APPLIED THERMAL DESIGNSTulsa, Oklahoma |
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Rotary Kiln refractory failure report I am pleased to submit this report on the inspection and evaluation of the ____ Plant kiln. My first inspection was performed Feb 12 while the kiln was operating. This consisted of observation of the external equipment, visiting with operators, conferences with ___ ____, and examination of the file and drawings. I performed another more thorough internal and external inspection, February 28 through March 1, after additional refractory failure forced a plant shut down. This purpose of this exercise is to eliminate the premature refractory failures in this kiln. In summary, I have concluded that the failures at center of the kiln can be attributed to two facts: * the brick was installed with mortar * there is significant mechanical interference between the metal lifters and the brick. A secondary failure occurred at the discharge end of the kiln. I believe that it was caused by a one or all of the following: * insufficient thermal expansion provisions * the discharge seal ring distorting the shell * high longitudinal thrust. The following discussion outlines my findings and details my logic and conclusions. I have examined the drawings and have done thermal and mechanical design calculations. To date, I have found no steel structural problems with the kiln. The calculated maximum longitudinal deflection at the center of the kiln is .021 inches. The maximum end deflections calculate to be .014 inches. These numbers are well within industry practice. I calculate the shell bending stress at the center of the kiln, between the supports, to be 1,200 psi. The allowable I normally use for 180 degree reverse cyclic loading for mild steel is 8000 psi. The shell thickness required to provide sufficient rigidity for the refractory is of more interest, in rotary kiln design, than the other structural consideration. The shell thickness of 3/4" for a 9' I.D. refractory lined kiln is per current industry practice. Also, measurements were taken to determine the roundness of the kiln shell. These numbers show two sections to be approximately 1" out of round. I would have been more pleased to have seen numbers ranging from 1/4" - 1/2". The tire sections, the most critical area, showed a maximum out of round of only 3/8". The fact that this is a recycling operation, leads to some concern about the acid gases formed during the drying and regen processes. The evidence that I have seen to date does not indicate the presence of much acid. I have been told that the analysis of the feed shows up to 1% chlorine and sodium with traces of various heavy metals and organics. The feed is checked for acidity and neutralized before being feed into the kiln. Visual inspection of the kiln feed hood does not show the corrosion I would expect if significant chlorine were present. I suggest that ultrasound tests be done on the kiln, feed hood, discharge hood, and the duct between the feed hood and Secondary Combustion Chamber. This will tell us the existing material thickness which we can compare with the original thickness and determine how much, if any, corrosion has occurred. There does appear some corrosion of the feed end kiln seals. One of the leaves should be removed and sent to Metlab for testing. My examination of the second refractory failure area and the kiln drawings leads me to believe the failures in that can be attributed to a combination of improper brick installation and incorrect lifter/refractory interface design. Apparently, mortar was used to install the brick liner. In my experience (and in the experience of the major refractory installers I talked to), mortar is not used when installing brick in rotary kilns. The mortar is sensitive to chemical attack and fails, letting the brick become loose. Even if chemical attack is not a problem, the mortar is not as strong as the brick, so it fails mechanically. The normal flexing of the shell of a kiln as it rotates breaks the mortar joint. The flexing then causes the brick joint to work, which grinds away the mortar. Finally enough mortar is ground away to allow a key brick to fall out, which in turn allows the whole ring to fail. Generally when one ring fails the remainder quickly follow. If each brick had a 1/32" mortar joint, the 38 brick ring would have a total of 1.19" of mortar. The 9" RKB for the 9'0" I.D. kiln only has 1" of taper. The key brick can have as little as a 4-1/2" chord with only a 1/2" of taper. The reason the failures to date have been confined to one area is because the lifters are holding some of the brick in place. As was previously reported to me, I found that thick (3/8") carbon steel shims were used in several places to tighten the RKB liner. If this is true that may have also contributed to the failures. Thick shims are a symptom of incorrect liner installation. In addition, the logic of using carbon steel inside a refractory lined vessel escapes me. Carbon steel oxidizes rapidly at temperatures over 900 F. If shimming is required it should be done with thin SS shims (1/16" max) and spread over several courses. Shims interfere with the ability of brick to form an arch and cause pinch spalling because the inside is tighter than the outside. Thick shims accentuate the problem. Concern has been expressed that the liner may not have been properly keyed. It is not physically possible to verify the key shape without removing several liner rings, therefore, I was unable to determine the key shape. It is also impossible at this time to determine the original ring tightness, the use of mortar radially between the RKB makes this irrelevant. A systematic inspection of 30 rings (groups of 10 rings @ 20' intervals) revealed no small keys. One ring has a key brick dropped down about 2", it was definitely shaped correctly or it would have fallen out. Most of the rings where the lifter attachment bolts protruded through the refractory had small brick pieces on either side of the spacer. This is contrary to good practice as no brick with less than a 4-1/2" chord should be used. I understand the original installation had ceramic fiber packed around the spacers. The ceramic fiber fell out and was replaced with plastic refractory. It is amazing that none of these rings failed before the plastic was installed. Equally amazing none of those rings have failed to date. The brick failure at the discharge end of the kiln appeared to be a crushing failure rather than a thermal failure. I saw no evidence of flame impingement or thermal shock. This failure affected only 3 rows of brick. I believe the failure can be attributed to the following three facts, any one of which could have causes the failure:
I have the following proposals for the refractory repair for the Pryor Plant kiln. I am proposing four possible solutions, please refer to the accompanying drawings. The first is the most expensive but has the highest expected life if chemical attack is not a factor. Solution #1: Replace all of the brick and SS lifters with castable refractory and lifters. * Advantages ** best value ** high expected life ** few cracks for material to migrate into ** oblivious to shell irregularities ** resistant to shell deformation ** lowest installed weight ** failures limited to particular spot * Disadvantages ** high initial cost ** does not take advantage of existing material ** does not have high chemical resistance If chemical resistance is not a major factor, then the best refractory life for your kiln will be obtained by a high quality castable liner. If more chemical resistance is desired, the slightly more expensive "low cement castables" exhibit good chemical resistance for a small additional cost. Drawing 1 shows my preferred refractory schedule based on your service. It uses castable refractory lifters with a castable lining. I would add an aircooled discharge end and use end tiles per sketch 4. Solution #2: Install castable liner in the lifter section, and retain the existing SS lifters. * Advantages ** lower initial cost ** high expected life ** fewer cracks for material to migrate into ** oblivious to shell irregularities in the area where the castable is installed ** takes advantage of existing material ** resistant to shell deformation where the castable installed ** lower installed weight than all brick lining * Disadvantages ** the brick section is sensitive to shell flex ** the brick section is difficult to fit to the existing shell deformations The existing brick liner maybe salvaged, but I expect 2/3 of it will be scrap. The salvaged liner can be used down stream of the 40' lifter section. Solution #3: Install refractory lifters with a brick liner. * Advantages ** high chemical resistance ** takes advantage of existing material, some existing brick maybe reused * Disadvantages ** high initial cost ** many cracks for material to migrate into ** sensitive to shell flex ** difficult to fit to existing shell deformations ** difficult to fit brick lining between lifters ** one brick failing will rapidly lead to the failure of the whole section ** high weight ** sensitive to thermal spalling ** subject to pinch spalling If you must use a brick liner, it would be best to use refractory lifters. Please refer to drawing 3 for installation recommendations if brick must be used. I expect the installed cost of a brick liner with refractory lifters will be at least 20% higher than a comparable castable liner. I cannot recommend brick in the cold end, due to the low operating temperatures. Castable will be much more reliable there, as it does not rely on thermal expansion to keep it in place. Solution #4: Install SS lifters with a brick liner. * Advantages ** high chemical resistance ** takes advantage of existing material, some existing brick liner and stainless steel lifters maybe reused. * Disadvantages ** high initial cost ** many cracks for material to migrate into ** sensitive to shell flex ** difficult to fit to existing shell deformations ** difficult to fit brick lining between lifters ** one brick failing will rapidly lead to the failure of the whole section ** high weight ** sensitive to thermal spalling ** subject to pinch spalling If you must have brick and SS lifters, the existing SS lifter design must be modified. The lifter angle cannot be allowed to contact the liner. Please refer to drawing 5 for my installation recommendations for brick and SS lifters. Note that the spacer pipe is to be solidly welded to the shell, and plastic refractory packed around it. The installer must take care to insure that the bricks are very tightly installed in the affected rings. I estimate that the installed cost of a brick liner with this lifter design will be at least 10% higher than a comparable castable liner. I cannot recommend brick in the cold end, due to the low operating temperatures. Castable will be much more reliable there, as it does not rely on thermal expansion to keep it in place. I am currently collecting information to prequalify the refractory installation contractors. In addition, for quality assurance, I suggest than an independent lab be contracted to prequalify the materials and installation procedures. They should also do random sample material testing during installation and verify that the installation procedures are followed. "Refractory Testing and Inspection, Inc". is a highly recommended local company that performs this service. I expect the above proposals will be modified and the selection narrowed during the meeting scheduled for March 5. At that point the specifications and drawings can be completed, and bid packages prepared. If you have any questions or require further clarifications please call or fax me. |
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