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Kiln alignment and shell ovality: a new approach


Kiln alignment and shell ovality: a new approach

Walter M. Gebhart General Manager, Vice-President of International Sales, Phillips Kiln Service Company Ltd, Sioux City, Iowa, USA.

Introduction

Analysis of kiln alignment and ovality characteristics are important tools which can help to improve or maintain the mechanical stability of a kiln. As cement plants are becoming more maintenance-oriented, the rotary kiln industry has responded with various techniques to determine this information. This report will examine various methods of measuring alignment and ovality and will describe the latest technological developments.

Internal vs. external methods

To overcome this difficulty, both internal and external methods have been developed. However, there are many limitations inherent in both methods. For internal methods to be successful, the kiln must be down. There is often very limited time to take measurements in working conditions that can be very hostile. There is often competition with other activities in and around a kiln during a shutdown period. The kiln must be repositioned at least three times to give some indication of the center of rotation rather than just the geometric center of the shell. An internal alignment analysis can offer accuracy but the data is not sensitive to the kiln's operating conditions and can be very expensive to obtain in terms of lost production. In addition, there is no chance for verification of the data until the next shutdown. Traditional external alignment measurement on an operating kiln offers a different set of problems. The heat and motion of the operating kiln can make difficult working conditions even worse. Because the kiln is in motion, more complex data collection procedures are required. Data collected can be inherently less accurate because assumptions must be made as to the creep or tire shell gap. In many cases, inappropriate measurement procedures and instruments may be used if those performing the analysis are inexperienced. Depending on the methods used, reasonable to questionable results can be obtained at a relatively low cost with no downtime. External alignment methods are sensitive to the kiln's operating conditions and can be verified any time the kiln is operating. Traditional external alignment procedures have various methods of data acquisition ranging from plumb bobs draped over moving tires and piano wire stretched along the roller bases to sophisticated computerized systems and theodolites. All traditional external alignment methods, however, have a common limitation. They all attempt to determine the kiln center based on component geometry (ring and roller diameters, tire/shell gap, and spacing between roller centers) (See Figure 1).

Figure 1


On a moving kiln, it may be very difficult to obtain precise component measurements. Assumptions about the tire/shell gap are often factored into the alignment calculations resulting in potentially inaccurate data. Some external alignment methods use other indirect phenomenon such as ovality as an indication of misalignment. Although ovality curves do show an indication of alignment, it is important to remember that the indication is only qualitative and that damaged shell plate can radically affect the ovality curves as do many other factors. The use of ovality in calculating alignment characteristics is largely a subjective interpretation. Consequently, traditional, internal or external alignment methods present many obstacles including high downtime costs and dubious results. Because of these problems, alignment checks are done infrequently, usually only when kiln damage is evident. The alignment is done after the problem manifests itself rather than as the preventative maintenance tool it could be. Therefore, most kilns run in less than optimum conditions. Accelerated wear, frequent component failure, and high long-term costs of kiln operation have become accepted norms in some industries.

New alignment technology

A new concept in alignment measurement has emerged which eliminates the major limitation of traditional external alignment methods. This procedure, based on new technology, determines the centers of rotation, independent of component geometry. Using linear measurements from an arbitrarily chosen frame of reference outside the kiln, the true center of rotation of the kiln is determined (see Figure 2).

Figure 2


The practical application of this new procedure has many advantages over traditional internal or external alignment methods:
•    The final report is presented to and reviewed with plant personnel before the crew leaves the   job site.
•    The procedure is comparatively simpler, with measurements taken directly on the kiln shell.
•    Diameters, surface conditions, gaps and spacing are not factors in determining the kiln alignment.
•    Electronic measurement allows for self checking data, perpendicular measurements (unaffected by thermal conditions of the kiln), and high frequency measurement rates.
•    Measurements are direct functions of the centers of shell rotation (not the geometric centers) and not related to features such as diameters or ovalities.
•    Statistical determination of the center of rotation of each support is typically based on over 2000 readings which are logged over the course of numerous revolutions.

•    There is no limit of applications. Kilns of all types, speeds and processes can be analyzed with no downtime. The patented alignment procedure, performed exclusively by Phillips Kiln Services, is successfully completed on over 50 kilns per year.

Ovality

In addition to new alignment technology, a breakthrough has recently been made in kiln shell ovality analysis. Although ovality is sometimes influenced by alignment, it is always of interest and concern with respect to the mechanical stability of refractory. Ovality, as it applies to an operating kiln shell, is the change of curvature or flexing of the shell during the course of each revolution.
The first practical device to actually measure ovality was patented in 1953 by E.R.S. Kareby of Stockholm, Sweden, and was assigned to Skanska, AB, also of Stockholm. In the early 1960s, Holderbank designed improvements for the device and made it commercially available as the THS Shell Test Apparatus. It has been very successful and up to now, the standard of the industry. There are some practical limitations to the device, however. If not handled properly, its weight can cause it to fall from the shell and damage the delicate mechanical leverage and tracing mechanisms. Proper attachment to the shell to get the best possible tracing is a challenge especially with larger diameter, fast moving kilns. It is not unusual to reduce the kiln speed to properly place the instrument. The Shell Test Apparatus produces circular chart recordings which resemble the cross section of a kiln shell, creating a visual impression of the degree of ovality. Actual mathematical calculation, however, requires measuring the curves using a simple draftsman's scale. Since the diameter of the graph is only 3 in. and the chart is drawn with a pencil, accuracy is compromised. Therefore, the circular graph cannot offer much detail.

Advances in ovality measurement

The first significant improvement in almost 30 years has been made with the development of the High Resolution Ovality Beam (HR Beam). The leverage and tracing mechanisms of the Shell Test Apparatus were replaced with an electronic unit and data logger which opened a whole new world of detail. The difference in the viewing capabilities of the two machines can be comparable to the naked eye versus a microscope. In addition to the enhanced ovality measuring capabilities, the HR Beam is also able to detect other aspects of kiln shell reactions. The HR Beam easily identifies the following shell abnormalities:
•    Heavily loaded piers or undersized support components
•    Excessive tire/shell gaps
•    Bowed shell
•    Under-loaded piers
•    Misalignment
•    Cracked shell
•    Damaged or distorted shell plate
As with the original Shell Test Apparatus, great care was exercised to design a High Resolution Beam that would be stable with respect to thermal expansion. The thermally stable design of the new beam has an added advantage. Any thermal effects on the beam are clearly reflected in the data, allowing for computer correction to eliminate the temperature gradient in the data and in the resulting ovality curves. The new beam offers high resolution in a more practical device. The replacement of the mechanical leverage and tracing system with an electronic package has resulted in a much lighter and more compact beam which is much easier to use. The time-consuming adjustments necessary with the Shell Test Apparatus (proper pendulum action, pencil position/pressure/sharpness, etc.) are not required with the HR Beam. To operate the HR Beam, just put it on the shell and push a button to start the electronic readings which will be digitally fed into the data logger. The data logger can hold an entire set of data for a kiln and keep it in subsets according to location. Downloading the data to a computer program then allows data processing and graphing. Optionally, a data logger to process data and send the report and graph to a standard printer is also available. Another departure from the Shell Test Apparatus is the appearance of the ovality graphs. The traditional ovality graphing method was on a circular graph or polar plots. The somewhat superficial appearances of a kiln cross-section can actually hide details and cloud the evaluation. The HR Beam charts the ovality data on an X-Y plot (See figure 3).

Figure 3


With a circular graph a change in curvature represents a deviation from the circle. In the X-Y plot, a change in curvature is a deviation from a straight line. The eye is much more adept at judging what is a straight line than what is a true circle. In interpreting an HR Beam ovality curve, it is important to understand the concept of kiln ovality. An ideal kiln shell is one that is 100% rigid, does not flex, and would display no ovality. If measured by an HR Beam, the ideal kiln shell would produce a straight line curve. All kiln tires flex, however, to a varying degree. Since the kiln shell is thinner than the tires, it is similar to a bag of water in a container. There is a tendency for the kiln shell to assume the shape of the tire. As a result, there are three distinct changes of curvature exhibited by a normal and well proportioned kiln shell:
1. Because the tire is supported primarily at the bottom, its own weight will cause it to sit flat. This flattening of the tire makes the horizontal axis a little longer than the vertical axis. As a result, the degree of curvature at the 3 and 9 o'clock positions is greater (more positive on the X-Y plot) compared to the 12 and 6 o'clock positions (See Figure 4(a)).
2. Each tire has two point loads where it sits on the two support rollers. The two point loads work to straighten out the tire. As a result, the degree of curvature decreases from the nominal at each roller. This decrease in curvature shows as a negative dip on the X-Y plot (See Figure 4(b)). 
3. As the shell flexes inside the tire, a gap forms between the shell and tire at the 12 o'clock position. Here the shell becomes more flat than the tire and another decrease in curvature is recorded as a negative dip on the X-Y plot.
Keeping the normal curve pattern in mind, deviations can be easily identified. The stylized curves in Figures 5(a) to 5(f) were taken from actual kiln readings and serve to illustrate the most common types of ovality problems. Although the normal ovality curve is easily identified by shape, it cannot be categorized to an absolute scale. There is, however, an overall ovality range within which curves should fall. This range has been empirically derived over many years and serves as an overall guide for ovality analysis. The considerable spread within that range allows for the fact that all kilns are not created equal nor are the loadings from tire to tire expected to be equal on the same kiln. These detailed aspects of ovality are significant for their qualitative or pictorial value. They serve to highlight and corroborate misalignment and other problems but could be very misleading if not used in conjunction with other data when trying to sort out the many factors affecting them. An example of this is a misaligned roller. The curve would indicate a misalignment but could not by itself be used to determine how much a roller should be moved to gain alignment. To align a kiln to arbitrarily 'balance' ovalities pier to pier, (which is nearly impossible with a bowed shell) ignores a host of other considerations and often creates other problems.

Summary

Ovality, or the degree of change of curvature of the kiln shell during rotation, has always been of interest and concern with respect to the mechanical stability of refractory. The Ovality beam, or 'Shell Test' rig, continues to be a popular mechanical device used to measure the degree of flexing a shell undergoes during normal operation. With the advent of digital technology, the simple pencil tracings produced by the shell test mechanism have now been replaced by a device which takes digital readings whose graphs reveal the microcosm of changes in shell curvature during rotation. For discerning kiln users who operate large and/or fast moving kilns, particularly the cement industry, the ability of this state-of-the-art ovality beam to spot overloaded piers, excessive tire/shell gaps, a bowed shell, under-loaded piers, misalignment, and even shell cracks, this state-of-the-art ovality beam will be of considerable interest.





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