that this is time consuming and, with the pressures to limit shut down time, the minimum procedures are usually accepted. Another limitation is that the kiln is necessarily measured in its cold condition. Cold, the kiln is not thermally expanded. This means that the centres are not in the same vertical locations, and the tyres are not in the same axial positions in relation to their respective pairs of support rollers, as they would be with the shell hot and expanded. Re- checking alignment after the roller adjustments have been made is not an option, at least until the next shut down.

4. Limitations of external procedures

To date, all external procedures (hot or cold), invariably take measurements to at least the tyre, and usually to the rollers as well. Whether these measurements are taken hot or cold does not matter to those procedures. These hot procedures are only variations and extensions of the standard cold procedure, and in principle they are all identical. Preference is given to measuring hot to avoid the limitations of the internal procedure as described. Although a great variety of techniques are used to measure a moving tyre diameter, and hence compute its centre position, such efforts are plagued with accuracy problems. Even when the location of the tyre centre has been computed, it is the shell that must be aligned. Most kilns have migrating tyres, which means their shell position within the tyre is eccentric. This phenomenon must be accounted for. The hot external methods therefore still have credibility problems with knowledgeable critics in some segments of the kiln user industry.

5. The Direct Method

It must be appreciated that the accuracy of the final alignment is only as good as the accuracy and reliability with which these "centre points" can be established. The direct method establishes these centre points in the same way as a bore sight alignment, that is by measurement to the shell rather than through measurement of the support components. The same principle of trammelling points from the inside of the shell is thus applied although now working from one of the shell. There are some differences. Namely there is no restriction to three (or any number) of discrete shell positions. Theoretically, any number of positions can be used provided the computer and measuring device can handle them. As a result the shell centre of rotation is obtained, and not just a physical centre of the shell. It goes without saying, when working externally, that there is no refractory, material coating or sometimes kiln chain to impede the work. The fact that there is a rotating object, the shell, allows its centre of rotation to be found irrespective of the fact that it may not be circular and that it may have planetary motion. To do this it is only necessary to find three mean shell positions. Properly selected, these three positions will reveal the centre of rotation (Fig. 2).

Fig. 2

The dashed lines represent the rotation of the shell. Although only eight positions are indicated in this way the lasers each take 180 discrete readings during the course of one shell rotation. Technically, 180 shell positions should be shown but this would obscure the illustration. The 180 readings are arbitrary but are considered numerous enough to represent the entire surface of the shell. Each laser's 180 readings are then averaged, producing three mean shell positions. Only one circle can be drawn through three points. The resulting circle does not really represent the shell, it is simply called the working diameter. The only significance of this circle is that its centre is the centre of rotation of the shell. If the shell had no component of planetary motion, which is virtually never the case given the nature of the shell, its physical centre would correspond to the centre of rotation. In practice the centre of rotation is always different from the shell's physical centre as illustrated. The only thing that varies is the degree to which these points are separated. The centre of rotation at one cross section of the shell is found in this way. Performing this procedure once on the discharge side of each tyre and again on the in feed side of each tyre allows the interpolation of the centre of rotation for the centre of each tyre support. At this juncture a set of centres have been obtained, just like the bore sight method, the only difference being that the points are coordinate addresses in a 3 dimensional space. This then is the basic concept of the Direct Method. By definition it is clear that unlike other methods it can only be performed with the kiln in operation.

6. The apparatus

Fig. 3

Figure 3

Figure 3 shows the apparatus. "Instrument" means the position of the Integrate Total Station which will have coordinates (Ni, Ei, Zi) where N, E, Z are in millimetres (45023, 9467, 6251) (Fig. 4).

Fig. 4

Figure 4

Prism 1 and Prism 2 denote the coordinate addresses of the two fixed prisms on the bridge, Pl (Npl, Epl, Zpl) and P2 (Np2, Ep2, Zp2) all in millimetres (Fig. 5). Ll, L2, L3 are variable dimensions com posed of a bridge constant, i.e. the physical location on the bridge which does not change from application to application, and the variable laser readings.

Figure 5

These laser readings are distance displacement measurements from the laser to the shell. The bridge assembly (Fig. 5) shows the three lasers, one centre and one at each end. Means 1, 2 & 3 are the computer calculated positions based on the respective averages of each of the 180 readings of Ll, L2 and L3. The three sets of 180 laser distance displacement readings are logged virtually simultaneously during the course of one shell rotation. The unique circle defined by the three means 1, 2 & 3 is shown to clarify the trigonometric calculation of the centre. Al is the angle the bridge makes to the horizontal. It is not necessary therefore that the technician sets the bridge level, although in most cases near level would be the most convenient for him. There are other criteria of placement which are adhered to. Adjustment capability ridge to facilitate this. A2 is an angle which will vary with kiln diameter. For the sake of brevity its derivation is not shown, but it is a simple trigonometric calculation given the bridge configuration and measured means 1, 2 & 3. Similarly, the derivations of g, h & i are not shown, but are also found by trigonometric calculation from the known points and distances. Centre; Given the bridge constants, that is the fixed relative positions of the three lasers and the two prisms, and the average of the three laser displacement readings it is therefore a straightforward trigonometric calculation to show that the centre coordinates are: Centre [Pl(N,,l), P1 where h=g sin(A1+A2) ,~d i = g-cos(Al+A2) The cross section of the kiln is at right angles to the "N" direction. In actuality this would be a rare coincidence. The kiln position is expected to be skewed to all axes since the coordinate grid is set before the kiln position is measured. The coordinates of the centres are therefore true three-dimensional addresses. The centre of the shell at the centre of the support frame is interpolated from the two centres straddling a support tyre. This then yields a single point for each support. Their positions with respect to a straight line of a selected slope is then easily assessed. All the calculations are computer based. Their details are not needed for an understanding of the basic precepts.

7. A note on accuracy

Every kiln installation provides its own challenges that compromise the inherent accuracy of the instrumentation brought to the task of alignment measurement. In this sense the instrumentation described here is no more or less susceptible than that associated with any competent procedure. Absolute accuracy is therefore site specific. It goes without saying that instrument accuracy is always greater than the accuracy of the final results. So far the only concern has been to find the shell position. Any misalignment of the kiln position must now be translated into roller adjustment to correct the position. This requires a knowledge of the roller and tyre diameters. The advantages of the Direct Method measurements are best explained by example, which will also demonstrate why this method is inherently more accurate. A convenient set of tyre (7,000 mm) and roller (2,000 mm) diameters will be used for the kiln taken as an example (Fig. 6).

Fig. 6

Figure 6

Consider first one point from a set of centres calculated using an external procedure. It will then be assumed that two errors were made. First, the tyre diameter is actually 6,980 mm, and secondly one roller diameter is actually 1,990 mm. The practitioner who does not know this but calculates his centre point through his support component measurements will mislocate the centre by 15 mm. Any variation of diameter errors or roller spacing mismeasurements will have similar results. The error in this case is 15 mm. Now to be fair the same error hypothesis will be imposed on the Direct Method. Having measured the alignment without having to know any of the diameters, a misalignment is indicated of 10 mm vertical direction and 10 mm horizontal direction. From this position it is necessary to calculate the appropriate roller moves to reposition the kiln, for which the diameters must now be introduced. For the horizontal correction both rollers can be moved to the right 10 mm (Fig. 7).

Fig. 7

Figure 7

Next, the vertical correction must be calculated. First using the measured values, then with the actual values.

Fig. 8

Figure 8

Shown in Fig. 8 is the calculation for the vertical adjustment. An actual value for DH of 10.16 mm was calculated backwards from having moved 17.41 but using the actual A, B & C values. The measurements of A, B & C are crude because of the nature of the kiln. However, it is clear that the error in placing the rollers is barely affected by using poor values for A, B and C. Using quick and simple measurements to verify these dimensions for the purpose of roller support movement calculations therefore hardly affects the results for the Direct Method. This is why the accuracy of obtaining an alignment measurement is totally independent of the condition of the rollers and tyres and the effect on roller adjustments is negligible. Unlike the simple external procedure which produced a 15 mm error in placing the centre, the Direct Method resulted in only a 0.16 mm error with the same mistaken roller and tyre dimensions. The inherent accuracy for this example is almost 100 times better. Even if the horizontal spacing is in error it can be clearly seen that the Direct Method has inherently better accuracy.

8. Final observations

The most important aspect to emphasize is that the positions of a series of support points have been obtained externally with the kiln in normal operation without knowing any details about the nature of the supports. This has been accomplished by finding centres of rotation as opposed to the physical centres of the tyres, shell, etc. In retrospect, it is considered that the centres of rotation are more representative of the dynamic position of the kiln. Aligning these points then puts the shell in the lowest state of stress that can be achieved by positioning the support rollers. Although there is always a measurable difference between the centre of rotation and the physical centre of the shell, this difference for all except kilns in poor mechanical condition is somewhat academic. In other words, most times it makes no significant difference. But, apart from not knowing in advance the kilns for which it does make a difference, the real advantage of focusing on the centres of rotation is that they represent the dynamic position of the shell during operation. Hence the name "direct". Although the Direct Method is principally the same as the internal bore sight method, but carried out in an "in side out" fashion, it is not restricted to discrete shell positions. In addition, there is not only a vastly increased volume of data for supporting the calculation of the centre positions, but also the option is available of repeating and verifying positions after roller adjustments or at any time for whatever reason. The Direct Method of alignment also includes ovality measurements. Ovality is the measure of the dynamic change of shell curvature caused by rotation. Ovality is of primary interest with regard to the mechanical stability of the refractory lining. However, a detailed analysis of the curves as presented by the electronic ovality beam often reveals information useful in completing an alignment analysis. Significant differential ovality from one pier to the next can be used to correct the alignment recommendations when appropriate.

Conclusion

The kiln must be aligned so that flexing and distortion of the kiln shell are minimized and that loads on the support bearings are properly shared. Flexing and distortion of the kiln shell vastly increases mechanical wear and tear and can severely reduce refractory brick life. Poor load sharing amongst the supports leads to roller and bearing problems as well. Measuring alignment, with the kiln in full operation, accurately and reliably, requires an innovative approach. The Direct Method is that innovation making it a truly preventative maintenance tool. Now it is possible to align an operating kiln precisely.


 


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