The continuity of operation of a lime sludge kiln requires strict maintenance control. The rotary kiln is among the largest type of moving machines made and is subjected to extreme temperatures, power failures, atmospheric conditions, varying loads, and other operating conditions which affect its wear and alignment. It should be erected under the supervision of an experienced erecting engineer.
Even though great care has been taken in the design and the construction of the concrete piers, in some cases settling or tipping of the foundation can occur, throwing the carrying mechanisms, bases, and rollers out of alignment. If this condition is not corrected, it will lead to continual trouble with the kiln shell and the riding rings and rollers. It is advisable when making the original installation of the kiln to establish bench marks away from the kiln foundation. The kiln alignment should then be checked from the bench marks within six months after initial installation, and annually thereafter.
The thrust rollers are designed to carry the full downhill thrust of the kiln, with the riding ring and roller faces lined up for full bearing across the width. Carrying rollers should be set parallel to the axis of the kiln or cut slightly to avoid excessive downhill thrust, which might be detrimental to the thrust roller.
In aligning the kiln by adjusting or cutting the rollers, it is necessary to cut all rollers equally rather than taking all of the cut on one set of rollers. If only one set of rollers is cut to move the kiln in a given direction, and another set of rollers cut to move the kiln in the opposite direction, such action, if continued, would cause various sets of mechanism rollers to work against each other. If this action is carried to extremes, misalignment could occur between any one set of rollers and the corresponding riding ring, resulting in unequal and aggravated wear between the riding ring and roller surfaces. Operators should be discouraged from adjusting only the most easily accessible rollers, which would be on the discharge end mechanisms. A record of all roller adjustments should be kept as an aid to maintaining proper alignment.
When a kiln with a hot charge is stopped for any reason, such as a power failure, it is imperative to keep the kiln rotating with the auxiliary drive. Failure to do so may result in a warped or distorted kiln shell. It is difficult to return a warped shell to its original condition, and operating a kiln with a bowed or warped condition will place an excessive load on various mechanism piers. This is particularly troublesome in multiple support kilns.
Sometimes a warped kiln can be returned to somewhat its original alignment by carefully re-heating the kiln on the side opposite from the warp to draw it back in line. Even at best, though, constant attention must then be given to the carrying mechanisms to provide an alignment which will not cause additional damage or excessive wear.
Sometimes the only way in which to correct a warped or bowed condition or misaligned shell is to cut out a portion of the shell, realign the ridng rings and carrying rollers, and weld the shell section back in. This might result in a slightly disjointed shell, but the items of major importance, namely the riding rings and rollers, are then realigned.
Some kilns are installed without auxiliary drives. This is false economy, since the small additional cost of the auxiliary drive in the initial installation provides good insurance against much more serious difficulties.
The main gear, usually a spur gear, is made in halves with full machined teeth to permit reversing of the gear to obtain a double life. This gear is bolted to a gear flange which is welded to the kiln shell. The driving pinion is mounted on a jackshaft which is coupled to the low speed shaft of an enclosed gear reducer. The gear reducer and jackshaft assembly is fixed to the foundation on the same slope as the kiln, and is provided with adjusting bolts and lugs on the base plate to provide for alignment of the drive. The driving motor is usually connected to the high speed shaft of the gear reducer through a multiple V-belt drive. The motor is also mounted on the same slope as the kiln. Ball bearing motors should always be used, since oil will run out of the bearings on a sleeve bearing motor.
The main gear and pinion should be maintained in proper mesh. Improper meshing of the teeth results in a jerky or vibrating motion of the kiln. Too small a clearance will cause bottoming of the main gear on the base of the pinion teeth. Proper adjustment of the carrying rollers to compensate for the wear on the tires and rollers should prevent this condition.
If a minimum adverse clearance is allowed to continue with a resulting scoring of gear teeth and peening of pinion teeth, it will be necessary to reverse the gear and pinion before such action is normally necessary and then reset the drive accordingly.
Large, slow moving equipment such as rotary kilns will have a low natural frequency of vibration which in some cases could coincide with a kiln speed. Were this to occur, there would be a pronounced vibration of the kiln on the supporting rollers, and knocking and pounding in the main gear and the gear reducer. If such a condition were allowed to continue, the foundations and the kiln could be severely damaged.
The design of the kiln installation insures a natural frequency of vibration well out of the range of recommended operating speeds. Consequently, the kiln speed should never be changed without first investigating the effect which the increased speed might have on the vibrational characteristics of the kiln.
Confirm the original centerline of carrying mechanism bases. To do this, establish an offset centerline, preferably with piano wire, along the side of the kiln from the first to the last support where visibility is unobstructed, as shown in Figure 1. This offset centerline should be equidistant from the centerline marks on the carrying mechanism bases at the two extreme supports of the kiln. By tramming from this offset centerline, determine if the centerline marks on the carrying mechanism bases are all in line.
If the intermediate piers are not in line, determine whether they or the end piers have settled before proceeding with the alignment work. Some changes may be required in order to bring either the bases or piers into line, depending on whether or not the settling has reached its final stages. If no further settling is anticipated, or in cases where no settling has taken place, the offset centerline should be permanently located by setting lead or brass markers into the piers or floor. The true centerline should be clearly marked on each carrying mechanism base by tramming from the offset centerline.
Check the setting of the kiln shell in relation to the true centerline of the bases. This can be done by stringing a cord with a plumb bob attached to each end over the top of the shell as near the riding rings as possible. In cases where there are irregularities in the shell, the cord should be strung over the wearing faces of the riding rings. This cord must be long enough to permit the plumb bobs to hang free beneath the kiln, as shown in Figure 1. The midpoint of the distance between these plumb bobs is the center of the kiln shell at that position. Mark this point on the bases and proceed with the same operation at the next support. Rotate the shell 90 degrees and repeat the markings at each support. The mean of the four marks at each quarter point through a complete revolution will then indicate the path from support to support of the true centerline of the kiln shell.
If condition (2) applies, the carrying rollers must be adjusted so that the true centerline of the kiln shell is made to coincide with the true centerline of the bases. The carrying rollers must be kept parallel to the centerline of the kiln shell and carrying mechanism bases, as shown in Figure 1. In making these adjustments care must be taken to maintain the proper clearance at the feed end and discharge end air seals and at the main drive gear and pinion.
This check should be made when the kiln is shut down and at a time when the shell is not distorted by either radiant heat from adjacent kilns or the sun. Tape the circumference of each riding ring so the distance from the outside diameter of the riding ring to the center of the kiln can be determined for each ring. Set up a transit or level on top of the kiln over the feed end riding ring. Adjust transit so the line of sight is parallel to centerline of the kiln at the feed end and discharge end riding rings. Check the distance from the riding ring to line of sight for each intermediate ring, repeating this check at quarter points around the circumference. Knowing the radius of each riding ring, the average misalignment at each mechanism can now be determined. Correct this misalignment by making the necessary adjustments to the carrying rollers. The slope of the kiln can be checked with the transit at this time.
With the kiln now in correct vertical alignment, a simple gauge can be constructed to check vertical alignment at each mechanism without having to shut down the kiln and go through the elaborate measurements outlined above. This consists of a gauge pin or tram just long enough to reach from the mark of the true centerline on the carrying mechanism frame to kiln shell. A pin should be made for each mechanism frame. Gauging the distance between the frames and the shell will indicate the extent of wear on the rollers and riding rings. The carrying rollers can then be moved in to return the kiln to its original elevation.
Since the shell may not be perfectly round at the planes where the measurements are taken, reference points should be established on the shell so that the vertical distances between the mechanism bases and the shell will always be gauged at the same points on the circumference of the shell. Reference points can be made by welding four -in. nuts to the shell in each plane where measurements are to be taken. These nuts should be spaced 90 degrees apart. A bolt can then be turned into each nut to a point where the head of the bolt will just touch the gauge pin. The bolt is then welded to the nut. Four of these bolts are used at each mechanism to provide an average reading.
The floating type riding rings should not wobble as the kiln rotates, since it is impossible to obtain full contact between the carrying rollers and riding ring under such conditions. Figure 2 shows a method for determining the amount of runout in a ring.
Two pointers are constructed of angle iron and placed as shown. By using two pointers the effect of any kiln float is eliminated. These pointers should be mounted on the kiln pier away from the carrying mechanism and should extend to the centerline of the kiln. The edge of each pointer should be approximately one inch from the machined outer edge of the riding ring.
Measurements are taken from reference marks on the pointers to the machined sides of the riding ring. A set of readings taken at 16 equally spaced points around the circumference of the ring will indicate the location and magnitude of maximum runout. If the runout at any location exceeds it must be corrected by relocating the retaining bands and riding rings.
When rollers are set parallel to the centerline of the kiln, the roller shafts should bear against the downhill bearing caps. This can be checked by tapping the bearing caps with a hammer. The loaded caps will emit a solid sound.
Check each downhill bearing cap to make certain there is no excessive downhill thrust on any cap. This is done by cutting the roller to just relieve the pressure of the roller shaft against the cap. Note how much the adjusting screw was turned. Then return the roller to a setting which will just produce a light roller shaft force against the downhill bearing cap. After all the rollers are adjusted the kiln will bear against the lower thrust roller. Care must be taken to avoid excessive thrust roller loading.
Sometimes, when starting up a new kiln with the carrying rollers set parallel to the axis of the kiln, the carrying rollers and riding rings will not make 100% contact throughout the complete revolution of the kiln. In such cases it is better to let the rolling surfaces wear in to obtain full contact rather than to adjust the rollers to obtain full contact immediately.
The thrust mechanisms on modern kilns are designed to carry the full thrust of the kiln. On many older kilns, however, it is necessary to carry much of the thrust by adjusting the carrying rollers, since the thrust mechanisms were not designed to take the full thrust load. To float the shell of such a kiln, thus reducing or even eliminatingcompletely the thrust on the downhill thrust roller, the carrying rollers are set at an angle as shown in Figure 3. The illustrations exaggerate the amount of adjustment to show the principle involved more clearly. Any such adjustments must be performed carefully with each roller to avoid excessive pressure with resultant wear from developing on any one roller.
The main gear is equipped with adjusting screws to facilitate centering of the gear on the shell. This gear must run true. Several points around the circumference of the gear should be checked with a feeler gauge for uniform contact across the full face of the teeth. Flange bolts should be inspected periodically to be sure they are tight at all times. A tight fit can be achieved by pulling each bolt up tightly, then heating the bolt to about 350 F and advancing the nut an additional 20 degrees.
The pinion and gear must mesh properly, as shown in Figure 5. Pitch lines are scribed on both sides of the gear and pinion at the factory. The pinion should be set so the pitch lines on the gear and pinion are 1/16 in. apart when the shell is cold. When the shell is hot the pitch lines should be from 0 to 1/16 in. apart. In no case should the pitch lines overlap, since this would cause excessive wear and overloading of the pinionshaft and bearings.
The mesh of the gear and pinion should be checked at regular intervals. Any wear or adjustment on the carrying rollers will change the mesh. If the pinion is meshing too deeply, the kiln should be raised to its original position by moving the carrying rollers in toward the centerline of the kiln. It should not be necessary to back the drive out to obtain the correct mesh. While the drive is equipped with adjusting screws, these are mainly for use in the initial alignment of the drive. Sometimes it becomes necessary to back the drive away from the kiln to relieve a serious condition which, if permitted to continue, would result in damage to the gear and pinion. This must be considered only a temporary expedient, and the drive should be returned to its correct location immediately upon returning the shell to its true centerline in the recommended manner.
Sometimes it becomes necessary to reface riding rings or rollers. A grinding rig, shown in Figure 6, can be constructed on the job and used to reface the riding rings while the kiln is in operation. The coil springs and adjusting bolts serve as a stop so that the high spots on the ring will be ground off first. In operating this refacing tool, a reference mark should be established on the side of the riding ring being ground, and the adjusting screw on the tool turned one revolution per revolution of the kiln to assure even grinding across the face of the ring. Any circumferential ridges on the ring must be ground off first.
If it is not feasible to operate the tool throughout a full 24-hour period, it is a simple matter to lower the grinding table on the adjusting bolts, thereby removing the carborundum blocks from contact with the riding ring. This tool can also be adapted for refacing the carrying roller by tipping the rig up on its side and setting it against the face of the roller. This can be more easily accomplished on the outer side of the carrying mechanism due to space limitations, especially at the thrust mechanism. These rollers can also be refaced by using a lathe and cutting tool arrangement, or it may be more expedient and economical to put the kiln on cribbing and reface the rollers in the machine shop. In either case, whether the ring and roller are to wear in over a period of time or whether they are to be refaced, it is essential to watch the following:
Use high grade lubricant with specifications shown in Figure 7. After a new kiln has been in service for one month, drain oil and clean reservoir. Refill with new oil. Thereafter change oil every six months.
Use high grade lubricant with specifications shown in Figure 8. The oil level should be checked at frequent intervals and maintained at the proper level. When starting a new kiln the level should be checked daily. The oil should be changed after the first month of operation and at six-month intervals thereafter.
Floating type riding rings should be lubricated between riding rings and filler bars with a graphited grease. Initially, apply lubricant to the inner surface of the riding ring at all spaces between filler bars. Subsequent applications of lubricant need be made only at four evenly spaced points around the circumference of the ring. The lubricant can be most easily applied with a hand gun with extended nozzle.
Rotary kilns are incredibly diverse in their processing capabilities, a characteristic that has pushed them into a growing number of applications. While rotary kilns are reliable machines, they can experience issues, especially when not properly designed, monitored, or maintained.
Knowing why such issues occur, as well as how to recognize and resolve them is critical to maximizing the longevity of a rotary kiln. And while issues are often specific to the unique parameters of the operation at hand, some of the most common challenges rotary kiln operators face are highlighted here, along with their causes, how to spot them, and potential avenues to resolution. Many of these issues can also be identified through a process or equipment audit.
Slag or dam ring formation in a kiln refers to a buildup around the circumference of the kilns interior in such a way that material is either prevented from passing through, or is significantly inhibited.
The formation of a material ring in the kiln has several implications, including affecting residence time and causing product quality issues, creating problems with material buildup in the feed-end seal, reducing throughput, and promoting material backup in the kiln. It can also significantly reduce throughput. Further, if the ring (or a portion of the ring) breaks free, it has the potential to completely block the kiln outlet, resulting in more severe problems.
The slagging temperature is the point at which the material will fuse together, allowing it to solidify. If a change in feed composition that reduces the slagging temperature is allowed to occur, ring formation will result.
The slag ring can either be manually removed, or dissolved by increasing the operating temperature of the system. If utilizing the temperature adjustment approach, once the ring has been broken down, the temperature can again be lowered to below the point at which slag can form.
To prevent additional slagging in the future, the combustion chamber thermocouples and monitoring systems should be checked to ensure they are functioning properly for adequate temperature monitoring. Feed material specifications should also be compared to original process parameters to ensure a change in feedstock is not to blame.
Material sticking in the kiln is a problem for a number of reasons. As with ring formation, it restricts the flow of material through the kiln, ultimately disrupting the loading and throughput capacity. It also has the potential to break loose and block the kiln discharge, as well as cause damage to the refractory.
Unfortunately, sticking can be caused by almost innumerable issues, making the source of it difficult to pinpoint. It is typically specific to how the materials unique chemical composition and physical characteristics interact with heat. Material may also become more sticky as it moves through the chemical or physical changes induced by the kiln.
For these reasons, rotary kiln testing is a critical part of process development; testing helps to identify such problems prior to production, allowing the kiln to be engineered to work around the material characteristics.
Some materials are likely to stick only between given temperature and moisture ranges. As such, by increasing the kiln temperature, the material may become less sticky, or could reduce in moisture enough to move out of the range at which it is likely to stick.
Knockers are a device used to knock the shells exterior in order to dislodge any potential buildup. They are available in different designs depending on the needs of the project. A wear plate on the kilns exterior provides protection to the shell while still allowing the energy from the knock to dislodge any buildup.
As mentioned, batch and pilot testing such as that carried out in the FEECO Innovation Center are crucial to developing an efficient and reliable high-temperature thermal process. When working with an existing kiln system, testing can be used to troubleshoot and resolve sticking or other performance inhibitors in the kiln.
Refractory protects the kiln shell from the high temperatures passing through the unit. When refractory is allowed to fail, the kiln shell can become damaged, even requiring total replacement in extreme cases. Further, frequent failures significantly increase downtime, adversely affecting processing capacity; at a minimum, refractory repair takes three to five days to correct a minor problem. In most applications, plant managers should plan on replacing up to 5% of the refractory system as a result of normal wear and tear on an annual basis.
While refractory is less commonly employed in indirect kilns (calciners), when it is used in these applications, a failure has the potential to reduce the thickness of the kiln shell, resulting in deflection, ovality, or even cracking.
The most common cause of refractory wear is the amount of cycling that the kiln undergoes. Cycling is the shutdown and startup of the kiln. This heating up and cooling down puts stress on the refractory, and if carried out frequently, substantially increases wear and decreases the life of the refractory.
Not far behind cycling, is improper material selection. The chemical and physical composition of refractory must take into account the material to be processed, as well as the specific operating parameters of the application.
Unfortunately there is no simple fix for resolving refractory wear. Refractory can be patched or repaired, but the underlying cause will continue to promote wear. As such, it is essential to ensure chemical and physical compatibility of the material and process parameters with the refractory, while also minimizing cycling as much as possible.
Misalignment is another commonly encountered issue when working with kilns. Proper alignment of the kiln affects every mechanical component of the unit; when a kiln falls out of alignment, it often results in premature wear on tires, trunnions, and thrust rollers, and puts added strain on other mechanical components.
Additionally, misalignment has the potential to impact the effectiveness of the seals. Because rotary kilns are a carefully controlled processing atmosphere, a reduction in seal effectiveness can change process conditions, causing process upset or under-processed material.
All rotary kilns gradually become misaligned over time as a result of normal operation, meaning kilns should be routinely realigned as part of a preventative maintenance program. Its important to recognize that any significant maintenance or repairs can also affect kiln alignment, so kilns should always be realigned after any major repair work.
Several factors can expedite the path to misalignment. This might include sinking foundations or changes in the structural integrity of the system, fugitive material allowed to build up on components, and even humidity, among other things.
Misalignment can present itself in a number of different ways, but is a common underlying problem of other issues. Signs of a misaligned kiln often include wear on load-bearing surfaces after resurfacing, audible vibrations or chattering, or damage to any of the following: tires/riding rings, thrust rollers, trunnion wheels, pinion/girth gear.
While misalignment is easily resolved, its essential to recognize that if the root cause of misalignment is not addressed, the kiln will immediately begin to fall out of alignment again. A full rotary kiln inspection should be conducted to determine the cause of the issue and assess any resulting damage.
Carryover is the entrainment of material in the exhaust gas system. While most kilns experience some level of carryover, an excessive amount of carryover is highly problematic. Carryover puts added stress on downstream components such as air pollution control equipment and often results in a reduction in processing capacity.
Since carryover is the picking up of material in the exhaust gas stream, the causes of excess carryover are fairly limited. A change in the amount of carryover is typically a result of a change in the bulk density or particle size of the material being processed, allowing it to be more easily picked up in the air flow. Similarly, an increase in the gas velocity could allow the gas to pick up particles that were previously too heavy to pick up.
Excess carryover can be discovered through a variety of indicators. A lower product output from the kiln could signal that more material is exiting with the exhaust gas. Similarly, an increase in the amount of material in the air pollution control equipment also points to increased entrainment.
Resolving excessive carryover in a kiln most often centers around adjusting either the feed material parameters (bulk density or particle size), or making changes to the gas velocity to prevent entrainment.
The capital expenditure that a rotary kiln represents demands that the investment be protected through proactive maintenance. For this reason, in addition to routine maintenance and walk-throughs, FEECO recommends a minimum of annual inspections be conducted by a qualified service provider.
Those experiencing process issues or inefficiencies may also benefit from a rotary kiln process audit, which looks at the system as a whole, along with process parameters, to identify any issues or opportunities for improvement.
Rotary kilns provide an efficient and reliable approach to a wide range of thermal processing applications. Recognizing where potential for issues exists, as well as how to identify and correct these issues, is essential to maximizing the life of the unit. Process and equipment audits, along with annual inspections, are powerful tools in a preventative maintenance program and can also help to improve kiln process efficiency.
FEECO Customer Service Engineers are highly skilled in the mechanical and operational aspects of rotary kilns and in addition to identifying issues, can also help to carry out repairs such as realignment, tire and trunnion grinding, retrofitting solutions, and so much more. For more information on our extensive rotary kiln parts and service support, contact us today!