Main rotary kiln inspection starts with the shell. Then it proceeds to each support, sealing and drive system (girth gear, pinion, coupling, gearbox, redactor, lubrication etc.). This basic (visual) inspection should be scheduled daily. All changes from previous control procedure should be recorded and analyzed by experienced diagnostician.
Three main areas of stress that you should focus during kiln shell inspection is: cracks, thermal crank (shell temperature distribution) and other kiln shell problem caused by misalignment.
The most serious sign of shell stress is crack appearance on steel shims area (chair pads). There are two kind of shell cracks:
Longitudinal cracks – running parallel to kiln axis. It is usually a result of concentration of stress caused at a weld attachment. This kind of cracks are generally found in the shell close to under-tire shell segment and steel shims welded to the shell. The stress on that part is very high. When stress is getting higher (due to kiln geometry, axis, rollers skewing, inclination, higher amperage etc.) all weld attachment should be inspected regularly. Another cause of longitudinal cracks could be also excessive flexing of the shell.
Circumferential cracks – generally appear close to circumferential weld connections (shell segments) or shell opening like manhole or repair patches. The reason of that cracks in this case is a poor quality welding or bad procedure of welding work.
Another reason of this kind of shell crack, which is difficult to identify during every day inspection is design errors – kiln shell can be undersized or segments thickness and connection between them is not correct. This design errors causes can be detected by FEM (Finite Element Method) analysis method (provided by HAVEC Engineering).
Very important issue of kiln shell crack is correct mechanical and geometrical alignment. Even small deviations can create high operating stress. As it makes countless revolutions, this stress will eventually cause the crack or weld problems (damage). Also periodic stoppages of rotary kiln is highly stressful for all machinery and equipment – rapid expansion and contraction of whole shell body.
Besides every day inspection which should be helpful to identified the crack reasons a more precise and accurate method of identification should be planned for next kiln stoppage. There are some non-destructive method available, like: ultrasonic, x-ray, dye penetrant etc.
Both Ultrasonic and X-ray methods identify subsurface cracks, which is very important to define the extent of cracks. It gives a full view of existing problems and allows to establish a repair scope.
In turn dye penetrant testing method is a simple method to perform. This procedure can by carry by site technician. It of course has many limitation but it is also a valuable tool in the existing weld repair process.
Each crack on the shell is a very important symptom that should never be ignored.