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Kiln maintenance: unnecessary expense or profit generator?

Kiln Maintenance: unnecessary expense or profit generator?

It may be argued that the expense of modern kiln maintenance procedures is quickly returned in profit increases

Rising energy costs, increasing labor costs, and lost production through equipment failure are serious problems that cement manufacturers are facing as they struggle to survive in today's competitive markets. These three maintenance-related areas seem to be some of the few avenues open to control the upward spiral of today's operating costs. Efficiently planned and executed maintenance schedules cannot only reduce operating costs but can also directly influence profit margins.

However, many cement plants are reluctant to spend money on kiln maintenance. The issue that faces today's plant manager is this: Is planned kiln maintenance just an operating expense that can be postponed if the budget is tight or should it be perceived as an investment that can increase the profitability of a company?

This article will illustrate how some cement plants have used two basic kiln maintenance procedures to decrease operating costs and reduce energy usage, while increasing production levels and profit margins. These kiln maintenance procedures are tyre and support roller resurfacing and in-production mechanical analysis ('hot kiln' alignments). Before describing three case studies that illustrate how these procedures impact profitability, some basic kiln-related terminology and concepts will be discussed.

Concave and convex wear

Concave and convex wear is a common occurrence on the surfaces of tyres and support rollers. As pictured in Figure 1, concave wear is typical on the support rollers and convex wear is more prevalent on the tyre. The wear develops when support rollers are over-skewed to control the axial thrust of the kiln. The severity of wear is proportional to kiln loads and environmental contamination near and around the rolling surfaces. Severe cases of convex and concave wear can develop which will significantly restrict the travel of the tyre as the kiln changes its direction of axial thrust. When certain conditions exist, it is possible to damage thrust collars, thrust plates and other related components of the bearing housing assemblies. Since the actual load surface of the tyre and rollers is reduced, wear will accelerate and greater bearing adjustments will be necessary to control the axial thrust of the kiln. This will result in a significant frictional drag between the bearings and the roller shaft that will increase loading on the kiln drive system.

Uneven wear

As shown in Figure 2, uneven wear is usually caused by product or dust contamination passing through the contact point of the tyres and support rollers. This type of wear can also develop when rollers run in product build-up in the pits of the support roller structural bases. Severe uneven wear causes a poor transition surface for the tyre and support rollers which will hinder the axial movement of the tyre. This wear condition generates a frictional drag on the kiln drive system and can cause high contact pressures on the rolling surfaces if sufficient wear exists.

Tapered or conical wear

Radial taper on the surfaces of the tyres and/or the support rollers is one of the most difficult types of wear to detect. One noticeable indication of this wear condition is the axial loading of the tyre against the retaining bands/blocks. Tapered wear is an undesirable condition because the tyre will move toward the loaded side of the contact surface. Figure 3 shows an example of conical wear. A taper on the tyre or roller will generate wear on the support pads and tyre ID and will make support roller adjustments difficult. The wear on the support pads and tyre ID will accentuate the gap between tyre and shell, subsequently increasing the percentage of shell ovality. When this wear condition exists, there is often a significant amount of roller misadjustment that will increase the frictional drag on the kiln drive system. The inclination of plant personnel is to adjust rollers to close the gaps between tyres and rollers. This method of adjustment will create a misalignment of the roller shafts with the tyre axis generating unequal axial thrust.

Diagonal marks

Diagonal marks are evident when there is incorrect adjustment between the support rollers and the tyre. The support rollers are adjusted so the axis of rotation on each roller is not parallel. Consequently, the tyre does not have a smooth rolling surface and frictional drag results, generating tapered wear on the surfaces of the tyre and support rollers. This condition creates a high torsional load on the shell that hinders the smooth operation of the kiln drive system.

Horizontal marks

Like diagonal marks, horizontal marks develop when the support rollers are incorrectly adjusted and/or misalignment of the drive gear and pinion is present, Figure 6. Although this condition is more common on two-pier higher speed kilns, it can also exist on the large, slower kilns. Sometimes the misadjustment does not occur between two rollers on the same pier. Misadjustment can be present when rollers on adjacent piers are adjusted to thrust in opposite directions. If this condition is present it will cause rough operation of the drive gear and pinion. Additionally, the torsional loading on the kiln will increase, resulting in a drag in the kiln drive system. The wear conditions are generated by many different circumstances which can not be covered at length in this article. Individually or collectively, these wear patterns will influence the overall efficiency of the kiln drive system and will require a greater allocation of maintenance expenditure and plant labor to effectively operate the kiln.

Tire and support roller resurfacing

Tyre and support roller resurfacing is the process of removing various types of surface wear. The actual grinding process is performed in-place with no production downtime so the work does not interrupt the normal operation of the kiln. As the wear is removed, adjustments are made to the support rollers to locate the roller shafts as close as possible to a neutral thrust position. The removal of this wear and the balancing of the support roller thrust will notably diminish the frictional drag placed on the kiln drive system, in addition to stabilizing the axial thrust of the kiln.

Hot kiln alignment

The method of alignment employed on a kiln, must achieve two fundamental principles. Accurate measurements must be taken when the kiln is in operation to determine pier loading. The operating axis of the kiln shell must be identified and defined in relationship to the slope of the support rollers. A straight shell axis during normal kiln operation will reduce the frictional drag on the drive system, decrease the shell deflection or ovality due to excessive pier loading, and greatly reduce the adjustments needed to control the axial thrust of the kiln. If these three improvements are realized, kiln drive amperage will be reduced, the likelihood of refractory failure will be diminished, and kiln maintenance costs will decrease. Two common kiln alignment variances can affect the straightness and stability of the shell, as well as its operational efficiency.

Incorrect elevation axis of the kiln

An incorrect elevation of the kiln axis exists when the kiln shell does not have an axis of rotation that is consistent with the slope of the structural steel bases, as illustrated in Figure 9. Shell slope will not match tyre and roller slope and high axial thrust will be present on the related piers. The tyre will have high axial loads against the retaining blocks/bands if this condition exists. Higher loads will be placed on the bearings which will create higher Hertz pressures on the roller shafts and the bearing liners. Frictional drag will eventually occur between the tyres and rollers causing torsional loading of the shell between the respective piers. Inevitably, the torsional drag will cause a significant increase in the amperage needed to operate the kiln.

Misalignment of the kiln in elevation is common because of the thermal expansion of the kiln. If the kiln is aligned while it is out of production and cold, consideration must be taken to position the kiln where the axis will be after thermal expansion of the shell, tyre and rollers. This position is difficult to determine because the shell is not uniform in temperature (it is higher in the burning zone than at the feed end). Brick thickness and environmental conditions can make it nearly impossible to prevent elevation misalignment of the kiln shell unless that alignment is performed while the kiln is operating. The elevation alignment affects the kiln drive amperage. Contrasting loads will be placed on the individual support rollers. It is evident in Figure 9 that there will be more loading of the #1 and #4 piers support rollers. Since these support rollers will carry more than their share of the design loads, the Hertz pressures will be higher between the roller shafts and the bearing liners. Consequently, a frictional drag will result between the bearing liners and the support roller shafts, initiating a load on the kiln drive system that will result in higher energy consumption to rotate the kiln. Furthermore, shell ovality will be higher on the loaded piers and for this reason, measuring shell ovality is a necessary function of a hot alignment, (Figure 10, item 2).

Incorrect plan view shell axis

An incorrect plan view axis of the kiln shell will result in loading problems similar to those found with elevation misalignment, with one exception. The vertical slope is not a consideration in the plan view. However, it is interesting to note that the elevation misalignment will influence or heighten the reaction caused by a plan view deviation from the design axis of the kiln. A deviation of the shell axis in the plan view will result in a reaction even if the elevation axis of the shell is correct.

With a plan view misalignment, the axis of the roller shafts will be affected by the plan view axis of the kiln shell. Figure 11 relates the effect of the kiln axis in the plan-view plane as it compares to the axes of the respective roller shafts. This diagram shows a kiln shell with a distinct curve to one side. Notice how the axis of the shell relates to the axis of the respective support rollers. Ovality readings will also demonstrate that one roller on this pier is taking more load than the opposite roller, (Figure 10, item 6). If the discharge end support rollers are adjusted to have full face contact with the tyre, the roller axis will correspond to the axis of the shell. The bearing on the feed side will have a shorter distance to travel to reach the desired axis than the bearings on the downhill side of the tyre. For this reason, it is best to swap the position of each roller shaft intermittently when making alignment moves on the support rollers. Shell alignment as outlined here will cause an unstable condition with the axial thrust of the unit, and again generate greater energy requirements to rotate the kiln.

In addition to the apparent problems created when elevation and plan-view shell misalignments place torsional load on the shell of the kiln, an additional concern is the effect of the torsion on the brick or refractory. The resistance of the kiln shell to rotate because of the tyre and roller wear and a shell misalignment condition puts the shell in high torsional stress, creating an unstable environment for the bricks. Since the installation of refractory brick does not allow for torsional movement, any significant amount of loading may result in premature refractory brick failure. When examining various conditions that will generate a torsional loading of the kiln shell and reduce the energy efficiency of the kiln drive system, it is evident that the following considerations affect the operating kiln.

Wear on the surfaces of the tyres and support rollers will generate high hertz pressures on the contact surfaces, induce frictional drag between the roller shafts and bearing liners and make it difficult to control the axial thrust of the kiln with minor roller adjustments. Kiln misalignment will place a bind on the kiln shell and higher loads on the respective support piers. Minor support roller adjustments will be difficult to perform. Because of component wear and the kiln shell misalignment, the torsional loading of the shell will decrease the energy efficiency of the drive system and generate wear on the drive components. Since the above conditions affect the ability to perform minor support roller adjustments for controlling the axial thrust of the kiln, the rollers will probably be misadjusted. This will create additional wear on the component parts and the amperage required to rotate the kiln will increase. It is important to note that as the energy increases to turn the kiln, the additional energy to overcome the frictional drag creates additional wear on the component parts of the drive system, the tyres, and support rollers. Furthermore, productivity will decline and maintenance requirements of the kiln will increase.

To achieve maximum efficiency of kiln production, it is evident that a hot kiln alignment should be performed in conjunction with the tyre and roller resurfacing. The following case studies of three manufacturing plants substantiate the effectiveness of these two maintenance procedures in increasing productivity and operating efficiency. These advantages are enhanced when one notes that both the resurfacing and alignment procedures can be achieved with no production downtime.
Resurfacing and Alignment case studies

Case One: A 6-pier, Allis-Chalmers wet-process cement kiln in the Western United States was not operating to design specifications or production standards. Amperage readings were high and at many times unstable. The refractory brick life of the kiln was unsatisfactory and the brick would seldom last through an annual campaign. Plant personnel were constantly called to make roller adjustments to control the axial thrust of the kiln. The kiln was designed with hydraulic thrust rollers on the downhill side of the middle piers. As the kiln moved uphill, production was halted until adjustments could be made to move the kiln downhill. Production was lost because of frequent kiln outages and maintenance costs were magnified. This was a distressing circumstance as the plant was sold-out and thus sales from lost production could not be regained. In simple terms, they were producing less clinker at a greater operating cost and losing profits from the lost sales.

An inspection of the unit indicated misalignment of the kiln axis and a significant amount of wear on the tyre and support roller surfaces. Further examination revealed that support rollers were adjusted sporadically throughout the length of the kiln shell so that some rollers were pulling the kiln uphill and the remaining rollers were pushing the kiln downhill. The recommended course of action was to repair the worn surfaces of the tyres and support rollers with the resurfacing procedure followed by an in-production mechanical analysis or 'hot kiln' alignment.

The resurfacing procedure was performed in conjunction with support roller adjustments to eliminate the volatile thrust condition of the kiln. Adjustments were made to each individual support roller to locate the neutral position of the roller shafts. After these adjustments were achieved, small incremental moves of the bearing housings were made to control the axial thrust of the kiln. After the wear conditions on the tyre and roller surfaces were eliminated and the axial thrust of the kiln was stabilized, the frictional drag on the component parts was significantly diminished.

The next phase of the project was to perform the mechanical analysis or 'hot kiln' alignment. The goal of the mechanical analysis was to identify the reason for the inferior brick life and to determine the exact location of the kiln shell operational axis of rotation. Ovality readings were performed and a complete survey of the elevation and plan view position of the kiln shell axis was computer analyzed. Structural steel support bases were measured for actual slope and compared with the actual slope of the support roller shafts. The kiln was found to be out-of-position in the plan view significantly on the discharge pier. In addition, some other alignment discrepancies were apparent.

Arrangements were made to perform support roller adjustments to position the kiln shell at the correct operational axis. The adjustments were made over a period of several weeks to maintain the delicate axial thrust of the kiln and to eliminate any production delays as a result of the process. Both the resurfacing and alignment procedures were completed without any delays in production. Maintenance department participation was held to a minimum.

The results of the combined procedures were excellent. The production of the kiln increased by 25% immediately after the work. Kiln drive amperage has remained an average of 15% lower since the project was completed. The life of the refractory brick increased allowing a full year's service without kiln-related brick failure. The overall plant efficiency increased by 5%. Support roller adjustments to control the axial thrust of the kiln were virtually eliminated. Effectively, the profits of the plant increased significantly.

Case Two: A 4-pier, F.L Smidth kiln at a plant in Quebec, Canada. Similar misalignment and wear circumstances existed which exhibited operational problems. In this plant there were three other kilns that had similar operating problems as the subject kiln. Control of the axial thrust was difficult, requiring an excessive amount of maintenance manpower. Brick life was inadequate and production output was reduced. All of the product produced at this plant was sold in advance so that any production down time associated with these maintenance problems resulted in lost profits.

After the resurfacing and 'hot kiln' alignment procedures were performed, the drive amps on the subject kiln dropped from 190 amps to 150 amps, the daily production increased from 80 tph to 110 tph, and the operating efficiency of the kiln increased to 95%, up from 75%. After realizing these results, plant personnel performed the same maintenance procedures on the three remaining kilns. Similar improvements in amperage, production, and operating efficiency resulted. Company officials saw a phenomenal increase in the operating profits of this plant.

Case Three: A plant in Minnesota, USA, had five large 22 ft. dia., Allis-Chalmers kilns for steel production that had serious operating problems. The drive system had serious frictional drag on the components generating high kiln drive amperages. Numerous hot bearings were causing lost production on a weekly basis, production capacities were unsatisfactory, and plant maintenance on the equipment was extensive. After analyzing the above conditions resurfacing and alignment were recommended and performed.

The results were impressive. The kiln drive amperage dropped from 595 amps to 345 amps on one unit, a 42% decrease. Since the additional amps were being employed in overcoming the frictional drag, a noticeable change was seen in the operating efficiency of the drive system. The hot bearing temperatures on the support rollers were eliminated completely. Consequently, an average of 12 hours per week of lost production on each drum was regained, increasing the productivity of the entire plant. Adjustments to the support rollers were no longer needed and maintenance on the kilns has been reduced to checking the bearings for thrust on a periodic basis. The initial cost of the maintenance work was recovered in a few months of trouble free operation at the design production capacities.

The documented results of the resurfacing and alignment procedures at these three sites clearly speak for themselves. Unfortunately examples can also be cited where the resurfacing and/or alignment procedures were done by in experienced crews and inferior or poorly designed equipment. The success stories like the ones mentioned here all have several things in common:

The technicians performing the work were highly trained and had years of experience in these kiln maintenance techniques.

The equipment used was specially designed and manufactured (and, in these examples, patented).

The procedures used were advanced (also patented), proven reliable, and extremely accurate.

The resurfacing process and the in-production mechanical analysis have been successfully performed in North America for many years. Cement companies in many parts of the world are now utilizing these procedures to assist with the mechanical maintenance of their equipment. Performing these procedures will make kiln maintenance simpler and more effective. That fact makes resurfacing and kiln mechanical analysis worth their cost. These maintenance procedures become more important to cement plants when their effects on company production levels and profit margins are considered.

The issue of whether cement plants consider maintenance an unnecessary, expensive evil or an investment for improving company efficiency and profitability has been the focus of this article. As demonstrated by the aforementioned case examples, resurfacing and in-production mechanical analysis can indeed be used to generate profits through decreased energy consumption, reduced maintenance costs, and increased plant efficiency.

The costs of maintenance procedures are quickly recovered in short term profit increases (with a payback of a few short months in the examples cited). Since the normal rate of kiln wear occurs over a number of years, the long term profitability of kiln maintenance is even more attractive. Equally important, the environment benefits through lower energy consumption and increased efficiency. Resurfacing and in-production mechanical analysis are maintenance procedures that can assist with all these goals.

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