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The New Bore Site Alignment


The New Bore Sight Alignment

Using the Centers of Rotation for Bore Sight Alignment of Rotary Kilns

Walter M. Gebhart P.Eng., Vice President, Phillips Kiln Services Ltd.
Ronald W.T. Birchard P.Eng. Consultant rwtbirch@cgocable.net
Dave Roberts Manager, Alignment Services, Phillips Kiln Services Ltd.

Introduction

Although "The Direct Method"1 for measuring the alignment of rotating kiln shells has become a procedure of choice for many kiln operators world wide, it has one limitation. The kiln must be in operation in order to make the measurements. For the most part this is a strong advantage but when major repair work is underway, such as changing a shell section, where aligning the new section is a critical step, a bore sighting may still be necessary. Techniques involving rotation to align the field joints of the new section(s) still have the advantage but there are situations where this may not be possible. An example of that may be a kiln with planetary coolers when one of the field joints is between the inlet area of the coolers and the supporting tire immediately up-hill of this area. Securing the shell joint sufficiently to permit turning for the purpose of alignment may be prohibitive. In such a case a "bore sighting" as has been done for decades is the obvious solution.

The adaptation of some standard alignment techniques can make bore sight measurements significantly easier to perform than what is conventionally done2 .

The Direct Method1 (performed while the kiln is in normal operation) utilizes the turning shell to identify the point about which it rotates at each roller support. A typical bore sight procedure2 (always performed with a cold, shut-down kiln) involves setting up physical target plates inside the kiln on which a center is determined by direct measurement from the inside of the shell plate. This requires removing areas of refractory at each roller support station to expose sufficient areas to do this. See Fig 45.2


The Direct Method, a hot alignment technique (performed while the kiln is in normal operation) was developed from the principle of the bore sight. The Direct Method does continuous scans on the outside of the moving shell to determine a center of rotation. The center of rotation is virtually always different from the mean shell center. This may be academic in most cases but significantly the center of rotation is more accurately and reliably identifiable. The bore sight using the center of rotation instead of the mean shell center, is less work as well. The reason for this is because there is no need to measure to the inside of the shell. It is also possible to fix the target stands without refractory removal. This means significant savings of time and labor. Additionally it is done with far fewer rotations than that necessary to "trammel in" the centers compared to the conventional method 2. Thirdly the center of rotation is established entirely independently of the shape of theshell. This last point may often be the most significant in terms of accuracy since a distorted shell introduces uncertainty in determining a mean shell center. Finally, if done carefully, the effects of ovality (shape distortion due to flexing) can also be all but eliminated.

This is an ironic development since the bore sight inspired the development of the external, hot alignment, "Direct Method" procedure, and in turn, the Direct Method is now used to redefine the bore sight technique.

The New Bore Sight Alignment

The following is not meant to be a definitive description of the new bore sight alignment methodology but a simple outline so that anyone not familiar with the procedure can grasp a general appreciation of it.

Establishing the axis of rotation is done after the kiln is shut down but before any cutting of the shell commences. The shell is then cut and the replacement section can be aligned to that axis without rotating the shell.

An alignment scope (one equipped with visible laser light is a very convenient tool in this case) is placed immediately outside of the kiln. This is typically at the discharge end on the burner floor with the hood set aside. A position is roughed in using a tape measure etc. to place the instrument’s optical center approximately near the center of the open ended shell. Two positions along the interior length of the shell are then selected as target locations. We can assume the typical scenario where, on a three pier kiln, the first 20 meters of shell, including the first tire section, will be replaced. We would then select the center of tires two and three as reference target locations. At each of these reference positions target fixtures are installed. This fixture is usually a robust post welded to the shell, reaching radially inward to the center where a target plate is attached. This requires some refractory removal, perhaps only two bricks at the base of each post.

Also possible, is using a fixture which spans the diameter of the kiln, one leg firmly anchored into the refractory, the other leg firmly held against the refractory by a spring of considerable force. At the center is its target plate. There are certain physical requirements for this target fixture such that it does not sag when suspended horizontally inside the kiln and such that its movement, as a result of kiln shell flexing, is minimized. Two such target fixtures are then placed at the desired locations with the target plate roughly centered in the open area of the interior of the kiln. The advantage of not having to remove any refractory with this type of fixture is offset by the need for having such correctly designed fixtures available. The soundness of the refractory in those locations is then also an issue to be dealt with. In either case the target stands should be in-line along the length of the shell. On a case by case basis, judgement needs to be exercised in deciding on which fixtures to use.

With targets now in place the shell is rotated to a position where the fixed end of the target stands are positioned at approximately 3 o’clock. The scope is then aimed at the farthest target, Fig A. The spot on which it is focused is then marked on that target. Then, sighting the near target, a spot is marked on it. The roughed in position of the alignment scope must be close enough to accomplish this.



The kiln is then rotated so the fixed ends of the stands are approximately in the 6 o’clock position. Without having adjusted the alignment scope in any way, a second mark is put on each target, Fig B. Again the kiln is rotated to a position where the fixed ends of the stands are now in the 9 o’clock position. Still without any adjustment to the scope a third point is marked on each target, Fig C. Naturally the rough adjustment of the position of the scope must be such that all six points land in the target areas. The accuracy of the results does not depend on the exact amount of rotation.

The similarity with the Direct Method is now evident. Having three points, with simple geometry, defines the center of rotation, Fig E. The two centers of rotation so derived, one on each target, now define the axis of rotation of the shell, Fig D. As confirmation, an additional rotation to repeat the procedure, can be made.

Given that the sighting scope is mounted with horizontal and vertical adjustment means, it should now be repositioned so that its line of sight is coincident with these two center points. With the instrument so positioned, monuments can then be set at convenient places outside the kiln so that the axis of shell rotation can be re-established at any time thereafter without having to re-introduce targets inside the kiln. This can have a variety of immediate or future uses. This last step is best accomplished using a total integrated station rather than a simple theodolite. If the external monuments are prisms, conventional re-sectioning and other standard surveying procedures can be used to accurately relocate the instrument in the future. For best results the instrument positionand the reference targets should be well barricaded so that they are never disturbed throughout the course of the kiln work at hand.

Limitations

In using this technique one must be aware of the significant effect of shell flexing and its resulting change of shape with rotation. This is similar to the change of shape of a car tire. It always has a flat spot where it contacts the road, even while underway. The change of shape of the kiln shell is more complex than that of a car tire, it having flat areas at the top, and to a lesser degree, in the vicinity of the rollers. The fixed end of the fixture should therefore not be positioned in those areas during the procedure. For that consideration the clock positions for the target stands as described above are chosen. However, when a tire is known to be abnormally loose on the shell, better results would be obtained if the upper locations are dropped below the 3 and 9 o’clock positions approaching 4 and 8 o’clock respectively.

When using a visible laser to strike the centerline another apparent limitation is beam divergence. Even with instruments that can focus the laser at set distances, divergence is unacceptable at 100+ meters. Some simple steps can be taken to restore its resolution to about 1 mm diameter even at these distances.

Conclusion

A very precise axis of rotation can be obtained in this way using simple, yet well established techniques, by removing little if any refractory and without any measurements to the kiln’s shell. The axis so established can now be used to position and align a new section of shell without further turning of the kiln.

Although the technique is simple in principle, its successful application is dependent on appropriate fixturing and suitable means for instrument installation / manipulation. None of these are mysterious but are usually not part of a plants normal maintenance facility. The specialist who performs these tasks on a routine basis, who has the fixtures available, is usually the most cost effective choice.

To conclude, as stated at the outset, external alignment procedures, based on finding centers of rotation, are superior to internal methods. This is also true for most cases of shell section replacement work. But when it becomes impractical to turn the shell for alignment trials, the advantages of using bore sight alignment is a formidable alternative.

1. ZKG International Nr. 11/1995 "The Direct Method for Aligning Rotary Kilns" (Direktverfahren zur Ausrichtung von Drehrohröfen), by W.M. Gebhart, Phillips Kiln Services Ltd.
2. Fuller Company "Recommended Procedures for Mechanical Analysis of Rotary Kilns" 1985, by R.P. Chapman, chapter G ‘Internal Alignment Test Procedure’


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