Rotary Kiln – mechanical issues.

Rotary kilns are very big and heavy rotating machines. They are characterized by different weights. As it looks in numbers ? For example (only mobile parts) weights:

  • 3-pier kiln, length 50m, weight 610 t.
  • 4-pier kiln, length 80m, weight 1250 t
  • 7-pier kiln, length 175m, weight 2600 t.

As we can see the numbers shows the immensity of this type of machines. Due to big dimensions the users are exposed for many different consequences. Because every think is big and solid also the supports must be very strong. The rollers, frames and concrete foundation must withstand the load.

Also big mechanical parts cause that the maintenance is difficult. The type of work like: tire or roller replacement, shell replacement or renovation work, girth gear adjustment etc. require a lot of effort, time and cost.

The possibility of mechanical components damage is very high as well. Any part getting between tires and rollers running surface can be destroyed or broken.

Except different mass, each kiln has different dimension. Everything depends on design. Generally kilns diameter vary from 3 to 6m, and length from 40 to 200m. From maintenance point of view very important parameter is shell thickness:

  • 20mm – small shell thickness
  • 40mm – standard shell thickness.

There is also some useful parameter of rotary kiln design – thickness / diameter ratio. For many kiln producers is around 0,6%.

Big dimension also creates many unwanted consequences. When the machine is bis there is always difficult to transport spare parts, move of rotate, perform adjustment job, have an overview, to inspect (visually or using non destructive testing methods – because of the surface).

Another and probably the most important inconvenience from maintenance point of view is temperature. Normal kiln shell temperature during operation vary from 40oC to 420oC.  As we can see – very big variation. High temperature is very dangerous for maintenance staff and can easily burn. This is very difficult to come close to running kiln.

With increasing temperature the mechanical characteristics changes. When the kiln becoming hotter and hotter steel lost the strength:

  • At 400, steel has lost 50% strength
  • At 500, steel has lost 60% strength
  • At6400, elasticity limit is exceeded ant it will never get back to its original dimension even if temperature decrease.

What is important? – The dimensions are depended from temperature. Then temperature is higher the diameter is bigger. The thermal expansion of kiln and kiln component should be always included at any maintenance work or analysis.

For example: the kiln (one of the Cement Plant) with average operation temperature on 250oC because of thermal expansion is longer by 140mm than in cold condition. Measurements of kiln geometry, this is quite important value. Temperature should always be considered on renovation operations (repair or replacement) and on adjustment (especially when procedure is performed in cold condition).

Another important issue linked to temperature parameter is application to tire/shell installation. Let’s check numbers again:

  • The inner diameter of tire is 4000mm and temperature is 150o
  • Shell outer diameter (with chair-pads) is 3990mm and the temperature is 300o

The question is: whether the kiln shell temperature can increase without any consequences or not?

When we recalculate this parameters: the thermal expansion for tire is 7,2mm and for shell is 14,4mm. So as a result we obtain the hot under-tire clearance parameter on 2,8mm level. Tolerance given by most kiln producers is from 3-6mm. So this situation is a warning to kiln operators. Temperature parameters has a very important impact of tire/shell operation. For initial clearance there is always a temperature limit for expansion.

Different thermal expansion of shell and tire can lead to some shrinkage (tire on shell). The reason can be too quick preheating or too small value of clearance in cold condition. The consequences can be very severe. Shell can be deformed permanently, failure of brick lining can appear, problems with component attachment. This kind of situation can also lead to shell crack, or at least to initiate this process in the future.

Next important point with temperature expansion background is application to girth gear and pinion adjustment. For example measured root clearance (at 3-pier kiln, 4,6m diameter with pre-heater) in cold condition will increase by 6mm. This is because of thermal expansion of tires and shell. When overall temperature increase, all dimensions getting bigger and all kiln component move up. Under-tire clearance is getting lower and root clearance of teeth is bigger.

The situation mention above is just an example. Every time analysis of this type should be consulted with technical center because there is many other factors linked with temperature expansion and root clearance value.  One of this factor is angular relation (designed position) between pinion and girth gear. Generally speaking depends on pinion position on the gear circumference.

  • Pinion position closer to kiln axis (more under the kiln) – high influence
  • Pinion position more on one of sides of kiln axis – less influence.